Hydrocarbons

ABYSS Learning

Acetylene tetrachloride is also called as westron.

When westron is treated with Ca(OH)2 then we get westrosol

Cl Cl CHCl

H C C H + Ca(OH)2 –HCl CCl2

Cl Cl Westrosol or triclene

Both westron and westrosol are used as solvents in cloth industries

Reaction with dilute Br or bromine water:
2

CH CH + Br H2O Br—CH CH—Br
2

CH CH + Br liquid  HH
2  Br C C—Br

Br Br

Reaction with Iodine :

CH CH + I ethanol   CHI CHI (addition reaction)
2 NH3 
I—C C—I + NH I (substitution reaction)
CH CH + I 4
2
Di iodoacetylene

( c ) Addition of halogen acids (H – X) : Addition according to Markowni-Koff's Rule.

Reactivity order of H – X : HI > HBr > HCl > HF

XH

R—C C—H HX  RCC H

XH

(Gem dihalides major product)

Mechanism : [HHX ] +
R—C CH
RCC H

H
(Intermediate is carbocation)

 X – X
R C CH2
R— C CH
2

Further

X X X
R C CH2
H R C CH2 + 

 R C CH2

H H
(only –I of X)
(more stable
due to +M of X)

so

X X X
R C CH3
R C CH3
+ X

(Major Product)

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( d ) Addition of HOX : Alkynes react with hypohalous acids according to markownikoff's rule and form
gem diol, which are unstable, lose a molecule of water and form halo aldehyde or halo ketones.

Reactivity order HOCl > HOBr > HOI

R C CH + HOCl  R C CHCl2

O

Mechanism :

R—C CH [HCOlCl]  R + C CH

C CH  R +

:C..l: Cl

(Cyclic cation)

OH



R C CH O H R C CH

+

Cl Cl

Further

OH Cl Cl
R C CH
Cl R C CH + 

R C CH

Cl OH Cl OH Cl

(more stable due (less + M of Cl - Atom)

to +M of OH group)

so

Cl OH
R C CHCl2 H2O R C CHCl2
R + 

C CH O H

OH Cl OH O
(Unstable) (Dichloro ketones)

( e ) Addition of BH / THF or B H (Hydroboration) : THF - Tetrahydrofurane is used as solvent.
3 26

3R—C C—R TBHHF3  (R C C )3B

HR
Since BH is not available as monomer so a solvent THF is used for the stability of BH .

33

R + – + H–  R CH C BH2
H–
C C R+ B H–

R
R CH C BH2+ 2R C C R (R CH C )3B

RR

H2O2( –OH) (R C C OH + B(OH)3  R CH2 C R
Basic oxidation

(R C C )3B HR O
HR (enol form) (Ketones)

H+/ H2O RR
C C + B(OH)3

HH
(cis-Alkene)

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ABYSS Learning

Example :

TBHHF3  

CH —C C—CH (CH3 C C )3B H2O2 /O H CH3 CH C OH CH3 CH2 C CH3
3 3

H CH3 CH3 O
(enol form (keto form

less stable) more stable)

( 2 ) Nucleophilic addition reaction : In these reactions some heavy metal cation like Hg+2, Pb+2, Ba+2 are used.

These cation attracts the – e– of alkynes and decrease the e– density and hence a nucleophile can attack an

alkynes.

( a ) Addition of dil. H SO (Hydration) : The addition of water takes place in the presence of Hg+2 and
24

H SO [1% HgSO +40% H SO ]. In this reaction carbonyl compounds are obtained .
24 4 24

CH  CH + H—OH  CH CH—OH  CH —CHO
2 3

enol unstable

OH H

CH —C CH + H—OH  CH3 C CH  CH3 C CH3
3

enol (unstable) O

A structure in which –OH group is attached to double bond carbon is called as enol (ene + – ol).

This reaction is used for preparation of aldehyde and ketone.

R—C  CH ddili.lH. Hg2SSOO44((410%%) ) R C CH3

O

Mechanism :

R—C CH Hg2  R  C CH

C CH  R +2
:Hg+
Hg

H O+ H OH OH

.. CH–H+ R CH–HHg++2 R

R C CH HO.. H R C C C CH R C CH3

+2

Hg Hg+ Hg+ H O

(enol) (Keto more stable)

OH O

Example : CH —C C—CH H H/gH22 O CH3 C C CH3 CH3 C CH2 CH3
3 3

H

2-butanone

( b ) Addition of HCN : The addition of HCN in presence of barium cyanide to form vinyl cyanide.

CH CH + HCN Ba(CN)2   CH CHCN
2

The vinyl cyanide is used for making polymers such as orlon and Buna-N rubber.

CH CHCN Polymerisation ( CH2 CH )n
2

CN

Orlon

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( c ) Addition of acetic acid : Acetate acetylene combines with acetic acid in presence of murcuricsulphate.
It first forms vinyl acetate and then ethylidene acetate.

CH H CH2 CH3
CH(OOCCH3)2
+ OOCCH3 Hg2  CHOOCCH3 CH3COOH
CH

vinyl acetate Ethylidene acetate

CH2 CHOC CH3Polymerisation( CH2 CH )n Hydrolysis( CH2 CH )n + nCH3COOH

O O C CH3 OH

Vinyl Acetate O Poly vinyl alcohol
(Monomer) Poly vinyl acetate
(Polymer : adhesive compound)

When ethylidene acetate is heated it give acetaldehyde and acetic anhydride

O

OC CH3 CH3 C

CH3 CH O   CH3 CHO + O

OC CH3 CH3 C
O O

( d ) Addition of alcohols : In presence of BF and HgO alkynes react with alcohols and form acetal and ketal
3

CH CH + CH OH HBgFO3   CH CHOCH CH3OH CH3 CH OCH3
3 2 3 OCH3

Methylal (acetal)

R—C CH + CH —OH HBgFO3  R OCH3 OCH3
3 C CH2 CH3OH R C CH3

OCH3

Ketal

Acetylene forms acetal while other alkynes form ketal.

( 3 ) Addition of AsCl : In presence of AlCl or HgCl acetylene combines with AsCl to yield Lewisite gas. It is
3 32 3

four times poisonous than mustard gas.

CH CH + Cl—AsCl  CH Cl
2

CHAsCl2

2-Chlorovinyl dichloro arsine (Lewisite gas)
The action of Lewsite may be checked by its antidote BAL (British Anti Lewisite). BAL combines with the
Lewisite to form a cyclic non toxic compound.

SH SH OH
2, 3–Dimercapto–1–propanol [BAL]

CH2 CH CH2

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( 4 ) Carbonylation : Reaction of alkynes with CO in presence of Ni(CO)
4

Alkyne + CO + H O Ni(CO)4  Olefinic acid
2

Alkyne + CO + R–OH Ni(CO)4  Olefinic ester

CH CH2
CHCOOH
+ CO + H O Ni(CO)4  Acrylic acid
2
CH

CH H Ni(CO)4  CH2
+ CO + CHCOOR
Alkyl acrylate
CH O R

 Oxidation reactions :
( a ) Combustion :

CnH2n-2 + 3n 1 nCO2 + (n-1) H2O + Heat
2 O2 

2HC CH + 5O  4CO + 2H O + 312 K.cal
2 22

The combustion of acetylene is used for welding and cutting of metals in which oxy-acetylene flame having high
temp (30000C) is produced.

( b ) Oxidation with alkaline KMnO : Oxidation with alkaline KMnO gives carboxylic acids.
44

CH 4[O] alk.KMnO4  COOH
COOH
+
CH

Acetylene Oxalic acid

CH —C CH + 4[O] alk.KMnO4  CH —COOH + HCOOH
3 3

( c ) Oxidation with acidic KMnO or K Cr O : In presence of acidic KMnO or acidic K Cr O . Alkynes
4 2 27 4 2 27

are oxidised to monocarboxylic acids.

R—C C—R' + 2[O]  R C C R' H2O[O] RCOOH + R'COOH
OO

CH  CHO H2 O [ O ] 2HCOOH
CHO
+ 2[O]
CH

glyoxal

CH —C CH + 2[O]  CH3 C O H2 O [ O ]  CH COOH + HCOOH
3 3

CHO

E x . An alkyne on oxidation with acidic KMnO , only acetic acid is obtained what is given alkynes ?
4

S o l . In Oxidation of alkynes two moles of mono carboxylic acids are obtained.

CH3 C OH + HO C CH3   CH3 — C — C — CH3
OO 2 Butyne

—

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( d ) Oxidation with selenium dioxide : Selenium dioxide oxidises alkynes to the dicarbonyl compounds.

CH S2e[OO2]  CHO
CH CHO
Glyoxal

CH3 C S2e[OO2]  CH3 CO
CH3 C CH3 CO
2–Butyne Butane - 2, 3 -dione

(e) Oxidation with ozone (O ) : In the ozonolysis both sp-C-atoms are converted into C C group.
3

OO

O

R C C R + O3  R C C R R C C R + H2O2  R C OH+ R C OH

OO OO OO

Ozonide (Addition of ozone) (Acids)

In this reaction H2O2 is oxidant which oxidise R C C R into acids.
OO

But if we use some amount of Zn as reductant with H O then it reduce H O so oxidation does not take place
2 22

HO + Zn  ZnO + HO
22 2

(i) O3 CH3 C C H+ H2O2  CH3 C OH + HCOOH
(ii) H2O

Example : CH3 C CH OO O

(i) O3 CH3 C C H+ ZnO
(ii) H2O/Zn OO

 Substitution Reaction : (Formation of metallic derivatives)

Only 1-alkynes give substitution reaction and show acidic characters – 

C— H

Acetylene is dibasic acid where as propyne is monobasic means acetylene can give two H+ where as propyne
can give one H+.

( a ) Formation of sodium acetylides : Acetylene and 1-alkynes react with sodamide to form acetylides

H—C C—H NaNH2 NaC C —H NaNH2  NaC CNa

Mono sodium Disodium acetylide

acetylide

Dry alkynides are generally unstable and explosive. These are easily converted in to original alkynes when

heated with dilute acids.

NaC CNa + 2HNO  HC CH + 2NaNO
3 3

This reaction can be used for the purification, seperation and identification of 1-alkynes.

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( b ) Formation of copper and silver acetylides : Copper and silver acetylides are obtained by passing

1-alkynes in the ammonical solution of cuprous chloride and silver nitrate (Tollen's reagent) respectivley.

R—C C—H + Cu Cl + NH OH  R—C CCu + NH Cl + H O
22 4 42

ammonical cuprous- copper acetylide

chloride [Red ppt.]

R—C C—H + AgNO + NH OH  R—C CAg + NH NO + H O
34 43 2

Tollen's reagent White ppt.

These reactions are used for detecting the presence of acetylenic hydrogen.These are test for distinguish

alkenes and alkynes or 1-alkynes and 2-alkynes.

 Isomerisation : When alkyne-1 is heated with alc. KOH alkyne-2 is obtained.

CH —CH —C CH alc.KOH  CH —C C—CH
32 33

1–Butyne 2–Butyne

When alkyne -2 is heated with NaNH alkyne -1 is obtained
2

CH —C C—CH NaNH2 CH —CH —C CH
3 3 32

2–Butyne 1–Butyne

 Polymerisation :

( a ) Linear polymerisation :

Dimerisation : When two molecules of acetylene passed through a solution of Cu Cl and NH Cl a
22 4

vinyl acetylene is obtained.

2HC C—H CNuH24CCl2l CH2 CH—C C—H

mono vinyl acetylene

When vinyl acetylene react with HCl then chloroprene is obtained.

CH CH—C C—H HCl  CH2 CH C CH2 Polymerisation Neoprene (Synthetic rubber)
2

Cl
2- chloro-1,3-butadiene
[chloroprene]

Trimerisation : 3 molecules of acetylene.

3CH CH CNuH24CCl2l CH CH—C C—CH CH
2 2

Divinyl acetylene

( b ) Cyclic polymerisation : When alkyne is passed through red hot metallic tube, cyclic polymerisation
takes place with the formation of aromatic compound

3CH CH iRroendthuobet 

Benzene

CH3

3 CH —C CH iRroendthuobet  H3C
3

CH3

Mesitylene

CH3 CH3
CH3

3 CH —C C—CH iRroendthuobet  CH3 CH3
33 CH3

Hexa methyl benzene

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3 HC CH (CN6iH(C5O)3)P4  benzene (90% yield)

(triphenyl–1–phosphene)

4CH CH Ni(CN )2  CH CH
HC CH

HC CH
CH CH

1,3,5,7–cyclooctatetraene

Other reactions of acetylene :

NH3 2HCN
Electric spark

HC CH NH3 CH CH
High temp.
CH.. CH
N

H Pyrole

HCN .. Pyridine
Red hot Fe N

CH2N2, CH2
–N2
CH CH CH2N2  CH——––––CH

–N2

CH2 CH2

Cyclo propene Bicyclo (1,1,0) butane

CH2N2 CH CH H2  CH2 CH
cold ether solution

CH N CH..2 N
..

NH NH

Pyrazoline

S CH CH
Al2O3, 400°C
CH.. CH
.S.

Thiophene

( v i ) Reaction with HCHO : This reaction is called ethynylation.

OH OH

CH CH + HCHO Cu  HC CCH —OH HCHO CH2 C C CH2
2

propargyl alcohol
(vii) Reaction with NaOCl : (Substitution reaction)

H — C C—H + NaOCl absence o0faCir andlight  H—C C—Cl + NaOH

H—C C—Cl + NaOCl absence o0faCir andlight  Cl—C C—Cl + NaOH
Dichloro acetylene

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ABYSS Learning

 Uses of Acetylene :
(i) Oxyacetylene flame used in welding and cutting
(ii) Acetylene is used as an illuminant
(iii) Acetylene is used for artificial ripenning of fruits
(iv) Used for manufacture of acetaldehyde, acetic acid, ethyl alcohol, westron, westrosol, PVC, PVA,
Chloroprene, butadiene, Lewisite etc.
(v) It is used as a general anaesthetic.

 L aborator y Te st for alkyne s :

Functional Reagent Observation Reaction Remarks
Group Hydroxylation

(1) Bayer's Pink Colour CH =CH +H O+O alk.KMnO4 
Reagent disappears 2 22 CH2–CH2
OH OH
alk.dil.Cold KMnO
4

–CC– (2) Br /H O Red Colour CH2–CH2
22 decolourises

Br +CH =CH  Br Br Bromination
222

White ppt

(3) O (ozone) Acid Formed R–CC–R' O3 RCOOH+R'COOH Ozonolysis
3

 Laboratory test of terminal alkynes :
When triple bond comes at the end of a carbon chain. The alkyne is called a terminal alkyne.

acetylenic hydrogen

H –C C–CH2CH3
1-Butyne, a terminal alkyne

Functional Group Reagent Observation Reaction

(1) Cuprous chloride Red ppt. R–CCH+CuCl NH4OH
+ NH OH
R–CC Cu(red)
4

R–CC–H (2) AgNO +NH OH White ppt. R–CCH+Ag'  R–C  C Ag  (white)
34
Colourless gas HC  CH + 2Na  Na – C  C – Na + H 
(3) Na in ether 2

(i) Decolourization of Br in CCl solution.
24

(ii) Decolourisation of 1% alkaline KMnO solution.
4

(iii) 1- alkynes give white ppt. with ammonical AgNO and red ppt with ammonical cuprous chloride
3
solution.

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Note : (i) and (ii) tests are used for determination of unsaturation (i.e, presence of double or triple bond in any
compound)
(iii) Test is used for distinguish between alkenes and 1-alkynes or 1-alkyne and 2-alkyne.

 Seperation of ethane, ethene and ethyne :
[C2H2 + C2H4 + C2H6]

Passed through ammonical Cu2Cl2

red ppt Gaseous mixture
CuC CCu (C2H6 + C2H4)

KCN solution conc. H2SO4

Ethyne

Ethyl sulphate Ethane (Gas)


Ethene

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SOLVED EXA MPLES

Ex. R—CH2—CCl2—R Reagent  R—C  C—R. The reagent is -

Sol. (A) Na (B) HCl in H2O (C) KOH in C2H5OH (D) Zn in alcohol.
Ex. Ans.(C)

Sol. Alcoholic KOH brings about dehyrohalogenation
Ex.
Acetylene when treated with dilute HCl at 600C (333 K) in presence of HgCl2 produces -

(A) Methyl chloride (B) Vinyl chloride (C) Acetaldehyde (D) Formaldehyde

Ans.(B)

(B) H–C  C – H + HCl H6g0Cl2  CH = CH – Cl
2

Vinyl chloride

When propyne is treated with aqueous H SO in the presence of HgSO , the major product is -
24 4

(A) Acetaldehyde (B) Propanal (C) 2-Propanol (D) Propanone Ans.(D)

Sol. CH3 – C  CH + H2O   CH3 C CH2 CH3 C CH3
Ex. O
OH

Alkaline KMnO4, oxidizes acetylene to -

(A) Acetic acid (B) Glyoxal (C) Oxalic acid (D) Ethylene glycol
Ans.(C)
Sol. COOH
Ex. (D) 2-Butyne Ans.(A)
Sol. H – C  C – H + 4[O]  (D) CH3CHO Ans.(C)
Ex.
COOH
Sol.
Which of the following is most acidic -

(A) Ethyne (B) Propyne (C) 1-Butyne
(C) Glyoxal
Because ethyne gives most stable anion.

Ozonolysis of acetylene gives -

(A) Oxalic acid (B) Ethylene glycol

HC  CH (ii) (Hi)2OO3/Zn H C C H

OO
Glyoxal

E x . Propyne on reaction with aqueous chlorine gives -

(A) 1, 1, 2, 2-Tetrachloropropane (B) 1, 2-Dichloropropene

(C) 1, 1-Dichloropropanone (D) 2, 2-Dichloropropanone Ans.(C)

Sol. CH –C  CH + 2HOCl  CH3 OH O
3 C CHCl2 H2O  CH3–C–CHCl2
OH

E x . Mesitylene can be obtained by polymerization of -

(A) Ethyne (B) Ethene (C) Propene (D) Propyne Ans.(D)

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H3C CH3

Sol. Propyne on trimerization yields mesitylene 3CH3C  CH 

CH3
Mesitylene

Ex. Excess of CH COOH is reacted with CH  CH in presence of Hg2+, the product is -
3

(A) CH3CH(OOCCH3)2 (B) CH2 = CH(OOCCH3)

(C) (CH COO)CH —CH (OOCCH ) (D) None of these
3 22 3

Sol. H–C  C–H + 2CH COOH  CH – CH(OCOCH ) Ans.(A)
Ex. 3 3 32 Ans.(A)

Both the protons go to same carbon atom

A compound is treated with NaNH2 to give sodium salt. Identify the compound -

(A) C H (B) C H (C) C H (D) C H
22 66 26 24

Sol. Ethyne is acidic in character

H–C  C–H + NaNH2  H – C  C–Na+ + 1
2 H2

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