Organic Chemistry for 11
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
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