(i) Oxidation by mild oxidising agents : Mild oxidising agents oxidise only aldehydes into carboxylic acids.
They do not oxidises ketones. Main oxidising agents are:
(a) Fehling solution : It is a mixture of two Fehling solution: Fehling solution No.1 : It contains CuSO4
solution and NaOH.
Fehling solution No.2 : It contains sodium potassium tartrate. (Roschelle salt).
(b) Benedict's solution : This solution contains CuSO4 , Na2CO3 and sodium or potassium citrate.
Reacting species of both solutions is Cu++ oxidation no. of Cu varies from 2 to 1.
These two oxidising agents oxidise only aliphatic aldehydes and have no effect on any other functional
groups:
CHO Cu+ + Redox → CH3COOH ⊕ (as Cu2O ) (red ppt.)
CH3 reaction Cu
− + +
CH 2 = CH − CHO Cu++ → CH 2 = CH − COOH + Cu2O
∆
CH 2OH − (CHOH)4 − CHO Cu++ → CH 2OH − (CHOH)4 − COOH + Cu2O
∆
Benedict's solution and Fehling solutions are used as a reagent for the test of sugar (glucose) in blood sample.
(c) Tollens reagent : Tollens reagent is ammonical silver nitrate solution. Its reacting species is Ag ⊕ .
It oxidises aliphatic as well as aromatic aldehydes.
R − CHO + Ag ⊕ Redox → RCOOH + Ag (as silver mirror)
reaction
C6 H5CHO + Ag ⊕ → C6 H5COOH + Ag
This reagent has no effect on carbon-carbon multiple bond.
CH 2 = CH − CHO + Ag ⊕ → CH 2 = CH − COOH + Ag
C6 H5 − CH = CH − CHO + Ag ⊕ → C6 H5 − CH = CH − COOH + Ag
In this reaction the oxidation no. of Ag varies from +1 to 0.
Note : Glucose, fructose give positive test with Tollen's reagents and Fehling solution.
C5 H11O5CHO + Cu2O (or) Ag 2O → C5 H11O5COOH
Gluconic acid
Fructose contain C = O (keto) group yet give positive test with Fehling solution due to presence
of hydroxyl group. Tollens reagent also gives positive test with terminal alkynes and HCOOH.
Reaction with mercuric chloride solution : R − C − H + HgCl2 + H 2O → R − C − OH + HCl + Hg 2Cl 2 (↓)
(White)
|| ||
O O
R − C − H+ Hg 2Cl2 + H 2O → R − C − OH+ HCl + Hg(↓)
|| (Black)
|| O
O
Schiff's reagent : Megenta dye SO2 → colourless soln CH3CHO → pink colour restored.
(ii) Oxidation by strong oxidising agents : Main strong oxidising agents are
KMnO4 / OH / ∆, KMnO4 / H ⊕ / ∆, K2Cr2O7 / H ⊕ / ∆ and conc HNO3 / ∆ . These agents oxidise aldehydes as
well as ketones.
(a) Oxidation of aldehydes : Aldehydes are oxidised into corresponding acids.
RCHO [O] → RCOOH ; C6 H5CHO KMnO4/ OH /∆ → C6 H5COOH
C=n C=n
(b) Oxidation of ketones : Ketones undergo oxidation only in drastic conditions. During the oxidation of
ketones there is breaking of carbon-carbon bond between α-carbon and carbonyl carbon. In this process both
carbons convert into carboxylic groups. This leads to the formation of two moles of monocarboxylic acids.
Case I : Oxidation of symmetrical ketones
O
||
CH 3 − CH 2 − CH 2 − C CH 2 − CH 2 − CH 3 [O] →
C↓OOH ↓α
C=7 COOH
CH3 − CH 2 − CH 2 − COOH+ CH 3 − CH 2 − COOH
C=4 C=3
Total number of C'S=4+3=7
Thus number of carbons in any product is less than the number of carbons in ketone.
Case II : Oxidation of unsymmetrical ketones : In case of unsymmetrical ketones α-carbon whose bond
breaks always belongs to the alkyl group which has more number of carbons. This rule is known as Poff’s rule.
O
||
CH 3 − CH 3 [O] → CH 3 − CH 2 − COOH + CH 3 − CH 2 − COOH
− CH 2 − CH 2 − C − CH 2
↓↓
COOH COOH
Case III : Oxidation of cyclic ketones : Formation of dibasic acid takes place from cyclic ketones. In this
case number of carbons in ketone and dibasic carboxylic acid is always same.
O
α [O] → COOH − (CH2)4 − COOH
Note : If both α-carbons are not identical then bond breaking takes place between carbonyl carbon and
α-carbon which has maximum number of hydrogens.
2H O α CH3 CH3
1H [O] → COOH − (CH 2 )3 − | − COOH
CH
(iii) Miscellaneous oxidation
(a) Haloform Reaction : In this reaction α-methyl carbonyl compounds undergo oxidation with X 2 / H .
O
||
R − − CH3 (i) X2 / OH → RCOOH + CHX3
C ⊕
(ii) H
O
||
− CH3 (i) X2 / OH → COOH + CHX3
C ⊕
(ii) H
O
||
C6 H5 − CH 3 (i) I2/ Na2CO3 → C6 H5COOH + CHI 3
C− ⊕
(ii) H
(b) Oxidation at α-CH2 or CH3 by SeO2 : SeO2 oxidises α − CH2 − group into keto group and
α − CH3 − group into aldehydic group.
In this oxidation reactivity of CH2 is more than the CH3 group and Oxidation is regio selective in nature.
OO
|| ||
CH 3 − CHO SeO2 → CHO − CHO ; CH 3 − − CH 3 SeO2 → CH 3 − − CHO
Glyoxal C C
Methylglyoxal
O OO
|| || ||
CH 3 − CH 2 − − CH3 SeO2 → CH 3 − − CH 3
C C− C
OO Dimethylglyoxal
SeO2 O
(c) Oxidation by organic peracids : Organic peracids oxidise aldehydes into carboxylic acids and ketones into
esters. This oxidation is known as Baeyer – Villiger oxidation.
OO OO
|| || || ||
R − − H C6H5COOOH → R − − O − H ; R − − R C6H5COOOH → R − − O − H
C C C C
In case of aldehyde there is insertion of atomic oxygen (obtained from peracid) between carbonyl carbon and
hydrogen of carbonyl carbon.
In case of ketone, insertion of oxygen takes place between carbonyl carbon and α-carbon. Thus the product is
ester. This is one of the most important reaction for the conversion of ketones into esters.
Symmetrical ketones : O
O
CF3COOOH O
ε-Lactone
OO
|| ||
CH 3 − − CH 3 C6H5COOOH → CH3 − O − CH 3
C C−
Unsymmetrical ketones : In case of unsymmetrical ketones preference of insertion in decreasing order is as
H > 3°R > 2°R > Ph > 1°R > CH3
OO
|| ||
CH 3 − − C6 H5 CF3COOOH → CH 3 − O − C6 H5
C C−
C| H3 O CH3 O
CH3 − C − || | ||
CF3COOOH → CH 3 C− O − C − CH3
| C − CH3 −
|
CH3 CH3
Note : Vic dicarbonyl compound also undergo oxidation & product is anhydride.
OO
|| ||
C6H5COOOH → R − C− O − C− R
R − C− C− R || ||
OO
Popoff's rule : Oxidation of unsymmetrical ketones largely take place in such a way that the smaller
alkyl group remains attached to the CO group during the formation of two molecules of acids. This is
known as Popoff's rule
Example : CH3 − CO − CH2 − CH3 [O] → CH3 − COOH + HOOCCH3
H
|
(d) Baeyer- villiger oxidation : H − C − H + O − O − C − H → H − C − OH
|| || ||
OOO
H
|
CH 3 C− H O− O C H CH 3 C OH
− + − − → − −
|| || ||
OO O
Note : Reaction will be held if the oxidation agent is performic acid.
(4) Reuction of carObonyl compounds
||
(i) Reduction of – C – group into –CH2 – group : Following three reagents reduce carbonyl group into
− CH2 − groups: (a) HI / P / ∆ (b) Zn / Hg / Conc. HCl and (c) NH2 − NH2 / OH .
HI/P/∆ R – CH2 – R'
O
|| Zn/Hg/Conc. HCl R – CH2 – R' (Clemmensen reduction)
∆
R – C – R'
(ii) Reduction of carbonyl NH2 – NH2 /OH R – CH2 – R' : Carbonyl group converts into
hydroxy compounds
compounds ∆into
− CHOH − group by LiAlH4, NaBH4, Na / C2H5OH and aluminium isopropoxide.
O OH
|| |
R − CHO (i) LiAlH4 → R − CH2OH ; R − R' (i) LiAlH4 → R − R'
(ii) NaBH 4 −C (ii) NaBH 4 − CH
(iii) Aluminium isopropoxide (iii) Aluminium isopropoxide
NaBH4 is regioselective reducing agent because it reduced only. CHO in the presence of other reducible
group.
Example : CH3 − CH = CH − CHO NaBH4 → CH3 − CH = CH − CH2OH
Hydride ion of NaBH4 attack on carbonyl carbon during reduction. OH
OD NaBD4 |
| H2O
Example : CH 3 C CH CH 3 ←NaBD4 2-Butanone CH3 – C – CH2 – CH3
− − 2 − D2O NaBH4 |
| D2O D
D
OD
|
CH3 – C – CH2 – CH3
|
H
(iii) Reductive amination : In this reduction − CO − group converts into − CH − NH 2 group as follows:
R RR
C = O + NH3 → C = NH H2 /Ni → CH − NH 2
R RR
Im ine
O NH2
|| |
CH 3 − CH 2 − CH 3 (i)NH3→ CH 3 − CH 3
−C (ii) H2 / Ni − CH 2 − CH
Primary -amine
(iv) Reduction of ketones by Mg or Mg/Hg : In this case ketones undergo reduction via coupling reaction
and product is vic cis diol.
OO OH OH
|| || | |
+ R (i) Mg / Hg → R − − − R
R−C C− C C
|| || (ii) HOH ||
RR
RR
Vic cis diol
When this reaction is carried out in the presence of Mg / Hg / TiCl4 , the product is vic trans diol.
2 O (i) Hg – Mg – TiCl4 HO
(ii) HOH OH
Vic trans diol
(v) Reduction of benzaldehyde by Na/C2H5OH : Benzaldehyde undergoes reduction via coupling
reaction and product is vic diol.
O O OH OH
|| || ||
C6 H5 C C− C6 H (i) Na/C2H5OH → − C6H5 (Bouveault-blanc reaction)
− + 5 (ii) HOH C6H5 − CH − C H
| | vic diol
H H
Note : Aldehydes are reduced to 1° alcohols whereas ketones to 2° alcohols. If carbon – carbon double
bond is also present in the carbonyl compound, it is also reduced alongwith. However, the use of the
reagent 9-BBN (9–borabicyclo (3, 3, 1) nonane) prevents this and thus only the carbonyl group is
reduced
Example : CH = CH − CHO 9−BBN → HOCH2CH2 NH2 → CH = CHCH2OH
Cinnamaldehyde Cinnamyl alcohol
If reducing agent is NaH, reaction is called Darzen's reaction, we can also use LiAlH4 in this reaction.
If reducing agent is aluminium iso propoxide (CH 3 −CH − O−)3 Al . Product will be alcohol. This
|
CH3
reaction is called Meerwein – pondorff verley reduction (MPV reduction).
The percentage yield of alkanes can be increased by using diethylene glycol in Wolf Kishner
reduction. Then reaction is called Huang – Millan conversion.
(vi) Hydrazones when treated with base like alkoxide give hydrocarbon (Wolf – Kishner reduction).
O N.NH2
|| ||
R − − R' NH2NH2 → RONa → R − CH 2 − R
C R − C − R' ∆
Hydrazone
(vii) Schiff's base on reduction gives secondary amines.
R − CH = O R'NH2 → R − CH = NR' H2 /Ni → R − CH 2 NHR
Schiff's base
(5) Reactions due to α-hydrogen
(i) Acidity of α-hydrogens :
(a) α-hydrogen of carbonyl compounds are acidic in character due to the presence of the electron withdrawing
− CO − group. α-Hydrogen is acidic due to strong –I group; – CO –.
HO
| ||
–C–C–
|
α-Carbon
(b) Thus carbonyl compounds having α-hydrogen convert into carbanions in the presence of base. This
carbanion is stabilised by delocalisation of negative charge.
O O
|| O
|| |
CH3 − C − R Base CH 2 − C − R CH2 = C − R
Carbanion ←→ Enolate ion
(less stable) (more stable)
(c) The acidity of α-hydrogen is more than ethyne. pKa value of aldehydes and ketones are generally 19 – 20
where as pKa value of ethyne is 25.
(d) Compounds having active methylene or methyne group are even more acidic than simple aldehydes and
ketones.
O OO pKa = 8.5
|| || ||
C6 H5 − CH2 − C − CH3 pKa = 15.9 ; C6 H5 − C − CH2 − C − CH3
(ii) Halogenation : Carbonyl compounds having α-hydrogens undergo halogenation reactions. This reaction
is catalysed by acid as well as base.
(a) Acid catalysed halogenation : This gives only monohalo derivative.
OO
|| ||
CH 3 − − CH3 Br2/ CH3COOH → CH3 − CH 2 Br
C C−
(b) Base catalysed halogenation : In the presence of base all α-hydrogens of the same carbon is replaced by
halogens.
O OX
|| || |
CH 3 CH 2 CH 3 CH 3 CH 2 C− C− CH 3
− − C− CH 2 − X2 / OH → − −
(Excess) |
X
X2 / OH
O
||
CH 3 CH C CH CH
− − − − 3
| |
XX
Carbonyl compounds having three α-hydrogens give haloform reaction.
R − O CH 3 R − O CX 3 + CHX 3
|| X2 /OH → || OH→ RCOO
C− C−
(iii) Deuterium exchange reaction : Deuterium exchange reaction is catalysed by acid (D⊕ ) as well as base
(OD) . In both the cases all the hydrogens on only one α-carbon is replaced by D.
O O OO
|| ⊕ ||
R − || CH 2 − R D2O/ OD → R − || CD2 − R; R − CH 2 − R R − CD2 − R
C− D2O/D → C−
C− C−
(iv) Racemisation : Ketones whose α-carbon is chiral undergo Racemisation in the presence of acid as well
as base.
O CH3 O CH3 CH3 O
|| | || | | ||
C6 H5 C2 H 5 H⊕or → C6 H C− C2 H C2 H 5 C − C− C6 H5
− C− C− 5 − C− 5 + −
| |
| H
H OH H
Racemic mixture
(v) Alkylation : Carbonyl compounds having α-hydrogens undergo alkylation reaction with RX in the
presence of base. This reaction is SN2 reaction. The best result is obtained with CH3 − X . Other halides undergo
elimination in the presence of strong base.
O CH 3 O CH 3 O CH3
CH 3 CH 3 || |
|| NaH → CH || CH3I → CH 3 C C CH
3 − − − 3
CH3 − C− CH (Small base) − C− C |
LDA CH3
(Bulky base) (Main product)
CH 3 O CH 3
CH 3
CH 2 − C − CH CH3I → CH3CH 2 − || CH
|| C−
O CH 3 (Main product)
(vi) Wittig reaction : Aldehyde and ketones undergo the wittig reaction to form alkenes.
Ph3 P = CHR1 + CHR 2 → Ph3 P ⊕ − CHR1 → Ph3 P− CHR1 → Ph3 P+ CHR1
|
|| O −CHR2 | | || ||
O − CHR2
O O CHR2
(6) Condensation reaction of carbonyl compounds : Nucleophilic addition reaction of compounds
having carbonyl group with those compounds which have at least one acidic hydrogen at α-carbon is known as
condensation reaction. In this addition reaction :
Substrate is always an organic compound having a carbonyl group, e.g.
O O OO
|| || || ||
H − C − H, C6 H5 − C − H, R − C − H, R − C − R etc.
Addition always takes place on the carbonyl group.
Reagents of the condensation reaction are also organic compounds having at least one hydrogen on α-carbon
and α-carbon should have –I group, e.g.
CHα 3 − NO2, CH 3 α CHO, α
CH
− − CH3 − CH2 − CN
|
CH3
Note : If substrate and reagent both are carbonyl compounds then one should have at least one α-
hydrogen and other may or may not have α-hydrogen.
Condensation reaction always takes place in the presence of acid or base as catalyst. Best result is obtained
with base at lower temp.
O OH
|| |
R − − R + CH 3 − Z H⊕or → R− − CH2 − Z
C C
|
OH R
Condensation is carried out at lower temperature (≤ 20°C) because product of the reaction is alcohol which
has strong –I group at β-carbon.
OH
|
R −C− CH 2 − Z
α| β
R
Such type of alcohols are highly reactive for dehydration. They undergo dehydration in the presence of acid
as well as base even at 25°C. They also undergo elimination even on strong heating.
OH R
| HO/ ∆ →
R −C− CH 2 − Z RDehydration C = CH − Z
α| β
R
(i) Aldol condensation
(a) This reaction takes place between two molecules of carbonyl compounds; one molecule should have at
least two α-hydrogen atoms. In this reaction best result is obtained when
Both molecule are the same or
One should have no α-hydrogen atom and other should have at least two α-hydrogens.
(b) These reactions are practical when base is NaOH and reaction temperature is high (≥ 100°) .
(c) The reaction is two step reaction. First step is aldol formation and second step is dehydration of aldol.
CH 3 − CHO + CH3 − CHO NaOH∆ /OH →CH3 OH Dehydration → CH3 − CH = CH − CHO
− CHO
| a, β −unsaturated aldehyde
− CH − CH2
Due to hyper conjugation in crotonaldehyde further condensed give conjugated alkene carbonyl compound.
CH3 – CH = CH – CHO + CH3 – CH = CH – CHO
NaOH
OH
|
CH3 – CH = CH – CH – CH2 – CH = CH – CHO
∆ –H2O
CH3 – CH = CH – CH = CH – CH = CH – CHO
CH3 – (CH = CH –)3 – CHO
Condensed compound
The net result can be written as follows]
CH3 – CHO + H2CH – CHO
– O – H2
OH/∆
CH3 – CH = CH – CHO
Crotonaldehyde
C6H5CHO + CH3 – CHO C6H5 – CH = CH – CHO
OH/∆
– H2O Cinnamaldehyde
OO
||
OH/∆ ||
C6H5 – CHO + H2CH – C – CH3
C6H5 – CH = CH – C – CH3
Note : Benzalacetophenone
If product is given then reactants can be known as follows :
Suppose structure of product is C6 H5 β = CαH − CHO
− CH
Break carbon-carbon double bond between α and β carbons and attach two hydrogens on α-carbon
β CHαH2 − CHO → C6 H5 − CHO + CH 3 − CHO .
and an oxygen on β-carbon, i.e. C6 H5 − COH
Mechanism : C6 H5 − CHO + CH 3 − CHO OH/ ∆ → C6 H5 − CH = CH − CHO + HOH
O
Step I : HO + H − CH 2 − CHO CH || O
HOH C− CH |
H C− H
+ 2 − ←→ 2 =
O O OH
|| | |
Step II : C6 H5 C CH 2 CHO C6 H5 C CH − CHO HOH → C6 H5 C− CH − CHO
− + − − − 2 − 2 + OH
| | |
H HH
OH
|
Step III : C6 H5 CH CH CHO C6 H5 − CH = CH − CHO + HOH
− − − →
|
H
OH
In aldol condensation, dehydration occurs readily because the double bond that forms is conjugated, both
with the carbonyl group and with the benzene ring. The conjugation system is thereby extended.
Crossed aldol condensation : Aldol condensation between two different aldehydes or two different ketones
or one aldehyde and another ketone provided al teast one of the components have α-hydrogen atom gives different
possible product
OH CH3
||
(a) CH3CHO+ CH3 − CH 2 − CHO dil NaOH → CH 3 − CHO + CH3 − CH 2 − CHOH − CH2 − CHO
− CH − CH
Ethanal Propanal
However crossed aldol condensation is important when only it the components has α-hydrogen atom.
CH2O + CH3CHO → CH2 − CH2 − CHO ∆ → CH 2 = CH − CHO
| −H2O
(Acrolein)
OH
(3-hydroxy propanal)
Intra molecular aldol condensation : One molecule Intramolecular condensed give aldol compounds
Example : O = CH − (CH2)5 − CHO NaOH → OH
CHO
(ii) Claisen – Schmidt reaction : Crossed aldol condensation between aromatic aldehyde and aliphatic
ketone or mixed ketone is known as Claisen – Schmidt reaction. Claisen – Schmidt reactions are useful when bases
such as sodium hydroxide are used because under there conditions ketones do not undergo self condensation.
Some examples of this reaction are :
O O
|| OH ||
C6 H5CHO CH 3 C − CH3 C6 H5 − CH = CH − C − CH
+ − 100°C → 3
4 −Phenyl − 3− buten- 2-one
C6 H5 − CHO + CH3 − O − C6 H5 − CH = CH − O − C6 H5
|| OH/∆ → C6 H5 ||
C C
1,3−Diphenyl − 2− propene-1-one
CHO O O
OH
|| ||
CH = CH – C – CH3
+ CH3 – C – CH3
Geranial Pesudoionone
Test of aldehydes and Ketones (Distinction).
Test Aldehydes Ketones
1. With Schiff's reagent Give pink colour. No colour.
2. With Fehling's solution Give red precipitate. No precipitate is formed.
3. With Tollen's reagent Black precipitate of silver mirror No black precipitate or silver
is formed. mirror is formed.
4. With saturated sodium bisulphite Crystalline compound (colourless) Crystalline compound (colourless)
solution in water is formed. is formed.
5. With 2 : 4-dinitrophenyl hydrazine Orange-yellow or red well defined Orange-yellow or red well defined
crystals with melting points crystals with melting points
characteristic of individual characteristic of individual
aldehydes. ketones.
6. With sodium hydroxide Give brown resinous mass No reaction.
(formaldehyde does not give this
test).
7. With sodium nitroprusside and A deep red colour (formaldehyde Red colour which changes to
few drops of sodium hydroxide does not respond to this test). orange.
Some commercially important aliphatic carbonyl compounds.
Formaldehyde : Formaldehyde is the first member of the aldehyde series. It is present in green leaves of
plants where its presence is supposed to be due to the reaction of CO2 with water in presence of sunlight and
chlorophyll.
CO2 + H 2O → HCHO + O2
Traces of formaldehyde are formed when incomplete combustion of wood, sugar, coal, etc., occurs.
(1) Preparation
(i) By oxidation of methyl alcohol 2CH 3OH + O2 Platinised asbestos → 2HCHO + 2H 2O
300−400°C
CH 3OH + [O] K2Cr2O7 → HCHO + H2O
H 2 SO4
(ii) By dehydrogenation of methyl alcohol CH 3OH Cu or Ag → HCHO + H2
300−400°C
(iii) By heating calcium formate : Ca(HCOO)2 Heat → CaCO3 + HCHO
Calcium formate
Formaldehyde
O
(iv) By ozonolysis of ethylene : CH 2 = CH 2 + O3 → H2C CH2 H2 → 2HCHO + H 2O
Pd
O Ozonide O
(v) Manufacture : CH4 + O2 Mo-oxide → HCHO + H2O
Catalyst
Methane Formaldehyde
It is also prepared by passing water gas at low pressure through an electric discharge of low intensity.
CO + H 2 Elec. discharge → HCHO
(2) Physical properties
(i) It is a colourless, pungent smelling gas.
(ii) It is extremely soluble in water. Its solubility in water may be due to hydrogen bonding between water
molecules and its hydrate.
(iii) It can easily be condensed into liquid. The liquid formaldehyde boils at – 21°C.
(iv) It causes irritation to skin, eyes, nose and throat.
(v) Its solution acts as antiseptic and disinfectant.
(3) Chemical properties : Formaldehyde is structurally different from other aldehydes as it contains no alkyl
group in the molecule. Though it shows general properties of aldehydes, it differs in
N
certain respects. The abnormal properties of formaldehyde are given below
(i) Reaction with ammonia : Like other aldehydes, formaldehyde does not H2C CH2 CH2
form additon product but a crystalline compound, hexamethylene tetramine, with
ammonia. N
6HCHO + 4 NH 3 → (CH 2 )6 N 4 + 6H2O CH2 CH2
Urotropine N
Formaldehyde N
(Hexamethylene tetramine)
C
H2
Urotropine
Hexamethylene tetramine has a cyclic structure. It is used as medicine in case of urinary troubles under the
name of Urotropine or hexamine.
(ii) Reaction with sodium hydroxide (Cannizzaro's reaction) : It does not form resin with sodium
hydroxide like acetaldehyde but when treated with a concentrated solution of sodium hydroxide, two molecules of
formaldehyde undergo mutual oxidation and reduction forming formic acid salt and methyl alcohol
(Disproportionation).
2HCHO + NaOH → HCOONa + CH 3OH
Sod. Formate
Formaldehyde Methyl alcohol
This transformation is known as Cannizzaro's reaction.
Tischenko's reaction : This is a modified form of cannizzaro's reaction. All aldehydes undergo cannizzaro's
reaction in presence of aluminium ethoxide. The acid and alcohol formed react together to give the ester.
[ ] [ ]2CH 3CHO (C2H5O)3 Al → CH 3COOH + C2 H5OH → CH 3COOC2 H5
Ethyl acetate
CH 3 CH − CHO Al +Butoxide → CH 3 CH − CH 2OH + HOOC − CH CH 3
CH 3 CH 3 CH 3
CH 3 ↓ CH 3
CH 3 CH − CH 2OOC − CH CH 3
(iii) Aldol condensation : Formaldehyde in presence of a weak base undergo repeated aldol condensation
to give formose (α- acrose).
6HCHO Ca(OH)2 → C6 H12O6
Formaldehyde Formose (hexose)
(iv) Condensation with phenol : Formaldehyde condenses with phenol to give a synthetic plastic, bakelite.
The condensation occurs in presence of dilute sodium hydroxide or ammonia at 80 – 90°C. Bakelite is used for
preparing electrical insulators, electric switches, toys, etc.
OH OH
OH O CH2 CH2
|| Base OH
H–C–H dil. K2CO3 OH
+ CH2 CH2
Formaldehyde
Phenol
CH2 CH2 OH
OH
Bakelite
Bakelite is electrical and thermal resistant so it is used in formation of electrical appliances. This reaction is
called Lederer- Manasse reaction.
(v) Condensation with urea : Formaldehyde also condenses with urea in acidic solution to form a plastic
like product.
mH 2 NCONH 2 + nCH 2O → − CH 2 N − CO − N |
CH2
||
CH 2 − N − CO − N −
Urea Formaldehyde − CH 2 CH 2 − N − CO − N−
| |
CH2
|
Formaldehyde- urea plastic
(vi) Reaction with alcohol : Formaldehyde reacts with methyl alcohol in presence of dry hydrogen chloride
or fused calcium chloride forming methylal which is used as soporific.
H OCH3 OCH 3
H2C = O H2C H2O
+ + → OCH 3 +
Formaldehyde H OCH3
Methylal
Methyl alcohol
(Dimethoxy methane)
(vii) Polymerisation : Formaldehyde readily undergoes polymerisation.
(a) Paraformaldehyde : When an aqueous solution of formaldehyde is evaporated to dryness, a white
crystalline solid with fishy odour is obtained. It is a long chain polymer.
nHCHO (CH2O)n n = 6 to 50
Formaldehyde Para-formaldehyde
On rapid heating it gives back gaseous formaldehyde.
When a formaldehyde solution is treated with con. H 2SO4 , a white solid, polyoxy methylenes (CH 2O)n.H 2O
are formed.
nHCHO Conc. H2SO4 (CH 2O)n .H 2O ; n > 100
heat Polyoxy methylene
This on heating gives back formaldehyde.
(b) Metaformaldehyde : On allowing formaldehyde gas to stand at room temperature, it slowly polymerises to
metaform, (HCHO)3 . It is a white solid (m.pt. 61 – 62°C). This on heating gives back gaseous formaldehyde.
3HCHO (HCHO)3 or CH 2 O − CH 2 O
Formaldehyde Meta- formaldehyde or trioxane O − CH 2
Trioxy methylene (trioxan)
(viii) Reaction with grignard reagent : Formaldehyde forms primary alcohols with Grignard reagent.
R OH
| I
H − C = O + RMgI Ether → H HOH → RCH 2OH + Mg
− C− OMgI Primary alcohol
| |
H
H
Formaldehyde does not react with chlorine and phosphorus pentachloride. It does not give iodoform test.
(4) Uses
(i) The 40% solution of formaldehyde (formalin) is used as disinfectant, germicide and antiseptic. It is used for
the preservation of biological specimens.
(ii) It is used in the preparation of hexamethylene tetramine (urotropine) which is used as an antiseptic and
germicide.
(iii) It is used in silvering of mirror.
(iv) It is employed in manufacture of synthetic dyes such as para-rosaniline, indigo, etc.
(v) It is used in the manufacture of formamint (by mixing formaldehyde with lactose) – a throat lozenges.
(vi) It is used for making synthetic plastics like bakelite, urea-formaldehyde resin, etc.
(vii) Rongalite – a product obtained by reducing formaldehyde sodium bisulphite derivative with zinc dust and
ammonia and is used as a reducing agent in vat dyeing.
(viii) As a methylating agent for primary and secondary amines, e.g.,
C2 H5 NH 2 + 2HCHO → C2 H5 NH − CH 3 + HCOOH
Ethylamine Ethyl methylamine
(ix) If aqeous solution of formaldehyde is kept with lime water in dark room for 5 – 6 days then it converts into
a sweet solution called formose or α-acrose. It is an example of linear polymer.
6HCHO Ca(OH)2 / Ba(OH)2 → C6 H12O6
Dark 5-6 days
Formose / α -acrose
Acetaldehyde
Acetaldehyde is the second member of the aldehyde series. It occurs in certain fruits. It was first prepared by
Scheele in 1774 by oxidation of ethyl alcohol.
(1) Preparation : It may be prepared by any of the general methods. The summary of the methods is given
below
(i) By oxidation of ethyl alcohol with acidified potassium dichromate or with air in presence of a catalyst like
silver at 300°C.
(ii) By dehydrogenation of ethyl alcohol. The vapours of ethyl alcohol are passed over copper at 300°C.
(iii) By heating the mixture of calcium acetate and calcium formate.
(iv) By heating ethylidene chloride with caustic soda or caustic potash solution.
(v) By the reduction of acetyl chloride with hydrogen in presence of a catalyst palladium suspended in barium
sulphate (Rosenmund's reaction).
(vi) By the reduction of CH3CN with stannous chloride and HCl in ether and hydrolysis (Stephen's method).
(vii) By hydration of acetylene with dil. H2SO4 and HgSO4 at 60°C.
(viii) By ozonolysis of butene-2 and subsequent breaking of ozonide.
(ix) Laboratory preparation : Acetaldehyde is prepared in the laboratory by oxidation of ethyl alcohol with
acidified potassium dichromate or acidified sodium dichromate.
K2Cr2O7 + 4 H2SO4 → K2SO4 + Cr2(SO4 )3 + 4 H2O + 3[O]
[CH3CH2OH + O → CH3CHO + H2O] × 3
K2Cr2O7 + 3CH3CH2OH+ 4H2SO4 → K2SO4 + Cr2(SO4 )3 + 3CH3CHO+ 7H2O
Potassium Ethyl alcohol Sulphuric acid Potassium Chromic Acetaldehyde Water
dichromate sulphate sulphate
To recover acetaldehyde, the distillate is treated with dry ammonia when crystallised product, acetaldehyde
ammonia, is formed. It is filtered and washed with dry ether. The dried crystals are then distilled with dilute
sulphuric acid when pure acetaldehyde is collected.
OH
|
CH 3 CHO + NH 3 → CH3 − − NH 2 H2SO4 → CH3CHO + (NH4 )2 SO4
CH
Acetaldehyde ammonia Acetaldehyde
(x) Manufacture : Acetaldehyde can be manufactured by one of the following methods:
(a) By air oxidation of ethyl alcohol : Ethyl alcohol vapours and limited amount of air are passed over heated
silver catalyst at 300°C.
2CH 3 CH 2OH + O2 Ag → 2CH 3CHO + 2H2O
300°C
(b) By dehydrogenation of alcohol : Vapours of ethyl alcohol are passed over heated copper at 300°C.
CH 3 CH 2OH Cu → CH 3 CHO
300°C
(c) By hydration of acetylene : Acetylene is passed through water containing 40% sulphuric acid and 1%
mercuric sulphate at 60°C when acetaldehyde is formed.
CH ≡ CH + H2O HgSO4,(1%),60°C → CH 3 CHO
H2SO4 (40%)
(d) From ethylene (Wacker process) : Ethylene is passed through an acidified aqueous solution of palladium
chloride and cupric chloride, when acetaldehyde is formed.
CH 2 = CH 2 + PdCl2 + H2O CuCl2 → CH 3 CHO + Pd + 2HCl
H+
Pd + 2CuCl2 → PdCl2 + 2CuCl
2CuCl + 2HCl + 1 O2 → 2CuCl2 + H2O
2
CH2 = CH2 + 1 O2 → CH 3 CHO
Ethylene 2
Acetaldehyde
(So H2C = CH2 + O2 PdCl2,CuCl2 → H 3C − CHO )
H2O
(2) Physical properties
(i) Acetaldehyde is a colourless volatile liquid. It boils at 21°C.
(ii) It has a characteristic pungent smell.
(iii) It is soluble in water, chloroform, ethyl alcohol and ether. Its aqueous solution has a pleasant odour. In
water, it is hydrated to a considerable extent to form ethylidene glycol.
CH3CHO + H2O → CH3CH(OH)2
(3) Chemical properties : It gives all characteristic reactions of aldehydes. Besides general reactions,
acetaldehyde shows the following reactions also.
(i) Haloform reaction : It responds to iodoform reaction due to the presence of CH3CO group.
(ii) Tischenko's reaction : It forms ethyl acetate in presence of aluminium ethoxide.
2CH3CHO (C2H5O)3Al → CH3COOC2H5
Ethyl acetate
(iii) Chlorination : Hydrogen atoms of the methyl group are substituted by chlorine atoms when
acetaldehyde is treated with chlorine.
CH3CHO + 3Cl2 → CCl3CHO+ 3HCl
Chloral
(iv) Polymerisation : Acetaldehyde undergoes polymerisation forming different products under different
conditions.
(a) Paraldehyde : It is formed, when anhydrous acetaldehyde is treated with conc. sulphuric acid.
CH
3CH 3CHO (CH 3CHO)3 3 O
Acetaldehyde
Paraldehyde, (trimer) O
CH HC CH
O
It is a pleasant smelling liquid (b.pt. 124°C). It has cyclic structure and when heated with dilute sulphuric acid it
changes again into acetaldehyde. It is used as a hypnotic and soporific (sleep producing).
H H H
|
|
Reaction with NH3 : CH3 − C = O + N H CH3 − C = NH
H − −H2O → Acetaldimine
CH3 NH CH3
CH–CH3+3H2O
| |
CH CH
NH HN NH
. 3H2O
||
HC CH
CH3 – CH
NH
CH3 HN CH3
Trimethyl hexa hydro triazine [Trihydrate]
(b) Metaldehyde : Acetaldehyde on treatment with hydrogen chloride or sulphur dioxide is converted into
metaldehyde (CH3CHO)4 . It is a white solid (m. pt. 246°C). On heating it sublimes but changes again into
acetaldehyde when distilled with dilute sulphuric acid. It is used as a solid fuel.
CH3 − CH − O − CH − CH3
||
4CH3CHO → O O
Acetaldehyde ||
CH3 − CH − O − CH − CH3
Metaldehyde (textramer)
It is used for killing slugs and snails.
(4) Uses : Acetaldehyde is used :
(i) In the preparation of acetic acid, acetic anhydride, ethyl acetate, chloral, 1,3-butadiene (used in rubbers),
dyes and drugs.
(ii) As an antiseptic inhalent in nose troubles.
(iii) In the preparation of paraldehyde (hypnotic and sporofic) and metaldehyde (solid fuel).
(iv) In the preparation of acetaldehyde ammonia (a rubber accelerator).
Comparative study of formaldehyde and acetaldehyde
S.No. Reaction Formaldehyde HCHO Acetaldehyde CH3CHO
1. Forms methyl alcohol Forms ethyl alcohol
Similarty HCHO + H2 → CH3OH
2. Addition of hydrogen CH3CHO + H2 → CH3CH2OH
(a) H2 in presence of catalyst, Ni, Forms methane Forms ethyl alcohol
Pd or Pt HCHO + 4H → CH4 + H2O Forms ethane
(b) LiAlH4 (ether)
Forms bisulphite addition product CH3CHO + 4H → C2H6 + H2O
(c) Amalgamated zinc + conc. HCl HCHO + NaHSO3 → CH2(OH)SO3Na
(Clemmensen reduction) Forms bisulphite addition
Addition of NaHSO3 solution product
CH3CHO + NaHSO3 →
3. Addition of HCN Forms formaldehyde cyanohydrin CH3CH(OH)SO3 Na
HCHO + HCN → CH2(OH)CN Forms acetaldehyde cyanohydrin
CH3CHO + HCN →
CH3CH(OH)CN
4. Addition of Grignard reagent Forms ethyl alcohol Forms isopropyl alcohol
followed by hydrolysis HCHO + CH3MgI → CH2 OMgI CH3CHO + CH3MgI →
CH3
H2O → CH 3CH 2OH CH3 −C HOMgI H2O →
|
− Mg(OH)I CH3 − Mg(OH)I
CH3 −CH − OH
|
CH3
5. With hydroxylamine NH 2OH Forms formaldoxime Forms acetaldoxime
CH 2 = O + H 2 NOH −H2O →
CH 3CH = O + H 2 NOH −H2O →
6. With hydrazine (NH 2 NH 2 ) CH 2 = NOH CH 3CH = NOH
Forms formaldehyde hydrazone Forms acetaldehyde hydrazone
CH 2O + H 2 N NH 2 −H2O → CH 3CH = O + H 2 NNH 2 −H2O →
CH 2 = NNH 2 CH3CH = NNH2
7. With phenyl hydrazine Forms formaldehyde phenyl Forms acetaldehyde phenyl
hydrazone hydrazone
(C6 H5 NHNH 2 ) CH 3CH = O + H 2 NNHC6 H5
CH 2 = O + H 2 NNHC6 H5 −H2O →
−H2O → CH 3CH = NNHC6 H5
CH 2 = NNHC6 H5
8. With semicarbazide Forms formaldehyde semicarbazone Forms acetaldehyde
semicarbazone
(H 2 NNHCONH 2 ) CH 2 = O + H 2 NNHCONH 2 −H2O → CH 3CH = O + H 2 NNHCONH 2
CH 2 = NNHCONH 2 −H2O → CH 3CH = NNHCONH 2
9. With alcohol (C2H5OH) in Forms ethylal Forms acetaldehyde diethyl acetal
presence of acid H 2C = O + 2C2 H5OH HCl → CH 3CHO + 2C2 H5OH HCl →
CH2 OC2H5 CH3CH OC2H5
OC2H5 OC2H5
10. With thioalcohols (C2 H5 SH) in Forms thio ethylal Forms acetaldehyde diethyl
thioacetal
presence of acid H 2C = O + 2C2 H5 SH →
CH 3CH = O + 2C2 H5 SH →
CH2 SC2H5 CH3CH SC2H5
SC2H5 SC2H5
11. Oxidation with acidified K2Cr2O7 Forms formic acid Forms acetic acid
HCHO + O → HCOOH CH3CHO + O → CH3COOH
12. With Schiff's reagent Restores pink colour of Schiff's Restores pink colour of Schiff's
reagent reagent
13. With Tollen's reagent Gives black precipitate of Ag or Gives black precipitate of Ag or
silver mirror silver mirror
Ag2O + HCHO → 2Ag + HCOOH Ag2O + CH3CHO →
2Ag + CH3COOH
14. With Fehling's solution or Gives red precipitate of cuprous Gives red precipitate of cuprous
Benedict's solution oxide oxide
2CuO + HCHO → Cu2O + HCOOH 2CuO + CH3CHO →
15. Polymerisation Undergoes polymerisation Cu2O + CH3COOH
Difference Evaporation Undergoes polymerisation
nHCHO 3CH3CHO H2SO4Conc.
16. With PCl5
Pa(rHafoCrmHaldOeh)nydReoom temp. dil. H2SO4. distill
3HCHO heat (CH3CHO)3
Paraldehyde
(HCHO)3
4CH3CHO
Metaformaldehyde (CH 3 CHO)4
Metaldehyde
No reaction Forms ethylidene chloride
CH 3CHO + PCl5 → CH 3CH
Cl
Cl
17. With chlorine No reaction + POCl 3
Forms chloral
CH3CHO + 3Cl2 → CCl3CHO
+3HCl
18. With SeO2 No reaction Forms glyoxal
CH3CHO + SeO2 → CHO.CHO
19. Iodoform reaction (I2+NaOH) No reaction +Se + H2O
Forms iodoform
20. With dil. alkali (Aldol No reaction CH3CHO + 3I2 + 4 NaOH →
condensation) CHl3 + HCOONa + 3NaI + 3H2O
Forms aldol
CH3CHO + HCH2CHO →
21. With conc. NaOH (Cannizzaro's Forms sodium formate and methyl CH 3 CH(OH)CH 2CHO
reaction) alcohol Forms a brown resinous mass
2HCHO + NaOH → HCOONa
22. With ammonia
+CH 3 OH tetramine Forms addition product,
23. With phenol Forms hexamethylene acetaldehyde ammonia
24. With urea (urotropine)
25. Condensation in presence of
6HCHO + 4 NH 3 →(CH 2 )6 N 4 + 6H 2O CH3CHO + NH3 →
Ca(OH)2
CH 3 CH OH
NH 2
Forms bakelite plastic No reaction
Forms urea-formaldehyde plastic
Form formose (a mixuture of sugars) No reaction
No reaction
Inter conversion of formaldehyde and acetaldehyde
(1) Ascent of series : Conversion of formaldehyde into acetaldehyde
(i) HCHO H2 /Ni → CH 3OH PCl5 → CH 3Cl Alc. → CH 3CN Na / Alcohol → CH 3CH 2 NH 2 NaNO2 →
Methyl Methyl KCN Methyl Ethyl amine HCl
Formaldehyde
alcohol chloride cyanide
CH3CH2OH H2SO4(dil.) → CH3CHO
Ethyl alcohol K 2Cr2O7
Acetaldehyde
(ii) HCHO CH3MgI → CH 3CH 2OMgI H3+O → CH 3CH 2OH Cu → CH 3CHO
Ether Ethyl alcohol 300°C
Formaldehyde Acetaldehyde
(iii) HCHO K2Cr2O7 → HCOOH Ca(OH)2 →(HCOO)2 Ca (CH3COO)2 Ca → CH 3CHO
H 2 SO4 Calcium formate heat
Formaldehyde Formic acid Acetaldehyde
(2) Descent of series : Conversion of acetaldehyde into formaldehyde
(i) CH 3CHO K2Cr2O7 → CH 3COOH NH3 → CH 3COONH4 Heat → CH 3CONH 2 Br2/ KOH →
H 2 SO4 Acetic acid Amm. acetate Acetamide
Acetaldehyde
CH 3 NH 2 NaNO2 → CH 3 OH Cu → HCHO
HCl 300°C Formaldehyde
Methyl amine
(ii) CH3CHO K2Cr2O7 → CH3COOH NaOH → CH3COONa Sodalime → CH4 Cl2 → CH3Cl AgOH →
H 2SO4 Acetic acid Sod.acetate heat hv
Acetaldehyde Methane
CH3OH Cu → HCHO
300°C
Formaldehyde
Acetone
It is a symmetrical (simple) ketone and is the first member of the homologous series of ketones. In traces, it is
present in blood and urine.
(1) Laboratory preparation : Acetone is prepared in laboratory by heating anhydrous calcium acetate.
(CH3COO)2 Ca → CaCO3 + CH3COCH3
Calcium acetate Acetone
The retort is heated slowly when acetone distills over and collected in the receiver.
The distillate is shaken with saturated solution of sodium bisulphite when colourless crystals are formed. These
are filtered and distilled with saturated solution of sodium carbonate. The aqueous solution of acetone is dried over
anhydrous calcium chloride and redistilled to obtain pure acetone. The fraction is collected between 55 to 57o C
(b.pt. pure acetone 56o C ).
CH 3 CH 3 OH
C = O + NaHSO3 → C
CH 3 CH 3 SO3 Na
Acetone Acetone sodium bisulphite
CH 3 OH CH 3
CH 3
C + Na2CO3 → C = O + NaHCO3 + Na2SO3
SO3 Na CH 3
Acetone
Aldehydes & Ketones
(2) Manufacture : Acetone is manufactured by following methods:
(i) By air-oxidation of isopropyl alcohol : The air oxidation occurs at 500o C .
2CH 3CHOHCH3 + O2 500oC → 2CH 3COCH 3 + 2H 2O
Isopropyl alcohol Acetone
(ii) By dehydrogenation of isopropyl alcohol : The vapours of isopropyl alcohol are passed over heated
copper at 300o C .
CH 3CHOHCH 3 Cu → CH 3 COCH 3 + H2
300o C
(iii) From propene
(a) Wacker's process : The mixture of propene and air under pressure is passed through palladium chloride and
cupric chloride solution when acetone is formed.
CH 3CH = CH 2 + PdCl2 + H 2O → CH 3COCH3 + Pd + 2HCl
Pd + 2CuCl2 → PdCl2 + 2CuCl
4CuCl + 4HCl + O2 → 4CuCl2 + 2H 2O
(b) Propene is absorbed in concentrated sulphuric acid and the resulting product is boiled with water when
isopropyl alcohol is formed. Isopropyl alcohol on dehydrogenation yields acetone.
CH 3CH = CH 2 + H 2SO4 → CH 3CH(HSO4 )CH 3 H2O → CH 3CH(OH)CH 3 Cu → CH 3 COCH 3
Isopropyl alcohol 300o C
Acetone
(iv) From ethyl alcohol : By passing a mixture of ethyl alcohol vapour and steam over a catalyst, zinc
chromite at 500o C , acetone is obtained. The yield is about 80%.
2C2H5OH + H2O Zn(CrO2)2 → CH3COCH3 + CO2 + 4 H2
(v) From acetylene : By passing a mixture of acetylene and steam over a catalyst, magnesium or zinc
vanadate at 420o C , acetone is obtained.
2CH ≡ CH + 3H2O → CH3COCH3 + CO2 + 2H2
(vi) From pyroligneous acid : Pyroligneous acid containing acetic acid, acetone and methyl alcohol is
distilled in copper vessel and the vapours are passed through hot milk of lime. Acetic acid combines to form
nonvolatile calcium acetate. The unabsorbed vapours of methanol and acetone are condensed and fractionally
distilled. Acetone distills at 56o C .
The acetone thus obtained is purified with the help of sodium bisulphite as described in laboratory preparation.
(3) Physical properties : (i) It is a colourless liquid with characteristic pleasant odour.
(ii) It is inflammable liquid. It boils at 56o C .
(iii) It is highly miscible with water, alcohol and ether.
(4) Chemical properties
ReducotrioLniAHl2H, 4Ni¸PdCH3CHOHCH3
Amalgamated Zn Isopropyl alcohol
CH 3CH 2CH 3
+ conc. HCl
Propane
NaHSO3 (CH3 )2 C(OH)SO3 Na
HCN
CH3MgI Acetone sodium bisulphite derivative
Ether (CH 3 )2 C OH CaOCl2
CN (Bleaching powder)
heat CHCl3
Acetone cyanohydrin
Chloroform
(CH3)3 COH CH3COCH3 K2Cr2O7 + H2SO4 CH3COOH + CO2 + H 2O
(Acetone)
Tertiary butyl alcohol
NH2OH (CH3)2 C = NOH CHCl3 (CH3 )2 C(OH)CCl3
Acetoxime Chloretone
CH3COCH3 NH2NH2 (CH3 )2 C = NNH2 Ba(OH)2 (CH3 )2 C(OH)CH 2COCH3
(Acetone) C6H5NHNH2
H2NNHCONH2 Acetone hydrazone HNO2 Diacetone alcohol
NH3
PCl5 (CH3)2 C = NNHC6 H5 Mg–Hg + H2O CH3COCH = NOH
Cl2 Schiff's reagent (Oximino acetone)
Acetone phenyl hydrazone Tollen's reagent
I2 +NaOH (CH3 )2 C(NH 2 )CH 2COCH3
(CH3)2 C = NNHCONH2
Diacetone amine
Acetone semicarbazone
OH OH
(CH3)2 C Cl
Cl ||
Isopropylidene chloride (CH3 )2 C — C (CH3 )2
CH 3 COCCl 3 Pinacol
Trichloro acetone No reaction
CHI 3 No reaction
Iodoform Fehling's solution No reaction
CH3
Conc. H2SO4 C
CH CH
C C CH3
CH3 CH
Mesitylene
(1, 3, 5-trimethyl benzene
If acetone would be in excess in ketal condensation or catalyst (ZnCl2 / dry HCl) is used then three moles of
acetone undergoes condensation polymerisation and form a compound called ‘Phorone’.
CH CH C=O
3 3
| |
CH 3 − C = O H CH 3 − C = CH
CH
H C=O ZnCl2 → CH 3 − C = CH
CH dry. HCl |
CH3
3
|
CH 3 − C = O H CH
H
Molecular mass of phorone = 3 mole of acetone – 2 mole of H 2O
Note : If two moles of acetone are used then Mesityl oxide (CH3 )2 C(OH)CH 2COCH3
Reformatsky reaction: This reaction involves the treatment of aldehyde and ketone with a bromo acid ester
in presence of metallic zinc to form β -hydroxy ester, which can be easily dehydrated into α, β -unsaturated ester.
(a) BrCH 2COOC2 H 5 + Zn Benzene → Br − Z⊕n− CH 2COOC2 H 5
Organo zinc compound
(b) Addition to carbonyl group
CH3 Zn+Br CH3 CH3
CH3 | | |
C O → CH3 C − CH2CH2COOC2H5 HOH / H+ → CH CH2
= + CH2COOC2H 5 − − Zn Br 3 − C−
| |
OZn⊕Br |
OH COOC2H5
OH
β -hydroxyesters
CH3
|
→ CH 3 − C − CH 2 − COOC2 H 5
|
OH
(5) Uses
(i) As a solvent for cellulose acetate, cellulose nitrate, celluloid, lacquers, resins, etc.
(ii) For storing acetylene.
(iii) In the manufacture of cordite – a smoke less powder explosive.
(iv) In the preparation of chloroform, iodoform, sulphonal and chloretone.
(v) As a nailpolish remover.
(vi) In the preparation of an artificial scent (ionone), plexiglass (unbreakable glass) and synthetic rubber.
(6) Tests
(i) Legal's test : When a few drops of freshly prepared sodium nitroprusside and sodium hydroxide solution
are added to an aqueous solution of acetone, a wine colour is obtained which changes to yellow on standing.
(ii) Indigo test : A small amount of orthonitrobenzaldehyde is added to about 2 ml. of acetone and it is
diluted with KOH solution and stirred. A blue colour of indigotin is produced.
(iii) Iodoform test : Acetone gives iodoform test with iodine and sodium hydroxide or iodine and ammonium
hydroxide.
Comparison between Acetaldehyde and Acetone
Reaction Acetaldehyde Acetone
Similarty Forms ethyl alcohol Forms isopropyl alcohol
1. Reduction with H2 and CH 3CHO + H 2 Ni → CH 3CH 2OH CH 3COCH 3 + H 2 → CH 3CHOHCH3
Ni or LiAlH4
2. Clemmensen's Forms ethane Forms propane
reduction CH 3CHO + 4H → CH 3CH 3 + H 2O CH 3COCH3 + 4H → CH 3CH 2CH 3 + H 2O
(Zn/Hg and conc. HCl)
Forms acetaldehyde cyanohydrin Forms acetone cyanohydrin
3. Addition of HCN
OH (CH 3 )2 CO + HCN →(CH 3 )2 C OH
CH 3CHO + HCN → CH 3CH CN
4. Addition of NaHSO3 CN White crystalline derivative
5. Grignard reagent White crystalline derivative OH
followed by hydrolysis (CH 3 )2 CO + NaHSO3 →(CH 3 )2 C
OH
CH 3CHO + NaHSO3 → CH 3CH SO3 Na
SO3 Na Forms tertiary butyl alcohol
Forms isopropyl alcohol
(CH 3 )2 CO + CH 3 MgI →(CH 3 )3 COMgI
CH3CHO + CH3 MgI →(CH3 )2 CH − OMgI H2O → (CH 3 )3 COH
H2O → CH 3CHOHCH 3
6. With hydroxylamine Forms acetaldoxime Forms acetoxime
(NH2OH) CH 3CHO + H 2 NOH → CH 3CH = NOH (CH 3 )2 CO + H 2 NOH →(CH 3 )2 C = NOH
Forms acetaldehyde hydrazone Forms acetone hydrazone
7. With hydrazine
( NH 2 NH 2 ) CH 3CHO + H 2 NNH 2 → CH 3CH = NNH 2
Forms acetaldehyde phenylhydrazone (CH 3 )2 CO + H 2 NNH 2 →(CH 3 )2 C = NNH 2
8. With phenyl hydrazine CH 3CHO + H 2 NNHC6 H5 → Forms acetone phenyl hydrazone
(C6H5 NHNH2)
CH 3CH = NNHC6 H 5 (CH 3 )2 CO + H 2 NNHC6 H 5 →
9. With semicarbazide Forms acetaldehyde semicarbazone (CH 3 )2 C = NNHC6 H 5
(H 2 NNHCONH 2 )
CH 3CHO + H 2 NNHCONH 2 → Forms acetone semicarbazone
10. With PCl5 CH 3CH = NNHCONH 2
(CH 3 )2 CO + H 2 NNHCONH 2 →
Forms ethylidene chloride
Cl (CH 3 )2 C = NNHCONH 2
CH3CHO + PCl5 → CH3CH Forms isopropylidene chloride
Cl
Cl
Forms chloral (CH3)2 CO + PCl5 →(CH3)2 C
CH3CHO + Cl2 → CCl3CHO Cl
11. With chlorine Forms acetal Forms trichloro acetone
12. With alcohols
OC2H5 CH3COCH3 + Cl2 → CCl3COCH3
CH3CHO + 2C2H5OH → CH3CH Forms ketal
13. With SeO2 OC2H5 OC2 H5
Forms glyoxal (CH3 )2 CO + 2C2 H5OH →(CH3 )2 C
CH3CHO + SeO2 → CHOCHO + Se + H2O OC2 H5
Forms methyl glyoxal
(CH3 )2 CO + SeO2 → CH3COCHO + Se + H 2O
14. Iodoform reaction Forms iodoform Forms iodoform
(I2 + NaOH)
Forms chloroform Forms chloroform
15. Bleaching powder Forms aldol Forms diacetone alcohol
16. Aldol condensation 2CH3CHO → CH3CHOHCH2CHO 2CH3COCH3 →(CH3)2C(OH)CH2COCH3
with mild alkali Undergoes polymerisation Does not undergo polymerisation but gives
condensation reaction
17. Polymerisation Forms diacetone ammonia
18. With NH3 Forms acetaldehyde ammonia
CH3CHO + NH3 → CH3CH OH (CH3)2CO + NH3 + OC(CH3)2 →
NH2 (CH3)2 C(NH2)CH2COCH3
19. With conc. NaOH Forms brownish resinous mass No reaction
20. With HNO2 Forms oximino acetone
No reaction CH3COCH3 + HNO2 → CH3COCH = NOH
21. With chloroform Forms chloretone
No reaction
OH
22. With alk. sodium Deep red colour (CH3)2CO + CHCl3 →(CH3)2C
nitroprusside Blue colour
21o C CCl3
23. With sodium Red colour changes to yellow on standing
nitroprusside + Pyridine
No effect
24. Boiling point
56o C
1. With Schiff's reagent Difference Does not give pink colour
2. With Fehling's solution No reaction
3. With Tollen's reagent Pink colour No reaction
Gives red precipitate
4. Oxidation with acidified Gives silver mirror Oxidation occurs with difficulty to form
K2Cr2O7 Easily oxidised to acetic acid acetic acid
CH3CHO + O → CH3COOH CH3COCH3 + O → CH3COOH + CO2 + H2O
Aromatic Carbonyl Compounds.
Aromatic aldehydes are of two types :
The compounds in which − CHO group is attached directly to an aromatic ring, e.g., benzaldehyde, C6H5CHO .
Those in which aldehyde (−CHO) group is attached to side chain, e.g., phenyl acetaldehyde, C6H5CH2CHO .
They closely resemble with aliphatic aldehydes.
Aromatic ketones are compounds in which a carbonyl group ( > C = O) is attached to either two aryl groups or
one aryl group and one alkyl group. Examples are :
CHO COCH3 COC6 H5 OH
CHO
Benzaldehyde Acetophenone Benzophenone Salicylaldehyde
(Methyl phenyl (Diphenyl ketone)
ketone)
CHO
Benzaldehyde, C6H5CHO or
Benzaldehyde is the simplest aromatic aldehyde. It occurs in bitter almonds in the form of its glucoside,
amygdalin (C20H27O11N). When amygdalin is boiled with dilute acids, it hydrolyses into benzaldehyde, glucose
and HCN
CN
|
C6 H5CHOC12H21O10 + 2H2O → C6 H5CHO+ 2C6 H12O6 + HCN
Amygdalin Benzaldehyde Glucose
Benzaldehyde is also known as oil of bitter almonds.
(1) Method of preparation
(i) Laboratory method : It is conveniently prepared by boiling benzyl chloride with copper nitrate or lead
nitrate solution in a current of carbon dioxide.
2C6 H5CH 2Cl+ Cu(NO3 )2 heat → 2C6 H5CHO+ CuCl 2 + 2HNO2 [2HNO2 → NO + NO2 + H2O]
Benzyl chloride Pb(NorO3)2 CO2 Benzaldehyde
(ii) Rosenmund reaction : C6H5COCl + H2 Pd /BaSO4 → C6H5CHO + HCl
(iii) By dry distillation of a mixture of calcium benzoate and calcium formate
O
C6 H5 COO ||
C6 H5 COO O CH
Ca + Ca CH heat → 2C6 H5CHO+ 2CaCO3
O Benzaldehyde
|| (Major product)
O
(iv) By oxidation of benzyl alcohol : This involves the treatment of benzyl alcohol with dil. HNO3 or acidic
potassium dichromate or chromic anhydride in acetic anhydride or with copper catalyst at 350o C .
CH2OH [O] → CHO
Benzyl alcohol Benzaldehyde
This method is used for commercial production of benzaldehyde.
(v) By hydrolysis of benzal chloride : OH
CHCl2 CH OH CHO
NaOH → (−H2O) →
Ca(OH ) 2
Benzyal Chloride Intermediate Benzaldehyde
(unstable)
This is also an industrial method.
(vi) By oxidation of Toluene
CH 3 CHO
+ H2O
+ O2 V2O5 →
350o C
Commercially the oxidation of toluene is done with air and diluted with nitrogen (to prevent complete
oxidation) at 500o C in the presence of oxides of Mn, Mo or Zr as catalyst.
Partial oxidation of toluene with manganese dioxide and dilute sulphuric acid at 35o C , also forms
benzaldehyde.
C6 H5CH 3 CrO3 → C6 H5CH(OCOCH 3 )2 H + / H2O → C6 H5CHO + 2CH3COOH
Toluene Benzylidene acetate
(CH 3CO)2 O
(vii) Etard's reaction : C6 H5CH3 + 2CrO2Cl2 → C6 H5CH3 2CrO2Cl2 H2O → C6 H5CHO
Brown addition product Benzaldehyde
(viii) Gattermann-koch aldehyde synthesis : Benzene is converted into benzaldehyde by passing a mixture
of carbon monoxide and HCl gas under high pressure into the ether solution of benzene in presence of anhydrous
aluminium chloride and cuprous chloride.
CHO
+ CO + HCl AlCl3 → + HCl
Benzene Benzaldehyde
(ix) Gattermann reaction
HC ≡ N + HCl + AlCl 3 → H C⊕ = NH + AlCl − ; ++
4
C6 H5 H + HC = NH → C6 H5CH = NH2
Benzene
+
C6 H5CH = NH2 + H2O + AlCl4− → C6 H5CHO + NH3 + AlCl3 + HCl
CHO
Thus, + HCN + HCl + H 2O AlCl3 → + NH4 Cl
(x) Stephen's reaction : Benzaldehyde is obtained by partial reduction of phenyl cyanide with stannous
chloride and passing dry HCl gas in ether solution followed by hydrolysis of the aldimine stannic chloride with water.
C6 H5C ≡ N HCl / SnCl2 → [C6 H 5 CH = NH]2 H 2 SnCl6 H2O → 2C6 H5CHO
Phenyl cyanide Ether aldimine complex
(xi) By ozonolysis of styrene
O
C6 H5CH = CH2 O3 → C6 H5 – CH CH2 H2O → C6 H5CHO + HCHO + H2O2
Vinyl benzene O
O
OO Br
|| || OC2 H5
(xii) Grignard reaction HCOC2H5 + BrMgC6 H5 → C6 H5C − H + Mg
Ethyl formate Benzaldehyde
Other reagents like carbon monoxide or HCN can also be used.
(xiii) From Diazonium salt
N = N − Cl + HCH = NOH → CH = NOH + HCl + N2
Formaldoxime
Benzaldoxime
H2O
CHO
Benzaldehyde
(2) Physical properties
(i) Benzaldehyde is a colourless oily liquid. Its boiling point is 179o C .
(ii) It has smell of bitter almonds.
(iii) It is sparingly soluble in water but highly soluble in organic solvents.
(iv) It is steam volatile.
(v) It is heavier than water (sp. gr. 1.0504 at 15o C ).
(vi) It is poisonous in nature.
(3) Chemical properties
(i) Addition reaction: The carbonyl group is polar as oxygen is more electronegative than carbon,
δC+ = δO−
Thus, The positive part of the polar reagent always goes to the carbonyl oxygen and negative part goes to
carbonyl carbon.
HCN OH OH
C6 H5CH H+ → C6 H5CH
CN H2O COOH
Benzaldehyde cyanohydrin Mandelic acid
NaHSO3 OH
CHO C6 H5CH
SO3 Na
Benzaldehyde sodium bisulphite
(White solid)
OMgI OH
(Benzaldehyde) CH3MgI C6 H5CH H+ → C6 H 5 CH
H2O
CH 3 CH 3
1-Phenyl -1-ethanol
(2o alcohol)
2[H] C6 H5CH2OH
LiAlH4 Benzyl alcohol
However on reduction with sodium amalgam and water, it gives hydrobenzoin,
C6 H5CH = O + 2H + O = HCC6 H5 Na−Hg → C6 H 5 CH − CH − C6 H 5
H2O
| |
OH OH
Hydrobenzoin
(ii) Reactions involving replacement of carbonyl oxygen
H2NNH2 C6 H5CH = NNH2 + H2O
Benzaldehyde hydrazone
CHO H2N.NHC6H5 C6 H5CH = N.NHC6 H5 + H2O
H2NOH
Benzaldehyde phenyl hydrazone
C6H5CH = NOH + H2O
Benzaldoxime
(Benzaldehyde) H2N.NHCONH2 C6 H5CH = NNHCONH2 + H2O
Benzaldehyde semicarbazone
H2NC6H5 C6 H5CH = NC6 H5 + H2O
PCl5
Benzylidine aniline (Schiff' s base)
2CH3OH
HCl C6 H5CHCl2 + POCl3
Benzal chloride
O| CH3
C6 H5CH + H2O
|
OCH3
Methyl acetal of benzaldehyde
(iii) Oxidation : Benzaldehyde is readily oxidised to benzoic acid even on exposure to air.
C6 H5CHO [O] → C6 H5COOH
Acidified K2Cr2O7 , alkaline KMnO4 and dilute HNO3 can be used as oxidising agents for oxidation.
(iv) Reducing properties : Benzaldehyde is a weak reducing agent. It reduces ammonical silver nitrate
solution (Tollen's reagent) to give silver mirror but does not reduce Fehling's solution.
C6 H5CHO+ Ag 2O → 2Ag + C6 H5COOH
Benzaldehyde Benzoic acid
(v) Clemmensen's reduction : With amalgamated zinc and conc. HCl, benzaldehyde is reduced to toluene.
C6 H5CHO + 4H Zn−Hg → C6 H 5 CH 3 + H2O
HCl
(vi) Schiff's reaction: It restores pink colour to Schiff's reagent (aqueous solution of p-rosaniline hydrochloride
decolourised by passing sulphur dioxide).
(vii) Tischenko reaction : On heating benzaldehyde with aluminium alkoxide (ethoxide) and a little of
anhydrous AlCl3 or ZnCl2 , it undergoes an intermolecular oxidation and reduction (like aliphatic aldehydes) to
form acid and alcohol respectively as such and react to produce benzyl benzoate (an ester).
2C6 H5CHO Al(OC2H5)3 → C6 H5CH 2OOCC6 H5
Benzaldehyde Benzyl benzoate (ester)
(viii) Reactions in which benzaldehyde differs from aliphatic aldehydes
(a) With fehling's solution : No reaction
(b) Action of chlorine : Benzoyl chloride is formed when chlorine is passed through benzaldehyde at its boiling
point in absence of halogen carrier. This is because in benzaldehyde there is no α -hydrogen atom present which
could be replaced by chlorine.
C6 H5CHO + Cl 2 170oC → C6 H 5 COCl + HCl
∆
(c) Reaction with ammonia
C6 H5CH O + H 2 N H + O HCC6 H5 → C6 H5CH = N CHC6 H5 + 3H 2O
C6 H5CH O + H 2 N H C6 H5CH = N
Hydrobenzamide
(d) Cannizzaro's reaction : 2C6 H5CHO KOH → C6 H5CH 2OH+ C6 H5COOK
Benzaldehyde Benzyl alcohol Potassium benzoate
The possible Mechanism is
First step is the reversible addition of hydroxide ion to carbonyl group.
C6 H5 − C = O + OH − (Fast) H
| |
H C6 H5 − C − O−
|
OH
Second step is the transfer of hydride ion directly to the another aldehyde molecule, the latter is thus reduced
to alkoxide ion and the former (ion I) is oxidised to an acid.
HH H
| | |
O C6 H5 C O− Hydride → O
C6 H5 C = + − ion transfer C6 H5C − O− + C6 H5 C =
| ||
OH (slow) H OH
Alkoxide ion acid
(H+ exchange) –H+
+H+
H
|
C6 H5 C− OH + C6 H5 C =O
− −
| |
H O−
Benzyl alcohol Benzoate ion
Third Step is exchange of protons to give most stable pair alcohol and acid anion.
So one molecule of aldehyde acts as hydride donar and the other acts as hydride acceptor. In other words,
Cannizzaro's reaction is an example of self reduction and oxidation.
Note : Two different aldehydes each having no α -hydrogen atom, exhibit crossed Cannizzaro's reaction
when heated in alkaline solution.
C6 H5CHO+ HCHO NaOH → C6 H5CH 2OH + HCOONa
Benzaldehyde heat Benzyl alcohol Sod. formate
Formaldehyde
Aldehyde which do not have α - hydrogen ( C6H5 − CHO, CCl3CHO,(CH3)3 C − CHO, CH2O etc.
undergoes Cannizzaro’s reaction.
Intramolecular cannizzaro reaction CH2 COOH
CHO CHO OH
NaOH/ 100oC →
H + / H2O
CHO CHO COOH CH2 OH
(e) Benzoin Condensation
HO Alc.KCN → HO ( β − hydroxy ketone)
| || | ||
− C + C− − C − C−
|| |
OH |
Two molecules of benzaldehyde
OH
Benzoin
Benzoin can also be reduced to a number of product i.e.,
[H] C6H5 − CHOH − CHOH − C6H5
Na-Hg/C2H5OH
Hydrobenzoin
OH O OH H
| || [H]
C6 H5 C C− C6 H5 Zn-Hg/HCl | | −H2O → C6H5CH
− − C6 H5 − C6 H5 = CHC6 H5
| CH − CH −
H Stilbene
Benzoin
H2 C6H5 − CH2 − CH2 − C6H5 + 2H2O
H2/Raney Ni Dibenzyl
Benzoin can be readily oxidised to a diketone, i.e, benzil.
C6 H5 − CH − C− C6 H5 + [O] CuSO4 → C6 H 5 − C− C− C6 H5
Pyridine
| || || ||
OH O H2O O O
Benzoin Benzil
(f) Perkin's reaction
C6 H5CH O+ H2 CHCOOCOCH3 CH3COONa → C6 H 5 CH = CHCOOCOCH3
Benzaldehyde Acetic anhydride −H2O
CHO CHO H2O → C6 H5CH = CHCOOH+ CH3COOH
Cinnamic acid Acetic acid
CH3
CHOHC2HC|O− CO
CH3
|
C6 H5CH = O + O CH3CH2COONa → C6 H5CH = − COOH + CH3CH2COONa
C
CH3 − CH 2CO α -Methyl cinnamic acid
Propionic anhydride
Mechanism
CH3CO.O.COCH3 + CH3COO− C H2CO.O.COCH3 + CH3COOH
O O OH CH 2CO.O.COCH–3H2O
|| | |
C6 H5 C+ C H2CO.O.COCH3 C6 H5 C− CH 2CO.O.COCH3H+ C6 H5
− − − C−
| |
H |
HH
CH 3 COOH + C6 H5CH = CHCOOH ←hyrolysis C6 H5CH = CHCO.O.COCH3
Cinnamic acid (H2O)
(g) Claisen condensation [Claisen-schmidt reaction]
C| H3 NaOH → C6 H 5CH = C| H3 H 2 O
C6 H5CHO + H2C − CHO (Dil.) C − CHO +
α -Methyl
Propionaldehyde cinnamic aldehyde
C6 H5CHO + H 2CHCOCH3 NaOH(Dil.) → C6 H5CH = CHCOCH3 + H 2O
Acetone Benzylidene acetone
(h) Knoevenagel reaction
C6H5CH = O + H2 C COOH Pyridine → C6H5CH = CHCOOH+ CO2 + H2O
COOH ∆ Cinnamic acid
Malonic acid
(i) Reaction with aniline : Benzaldehyde reacts with aniline and forms Schiff's base
C6 H5CH = O + H2 NC6H5 Warm → C6 H5CH = NC6 H5
Aniline (− H 2O) Benzylidene
aniline
(Schiff's base)
Reaction with Dimethylaniline
H N(CH3 )2 N(CH3 )2
CH = O + + Conc. H2SO4 → CH
H (− H 2O)
N(CH3 )2
N(CH3 )2
Tetramethyl diamino
Dimethyl aniline triphenyl methane
(Malachite green)
(j) Reaction with Ammonia : Benzaldehyde reacts with ammonia to form hydrobenzamide aldehyde other than
CH2O give aldehyde ammonia while CH2O forms urotropine.
C6 H5 − CHO + H2 NH O=CH−C6H5 → C6 H 5 − CH = N CH − C6 H5
C6 H5 − CHO H2 NH C6 H 5 − CH = N
Hydrobenzamide
(k) Reformatsky reaction
C6 H5CH = O+ Zn + Br Cα H 2COOC2 H5 → C6 H5CHCH 2COOC2 H5 H2O → C6 H 5 − CH − CH 2 COOC2 H 5
Benzaldehyde Bromo ethylacetate | |
OZnBr OH
β -hydroxy ester
(l) Reaction of benzene ring HNO3(conc.) CHO
H2SO4 (conc.)
m − NitrobenNzaOld2ehyde
CHO H2SO4 CHO
fuming
Benzaldehyde SO3H
m − Benzaldehyde
Sulphonic acid
Cl2 CHO
FeCl3
Cl
(4) Uses : Benzaldehyde is used,
(i) In perfumery m−Chlorobenzaldehyde
(ii) In manufacture of dyes
(iii) In manufacture of benzoic acid, cinnamic acid, cinnamaldehyde, Schiff's base, etc.
(5) Tests : (i) Benzaldehyde forms a white precipitate with NaHSO3 solution.
(ii) Benzaldehyde forms a yellow precipitate with 2 : 4 dinitrophenyl hydrazine.
(iii) Benzaldehyde gives pink colour with Schiff's reagent.
(iv) Benzaldehyde forms black precipitate or silver mirror with Tollen's reagent.
(v) Benzaldehyde on treatment with alkaline KMnO4 and subsequent acidification gives a white precipitate of
benzoic acid on cooling.
Acetophenone, C6H5COCH3, Acetyl Benzene
(1) Method of preparation
(i) Friedel-Craft's reaction : Acetyl chloride reacts with benzene in presence of anhydrous aluminium chloride
to form acetophenone.
C6H5 H + Cl COCH3 AlCl3 → C6H5COCH3 + HCl
Benzene Acetyl chloride Acetophenone
(ii) By distillation of a mixture of calcium benzoate and calcium acetate.
O
||
C6H5 COO Ca + Ca O
C6H5 COO O CCH3
||
Calcium benzoate ∆ → 2C6H5 CCH3 + 2CaCO3
O CCH3 Acetophenone
||
O
Calcium acetate
(iii) By methylation of benzaldehyde with diazomethane.
C6H5CHO + CH2 N2 → C6H5COCH3 + N2
(iv) By treating benzoyl chloride with dimethyl cadmium.
2C6H5COCl + (CH3)2 Cd → 2C6H5COCH3 + CdCl2
(v) By Grignard reagent
(a) CH3C ≡ N + C6H5MgBr → CH3C| = NMgBr H2O
C6 H5
C6H5COCH3 + NH3 + Mg(OH)Br
OO Br
OC2 H 5
|| ||
(b) C6 H5 MgBr + H5C2O C CH 3 → C6 H5 C CH3 + Mg
Ethyl acetate
(vi) Commercial preparation : Ethylbenzene is oxidised with air at 126o C under pressure in presence of a
catalyst manganese acetate.
CH 2CH3 COCH 3
+ O2 Catalyst → + H2O
126o C pressure
(2) Physical properties : It is a colourless crystalline compound with melting point 202o C and boiling point
20o C . It has characteristic pleasant odour. It is slightly soluble in water. Chemically, It is similar to acetone.
(3) Chemical properties :
HCN OH
H2NOH |
Clemmensen C6 H5 − C − CH3
reduction
|
Zn(Hg)/HCl
Reduction CN
Na/C2H5OH
Oxidation Acetophenone cyanohydrine
Cold KMnO4
CH3
|
= NOH Rearrangement → C6 H5 NHCOCH3
C6 H5 − C H 2SO4 Acetanilide
Acetophenone oxime or
(Methylphenyl ketoxime)
C6H5COCH3 C6 H5CH 2CH3
(Acetophenone) Ethyl benzene
C6 H5 CH OH
|
CH3
Methyl phenyl carbinol
(2o alcohol)
C6 H5COCOOH [O] → C6 H5COOH
Phenyl glyoxylic acid Benzoic acid
Oxidation C6 H 5COCHO
SeO2 Phenyl glyoxal
PCl5 C6 H 5CCl 2CH 3
2, 2-Dichloroethylbenzene
C6H5COCH3 Cl2 C6 H5COCH 2Cl It is relatively harmless but powerful
lachrymator or tear gas and is used
(Acetophenone) Iodoform reaction Phenacyl chloride
I2/NaOH (Used as a tear gas) by police to disperse mobs.
C6 H5COONa + CHI3
Iodoform
Aldol type C| H3 O
condensation
Alter-butoxide ||
C6 H5 − C = CH − C− C6 H5
Dypnone (It is used as hypnotic)
Nitration NO2C6 H 4 COCH 3
m- Nitroacetophenone
HNO3/H2SO4
conc. H2SO4 HSO3C6 H4COCH3
Acetophenone
m− sulphonic acid
(4) Uses : It is used in perfumery and as a sleep producing drug.
Quinones
Quinones are unsaturated cyclic diketones. Two quinones of benzene are possible (m-benzoquinone is not
possible as it is not possible to construct such formula by maintaining tetravalency of carbon).
Note that quinones are non-aromatic conjugated cyclic diketones. Since they are highly conjugated they are
highly coloured substances.
p-Benzoquinone, being the most important, is commonly known as quinone. It is prepared by the oxidation of
hydroquinone or aniline.
O O OH O NH2 O
O FeCl3 → MNO2 → O
; ;
o - Benzoquinone O H2SO4 p − Benzoquinone
OQuHinol O
p - Benzoquinone Aniline
p − Benzoquinone
[Laboratory method]
α, β-Unsaturated carbonyl compounds
α, β-Unsaturated carbonyl compounds. As the name represents these compounds contain unsaturation between
O
| | ||
α-and β-carbon atoms with respect to carbonyl group, i.e., − C = C− C− . Such molecules are quite stable due to
the presence of conjugated system of double bond. Such molecules give properties of the double bond, carbonyl
group and some additional properties due to the interaction of the two groups. Due to electron withdrawing nature
of the > C = O group, the reactivity of C = C towards electrophilic reagents decreases as compared to an isolated
double bond. On the other hand, C = C group undergoes nucleophilic addition reactions which are uncommon for
simple alkenes.
Two important addition reactions of α, β-unsaturated carbonyl compounds are Michael reaction and Diels-Alder
reaction.
Michael reaction: C6H5CH = CHCOC6H5 + CH2(COOC2H5)2 Piperidine → C6H5 CH.CH2.COC6H5
Benzal acetophenone |
CH(COOC2H5)2
Diel's-Alder reaction
CH 2 CH.CHO CHO
CH CH 2
100oC →
+ Acrolein
CH 1, 2, 3, 6 - Tetrahydrobenzaldehyde
CH 2
1,3 butadiene
Carboxylic acids and their derivatives
Carboxylic Acids
Carboxylic acids are the compounds containing the carboxyl functional group – C– OH
||
O
The carboxyl group is made up of carbonyl ( C = O) and hydroxyl (–OH) group.
Classification, structure, Nomenclature, & Isomerism.
(1) Classification
(i) Carboxylic acids are classified as monocarboxylic acids, dicarboxylic acids, tricarboxylic acids etc.
depending on the number of – COOH groups present in the molecule.
CH 3COOH C H 2 COOH CH2COOH
monocarboxylic acid
| |
CH2COOH
C HCOOH
Dicarboxylic acid
|
CH2COOH
Tricarboxylic acid
(ii) Monocarboxylic acids of aliphatic series are commonly known as fatty acids such as palmitic acid
(C15 H 31COOH) and stearic acid (C17 H 35 COOH) .
(iii) The general formula for monocarboxylic acids is Cn H 2n+1COOH or Cn H 2nO2 . Where n = number of
carbon atoms.
(iv) The carboxylic acids may be aliphatic or aromatic depending upon whether – COOH group is attached to
aliphatic alkyl chain or aryl group respectively.
Aliphatic acids
HCOOH CH 3COOH CH3 –CH COOH
Acetic acid |
Formic acid CH3
Isobutyric acid
Aromatic acids
COOH COOH COOH
CH3
Benzoic acid NO2 o-Toluic acid
m-Nitrobenzoic acid
(2) Structure :
(i) The name carboxyl is derived from carbonyl C = O and hydroxyl (– OH) because both carbonyl and
hydroxyl groups are directly linked to each other.
(ii) The carboxylic carbon atom and two oxygen atom in carboxylic acid are sp2 hybridized.
Pure p- orbital sp2 O
R sp2
sp3 C
O H
Pure p-orbital
1.23 Å H 1.43Å 1.20Å
O | R–C=O
R–C |
1.36 Å → | R − C − OH R
|
O–H H
Delocalized π-electron cloud
(iii) The shorter bond (c – o) and longer bond (c = o) of carboxylic acid than alcohol and ketone is due to
delocalization of π electrons.
(3) Nomenclature : The monocarboxylic acids are named according to following systems.
(i) Common or trivial names : The names of lower members are derived from the Latin or Greek word that
indicates the source of the particular acid. The common names have ending –ic acid.
Formula Source Common name
HCOOH Red ant (Latin, ant = Formica) Formic acid
CH 3COOH Vinegar (Latin; vinegar = Acetum) Acetic acid
C2 H5COOH
Proton-pion (Greek; Proton = first, Pion = Propionic acid
C3 H7COOH Fat)
C4 H9COOH Butter (Latin ; Butter = Butyrum) Butyric acid
Valeric acid
Root of valerian plant
(ii) Derived system : Monocarboxylic acids may be named as alkyl derivatives of acetic acid.
CH 3CH 2COOH CH 3 − C H − COOH
Methyl acetic acid
|
CH3
Dimethyl acetic acid
(iii) IUPAC system : Acids are named as alkanoic acids (Alkane – e + oic acid). The name is derived by
replacing 'e' of the corresponding alkane by –oic acid.
HCOOH Methanoic acid (Methane – e + oic acid)
CH 3COOH Ethanoic acid (Ethane – e + oic acid)
In case of substituted acids,
Br
5 4 3 21 4 3| 2 1
C H3 C H C H C H2 C OOH; C H3 C H− C H C OOH;
− − − − −
| | |
CH3 CH3 CH3
3,4-Dimethylpentanoic acid 3-Bromo- 2-methyl butanoic acid
(4) Isomerism CH3
(i) Chain isomerism : CH3 − CH 2 − CH 2 − CH 2 − COOH ;
|
Pentanoic acid
CH 3 − CH 2 − C H − COOH
2-methyl butanoic acid
(ii) Position isomerism : CH3 − C H − CH2 − COOH ; CH 3 − CH 2 − C H − COOH
| |
CH3 CH3
3-methyl butanoic acid 2-methyl butanoic acid
(iii) Functional isomerism : CH3 − CH 2 − COOH ; CH 3COOCH3 ; HCOOC2 H5
Propanoic acid Methyl acetate Ethyl formate
(iv) Optical isomerism
C2H5 C2H5
| |
CH 3 C− C3 H7 C3 H7 − C− CH 3
− |
|
COOH COOH
2-Ethyl-2Methyl Pentanoic Acid
Methods of Preparation of Monocarboxylic acid.
(1) By oxidation of alcohols, aldehydes and ketones
RCH 2OH [O} → RCHO [O] → RCOOH
alcohol K2Cr2O7
K2Cr2O7 Carboxylic acid
RCHO [O] → RCOOH
Aldehyde
Ketones and secondary alcohols form acid with fewer carbon atoms.
R CHOH [O} → R C = O K2Cr2O7 → RCOOH + R′COOH
R′CH 2 R′CH2 H2SO4
Sec. Alcohol Ketones
Note : Aldehyde can be oxidized to carboxylic acid with mild oxidising agents such as ammonical silver
nitrate solution [Ag2O or Ag(NH3 )+2 OH − ]
Methanoic acid can not be prepared by oxidation method.
Ketones can be oxidized under drastic conditions using strong oxidising agent like K2Cr2O7 .
Methyl ketones can also be converted to carboxylic acid through the haloform reaction.
R – C− CH 3 + 3I 2 + 3 NaOH ∆ → R − C− OH + CHI 3 + 3 NaI + 3H2O
H2O
|| ||
O O
(2) By Hydrolysis of nitriles, ester, anhydrides and acid chloride
(i) Hydrolysis of nitriles
R − C ≡ N + HOH orHNCaOl →H R − C OH Rearrangement → R − C O H2O → RCOOH + NH4Cl
NH NH2 HCl
(ii) Hydrolysis of Esters : RCOOR'+ HOH HCl → RCOOH+ R' OH
Ester OH − Acid Alcohol
O
||
(iii) Hydrolysis of Anhydrides : CH3 − C
CH3 − C O + HOH H+ / OH− → 2CH3COOH
Ethanoic acid
||
O
Ethanoic anhydride
(iv) Hydrolysis of acid chloride and nitro alkane
R − C− Cl + HOH H+ / OH− → RCOOH + HCl
||
O
R − CH 2 − NO2 85%H2SO4 → RCOOH
X OH O
X + 3NaOH → R − C OH
(v) Hydrolysis of Trihalogen : R − C X OH −H2O → R − C + 3 NaX
OH
(3) From Grignard Reagent
O
RδM−δg+X ||
O = C = O + Dryether → R − OMgX H + / H2O → RCOOH + Mg(OH)X
C−
Carbon dioxide Grignard reagent (R = CH3,C2H5,(CH3)2 CH −,(CH3)3 C −
(4) From Alkene or Hydro-carboxy-addition (koch reaction)
When a mixture of alkene, carbon monoxide and steam is heated under pressure at 350°C in presence of
phosphoric acid (H3 PO4 ) monocarboxylic acid is formed.
CH 2 = CH 2 + CO + H 2 O H3PO4 → CH 3 CH 2 COOH
500−1000atm
& 350°C
Mechanism :
OH
H H C| = O
H⊕ H +
(i) C = C + H + → − C − C− | C− | | |
(ii) − − C≡O → H2O → −
| C − C− C C− C−
Carbocation || || −H⊕ ||
Carboxylic acid
C=O
⊕
Acyl cation
(5) Special Methods
(i) Carboxylation of sodium alkoxide : RONa + CO → RCOONa HCl → RCOOH
Sod. alkoxide Sod. salt Acid
(ii) Action of heat on dicarboxylic acid : R − CH COOH −CO2 → R − CH2COOH
COOH heat Monocarboxylic acid
Substituted malonic acid
(iii) From Acetoacetic ester : CH 3 CO CHRCO O C2H5 Hydrolysis → CH 3 COOH
OH H OH H
+ RCH COOH + C H OH
2 25
(iv) Oxidation of alkene and alkyne
RCH = CHR′ [O] → RCOOH + R′COOH
Alkene Hot alkalne
KMnO4
R − C ≡ C − R′ (i)O3 → R − COOH + R′COOH
Alkyne (ii) H 2O
(v) The Arndt-Eistert Synthesis : R − C− Cl + CH 2 N 2 → R − C− CHN 2 H2O → R − CH 2 − COOH
|| Ag 2O
O ||
O
(vi) From acid amides : RCONH 2 + H2O Acid → RCOOH + NH 3
Amide or Alkali Acid
RCONH 2 + HNO2 → RCOOH + N 2 + H 2O
Amide Nitrous acid
Physical properties of monocarboxylic acids.
Important physical properties of carboxylic acids are described below :
(1) Physical state : The first three members (upto 3 carbon atoms) are colourless, pungent smelling liquids. The
next six members are oily liquids having unpleasant smell. The higher members are colourless and odourless waxy solids.
(2) Solubility : The lower members of the aliphatic carboxylic acid family (upto C4) are highly soluble in
water. The solubility decreases with the increase in the size of the alkyl group. All carboxylic acids are soluble in
alcohol, ether and benzene etc.
Note : The solubility of lower members of carboxylic acids is due to the formation of hydrogen bonds
between the – COOH group and water molecules.
Acetic acid exists in the solution in dimer form due to intermolecular hydrogen bonding. The
observed molecular mass of acetic acid is 120 instead of 60.
(3) Melting point
(i) The melting points of carboxylic acids donot vary smoothly from one member to another.
(ii) The melting point of the acids having even number of carbon atoms are higher than those containing an
odd number immediately above and below them.
50
30
Melting point/°C 10
–1 0
–3 0
–5 0 3 5 7 9 11
1
No. of C-atoms/molecule
(iii) The acids with even number of carbon atoms have the – COOH group and the terminal – CH3 group on
the opposite side of the carbon chain.
(iv) In the case of odd numbers, the two groups lie on the same side of the chain.
CH2 COOH CH2 CH2
CH3 CH2 CH3 CH2 COOH
the two terminal groups lie on the two terminal groups lie on
the opposite sides of the chain the same side of the chain
When the terminal groups lie on the opposite sides the molecules fit into each other more closely. More
effective packing of the molecule in the lattice. Therefore, results into higher melting point.
(4) Boiling point : Boiling point of carboxylic acids increase regularly with increase of molecular mass.
Boiling points of carboxylic acids are higher than those of alcohols of same molecular mass. This is due to
intermolecular hydrogen bonding between two acid molecules.
Hydrogen bonding
CH3 – C O H–O C – CH3
O–H O
Hydrogen bonding Acetic acid dimer
Acidic nature of monocarboxylic acids.
Among organic compounds, carboxylic acid are the most acidic and ionize in aqueous solution. It is expressed
in term of dissociation constant (Ka)
R − COOH+ H2O ⇌ RCOO− + H3O+
Carboxylic acid Carboxylate ion Hydronium ion
Ka = [RCOO− ][H3O+ ]
[RCOOH]
Note : Greater the value of Ka or lesser the value of pKa stronger is the acid, i.e. pKa = – log Ka
Acidic nature ( Ka ) α 1/molecular weight
HCOOH > CH3COOH > C2H5COOH
Ka Value 17.7 × 10−5 1.75 × 10−5 1.3 × 10−5
The formic acid is strongest of all fatty acids.
Acetic acid is less weak acid than sulphuric acid due to less degree of ionisation.
(1) Cause of Acidic Nature
(i) A molecule of carboxylic acid can be represented as a resonance hybrid of the following structures.
.. O..:
O:
.. |⊕
||
R − C − O..− H ↔ R − C = O..− H
(I) (II)
(ii) Due to electron deficiency on oxygen atom of the hydroxyl group (Structure II), their is a displacement of
electron pair O–H bond toward the oxygen atom. This facilitate the release of hydrogen as proton (H+).
R−C O − O⊕ ← H ↔ − C O ↔ R−C O− ≡ R −C O
R O− O 1.27 A°
1.27 A°
O
Resonance hybrid
(iii) The resulting carboxylate ion also stabilized by resonance (As negative charge is dispersed on both the
oxygen atom). This enhance the stability of carboxylate anion and make it weaker base.
(2) Effect of substituent on acidic nature
(i) An electron withdrawing substituent (– I effect) stabilizes the anion by dispersing the negative charge and
therefore increases the acidity.
G←C O − G→C O −
O O
(I) (II)
(ii) An electron releasing substituent (+ I effect) stabilizes negative charge on the anion resulting in the
decrease of stability and thus decreased the acidity of acid.
Electron with drawing nature of halogen : F > Cl > Br > I
Thus, the acidic strength decreases in the order :
FCH 2COOH > ClCH 2COOH > BrCH 2COOH > ICH 2COOH
similarly : CCl3COOH > CHCl2COOH > CH 2ClCOOH > CH 3COOH
(iii) Inductive effect is stronger at α-position than β-position similarly at β-position it is more stronger than at γ -position
Example: CH 3 − CH 2 − C H− COOH > CH 3 − C H − CH 2 − COOH > C H 2 − CH 2 − CH 2 − COOH
|
| |
Cl Cl Cl
(iv) Relative acid strength in organic and inorganic acids RCOOH > HOH > ROH > HC ≡ CH > NH3 > RH
Chemical properties of Monocarboxylic acids.
(1) Reaction involving removal of proton from –OH group
(i) Action with blue litmus : All carboxylic acids turn blue litmus red.
(ii) Reaction with metals : 2CH 3COOH + 2Na → 2CH3COONa+ H 2
Sodium acetate
2CH 3COOH + Zn → (CH 3COO)2 Zn + H 2
Zinc acetate
(iii) Action with alkalies : CH3COOH+ NaOH → CH3COONa+ H 2O
Acetic acid Sodium acetate
(iv) Action with carbonates and bicarbonates
2CH 3COOH + Na2CO3 → 2CH 3COONa+ CO2 + H 2O
Sod. acetate
CH 3COOH + NaHCO3 → CH 3COONa+ CO2 + H 2O
Sod. acetate
Note : Reaction of carboxylic acid with aqueous sodium carbonates solution produces bricks
effervescence. However most phenols do not produce effervescence. Therefore, this reaction may be used to
distinguish between carboxylic acids and phenols.
(2) Reaction involving replacement of –OH group
(i) Formation of acid chloride : CH3COOH+ PCl5 → 3CH 3COCl+ POCl3 + HCl
Acetic acid Acetyl chloride
3CH 3COOH+ PCl3 → 3CH 3COCl+ H 3 PO3
Acetic acid Acetyl chloride
CH 3COOH+ SOCl2 → CH 3COCl+ SO2 + HCl
Acetic acid Acetyl chloride
(ii) Formation of esters (Esterification)
CH 3CO OH + H OC2 H 5 Conc.H2SO4 CH 3COOC2 H5 + H 2O
Acetic acid Ethyl alcohol ∆ Ethyl acetate
(Fruity smelling)
(a) The reaction is shifted to the right by using excess of alcohol or removal of water by distillation.
(b) The reactivity of alcohol towards esterification.
tert-alcohol < sec-alcohol < pri-alcohol < methyl alcohol
(c) The acidic strength of carboxylic plays only a minor role.
R3CCOOH < R2CHCOOH < RCH 2COOH < CH 3COOH < HCOOH
Mechanism of Esterification : The mechanism of esterification involves the following steps :
Step I : A proton from the protonic acid attacks the carbonyl oxygen of acetic acid.
CH 3 − C O + H+ CH 3 − C O+ − H CH 3 − C + OH
OH OH OH
Acetic acid Protonated acetic acid
Step II : The electron rich oxygen atom of the ethyl alcohol attaches itself at positively charged carbon atom.
OH H OH H
OH | ||
CH3 − C : O − C2 H 5 CH − C− O− C2 H5
+ 3 |
•• OH +
Ethyl alcohol
Step III : From the resulting intermediate, a proton shifts to OH group as :
.. + O| H2
:OH H
||
CH3 − C− O C2 H5 Proton transfer CH3 − C− OC2 H5
−
| + |
OH OH
Step IV : The intermediate obtained in Step III loses a water molecule to form a carbocation.
+|OH2 CH 3 C+ − OC2 H5 H2O
CH 3 C O C2 H
− − 5 − | +
|
OH OH
Carbocation
Step V : The carbocation loses a proton to form an ester.
CH 3 − C+ − OC2 H5 – H+ CH 3 − C− OC2 H5
| ||
O−H O
Ethyl acetate
Note : The OH group for making H2O comes from acid.
(iii) The mechanism is supported by labelling of ethanol. Isotopic oxygen as :
O H+ O
|| 18 || 18
CH 3 − C− OH + CH 3CH 2 O H CH 3 − C− O C2 H5 + H 2O
When methanol is taken in place of ethanol. then reaction is called trans esterification.
(iv) Formation of amides : CH 3COOH+ NH 3 heat → CH 3COONH4 ∆ → CH 3CONH 2 + H 2O
Acetic acid Amm. acetate Acetamide
(v) Formation of acid anhydrides : CH3COO H Heat → CH 3 CO O + H2O
CH3C+O OH P2O5 CH 3 CO
Acetic acid Acetic anhydride
(vi) Reaction with organo-metallic reagents : R' CH2MgBr + RCOOH ether → R' CH3 + RCOOMgBr
Alkane
(3) Reaction involving carbonyl (>C = O) group : Reduction : R − C− OH LiAlH4 → R − CH 2 − OH
||
O
Carboxylic acid are difficult to reduce either by catalytic hydrogenation or Na C2 H 5 OH
(4) Reaction involving attack of carboxylic group (– COOH)
O
||
(i) Decarboxylation : R − OH (−CO2 ) → R − H
C−
When anhydrous alkali salt of fatty acid is heated with sodalime then :
RCOONa+ NaOH CaO → R − H+ Na 2CO3
Sodium salt heat Alkane
Note : (Exception) When sodium formate is heated with sodalime H2 is evolved.
HCOONa + NaOH CaO → H 2 + Na2CO3
(ii) Heating of calcium salts : (RCOO)2 Ca heat → RCOR+ CaCO3
Sodium salt Ketone
(iii) Electrolysis : (Kolbe's synthesis) : RCOONa ⇌ RCOO− + Na +
At anode 2RCOO − → R − R + 2CO2 + 2e −
At cathode 2Na + + 2e − → 2Na 2H2O → 2NaOH + H 2
2CH 3 COOK+ 2H 2O Electrolysis → CH 3 − CH 3 + 2CO2 + 2KOH + H 2
Potassium acetate Ethane
(iv) Formation of Alkyl halide (Hunsdiecker's reaction) :
CH 3COOAg+ Br2 heat → CH 3 Br + AgBr + CO2
Silver acetate CCl 4
Methyl bromide
Mechanism : Two step process –
OO
|| ||
Step – I: + Br2 CCl4 → R − OBr + AgBr
R − C− OAg C−
. .O O
|| ||
Step – II : (ii) R − C− OBr + Br2 → R − C− O+ Br
.O .
||
R − C− O → R+ CO2
. .
R+ Br → R − Br
Note : In Hunsdiecker reaction, one carbon atom less alkyl halide is formed from acid salt.
(v) Formation of amines (schmidt reaction) : RCOOH + N3H H2SO4(conc.) → RNH 2 + CO2 + N2
Acid
Hydrazoic Primary
acid amine
In schmidt reaction, one carbon less product is formed.
Mechanism :
O OH OH O O
|| || N+
|| || HN3 → | − H2O → R − − N2 → R − − H
C C−
R−C H+ → R − C+ R − C− OH
|| |
OH OH H − N − N+ ≡ N
NH − N+ ≡ N
− H+ → R − N = C = O H2O → RNH2 + CO2
(vi) Complete reduction : CH 3COOH+ 6HI P → CH 3CH 3 + 2H 2O + 3I 2
Acetic acid Ethane
In the above reaction, the – COOH group is reduced to a CH3 group.
(5) Reaction involving hydrogen of α-carbon
Halogenation
(i) In presence of U.V. light
H Cl
| |
− COOH + Cl 2 U.V.∆ → − + HCl
C− C− COOH
| |
α -chloro acid
(ii) In presence of Red P and diffused light [Hell Volhard-zelinsky reaction]
Carboxylic acid having an α-hydrogen react with Cl2 or Br2 in the presence of a small amount of red
phosphorus to give chloro acetic acid. The reaction is known as Hell Volhard-zelinsky reaction.
CH 3COOH Cl2,redP4 → ClCH 2COOH Cl2,redP4 → Cl 2 CHCOOH Cl2, redP4 → Cl 3 CCOOH
Acetic acid − HCl − HCl Dichloro acetic acid − HCl
Chloro acetic acid Trichloro acetic acid
Mechanism :
Step – I : R − CH2C O P+Br2 → R − CH2 − C O enolisation R − CH = C OH
O (PBr3 ) Br Br
− H
.. Br O
O.. H | Br
Step – II : R − CH = C + Br − Br −HBr → R − H− C
C
Br
Step – III : R − C H− C O + RCH 2 − C O → R − CH − C O + R − CH 2C O
| Br OH OH Br
Br |
Second molecule of acid Acid bromide (IV)
Br
α -bromocarboxylic acid
Individual members of Monocarboxylic acids
Formic Acid or Methanoic acid (HCOOH)
Formic acid is the first member of monocarboxylic acids series. It occurs in the sting of bees, wasps, red ants,
stinging nettles. and fruits. In traces it is present in perspiration, urine, blood and in caterpillar's.
(1) Methods of preparation : The following methods can be used for its preparation
(i) Oxidation of methyl alcohol or formaldehyde :
CH 3OH + 1 O2 Pt → HCHO + H2O
2
HCHO + 1 O2 → HCOOH
2
CH 3OH + O2 → HCOOH + H 2O
Formic acid
(ii) Hydrolysis of hydrocyanic acid : Formic acid is formed by the hydrolysis of HCN with acids or alkalies.
HCN + 2H 2O HCl → HCOOH + NH 3 ; HCN + H 2O NaOH → HCOONa + NH 3
(iii) Laboratory preparation : Formic acid is conveniently prepared in the laboratory by heating glycerol
with oxalic acid at 100-120°C. In actual practice, glycerol is first heated at 105o C and then hydrated oxalic acid is
added and the temperature is raised to 110°C. Glycerol monoxalate is first formed which decomposes into glycerol
monoformate and carbon dioxide. When the evolution of carbon dioxide ceases, more of oxalic acid is added. The
monoformate gets hydrolysed to formic acid regenerating glycerol which reacts with fresh oxalic acid. Thus, a small
quantity of glycerol is sufficient to convert large quantities of oxalic acid into formic acid.
CH2OH HO OC−COOH CH2OOC COO H CH2OOCH CH2OH
|
| Oxalic acid −H2O → −CO2 → | (COOH)2 2H2O → HCOOH + |
CHOH
CHOH CHOH CHOH
| | 110°C | Formic acid |
CH2OH CH2OH CH2OH CH2OH
Glycerol Glycerol Glycerol Glycerol
monoxalate monoformate
The following procedure is applied for obtaining anhydrous formic acid.
2HCOOH + PbCO3 → (HCOO)2 Pb+ CO2 + H 2O ; (HCOO)2 Pb + H2S → Pbs+ 2HCOOH
Lead formate Formic acid
ppt.
(iv) Industrial preparation : Formic acid is prepared on industrial scale by heating sodium hydroxide with
carbon monoxide at 210°C under a pressure of about 10 atmospheres.
CO + NaOH → HCOONa
Sodium formate
Sodium formate thus formed is distilled with sodium hydrogen sulphate, when anhydrous formic acid distils over.
HCOONa + NaHSO4 → HCOOH + Na2 SO4
(2) Physical properties
(i) It is a colourless pungent smelling liquid.
(ii) It melts at 8.4°C and boils at 100.5°C.
(iii) It is miscible with water, alcohol and ether. It forms azeotropic mixture with water.
(iv) It is strongly corrosive and cause blisterson skin.
(v) It exists in aqueous solution as a dimer involving hydrogen bonding.
O H−O Hydrogen bonding
H −C
C−H
O−H O
Hydrogen bonding
(3) Chemical properties : Formic acid is the strongest acid among all the members of the homologous
series. It exhibits some characteristics which are not shown by other members. This unique nature is due to the fact
that it contains both aldehyde group and carboxyl group.
O O O
OH ||
H −C OH H ||
H−C
C− OH
Aldehyde
group Carboxyl
Formic acid group
(i) Acidic properties
(a) It is a monobasic acid. Its dissociation constant value is 18 × 10–5 at 25°C. It's acidic properties are due to
its ionisation in aqueous solution.
HCOOH ⇌ HCOO− + H +
Formic acid Formate ion
(b) It reacts with carbonates and bicarbonates evolving carbon dioxide.
HCOOH + NaHCO3 → HCOONa + H2O + CO2 ↑
2HCOOH + Na2CO3 → 2HCOONa + H2O + CO2 ↑
(c) It reacts with alkalies to form corresponding salts. The salts of formic acid are termed as formates. Most of
the formates are soluble in water but lead and silver formates are insoluble.
HCOOH + NaOH → HCOONa + H2O
HCOOH + NH4OH → HCOONH4 + H2O
Amm. formate
(d) Highly electropositive metals evolve hydrogen when react with formic acid.
2HCOOH + 2Na → 2HCOONa + H2
(e) It combines with alcohols to form esters. It is not necessary to use a mineral acid as to catalyse the reaction
since the formic acid itself acts as a catalyst.
HCOOH + CH3OH ⇌ HCOOCH3 + H2O
Methyl formate
(f) It reacts with PCl5 or SOCl2 to give formyl chloride which is not a stable compound. It decomposes at once
into hydrogen chloride and carbon monoxide.
HCOOH + PCl5 → HCOCl + POCl3 + HCl
Formyl chloride
HCOCl → HCl + CO
(ii) Action of heat : When heated above 160°C, it decomposes to give carbon dioxide and hydrogen.
HCOOH → CO2 + H 2
(iii) Action of heat on formates
(a) When sodium formate is heated to 360°C. It decomposes to form sodium oxalate and hydrogen.
COONa H2
2HCOONa → | +
COONa
Sodium oxalate
(b) It does not form a hydrogen when sodium formate is heated with sodalime or its aqueous solution is
electrolysed.
HCOONa + NaOH CaO → Na2CO3 + H 2
(c) Formaldehyde is formed when dry calcium formate is heated.
(HCOO)2 Ca → HCHO + CaCO3
formaldehyde
(iv) Reducing properties
(a) Like aldehyde formic acid behaves as reducing agents, it is oxidised to an unstable acid, carbonic acid,
which decompose into CO2 and H2O
O
||
H − COOH [O] → → CO2 + H2O
HO − C− OH
Carbonic acid
(b) It decolourises acidified KMno4.
2KMnO4 + 3H 2 SO4 → K 2 SO4 + 2MnSO4 + 3H 2O + 5[O]
[HCOOH + O → CO2 + H 2O] × 5
2KMnO4 + 3H 2 SO4 + 5HCOOH → K 2 SO4 + 2MnSO4 + 5CO2 + 8H 2O
(c) It reduces mercuric chloride to mercurous chloride to mercury black
HCOOH + 2HgCl2 → Hg2Cl2 + CO2 + 2HCl
HCOOH + Hg 2Cl 2 → CO2 + 2HCl + 2Hg
(d) It reduces ammonical silver nitrate (Tollen reagents)
HCOOH + Ag 2O heat → 2Ag + CO2 + H 2O
Silver mirror
(e) It reduces fehling solution give red precipitate of Cu2O
HCOOH + 2CuO → Cu2O + CO2 + H2O
(Red ppt.)
(4) Uses : Formic acid is used.
(i) In the laboratory for preparation of carbon monoxide.
(ii) In the preservation of fruits.
(iii) In textile dyeing and finishing.
(iv) In leather tanning.
(v) As coagulating agent for rubber latex.
(vi) As an antiseptic and in the treatment of gout.
(vii) In the manufacture of plastics, water proofing compounds.
(viii) In electroplating to give proper deposit of metals.
(ix) In the preparation of nickel formate which is used as a catalyst in the hydrogenation of oils.
(x) As a reducing agent.
(xi) In the manufacture of oxalic acid.
(5) Tests of Formic Acid
(i) It turns blue litmus red.
(ii) Its aqueous solution gives effervescences with sodium bicarbonate.
(iii) Its neutral solution gives red precipitate with Fehling's solution.
(iv) Its neutral solution with Tollen's reagent gives silver mirror or black precipitate.
(v) It gives white precipitate with mercuric chloride which changes to grey.
HgCl2 → Hg2Cl2 → Hg
White ppt. Grey
Acetic Acid (Ethanoic Acid) (CH3COOH)
Acetic acid is the oldest known fatty acid. It is the chief constituent of vinegar and hence its name (Latin
acetum = vinegar)
(1) Preparation
(i) By oxidation of acetaldehyde (Laboratory-preparation) : CH 3 CHO Na2cr2o7 → CH 3COOH
H2So4 (O)
(ii) By hydrolysis of methyl cyanide with acid : CH3CN + 2H2O HCl → CH3COOH + NH3
(iii) By Grignard reagent : CH 3 MgBr + CO2 → CH 3 − O OMgBr H2OH+ → CH 3 − O OH
|| ||
C− C−
(iv) By hydrolysis of acetyl chloride, acetic anhydride or acetamide and ester
(a) CH3COOC2H5 + H2O H2SO4(conc.) → CH3COOH + C2H5OH
Ester
(b) CH3COCl + H2O dil.HCl → CH3COOH + HCl
acetylchloride
(c) (CH3CO)2 O + H2O dil.HCl → 2CH3COOH
(v) Manufacture of acetic acid
(a) From ethyl alcohol (Quick vinegar process) : Vinegar is 6-10% aqueous solution of acetic acid. It is
obtained by fermentation of liquors containing 12 to 15% ethyl alcohol. Fermentation is done by Bacterium
Mycoderma aceti in presence of air at 30-35°C. The process is termed acetous fermentation.
CH 3 CH 2 OH + O2 Mycodermaaceti → CH 3 COOH + H2O
Ethyl alcohol Bacteria Acetic acid
It is a slow process and takes about 8 to 10 days for completion.
In this process, the following precautions are necessary:
• The concentration of the ethyl alcohol should not be more than 15%, otherwise the bacteria becomes
inactive.
• The supply of air should be regulated. With less air the oxidation takes place only upto acetaldehyde stage
while with excess of air, the acid is oxidised to CO2 and water.
• The flow of alcohol is so regulated that temperature does not exceed 35°C which is the optimum
temperature for bacterial growth.
Acetic acid can be obtained from vinegar with the help of lime. The calcium acetate crystallised from the
solution is distilled with concentrated sulphuric acid when pure acetic acid distils over.
(b) From acetylene : Acetylene is first converted into acetaldehyde by passing through 40% sulphuric acid at
60°C in presence of 1% HgSO4 (catalyst).
CH ≡ CH+ H2O H2SO4(dil.) → CH 3 CHO
Acetylene HgSO4
Acetaldehyde
The acetaldehyde is oxidised to acetic acid by passing a mixture of acetaldehyde vapour and air over
manganous acetate at 70°C.
2CH 3CHO + O2 Manganousacetate → 2CH 3 COOH
70°C
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