Chemistry

Table 15.3 : Melting point and boiling point of alkenes

Fornula Name Molecular mass (u) b.p. (K) m.p. (K)
104
CH2=CH2 Ethene 28 171 88
CH2 = CHCH3 Propene 42 225
--
CH2 =CHCH2CH3 But-1-ene 56 267 --
Pent-1-ene 70 303 135
CH2 = CH(CH2)2CH3 Hex-1-ene 84 337 134
cis-But-2-ene 56 277 167
CH2 =CH(CH2)3CH3 trans-But-2-ene 56 274 132
cis CH3CH = CH CH3 Isobutylene 56 266
trans CH3CH = CHCH3
CH2 = C(CH3)2

15.2.3 Physical properties of alkenes : Alkenes H H CCl4 HH
are nonpolar or weakly polar compounds those H3C C room temp H3C C C H
are insoluble in water, and soluble in non- C H + Br2
polar solvents like benzene, ether, chloroform. Br Br
They are less dense than water. From Table red brown 1,2- Dibromopropane
15.3, it is clear that the boiling point of alkene Propene
rises with increasing number of carbons.
Branched alkenes have lower boiling point (Colourless)
than the corresponding straight chain alkane.
The boiling point of alkene is very nearly the Addition of bromine is useful test for
same as that of alkane with the same carbon
skeleton. detection C = C in unknown compounds. Red
15.2.4 Chemical properties of alkenes :
brown colour of bromine rapidly disappears in
Alkenes are more reactive than alkene
due to the presence of pi (p) electrons. They carbontetrachloride solution if a C = C double
undergo electrophilic addition reactions. The
bond is present in the compound.

3. Addition of hydrogen halides/

hydrohalogenation : Addition of hydrogen

halides (HX) like hydrogen chloride, hydrogen

bromide and hydrogen iodide converts alkene

into the corresponding alkyl halides.

different reactions of alkenes are given below: C C +H−X CC
1. Addition of dihydrogen/hydrogenation ( see

section 15.1.4). Alkene Hydrogen halide AlkHyl haXlide
2. Addition of halogens/halogenation.

Alkenes are converted into the corresponding The order of reactivity of hydrogen
vicinal dihalides by addition of halogens halides is HI > HBr > HCl
(X2 = Cl2 or Br2) . Addition reaction of HBr to symmetrical

C C + X2 CCl4 CC alkenes : The addition of HBr to symmetrical
room temp alkenes yield only one product.

Alkene VicXinal dXihalide H2C = CH2 + H − Br H3C − CH2 − Br

Ethene Bromoethane

Iodine generally fails to react. H2C = CH2 + H − Br Br − CH2 − CH3

HH Ethene Bromoethane

H + Cl2 CCl4 HH
room temp
H C C HC CH

Cl Cl
Ethene 1,2-Dichloroethane

242

Addition reaction of HBr to unsymmetrical

alkenes : Addition of HBr to unsymmetrical Remember

alkenes yield two isomeric products.
H3C -1C-BHro2 m- CopHro2p-aBner 1. The orientation of addition of HBr to
A (minor) unsymmetrical alkene is determined by
the presence or absence of peroxide.
H3C -PCroHpe=neCH2 + H - Br
2. The peroxide has no effect upon the

B 2-Bromopropane addition of HCl and HI.

(major) 4. Addition of sulfuric acid
H3C − CH − CH3

Br Alkenes react with cold concentrated

Experimentally it has been found that sulphuric acid to form alkyl hydrogen sulfate

2-Bromopropane is the major product. ( ROSO3H). The addition takes place according
Russian chemist Markovnikov studied to Markovnikov's rule.

hydrohalogenation of a number of an C C + H OSO3H
unsymmetrical alkenes and formulated a
geneneral rule (1869) as follows : Alkene Cold conc.

Markovnikov's rule : When an unsymmetrical C C
reagent is added to unsymmetrical alkene, the
negative part (X ) of the reagent gets attached H OSO3H
to the carbon atom which carries less number
of hydrogen atoms. Alkyl hydrogen sulfate

HH

Anti- Markovnikov addition Or peroxide H C C H + H OSO3H
effect or Kharasch - Mayo effect : In 1933,
ethene Cold conc.

M. S. Kharasch and F. R. Mayo discovered that HH
the addition of HBr to unsymmetrical alkene H CCH
in the presence of organic peroxide (R-O-O-R)

takes place in the opposite orientation to that H OSO3H

suggested by Markovnikov 's rule. ethyl hydrogen sulfate

H3C − CH = CH2 + H − Br (C6H5CO)2 O2 CH3 - CH = CH2 + H2SO4 CH3 - CH - CH3
Benzoyl peroxide OSO3H

Propene isopropyl hydrogen sulfate

H3C − CH2 − CH2 − Br If alkyl hydrogen sulfate is diluted with

1-Bromopropane water and heated, then an alcohol having the

H 23C-M−ethCC ylH=pr3oCpH-12- +enHe − B r (BCe6nHz5oCyO l p)2eOro2xide fssoaurlmftaehteaellikasyroglbgetrasoicunapeldea.smTthahenisuoifrsaigacintnuaerelxacolekfllyaellnchtoymhdoreoltshg.oend
CH3
H3C − C − CH2 − Br
H CH3 - CH2 - OSO3H H - OH
∆

1-Bromo-2-methylpropane Ethyl hydrogen sulfate HH

Internet my friend H C C H + H2SO4
https://www.britanicacom>science>hydrocarbon H OH

Ethanol

243

H H - OH HH HH O3 H OH
∆ C C CH3 HCCH CCl4 H CO OC H
H3C − C − CH3 H H OH

OSO3H propan-2-ol (Ethene) (Ethene ozonide)
HH
Isopropyl hydrogen sulphate H2O C = O + O = C + ZnO
Zn dust
+ H2SO4 HH
Reactive alkenes on adding water

molecules in the presence of concentrated

sulphuric acid, form alcohol.The addition of (Formaldehyde) (Formaldehyde)

water takes place according to Markovnikov's H3C HH H OH

rule.This reaction is known as hydration of C C H O3 H3C 1 CO OC2 H

alkenes. 1 2 CCl4

CH3H (Propene) (Propene ozonide)
2H-M3CethyClpropC-1-enHe + H − OH H+ H
C H3 H2O H3C
Zn dust H C = O + O =2C + ZnO

1 H

H3C − C − CH3 (Acetaldehyde) (Formaldehyde)
OH
Remember
2-Methylpropan-2-ol

Can you tell? 1. In the cleavage products a carbonyl group
( C =O) is formed at each of the original
Why are Propan-1-ol and 2-Methypropan-1- doubly bonded carbon atoms.
ol are not prepared by this method ?
2. Knowing the number and arrangement
5. Ozonolysis of alkenes : The C = C double of carbon atoms in these aldehydes and
ketones produced, we can identify the
bond gets cleaved on reaction with ozone structure of original alkene.

followed by reduction. This overall process of 3. The role of zinc dust is to prevent the
formation of hydrogen peroxide which
formation of ozonide by reaction of ozone with oxidizes aldehydes to corresponding
acids.
alkene in the first step and then decomposing
4.This reaction is used to locate the position
it to the carbonyl compounds by reduction in and determine the number of double
bonds in alkenes.
the second step is calledOozonolysis.
O3 H2O
CC CCl4 CO OC Zn dust

12

Alkene alkene ozonide

1 = O + O = C2 + ZnO Use your brain power

C On ozonolysis an alkene forms the
following carbonyl compounds. Draw the
Carbonyl compounds structure of unknown alkene from which
these compounds are formed.
Ozone gas is passed into solution of HCHO and CH3COCH2CH3
the alkene in an inert solvent like carbon
tetrachloride, unstable alkene ozonide is 6. Hydroboration- oxidation of alkene
obtained. This is subsequently treated with Alkenes with diborane in tetrahydrofuran
water in the presence of a reducing agent zinc (THF) solvent undergo hydroboration to
dust to form carbonyl comounds, namely, form trialkylborane, which on oxidation with
aldehydes and/or ketones .

244

alkaline peroxide forms primary alcohol. The H C = C H O2 Heat, pressure
n polymerization
overall reaction gives Anti-Markovnikov's
HH
product from unsymmetrical alkenes HHHH
(Ethene)
( (6 ( C (C + (BH3)2 THF 2 C C
B (Monomer)

3 C − C − C − C
HH H H
(Alkene) (Diborane) (Trialkylborane) ( (H H

HO - OH Or C−C n
HH
OH + C C +B(OH)3

OH (Polyethene)

(Alcohol)

H ( (H (Polymer)
6H C
C H+ H THE 2(CH3- CH2-)3B Here n represents the number of repeating units
HB solvent
H and is a large number.
2

(Ethene) (Diborane) Use your brain power

( (2 Write the structure of monomer from
H3C - CH2 3 HO-OH
H3C - CH2 B NaOH which each of the following polymers are

H3C - CH2 obtained.

Teflon (CF2 CCFH2)3H
Polypropene C−C
(Triethylborane) ( (a. −

+6CH3-CH2-OH + 2B(OH)3 b. n

(Ethanol)

H ( (H H H Cl
6H3C C HB
C H+ H 2 THE ( (c. Polyvinyl chloride HH
solvent C−C
H Cl n
(Propene) (Diborane)

( (H3C - CH2 - CH2 2(CH3-CH2- CH2-)3B
H3C - CH2 - CH2
B 3 HO-OH 8. Hydroxylation : Alkenes react with cold
NaOH
H3C - CH2 - CH2 and dilute alkaline potassium permanganate

(Tri-n-propylborane) to form glycols.

3CH3−CH2−CH2− OH + B(OH)3 HH alkaline
C = C + H - OH + (O) KMnO4
(Propan-1-ol)
HH

7. Polymerization : The process in which (Ethene)
large number of small molecules join together
HH

and form very large molecules with repeating H C C H
units is called polymerization. The compound
having very large molecules made of large OH OH
number repeating small units is called polymer
and the simple compound forming the repeating H3C H (Ethane-1,2-diol)
units in the polymer is called monomer. +
C = C H − OH + (O) alkaline

HH KMnO4 H

H

For example, ethene at high temperature and (Propene) H3C C C H
under high presssure interacts with oxygen, OH OH
and undergoes polymerization giving high
molecular weight polymer called polyethene. (Propane-1,2-diol)

245

Hydroxylation of alkenes is the most Table 15.4 : Lower alkynes

important method for the synthesis of 1,2-diols n Molecular Structural formula IUPAC
and
(Glycols). During this reaction the purple formula Common name name

colour of KMnO4 disappears. Hence such 2 C2H2 H-C ≡ C-H Ethyne
reaction serves as qualitative test for detecting Acetylene

the presence of double bond in the compound 3 C3H4 CH3-C≡ CH Methyl Propyne

under test. This is known as Baeyer's test. acetylene

9. Oxidation : Alkenes on oxidation with 4 C4H6 CH3-CH2-C≡CH But-1-yne
Ethyl acetylene
acidic KMnO4 or acidic potassium dichromate
form ketones and / or acids.
O 5 C4H6 C H 3 - C ≡ C - C H 3 But-2-yne

H3C − CH = CH2 KMnO4, H⊕ H(E3Ctha-nCoic- OH Dimethyl acetylene
(O)
(Prop-1-ene) acid)

CH3 + CO2 + H2O Do you know ?
H3C - C = CH - CH3
KMnO4, H⊕ 1. Simplest alkyne is ethyne which is known
(O) as acetylene.

(2-Methylbut-2-ene) CH3 O 2. The carbon-carbon triple bond is a
functional group.
PrHop3aCn--2-Con=e O+ H3C − C − OH
3. The aliphatic unsaturated hydrocarbons
Ethanoic acid containing two or three carbon-carbon triple

15.2.5 Uses of alkenes bonds are called alkadiynes and alkatriynes,
respectively.
1. Alkenes, are used as starting materials for
4. Cicutoxin from poisonous plants 'water
preparation of alkyl halides, alcohols, hemlock'.

aldehydes, ketones , acids etc. HO - (CH2)3 - C ≡ C - C ≡ C - (C10H14) -OH

2. Ethene and propene are used to manufacture

polythene, polypropylene those find use in

bags, toys, bottles, etc.

3. Ethene is used for artificial ripening of

fruits, such as mangoes.

Can you tell? 15.3.2 Preparation of alkynes

• What are aliphatic hydrocarbons? a. Industrial sources :
• Compare the proportion of carbon i. Ethyne is industrially prepared by controlled,
high temperature partial oxidation of methane.
and hydrogen atoms in ethane, ethene
and ethyne. Which compound is most M6CeHth4an+e2 O2 1 773K 2H - C≡ C- H + 2 CO2 + 10 H2
unsaturated with hydrogen ? Ethyne

15.3 Alkynes : Alkynes are aliphatic ii. From calcium carbide : Industrially the
unsaturated hydrocarbons containing at least
one C ≡ C.The number of hydrogen atoms is alkyne ethyne is prepared by reaction of
still less in alkynes as compared to alkenes.
calcium carbide with water.

CaC2 + 2H2O C2H2 + Ca (OH)2

Their general formula is CnH2n-2 . Table 15.4
shows names and formulae of lower alkynes.

15.3 Isomerism in alkynes : Alkynes show

position isomerism which is a type of structural

isomerism. For example 1-butyne and

2-butyne. 1-Alkynes are also called terminal

alkynes.

246

b. Methods of preparation of alkynes H−C≡C−H
1. By dehydrohalogenation of vicinal R−C≡C−H
dihalides : Removal of H and X from adjacent Such hydrogen atom shows appreciable
carbon atoms is called dehydrohalogenation. acidity and can be given away as proton on
Vicinal dihalides react with alcoholic solution reaction with sufficiently strong base.
of potassium hydroxide to form alkenyl halide In this method a smaller terminal alkyne first
which on further treating with sodamide forms reacts with a very strong base like lithium
alkyne. amide to form metal acetylide (Lithium amide
is easier to handle than sodamide).
HH Higher alkynes are obtained by reacting
metal acetylides (alkyn-1-yl lithium) with
C C + KOH - KX primary alkyl halides.
XX - H2O

Vicinal dihalide (Alcoholic)

H R - C ≡ C - H +Li+ NH2- R - C ≡ C - Li + NH3

CC X Na+ NH2- -C≡C- Terminal alkyne Lithium amide Alkyn-1-yl lithium
- NaX
Alkenyl halide - N H3
alkyne R - C ≡ C - Li + R1 - X R - C ≡ C-R1 + LiX

HH Alkyn-1-yl lithium Primary alkyl halide Higher alkyne

H C C H + KOH - KBr H-C≡C-H Li+NH2- H - C C - Li H3C − CH2 − Br
Br Br - H2O -NH3 Bromoethane
(Alcoholic) Ethyne ≡

1,2-Dibromoethane Ethynyllithium

HH H - CB≡ut-C1-y-nCeH2 - CH3 + LiBr

H C C Br Na+ NH2- H-C≡C-H
- NaX
- NH3 Ethyne

Bromoethene H3C - C ≡ C - H Li+NH2- H3C - C ≡ C - Li
-NH3
HH Propyne Prop-1-yn-1-yl lithium

H3C C C H + KOH - KCl H 3BCr−omCoHe2th−aBner HP3eCnt--2C-yn≡e C - CH2 - CH3 + LiBr
(Alcoholic) - H2O
Cl Cl
1,2-Dichloropropane

HH Use your brain power
Convert : 1-Bromobutane to Hex-1yne.
H3C C C Cl Na+ NH2- H3C - C ≡ C - H
- NaCl
- NH3

1-Chloropropanene Propyne

Can you tell? 15.3.3 Physical properties of alkynes : The
physical propeties of alkynes are similar to
Why sodamide is used in second step those of alkanes and alkenes. They are less
to remove HX from alkenyl halide in place dense than water. They are insoluble in water
of alcoholic KOH? and quite soluble in less polar organic solvents
like ether, benzene, carbon tetrachloride. The
2. From terminal alkynes : Terminal alkynes melting points and boiling points of alkynes
are the compounds in which hydrogen atom is increase with an increase in molecular mass
directly attached to triply bonded carbon atom. (Table 15.5).

247

Table 15.5 : Melting point and Boiling point of alkynes

Fornula Name Molecular Physical b.p.(K) m.p.(K)

mass/u State 191
171.5
CH =CH Ethyne 26 Gas 198 151

CH = CCH3 Propyne 40 Gas 250 175
CH =CCH2CH3 But-1-yne 54 Gas 282 149

CH = C(CH2)2CH3 Pent-1-yne 68 Liquid 313

CH =C(CH2)3CH3 Hex-1-yne 82 Liquid 345

15.3.4 Chemical properties of alkynes : Use your brain power

1. Acidity of alkynes : Arrange following hydrocarbons in the
increasing order of acidic character.
The hydrogen bonded to C ≡ C triple propane, propyne, propene.
bond has acidic character. Lithium amide (

LiNH2) is very strong base and it reacts with
terminal alkynes to form lithium acetylides

with the liberation of hydrogen indicating Do you know ?

acidic nature of terminal alkynes. Why is

it so ? In terminal alkynes, hydrogen atom Acidic alkynes react with certain
heavy metal ions like Ag+ and Cu+ and form
is directly attached to sp hybridized carbon insoluble acetylides. On addition of acidic
alkyne to solution of AgNO3 in alcohol
atom. In sp hybrid orbital, the percentage of form a precipitate which indicates that the
hydrogen atom is attached to triply bonded
s- character is 50%. An electron in s-orbital is carbon. This reaction is used to differentiate
terminal alkynes and non-terminal alkynes.
very close to the nucleus and is held tightly. 2. Addition of dihydrogen (See method of
preparation of alkenes from unsaturated
The sp hybrid carbon atom in terminal alkynes hydrocarbons)
3. Addition of halogens
is more electronegative than the sp2 carbon in - C ≡ C - + X - X CCl4 - C = C -

ethene or the sp3 carbon in ethane. Due to high

electronegative character of carbon in terminal

alkynes, hydrogen atom can be given away as

proton ( H+ ) to very strong base.

Examples

H−C≡C−H Li+NH2- H − C ≡ C − Li
-NH3
Ethyne Monolithium ethynide XX
Li+NH2-
Li − C ≡ C − Li + NH3 Alkyne (X2 = Cl2, Br2) 1, 2 - Dihaloalkene

Dilithium ethynide XX

PHr3oCp-1--Cyne≡ C - H Li-+NNHH32- LHit3hCium- Cpr≡opCyn-idLe i X−X −C−C−

The relative acidity of alkanes, alkenes and XX
alkynes follows the order
H − C ≡ C − H > H2C = CH2 > H3C − CH3 1, 1, 2, 2 - Tetrahalokane

Ethyne reacts with bromine in inert solvent

such as carbon tetrachloride to give

tetrabromoethane.
H - C ≡ C - H + Br - Br CCl4

Can you tell? Ethyne Bromine Br Br

Alkanes and alkenes do not react H - C = C - H Br − Br H − C - C − H
with lithium amide. Give reason.
Br Br Br Br

1,2-Bibromoethene 1,1,2,2-Tetrabromoethane

248

Red-brown colour of solution of bromine Alkynes react with water in presence of
40% sulphuric acid and 1% mercuric sulphate
in carbon tetrachloride disappears. This to form aldehydes or ketones i.e. carbonyl
compounds.
test can be used to detect the presence of

unsaturation in given compound.

4. Addition of hydrogen halides 40% H2SO4
1% HgSO4
−C≡C−+H−X −C=C− [ [H − C ≡ C − H + H − OH

HX Ethyne
H
Alkyne (HX = HCl , HBr,HI)
H C C H Tautomerisation
HX
H−X −C-C− OH

HX Vinyl alcohol H

Geminal dihalides H − C − C− H
OH
Hydrogen halides (HCl, HBr and HI) add
Ethanal
to alkynes across carbon-carbon triple bond in
40% H2SO4
two steps to form geminal dihalides (in which [ [HPr3oCpy−neC ≡ C − H + H − OH1% HgSO4

two halogen atoms are attached to the same

carbon atom).The addition of HX in both the H

steps takes place according to Markovnikov's H3C C C H Tautomerisation

rule. OH

H − C ≡ C − H + H − Br H −C=C−H

Ethyne Hydrogen bromide H Br H3C − C − CH3
O
1-Bromoethene
Propanone
H Br

H − Br H −C-C−H Use your brain power

H Br Convert : 3-Methylbut-1-yne into
3-Methylbutan-2-one.
1,1-Dibromoethane50%
15.3.5 Uses of acetylene :
H3C − C ≡ C − H + H−Br H3C − C = C − H 1. Ethyne (acetylene) is used in preparation of
Ethanal (acetaldehyde), Propanone (acetone),
Br H ethanoic acid (acetic acid).
2. It is used in the manufacture of polymers,
Propyne Hydrogen bromide 2-Bromopropene synthetic rubber, synthetic fibre, plastic etc.
3. For artificial ripening of fruits.
H − Br Br 4. In oxy-acetylene ( mixture of oxygen and
acetylene) flame for welding and cutting of
H3C − C − CH3 metals.
Br 15.4 Aromatic Hydrocarbons :

2,2-Dibromopropane Can you recall?

The order of reactivity of hydrogen halides is • What are aromatic hydrocarbons?
• What are benzenoid and non-benzenoid
HI > HBr > HCl
aromatics ?
5. Addition of water [ [C C
H OH
- C ≡ C - + H - OH 40% H2SO4
1% HgSO4

Alkyne Unstable

H

Tautomerism − C − C −

HO

Carbonyl compound

249

Many organic compounds obtained Remember
from natural sources like resins, balsams,
oil of wintergreen, etc. possessing pleasant 1. Aromatic compounds contain planar
fragrance (aroma = smell) are named as cyclic rings.
aromatic compounds. Aromatic hydrocarbons
(also called arenes) contain only carbon and 2. Not all compounds those resemble
hydrogen. Benzene is the simplest aromatic benzene have pleasant odour (smell).
compound.
Benzene and all compounds that have Anthracene is
structures and chemical properties resembling odourless.
benzene are classified as aromatic compounds.
Examples are : 3. Many compounds having pleasant odour
do not resemble benzene.
CH3 Br
O
H3C C O CH3

(Methyl acetate)

(Benzene) (Methylbenzene) (Bromobenzene) O
OH
H3C C O C2H5

(Ethyl acetate)

Cl

(Phenol) (Naphthalene) H C Cl
Cl
15.4.1 Benzene : The molecular formula of
(Trichloromethane or chloroform)

benzene is C6H6. Benzene is parent compound It is a colourless liquid having characteristic
of most of the aromatic compounds. odour. Its boiling point is 353K.

Coal-tar and petroleum are the two It was synthesized by Berthelot (1870)
large-scale sources of benzene (and aromatic from acetylene. Benzene was originally called
compounds like toulene , phenol, naphthalene.). phene and hence C6H5 is called phenyl group.

Table 15.6 : Difference between aromatic and aliphatic compounds

Aromatic compounds Aliphatic compounds

1. Aromatic compounds contain higher 1. Aliphatic compounds contain lower
percentage of carbon. percentage of carbon.

2. They burn with sooty flame. 2. They burn with non-sooty flame.

3. They are cyclic compounds with alternate 3. They are open chain compounds.
single and double bonds.

4. They are not attacked by normal oxidizing 4. They are easily attacked by oxidizing and

and reducing agents. reducing agents.

5. They do not undergo addition reactions 5. Unsaturated aliphatic compounds undergo

easily. They do not decolourise dilute alkaline addition reactions easily.They decolourise

aqueous KMnO4 and Br2 in CCl4, though double dilute aqueous alkaline
bonds appear in their structure. KMnO4 and Br2 in CCl4.

6. They prefer substitution reactions. 6. The saturated aliphatic compounds give

substitution reactions.

250

15.4.2 Structure of benzene : H
HC C CH
1. Molcular formula of benzene, C6H6, Usually
indicates the high degree of unsaturation. HC C CH written as

2. Open chain structure NOT possible : Open H

chain or cyclic structure having double and The Kekulé structure indicates

triple bonds can be written for C6H6. But the possibility of two isomeric
benzene does not behave like alkenes or
1,2-dibromobenzenes. In one of the isomers,
alkynes (see Table 15.7). This indicates that
the bromine atoms would be attached to the
benzene can not have the open chain structure.
Table 15.7 : Comparative reactivity of alkenes doubly bonded carbon atoms whereas in the

and benzene other, they would be attached to single bonded

Reaction Alkene Benzene carbons.
Br Br
With dil. Decolourisation No Br Br
alka. of purple colour of decolourisation
KMnO4 KMnO4 +
With Br2 in Decolourisation of No
CCl4 red brown decolourisation However, benzene was found to form only
colour of bromine one ortho-disubstituted benzene. This problem
With H2O Addition of H2O No reaction was overcome by Kekule' by suggesting the
in acidic molecule concept of oscillating nature of double bonds
medium in benzene as given below.

3. Evidence of cyclic structure : Benzene yields Even with this modification, Kekule' structure
of benzene failed to explain unusual stability
only one and no isomeric monosubstituted and preference to substitution reactions rather
than addition reactions, which was later
bromobenzene (C6H5Br) when treated with explained by resonance.
equimolar bromine in FeBr3. This indicates 5. Stability of benzene:
that all the six hydrogen atoms in benzene are
Benzene is a hybrid of various resonance
identical. C6H5Br + HBr srtuctures.The two structures, A and B given
C6H6 + Br2 FeBr3 by Kekulé are the main contributing structures.
The resonance hybrid is represented by
This is possible only if benzene has cyclic inserting a circle or a dotted circle inscribed
in the hexagon as shown in (C). The circle
structure of six carbons bound to one hydrogen represents six electrons delocalized over the
six carbon atoms of benzene ring.
atom each.
Or
Benzene on catalytic hydrogenation gives
(A) (B) (C)
cyclohexane. Ni C6H12 A double headed arrow between the
∆
C6H6 + 3H2 (Cyclohexane) resonance structures is used to represent the
resonance phenomenon.
(Benzene) Stability of benzene : The actual molecule is
more stable than any of its resonance structures.
This confirms the cyclic structure of benzene For benzene, the stability due to resonance

and three C = C in it.

4. Kekulé structure of benzene :

August Kekulé in 1865 suggested the

structure for benzene having a cyclic planar

ring of six carbon atoms with alternate single

and double bonds and hydrogen atom attached

to each carbon atom.

251

is so high that p-bonds of the molecule resist atom energies. P-orbital
breaking. This explains lack of reactivity of
benzene towards addition. π MO

The orbital overlap gives us better picture Fig 15.6 : Representative p molecular
of structure of benzene. All six carbon atoms orbital in benzene
in benzene are sp2 hybridised. Two sp2 hybrid
orbitals of carbons overlap and form carbon- Remember
carbon sigma (s) bond and the remaining third
sp2 hybrid orbital of each carbon overlaps with
s orbital of a hydrogen atom to form six C-H
sigma bonds.

Hσ σ σ H

σ
σ
σ In benzene , a. All carbon and hydrogen
σ
Hσ σH atoms lie in the same plane.

Hσ σ σ b. Six sigma ( s ) bonds lie in the same
H
plane.

The unhybrid p orbitals of carbon atoms c. All bond angles are 1200 H
overlap laterally forming p bonds. There are H
two possibilities of forming three p bonds by
overlap of p orbitals of C1-C2, C3-C4, C5-C6 or H 120° 120°
C2-C3, C4-C5, C6-C1, respectively, as shown in H
Fig. 15.5; both are equally probable. According
to resonance theory (Chapter 5) these are two HH
resonance structures of benzene.
The six electrons of the p orbitals cover
12 3 Or all the six carbon atoms and are said to be
6 delocalized. Delocalization of p electrons
results in stablity of benzene molecule.
54 6. Bond parameters of benzene : X-ray
diffraction data indicate that all C-C bond
Or lengths in benzene are equal ( 139 pm) which
is an intermediate between C-C (154 pm)
Fig. 15.5 Overlap of p orbitals in benzene and C=C bond (133pm). Thus absence of
According to molecular orbital (MO) pure double bond in benzene accounts for its
reluctance to addition reactions under normal
theory (Chapter 5) the six p orbitals of six conditions, which explains unusual behaviour
carbons give rise to six molecular orbitals of of benzene (Refer to sec.14.6.5).
benzene. Shape of the most stable MO is as
shown in Fig. 15.6. CC CC
1.39 A°
Three of these p molecular orbitals lie 1.54 A°
above and the other below those of free carbon Benzene-bond length
Single-bond lenght

252

4n + 2 = 6, ∴ 4n = 6-2 =4

CC n = 4/4 = 1, Here 'n' comes out to be an integer.
Hence benzene is aromatic.

1.34 A° 2. Naphthalene : It is cyclic and planar. It

Double-bond length has 5 double bonds and 10 p electrons.It has
15.4.3 Aromatic character ( Huckel Rule) :
Benzene undergoes substitution reaction p orbital on each carbon atom of the ring.
rather than addition reactions. This property
is common to all aromatic compounds and Hence a continuous overlap around the ring is
is referred to as aromaticity or aromatic
character. The aromatic character of benzene posssible. This is in accordance with Huckel
is correlated to its structure.
rule.
Aromaticity is due to extensive cyclic
delocalization of p electrons in planar ring 4n + 2 = Number of p electrons
structures.
4n + 2 = 10, ∴ 4n = 10 -2 = 8
The following three rules of aromaticity
are useful in predicting whether a particular n = 8/4 = 2, Here 'n' comes out to be an integer.
compound is
Hence napthalene is aromatic.
aromatic or non-aromatic.
3. Pyridine : Pyridine has three double bonds

and 6 p electrons. The six p orbital containing

six electrons form delocalized p molecular

orbital. The unused sp2 hybrid orbital of

nitrogen containing two non-bonding electrons

is as it is.

1. Aromatic compounds are cyclic and planar

(all atoms in ring are sp2 hybridized). N
2. Each atom in aromatic ring has a p-orbital.

The p-orbitals must be parallel so that Non bonding
continuous overlap is possible around the ring. electron pair
3. Huckel Rule : The cyclic p molecular
orbital formed by overlap of p-orbitals must 4n + 2 = Number of p electrons
contain ( 4n + 2) p electrons, where n= integer
0,1,2,3 ...etc. 4n + 2 = 6, ∴ 4n = 6 -2 = 4
Let us apply these rules to the following
compounds n = 4/4 = 1, Here 'n' comes out to be an integer.

Hence pyridine is aromatic.

4. Cycloheptatriene : It is cyclic and planar. It

has three double bonds and 6 p electrons. But

one of the carbons is saturated (sp3 hybridized)

Benzene Naphthalene and does not possess a p orbital. Hence a
continuous overlap around the ring is not

1. Benzene : It is cyclic and planar. It has possible.Therefore, it is non- aromatic.

three double bonds and six p electrons. It has sp3 hybridized
a p orbital on each carbon of the hexagonal

ring. Hence a continuous overlap above and

below the ring is posssible. 15.4.4 Preparation of aromatic compounds

a. Industrial source of aromatic compounds

Coal tar and petroleum are major sources of

aromatic compounds.

b. Methods of preparation of benzene

1. From ethyne ( By trimerization) : Alkynes

This compound is aromatic, 4n + 2 = Number when passed through a red hot iron tube at 873
of p electrons. K, polymerize to form aromatic hydrocarbons.

253

Ethyne when passed through a red hot iron to give benzene hexachloride .

tube at 873 K undergoes trimerization to form H CHHlClCCCHlCCHCClCCHlCHl
H
benzene. H H
H
CH H + 3Cl2 UV light

CH CH Red hot iron tube Or

CH 873k Benzene Benzene hexachloride (BHC)

CH

CH g - isomer of benzene hexachloride is called

Can you recall? gammexane or lindane which is used as
What is decarboxylation ? insecticide.
ii. Addition of hydrogen : When a mixture

2.From sodium benzoate : of benzene and hydrogen gas is passed
(By decarboxylation) over heated catalyst nickel at 453 K to 473
When anhydrous sodium benzoate is heated K,cylohexane is formed.

with soda-lime it gives benzene. + 3H2 Ni
453 - 473 K
O
C ONa Benzene (C6H6) Cyclohexane

+ NaOH CaO +Na2CO3 (C6H12)
∆ iii. Addition of ozone : When benzene is treated

Sodium benzoate Benzene with ozone in presence of an inert solvent

3. From phenol ( By reduction) : When carbon tetrachloride, benzene triozonide is

vapours of phenol are passed over heated zinc formed which is then decomposed by zinc dust

dust, it gives benzene. and water to give glyoxal. HO

OH H CO

+ Zn ∆ + ZnO + 3O3 O C O CH
O
CH
Phenol Benzene O CO O

15.4.5 Physical properties of benzene H CO
1. Benzene is colourless liquid.
2. Its boiling point is 353K and melting point Benzene H

is 278.5 K. Zn / H2O 3 CHO
3. It is isoluble in water. It forms upper layer CHO + 3H2O2

when mixed with water. Ethanedial or glyoxal
4. It is soluble in alcohol, ether, and chloroform.
5. Benzene vapours are highly toxic which on II. Substitution reactions : Benzene shows

inhalation lead to unconsciousness. electrophilic substitution reactions, in which
15.4.6 Chemical properties of benzene
Aromatic compounds are characterised by one or more hydrogen atoms of benzene ring
electrophilic substitution reactions. However,
they undergo addition and oxidation reactions are replaced by electrophilic groups like -Cl,
under special conditions. Some reactions of
benzene are discussed below. -Br, -NO2, -SO3H, -R (alkyl group) , -COR
I. Addition reactions (Acyl group) etc. (see Chapter 14).

i. Halogenation : In this reaction, hydrogen

atom of benzene ring is replaced by halogen

atom .

i. Addition of chlorine : Benzene when treated

with chlorine in presence of bright sunlight or

UV light, adds up three molecules of chlorine

254

Chlorination Cl NO2
H H
+ Cl2 Fe + HCl + H2O
H H
in dark H

Benzene Chlorobenzene Nitrobenzene

Chlorine reacts with benzene in dark in the When benzene is heated with a mixture

presence of iron or ferric chloride or anhydous of concentrated nitric acid and concentrated

aluminium chloride or red phosphorous as sulfuric acid (nitrating mixture) at about

catalyst to give chlorobenze . 313 K to 333 K , it gives nitrobenzene.

Electrophile : Cl⊕ , Chloronium ion Electrophile : NO2 ⊕ , nitronium ion.

Formation of the electrophile : Formation of the electrophile : HO NO2 +

Cl Cl + FeCl3 Cl⊕ + [FeCl4] 2H2SO4 2HSO4 + H3O⊕ + NO2⊕
Bromination of benzene is similar to
iii. Sulfonation :
chlorination : H

Br H H + HO -SO3H fumimg H2SO4
H H 373 K
+ Br2 FeBr3
+ HBr H

Benzene Bromobenzene Benzene

Electrophile : Br⊕, SO3H
H H + H2O
Formation of electrophile : HH

Br Br + FeBr3 Br⊕ + [FeBr4] H
Iodination of benzene is not possible as it is

reversible process. Benzenesulfonic acid

With excess of chlorine, benzene gives When benzene is heated with fuming

hexachlorobenzene. sulfuric acid (oleum) at 373 K, it gives benzene
H
sulfonic acid.
H H
H H + 6 Cl2 anhy.AlCl3 Electrophile : SO3 , free sulfur trioxide
Formation of the electrophile :

H 2H2SO4 H3O⊕ + HSO4 + SO3
iv. Friedel-Craft's alkylation reaction :
Benzene

Cl + CH3 - Cl anhydrous AlCl3
Cl Cl
Methyl Chloride
+ 6 HCl
Cl Cl Benzene

Cl CH3

Hexaclorobenzene + HCl

ii. Nitration : Methylbenzene
H (Toluene)

HH + HO-NO2 Conc. H2SO4 When benzene is treated with an alkyl
Heat halide like methyl chloride in the presence
H H Conc. of anhydrous aluminium chloride, it gives
toluene.
H
255
Benzene

The reaction is used to extend the chain outside In benzene, all hydrogen atoms are equivalent.

the benzene ring. Therefore, only one product is possible when it

Electrophile : R⊕ undergoes electrophilic substitution reactions.

Formation of the electrophile : Monosubstituted benzene :

R Cl + AlCl3 R⊕ + AlCl4 substituent
H C6 CS1 C2 H
v. Friedel-Craft's acylation reaction : H
HC C CH
anhy. AlCl3 -H C 4 C3H
heat
+ CH3- CO Cl HC C CH +S H C
5

Benzene Acetyl Chloride HH
Monosubstituted benzene
COCH3
Positions of carbon atoms in mono substituted

benzene :

+ HCl The positions 2 and 6 are equivalent and give

ortho (o-) products.

1- Phenylethanone or Acetophenone The position 3 and 5 are equivalent and give

+ (CH3- CO)2O anhy. AlCl3 meta (m-) products.
heat
The position 4 is unique and and gives para

Benzene Acetic anhydride (p-) product.

COCH3 Now in benzene , five positions are available

for electrophilic substitution.

+ CH3COOH When monosubstituted benzene is

subjected to further electrophilic substitution,

1- Phenylethanone or Acetophenone the second substituent i.e. electrophile (see

When benzene is heated with an acyl Chapter 14) or incoming group (E) can

halide or acid anhydride in the presence occupy any of these positions and give three

of anhydrous aluminium chloride, it gives disubstituted products. But these products are

corresponding acyl benzene. not formed in equal amounts.
SS
Electrophile : R-C⊕=O acylium ion
EE
Formation of the electrophile : R COCl +

AlCl3 R-C⊕ =O + AlCl4 S S S ortho-product
6. Combustion : When benzene is heated in
-H
air , it burns with sooty flame forming carbon E+

dioxide and water.

C6H6 + 15/2 O2 6 CO2 + 3 H2O EE meta-product
General combustion reaction for any S

hydrocarbon (CxHy) can be represented as para-product
follows:

CxHy + (x + y/4) O2 x CO2 + y/2 H2O E
15.4.7 Directive influence of a functional
Two types of behaviour are observed.
group in monosubstituted benzene a. ortho- and para - products or b. meta-
products are found as major products :
Structure of benzene :
This behaviour mainly depends on the
H nature of the substituent (S) already present
in the benzene ring and not on the nature of
HC C CH second substituent (E) i.e. incoming group.

HC C CH
H

256

Ortho and para directing groups : it has no nonbonding electron pair on the key

: :: atom.This is explained on the basis of special
: ::

::
::
Cl : , Br : , O H , O CH3 , type of resonance called hyperconjugation or

no bond resonance (see Chapter 14).
NH2 , NHR
CH3 , C2H5 , R etc. In case of aryl halides, halogens are

moderately deactivating. Because of their

strong -I effect, overall electron density on

The groups which direct the incoming the benzene ring decreases. It makes the
group to ortho and para positions are called
ortho and para directing groups. electrophilic substitution difficult. However,
Ortho and para directive influence of -OH
group : The resonance theory clearly explains due to resonance the electron density on
why certain substituents are ortho/para or meta
directing. Let us study the various resonance ortho and para positions is greater than
structures of phenol (see Chapter 14).
meta positions. Halogens are ortho and para

directing.

Let us study the various resonanting structures

of chlorobenzene.

::
:

:
::
:

:
:

:
::
:OH + OH + OH :Cl: O Cl: O Cl:

: :

Resonating structures Resonating structures

+ O   H : O   H O Cl : :Cl:

: :

It is clear from the above resonance Meta directing and deactivating groups
structures that the ortho and para positions H
have a greater electron density than the meta ⊕N O ⊕N H δC⊕ δN ,
positions. Therefore, -OH group activates O, H,
the benzene ring for the attack of second δO
substituent E at these electron -rich centres.
C H, δO
Remember
δ⊕
Due to -I effect of -OH group (see
Chapter 14), the electron density on ortho δO
positions of the benzene ring gets slightly
reduced. Thus resonance effect and C R, CO H,
inductive effect of OH group act opposite
to each other but +R effect of -OH is more δ⊕ δ⊕
powerful than -I effect.
δO
All ortho and para directing groups
possess nonbonding electron pair on the atom δ⊕ S O H
which is directly attached to aromatic ring. O etc.
Methyl group is an exception : The only
exception to above rule is methyl or alkyl δ
groups. It is ortho and para directing, although
All meta directing groups have positive
(or partial positive) charge on the atom which
is directly attached to an aromatic ring.

The groups which direct the incoming
group to meta positions are called meta
directing groups.

257

Metadirective influence of -NO2 group can It is clear from the above resonance structures
be explained by resonance theory : Meta that the ortho and para positions have
directing group withdraws electrons from the comparatively less electron density than at
aromatic ring by resonance, making the ring meta positions. Hence, the incoming group/
electron-deficient. Therefore, meta groups are electrophile attacks on meta positions.
ring deactivating groups. Due to -I effect , 15.4.6 Carcinogenicity and Toxicity :
-NO2 group reduces electron density in benzne
ring on ortho and para positions.So the attack Benzene is both toxic and carcinogenic
of incoming group becomes difficult at ortho (cancer causing). In fact, it might be considered
and para positions. Incoming group can attack "the mother of all carcinogens" as a large
on meta positions more easily. Let us study the number of carcinogens have structures those
various resonance structures of nitrobenzene. include benzene rings. Several polycyclic
aromatic compounds (containing more than
:: ::O ⊕N O:O ⊕N O two fused benzene rings) are produced by
:: incomplete combustion of tobacco, coal and
⊕ petroleum. In liver, benzene is oxidized to an
:: epoxide. Benzopyrene is converted into an
: epoxy diol. These substances are carcinogenic
Resonating structures and can react with DNA which can induce
mutation leading to uncontrolled growth of
:O ⊕N O :O ⊕N O :O ⊕ O cancer cells.
N

⊕

⊕

Internet my friend

1. chemed.chem.purdue.edu>1organic>Organic Chemistry
2. www.ncert.nic.in>ncerts>kech206(pdf)
3. https://www.britannica.com>science>benzene
4. https://pubchem.ncbi.nlm.nih.gov>Benzene

258

Exercises

1. Choose correct options E. Arrange the three isomers of alkane
A. Which of the following compound with malecular formula C5H12 in
has highest boiling point ? increasing order of boiling points and
a. n-pentane write their IUPAC names.
b. iso-butane
c. butane F. Write IUPAC names of the products
d. neopentane obtained by the reaction of cold
B. Acidic hydrogen is present in : concentrated sulphuric acid followed by
a. acetylene water with the following compounds.
b. ethane
c. ethylene a. propene b. but-l-ene
d. dimethyl acetylene G. Write the balanced chemical reaction
C. Identify 'A' in the following reaction:
CH3 - C = CH2 A CH3- C - CH3 for preparation of ethane from
CH3 O a. Ethyl bromide
b. Ethyl magnesium iodide
+ CO2 + H2O H. How many monochlorination products
a. KMnO4/H+
b. alkaline KMnO4 are possible for
c. dil. H2SO4/1% HgSO4 a. 2-methylpropane ?
d. NaOH/H2O2 b. 2-methylbutane ?
D. Major product of chlorination of Draw their structures and write their

ethyl benzene is : IUPAC names.
a. m-chlorethyl benzene I. Write all the possible products for
b. p - chloroethyl benzene
c. chlorobenzene pyrolysis of butane.
d. o - chloroethylbenzene J. Which of the following will exhibit
E. 1 - chloropropane on treatment with
geometical isomerism ?
alc. KOH produces : a. CH3- CH2 - C - CH3
a. propane
b. propene CH2
c. propyne
d. propyl alcohol b (CH3)2 C = CH2
2. Name the following :
A. The type of hydrocarbon that is used c. CH3- C = C - CH3
as lubricant. C2H5 C2H5
B. Alkene used in the manufacture of
polythene bags. K. What is the action of following on
C. The hydrocarbon said to possess ethyl iodide ?
carcinogenic property.
D. What are the main natural sources of a. alc, KOH
alkane? b. Zn, HCl
L. An alkene ‘A’ an ozonolysis gives 2
259
moles of ethanal. Write the structure
and IUPAC name of ‘A’.
M. Acetone and acetaldehyde are the
ozonolysis products of an alkene.
Write the structural formula of an
alkene and give IUPAC name of it.
N. Write the reaction to convert
a. propene to nypropylalcohol.
b. propene to isoproyl alcohol.

O. What is the action of following on 6. Identify giving reason whether the
following compounds are aromatic or
but-2-ene ? not.

a. dil alkaline KMnO4 reaction A.
b. acidic KMnO4
P. Complete the following

sequence : B.
HC ≡ CH LiNH2 ACH3 CH2Cl B HBr C

Comment on the acidic nature of B.

Q. Write the balanced chemical reactions C.

to get benzene from

a. Sodium benzoate. D.

b. Phenol. N

R. Predict the possible products of the 7. Name two reagents used for acylation
of benzene.
following reaction.
8. Read the following reaction and
a. chlorination of nitrobenzene, answer the questions given below.

b. sulfonation of chlorobenzene,

c. bromination of phenol,

d. nitration of toluene.

3. Identify the main product of the anhydrous
AlCl3
reaction + CH3Cl

a. CH3-CH2-CH3 O2
∆

b. CH3 - CH - CH3 60% H2SO4 CH3
OH 373K + HCl

A. Write the name of the reaction.
B. Identify the electrophile in it.
c. HC ≡ C-CH3 H2 C. How is this electrophile generated?
Pd-C/quinoline
Activity :
d. H-C≡CH3+H2O 40% H2SO4
1% HgSO4 Prepare chart of hydrocarbons and
note down the characteristics.
4. Read the following reaction and

answer the questions given below.
benzoyl
CH3 - C = CH2 + HBr peroxide

CH3

CH3 - CH- CH2Br
a. Write IUPAC name of the product.
b. State the rule that governs formation
of this product.

5. Identify A, B, C in the following
reaction sequence :

CH3 - CH = CH2 Br2/CCl4 A
room temperature

Zn B dil. alka. C
KMnO4

260

16. Chemistry in Everyday Life

Can you recall? 16.1 Basics of food chemistry
Food provides nutrients these are used
1. What are the components of balanced
diet ? by the body as the source of energy. These
nutrients also regulate growth, maintain
2. Why is food cooked ? what is the difference and repair body tissues. The nutrients
in the physical states of uncooked and comprise carbohydrates, lipids, proteins,
cooked food? vitamins, minerals and water. Grains, fruits
and vegetables provide carbohydrates and
3. What are the chemicals that we come vitamins; meat, fish, eggs, dairy products and
across in everyday life ? pulses provide proteins and vitamins. Lipids
are provided by vegetable oils, dairy products
The life, the atmosphere, the earth and the and animal fats.
universe, all have evolved over billions of years
to the present state. The evolution continues Most nutrients are organic
progress and accompanied by a variety of macromolecules. Proteins and carbohydrates
chemical changes. Natural phenomena such as are polymeric materials. As a result of
weathering, lightening, irruption of volcanoes, food digestion, the polymeric proteins and
photosynthesis, ripening of fruit, fermentation, carbohydrates ultimately break down into
release of fragrance by blooming flowers monomers, namely, α- amino acids and
and many others take around us involve glucose, respectively, under the influence of
intricate chemistry. Chemistry is involved in enzymes. Cooking makes food easy to digest.
a variety of life processes those occur within During the cooking process, high polymers of
and across our body. Human civilization carbohydrates or proteins are hydrolysed to
in different regions of the world discovered smaller polymers. The uncooked food mixture,
uses of various plant, animal and mineral described as heterogeneous suspension,
products for benefits of human life. With the becomes a colloidal matter on cooking.
advent of modern science, scientist discovered Because of smaller size of the resulting
structures of various constituent chemicals in constituent nutrient molecules, cooked food is
natural materials. Synthetic organic chemistry easier to digest than the uncooked food.
has led to advancement in science. Synthesis 16.1.2 Food quality chemistry :
of natural molecules and new molecules with
structural variation revolutionalized materials Just think
are used in all the walks of human life. This
influence is seen in all aspects of the basic 1. Why is food stored for a long time ?
needs, such as food, clothing, shelter and 2. What methods are used for preservation
beyond.
of food ?
In this chapter, we consider some aspects 3. What is meant by quality of food ?
of food chemistry, medicinal chemistry
and chemistry of cleansing materials with Quality of food is an important aspect
reference to compounds having simple of food chemistry. Food quality is described
structural features. in terms of parameters such as flavour,
smell, texture, colour and microbial spoilage.
Enzymes are present naturally in all foods.

261

Quality of foods changes on shelving O -H 1 O
mostly due to enzyme action, chemical 2
reactions with the environment and the action + O2 enzyme +H2O
of microorganisms. Some of these effects O
are beneficial. For example, setting of milk O -H
into curd and raising flour dough to make
bread is brought about by deliberate action of a poly phenol a quinone
microorganisms. Most changes brought about
by microorganisms and interaction with the Quinones undergo further reactions
environment however, adversely affect the
food quality. including polymerization giving brown

Problem 16.1 coloured products named tannins. This
How are the chemical reactions of food
stuff with the environment controlled browning reaction can be slowed down using
during storage ?
Solution : reducing agents such as SO2, ascorbic acid
Primarily the oxygen and microorganisms (vitamin C) or by change of pH by adding
in air are responsible for adverse effects on
stored food. The exposure of stored food to edible acid such as lemon juice (citric acid) or
atmosphere is minimized by storing them in
air tight container, evacuation or filling the vinegar.
container with N2 gas. Rate of a chemical
reaction decreases with the lowering of ii. Rancidity of oils and fats : On keeping for
temperature. Thus refregeration is useful
for controlling chemical reaction of food long time, oils and fats develop an unpleasant
stuff with envrionment. The reactions of
food stuff with environment are catalyzed or rancid smell and disagreeable taste.
by enzymes. Due to boiling, the enzymes
become denatured and the reactions are Fats are triesters of fatty acids (long chain
controlled.
carboxylic acids) and glycerol (propane - 1, 2,
Food preservation and food processing
methods aim at prevention of undesirable 3 - triol). One cause of rancidity is release of
changes and attempt about desirable changes
in food. The following cases illustrate some fatty acids produced during hydrolysis of fats
aspects of food quality and the underlying
chemisty. brought about by water present in food.
i. Browning of cut fruit/vegetables : When
fruits such as banana, apple or vegetables O
such as potato, bottelgourd are peeled and
sliced, sooner or later they turn brown. Cutting H2C - O - C- R1
action damage the cells resulting in release O
of chemicals. With the pH prevailing in fruit/ Hydrolysis
vegetables, the polyphenols released are HC - O - C - R2 + 3H2O
oxidised with oxygen in air owing to action O
from an enzyme to form quinones.
H2C - O - C - R3 O

Triglyceride

H - O - C - R1
+ O
H2C - OH
HC - OH + H - O - C - R2
H2C - OH +O

Glycerol H - O - C - R3

Free Fatty Acids

The hydrolysis of fats occurs rapidly in
the presence of certain microorganisms and
is an enzyme catalysed reaction. Rancidity of
milk and butter is due to the release of four,
six and eight carbon fatty acids (butanoic,

hexanoic and octanoic acids) on hydrolysis.
Chocolate develops oily or fatty flavour due to
release of palmitic, stearic and oleic acids on
hydrolysis. Lauric acid on hydrolysis gives a
soapy flavour to coconut oil.

262

The second cause of rancidity of oils and The long carbon chains of unsaturated
fats is oxidation by molecular oxygen in the fatty acids contain one or more C=C double
air. Many vegetable oils have one or more bonds. This produces one or more 'kinks' in
C=C double bonds in the fatty acid part of the chain, (see the Fig. 16.1) which prevent the
their structure. These are called mono or poly molecules from packing closely together. The
unsaturated fats. The unsaturated fat molecules van der Waals forces between the unsaturated
break down during the oxidation and form chains are weak. The melting points of
volatile aldehydes and carboxylic acids unsaturated fats therefore, are lower.
which give the unpleasent rancid taste.This
is called oxidative rancidity. It is caused by Natural fats are mixtures of triglycerides.
free radical reaction initiated by light (photo They do not have sharp melting points, and
oxidation) or catalysed by either enzymes or usually melt over a range of temperatures. The
metal ions. Polyunsaturated oils containing more unsaturated the fat lower is its melting
greater number of C=C double bonds and point and less crystalline it is. Some examples
usually become rancid very quickly. High of fats are given in Table 16.1.
temperature increases the rate of air oxidation
of unsaturated fats. Extensive oxidation can A C=C can have geometrical isomers cis
lead to some polymerization with consequent and trans. In the cis form of an unsaturated
increase in viscosity and browning. fatty acid the two hydrogens on the two double
iii. Saturated, unsaturated and trans fats : bonded carbons are on the same side of the
double bond, whereas they are on the opposite
Can you recall? sides in the trans isomer. The cis isomer is the
most common form of unsaturated fats. The
1. How is Vanaspati Ghee made ? trans form occurs only in animal fats and
2.What are the physical states of peanut processed unsaturated fats. Trans fats are
difficult to metabolize and may build up to
oil, butter, animal fat, vanaspati ghee at dangerous levels in fatty tissue.
room temperature ?
Fats in the form of lipoprotein are
You have noted earlier that fats are used in the body for transport of cholesterol.
triglycerides of fatty acids. Animal fats Excessive low density lipoprotein (LDL)
mostly contain saturated fatty acids, while results in deposition of cholesterol in blood
vegetable oils contain unsaturated fatty acids vessels, which in turn, results in the increased
as well. Long chains of tetrahedral carbon risk of cardio vascular disease. There is
atoms in a saturated fatty acid get packed some evidence that eating large amounts of
closely together. Moreover, van der Waal's saturated or trans unsaturated fats, increase
forces between the long saturated chains are the tendency of cholesterol getting deposited in
sufficiently strong to convert saturated fats blood vessels. Cis fats do not cause formation
into solid form at room temperature. of such deposits and decrease chance of
developing coronary heart disease.

O O
OO OO
OO OO
O O

a. A saturated fat molecule b. An unsaturated fat molecule.

Fig. 16.1 Molecular shapes of fats (A schematic representation)

263

Table 16.1 : Natural fats and their physical states

Mainly Saturated fats Mainly mono-unsaturated fats Mainly poly-unsaturated fats

Coconut fat/oil, butter fat, lard, Olive oil, peanut oil, canola oil Safflower oil, sunflower oil,

margarine, vanaspati ghee soyabean oil, corn oil, fish oil

solid liquid liquid

iv. Omega-3 : Two categories of the natural saturated carbons makes it fat soluble. It is
unsaturated fats of concern include those found in foods such as wheat germ, nuts, seeds,
containing either Omega-3 or Omega-6 fatty green leafy vegetables and oils like safflower
acids. These names are given for the position oil.
of the double bond in a long carbon chain of
the fatty acid. Omega denotes the last carbon For economic reason synthetic
of the carbon chain. Omega-3 fatty acids have antioxidants are used as additives to increase
C=C bond between the third and fourth carbon the shelf-life of packed foods. Common
from the end of a carbon chain. structural units found in synthetic antioxidants
For example : Linolenic Acid are phenolic OH group and tertiary butyl
group. For example BHT, which is 3, 5-di-tert-
(9,12,15-octadecatrienoic acid) butyl-4-hydroxytoluene.
O

HO C 9 12 15 ω
α 3

Omega-3 fats are found to raise the High

density lipoprotein HDL (good cholesterol) BHT

level of blood. On the contrary, Omega-6 16.2 Compounds with medicinal properties
fats are considered to have risk of high blood

pressure. Foods such as walnuts, flaxseeds, Can you tell?
chia seeds, soyabeans, cod liver oil are rich

source of Omega- 3 fatty acids. 1. What type of medicine is applied to a

v. Antioxidants as food additives : An bruise ?

antioxidant is a substance that delays the 2. When is an antipyretic drug used ?
onset of oxidation or slows down the rate of 3. What is meant by a broad spectrum
oxidation of food stuff. It is used to extend
antibiotic ?
the shelf life of food. Antioxidants react with 4. What is the active principle of cinnamon
oxygen-containg free radicals and thereby
bark ?
prevent oxidative rancidity.
For example, vitamin E (tocopherol) is A chemical which interacts with
biomolecules such as carbohydrates, lipids,

CH3 proteins and nucleic acid and produces a
HO
biological response is called drug. A drug
CH3 CH3 CH3 having therapeutic and useful biological

H3C O CH3 response is used as medicine. A medicine
CH3 contains a drug as its active ingredient.
CH Besides it contains some additional chemicals
3 which make the drug suitable for its use as
medicine. Medicines are used in diagnosis,
Vitamin E (Tocopherol) prevention and treatment of a disease.
a very effective natural antioxidant which is
added to pack edible oils. The phenolic OH Drugs being foreign substances in a body,
group in its structure is responsible for its often give rise to undersirable, adverse side
antioxidant activity, while the long chain of

effects.

264

Drug design is an important branch of Molecular structure of paracetamol
medicinal chemistry which aims at synthesis
of new molecules having better biological 16.2.2 Antimicrobials : The name
response. There is an increasing trend in antimicrobial is an umbrella term for any drug
the current research in medicinal chemistry that inhibits or kills microbial cells that include
to take cognizance of traditional medical bacteria, fungi and viruses. Disinfectants are
knowledge such as Ayurvedic medicine or non-selective antimicrobials, which kill a wide
natural materials to discover new drugs. range of microorganisms including bacteria.
Disinfectants are used on non-living surfaces
In this section, we consider some simple for example, floors, instruments, sanitary
compounds having medicinal properties and ware and many others. Antiseptics are used
active ingredients of some natural materials to sterilise surfaces of living tissue when the
those are traditionally known to possess risk of infection is very high, such as during
medicinal properties. surgery, on wounds and so on. Antibiotics
are a type of antimicrobial designed to target
Do you know ? bacterial infections within or on the body.
a. Antiseptics and disinfectants : Commonly
The drug manufacturing companies used antiseptics include inorganics like iodine
usually have a patent for drugs which are and boric acid or organics like iodoform
sold with the brand name. After the expiry and some phenolic compounds. Tincture of
of patent the drug can be sold in the name iodine (a 2 - 3 percent solution of iodine in
of its active ingredient. These are called alcohol- water mixture) and iodoform serve
generic medicines. as powerful antiseptics and find use to apply
on wounds. A dilute aqueous solution of
16.2.1 Analgesics and antipyretics boric acid is a weak antiseptic used for eyes.
'Algesis' is a Greek word meaning 'feeling Various phenols are used as antiseptics and
disinfectants. A dilute aqueous solution of
of pain.' Drugs which give relief from pain are phenol (known as carbolic acid) was one of
called analgesics. About half of the analgesics the first antiseptic used in medicine in the late
are anti-inflammatory drugs, which kill nineteenth century. It was however, found to be
pain by reducing inflammation or swelling. corrosive. Many chloro derivatives of phenols
Antipyretics are used to reduce fever. have been realized as more potent antiseptics
than the phenol itself. They can be used with
Pain-killing and fever reducing properties much lower concentrations, which reduce their
of an extract of bark of willow plant corrosive effects. Two of the most common
(commonly found in Europe) have been known phenol derivatives in use are trichlorophenol
for centuries. In nineteenth century its active (TCP) and chloroxylenol. The latter is the
ingredient, salicylic acid (2-hydroxybenzoic active ingredient (4.8 % W/V) of the popular
acid) was isolated and purified. The compound antiseptic dettol. The other ingredients of dettol
aspirin which is acetyl derivative of salicylic are isopropyl alcohol, pine oil, castor oil soap,
acid was found to have a fewer side effects caramel and water. Thymol obtained from oil
than salicylic acid. It is one of the most widely of thyme (a spice plant) is an excellent non-
used analgesic to date. It however, retains toxic antiseptic. The p-chlorobenzyl phenol is
stomach irritating side effects of salicylic acid. used as disinfectant in all purpose cleaners.
A less problematic pain-killer is paracetamol,
which is a phenol.

Salicylic acid Aspirin (prodrug)

265

Prontosil

Chloroxylenol 2, 4, 6 Trichlorophenol
OH

Sulphapyridine
Cl

Thymol p - Chloro - o - benzylphenol

b. Antibiotics : Antibiotics are purely synthetic

or obtained from microorganisms (bacteria,

fungi or molds). Arsenic compounds were Sulphanilamide

known to be highly poisonous to humans since In 1929, Alexander Fleming
discovered the antibacterial properties of
long. Paul Ehrlich, German bacteriologist a penicillium fungus. The clinical utility of
the purified active ingredient penicillin as
investigated arsenic based organic compounds antibiotic drug was established in the next
thirteen years. This is the first antibiotic
in order to produce less toxic substances for of microbial origin. Chloramphenicol,
isolated in 1947 is another antibiotic of
the treatment of syphillis. He discovered microbial origin.

the first effective treatment of syphilis, the

synthetic antibiotic named salvarsan. He was

awarded the Nobel prize for medicine (1908)

for this discovery. Ehrlich noticed similarity

in the structures of salvarsan and azodyes. O
R - C - NH
With further investigations he succeded in S CH3
O
synthesis of an effective diazo antibacterial, N CH3
COOH
prontosil, in 1932. Subsequently it was

discovered that prontosil gets converted into General structure of penicillin

a simpler compound sulphanilamide in our NH - CO - CHCl2
O2N CH - CH - CH2OH
body. This gave further direction to research
OH
in drug design which led to discovery of a Chloramphenicol

wide range of sulpha drugs, analogues to

sulphanilamide. One of the most effective Antibiotics can be of three types : broad
spectrum (effective against wide range of
being sulphapyridine. bacteria), narrow spectrum (effective against
one group of bacteria) or limited spectrum
Salvarsan (effective against single organism).
Azodye
A disadvantage of broad spectrum
antibiotics is that they also kill the useful
bacteria in the alimentary canal. Today many
broad spectrum, narrow spectrum and limited
spectrum antibiotics are known. They are
synthetic, semisynthetic or of microbial origin.

266

6.2.3 Traditional knowledge in medicine Since ancient times, in India,

many grandma remedies are

Do you know ? practised for curing ailments. The
ancient medicinal system of India
The turmeric patent battle : has documented medicinal uses of
India won the legal battle against US patent and innumerable Indian plants. There is
an increasing trend in the modern
Trademark office (PTO) in 1997 and protected its medicinal chemistry to make use
intelectual property of traditional Indian knowledge of the traditional knowledge from
about turmeric against patenting. Dr. Raghunath various parts of world, to isolate
Mashelkar, the then Director General of the Council of active ingredients from medicinal
Scientific and Industrial Research, New Delhi, India, plants and further develop new drugs.
led this case and upheld the national pride. In this long Table 16.2 enlists a few medicinal
battle the CSIR argued that turmeric, a native Indian plants, their medicinal property and
plant, had been used for centuries by its people for active ingredients therein.
wound healing.

Table 16.2 Active ingredients of some medicinal plants

Plant Medicinal property Name and structure of the active ingredient

Turmeric antiseptic Curcumin

Wintergreen analgesic Methyl salicylate
O
Cinnamon antimicrobial for colds C O CH3
OH
Clove antimicrobial, analgesic
Cinnamaldehyde
Citrus fruits antioxidant CH = CH - CHO

Eugenol

Vitamin C (ascorbic acid)

Indian gooseberry antidiabetic, antimicrobial Vitamin C, Gallic acid

(amla) antioxidant

267

16.3 Cleansing Agents : Cleansing agents are Hard water and soap : Soaps are water
substances which are used to remove stain, dirt soluble. They form scum in hard water
or clutter on surfaces. They may be natural or and become inactive. This is because hard
synthetically developed. water contains dissolved salts of calcium
and magnesium, which react with soap,
Can you tell? precipitating calcium or magnesium salt of
fatty acid (scum) which sticks to fabric.
• Can we use the same soap for bathing 2 R - COONa (aq) + CaCl2(aq)
as well as cleaning utensils or washing
clothes ? Why ? (R - COO)2Ca(s) + 2NaCl(aq)
Washing soda (Na2CO3) precipitates the
• How will you differentiate between dissolved calcium salts as carbonate and helps
soaps and synthetic detergent using the soap action by softening of water.
borewell water? b. Synthetic detergents : Synthetic detergents
are man made cleansing agents designed to
16.3.1 Types of cleansing agents : use even in hard water. There are three types
Commercially cleansing agents are mainly of synthetic detergents, anionic detergents,
of two types depending upon chemical cationic detergents and nonionic detergents.
composition : soaps and synthetic detergents. i. Anionic detergents are sodium salts of
a. Soaps : Soaps are sodium or potassium long chain alkyl sulfonic acids or long
salts of long chain fatty acids. They are chain alkyl substituted benzene sulphonic
obtained by alkaline hydrolysis of natural acids. ii. Cationic detergents are quaternary
oils and fats with NaOH or KOH. This is ammonium salts having one long chain alkyl
called saponification reaction. Chemically group. iii. Nonionic detergents are ethers
oils are triesters of long chain fatty acids and of polyethylene glycol with alkyl phenol or
propan-1,2,3-triol commonly known as esters of polyethylene glycol with long chain
glycerol or glycerin. Saponification of oil fatty acid. Table 16.3 displays some syntheic
produces sOoap and glycerol. detergents.
Mechanism of cleansing action : Soaps and
CH2 - O - CO- R detergents bring about cleansing of dirty,
greasy surfaces by the same mechanism.
CH - O - CO - R + 3 NaOH saponification Dirt is held at the surface by means of oily
matter and, therefore, cannot get washed with
CH2 - O - C - R water. The molecules of soaps and detergent
have two parts. One part is polar (called head)
oil/fat O and the other part is long nonpolar chain of
carbons (called tail).
CH2 - OH + 3 R - C - ON⊕a
CH - OH

CH2 - OH

glycerin soap Foam

The quality of soap depends upon the Greasy dirt particle
nature of oil and alkali used. Potassium

soaps are soft to skin. Therefore toilet soaps

are prepared by using better grades of oil

and KOH. Care is taken to remove excess of Hydrophobic tail
alkali. Laundary soaps are made using NaOH. Hydrophilic head

These also contain fillers like sodium rosinate

(a lathering agents), sodium silicate, borax, Fig. 16.2 : Soap micelle at work
sodium and trisodium phosphate.

268

The polar head (hydrophilic) can dissolve in water which is polar solvent. The nonpolar tail

(hydrophobic) dissolve in oil/fat/grease. The molecules of soap/detergent are arranged around

the oily droplet such that the nonpolar tail points towards the central oily drop while the polar

head is directed towards the water. (see Fig. 16.2) Thus, micelles of soap/detergent are formed

surrounding the oil drops, which are removed in the washing process.

Table 16.3 Synthetic detergents

Type Example Use

Anionic detergent (sodium lauryl sulphate) Household detergent, additive in
CH3(CH2)10CH2OSO3 Na⊕ toothpaste

Cationic detergent CH3(CH2)15-N⊕(CH3)3Br hair conditioner, germicide
(ethyltrimethyl ammonium bromide)

CH3(CH2)16- COO (CH2CH2O)n CH2 CH2OH liquid dishwash
(an ester)

Nonionic detergent C9H19 liquid detergent
O - (CH2CH2O)nCH2CH2OH

(an ether)

Exercises

1. Choose correct option H. Health benefits are obtained by
consumption of
A. Oxidative Rancidity is reaction
a. Saturated fats
a. addition b. subtitution b. trans fats
c. mono unsaturated fats
c. Free radical d. combination d. all of these
2. Explain the following :
B. Saponification is carried out by A. Cooking makes food easy to digest.
B. On cutting some fruits and vegetable
a. oxidation b. alkaline hydrolysis
turn brown.
c. polymarisation C. Vitmin E is added to packed edible oil.
D. Browning of cut apple can be prolonged
d. Free radical formation
by applying lemon juice.
C. Aspirin is chemically named as E. A diluted solution (4.8 % w/v) of

a. Salicylic acid 2,4,6-trichlorophenol is employed as
antiseptic.
b. acetyl salicylic acid F. Turmeric powder can be used as
antiseptic.
c. chloroxylenol d. thymol 3. Identify the functional grops in the
following molecule :
D. Find odd one out from the following A. Aspirin

a. dettol b. chloroxylenol OO
C CH3
c. paracetamol d. trichlorophenol
COOH
E. Arsenic based antibiotic is

a. Azodye b. prontosil

c. salvarsan d. sulphapyridine

F. The chemical used to slow down the

browning action of cut fruit is

a. SO3 b. SO2
c. H2SO4 d. Na2CO3
G. The chemical is responsible for the rancid

flavour of fats is

a. Butyric acid b. Glycerol

c. Protein d. Saturated fat

269

B. Paracetamol D. Anionic detergents
HO
H E. Non-ionic detergents
N O CH3
8. Explain : mechnism of cleansing Action

of soap with flow chart.

9. What is meant by broad spectrum

antibiotic and narrow spectrum

C. Penicillin antibiotics?
O
10. Answer in one senetence
R - C - NH
S A. Name the painkiller obtained from
O N COOH
acetylation of salicyclic acid.

B. Name the class of drug often called as

D. Chloramphenicol painkiller.
NH - CO - CHCl2
C. Who discovered penicillin?
O2N CH - CH - CH2OH
OH D. Draw the structure of chloroxylenol

and salvarsan.

E. Write molecular formula of Butylated

E. Sulphanilamide hydroxy toulene.
SO2NH2
F. What is the tincture of iodine ?

G. Draw the structure of BHT.

I. Write a chemical equation for

saponification.

NH2 J. Write the molecular formula and

F. Glycerin name of COOH

CH2- OH OCOCH3
CH - OH
11. Answer the following
CH2- OH A. Write two examples of the following.
a. Analgesics
4. Give two differences between the c. Antiseptics d. Antibiotics
e. Disinfectant
following B. What do you understand by
antioxidant ?
A. Disinfectant and antiseptic

B. Soap and synthetic detergent

C. Saturated and unsaturated fats

D. Rice flour and cooked rice

5. Match the pairs.
A group B group

A. Paracetamol a. Antibiotic

B. Chloramphenicol b. Synthetic Activity :

detergent Collect information about different
chemical compounds as per their applications
C. BHT c. Soap in day-to-day life.

D. Sodium stearate d. Antioxidant

e. Analgesic

6. Name two drugs which reduce body

pain.

7. Explain with examples

A. Antiseptics

B. Disinfectant

C. Cationic detergents

270

Modern Periodic Table

Periods Groups 18
1 13 14 15 16 17

12 3 4 5 6 7 8 9 10 11 12
2
271
3

4
5

6

7

Series

Notes

272

I

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