Organic Chemistry for 11

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  • Topic: Acetylene, Alkyne, Alkene
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  • Published : May 7, 2013
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13. ORGANIC CHEMISTRY
IV) ALKYNES
SYNOPSIS

Alkynes are commonly known as acetylenes. Acetylene is the simplest and important member of the alkyne series. • Alkynes contain – C ≡ C – • General molecular formula of alkynes is CnH2n–2 • Triple bonded carbons are ‘sp’ hybridized and greater s – character is associated with it. • Alkynes will exhibit chain, position and functional isomerism. • Their functional isomers are alkadienes and cyclo alkenes. • The minimum number of carbons required to exhibit chain isomerism by an alkyne is five. Preparation of Acetylene or ethyne : 1. Industrial method : By the hydrolysis of calcium carbide CaC2 + 2H2O → HC ≡ CH + Ca(OH)2

C C H − OH H − OH C −H C −H



Ca 2.

|||+

→ |||

+ Ca(OH)2

Even, other carbides like, MgC2, SrC2, BaC2 on hydrolysis will give acetylene. By the dehydrohalogenation of gem or vicinal dihalides. alc KOH i) CH2 − CH2 + KOH ⎯⎯⎯→ CH2 = CH − Br ⎯⎯.⎯⎯→ CH ≡ CH − HBr | Br | Br

(alc.)

− H Br

ii) 3. 4. 5. •

−HBr CH 3 − CHBr2 + KOH ⎯⎯⎯→ CH 2 = CH − Br (alc.)

⎯⎯ ⎯⎯→ CH ≡ CH
− HBr

alc.KOH

By the dehalogenation of acetylene tetrabromide
Δ CHBr2 − CHBr2 + 2Zn ⎯⎯→ CH ≡ CH + 2ZnBr2

Iodoform reacts with silver powder to give acetylene.
2CHI3 + 6 Ag ⎯⎯→ C2H2 + 6 AgI
Δ

Kolbe’s electrolysis : Sodium or potassium salts of unsaturated dicarboxylic acids, on electrolysis will give alkynes. Potassium maleate or potassium fumerate on electrolysis gives acetylene CH − COOK HO − H electrolysis CH || + ⎯⎯ ⎯ ⎯⎯ ⎯→ ||| + 2CO 2 + 2KOH + H2 ⎯ CH − COOK HO − H CH

at anode at cathode Physical properties : Pure acetylene is colorless, odourless gas, presence of impurities such as H2S, PH3 will give garlic odour. • It is sparingly soluble in polar solvents like H2O, but readily soluble in non – polar solvents like alcohol, ether etc. • It is less volatile than ethylene and ethane. Chemical properties :Acetylene undergoes addition reactions because it is unsaturated. • Acetylene undergoes both electrophilic additions and nucleophilic additions while ethylene undergoes only electrophilic additions. 1

Organic Chemistry • • Reactivity : Acetylene is more reactive due to the presence of triple bond or 2 bonds. But, acetylene is less reactive than ethylene due to shortening of C – C bond length –electrons are strongly held. Reactivity order : H2C = CH2 > HC ≡ CH > H3C – CH3 Addition reactions: In these additions, two molecules of the reagent will be added to acetylene 2 2 ⎯⎯ ⎯⎯→ H2C = CH2 ⎯⎯ ⎯⎯→ H3 C − CH3 4 4 ⎯⎯2 ⎯ ⎯ → HC = CH ⎯⎯2 ⎯ ⎯ → Cl2HC − CHCl2 ⎯ ⎯ ⎯ ⎯

I)

H / Ni

H / Ni

Cl / CCl

Cl / CCl

| Cl

| Cl

(westron )

⎯⎯2⎯ ⎯4 →, BrHC = CHBr ⎯⎯2⎯⎯4 → Br2HC − CHBr2 ⎯ ⎯

Br / CCl

Br / CCl

H C
|||

⎯⎯⎯→ CH2 = CH − Cl ⎯⎯⎯→
Vinylchloride
H2O ⎯⎯⎯ →

HCl

HCl

(1,1−dichloroethane )
rearrangement

CH3 − CHCl2

C H

1%HgSO4 + 30%H2SO4

CH2 = CH − OH
vinylalcohol (Unstable )

CH3 − CHO
( Acetaldehyde )

CH3COOH CH COOH ⎯⎯⎯⎯⎯⎯→ H2C = CH − OCOCH3 ⎯⎯3 ⎯ → H3C − CH ⎯ ⎯⎯ | Hg2 + Vinylacetate (OCOCH3 )2 ethylidene acetate ⎯⎯⎯ → ⎯

HCN H2 C = CH − CN CuCN vinyl cyanide or Acrylonitrile
AsCl

3 ⎯⎯⎯⎯ → ClHC = CH − AsCl 2 ⎯

lewisite

Reaction with H2O and HCN are nucleophilic additions. II. Oxidation : i) Acetylene is oxidised to oxalic acid with alkaline KMnO4 . Pink colour of KMnO4 is declorised . 4 HC ≡ CH + 4(O ) ⎯⎯ ⎯⎯→ HOOC − COOH

KMnO

OH− ,25 0 C

ii) Acetylene is oxidised to acetic acid in presence of chromic acid. HC ≡ CH + (O ) + H2O ⎯⎯ ⎯ ⎯ ⎯ → CH3COOH ⎯
chromic acid

III. Combustion : Acetylene burns in air to give oxy- acetylene flame, which is used in weldings and cuttings. CH ≡ CH + 5 O 2 → 2CO 2 + H 2 O + oxyacetylene flame 2

⎛ 3n − 1 ⎞ C n H 2n−2 + ⎜ ⎟O 2 → nCO 2 + (n − 1)H 2 O ⎝ 2 ⎠

IV) Polymerisation : Acetylene when passed through red hot iron tubes, it trimerises to give benzene. 3C 2H2 ⎯⎯→ C 6H6
Δ

In the above trimerisation, 3

bonds are broken and 3 2

bonds...
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