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Preparation of Alkynes | chemca

Hydrocarbons

Preparation of Alkynes

Synthesis from Calcium Carbide, Dihalides, and Higher Alkynes.

By chemca Team • Updated Jan 2026

Alkynes are unsaturated hydrocarbons containing a carbon-carbon triple bond ($C \equiv C$). They are prepared industrially from calcium carbide and in the laboratory by elimination reactions.

1. From Calcium Carbide (Industrial Method)

Hydrolysis of Carbide

On an industrial scale, Ethyne (Acetylene) is prepared by treating calcium carbide ($CaC_2$) with water.

$$ CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2 $$

Preparation of $CaC_2$: Calcium carbide is prepared by heating quick lime ($CaO$) with coke. $$ CaCO_3 \xrightarrow{\Delta} CaO + CO_2 $$ $$ CaO + 3C \xrightarrow{\Delta} CaC_2 + CO $$

2. From Vicinal Dihalides (Dehydrohalogenation)

Double Dehydrohalogenation

Vicinal dihalides undergo dehydrohalogenation on treatment with alcoholic KOH to form alkenyl halides. Further treatment with a stronger base like Sodamide ($NaNH_2$) yields alkynes.

$$ \underset{\text{1,2-Dibromoethane}}{CH_2Br-CH_2Br} \xrightarrow{KOH(alc), \Delta} \underset{\text{Vinyl Bromide}}{CH_2=CHBr} \xrightarrow{NaNH_2} \underset{\text{Ethyne}}{CH \equiv CH} $$

Why $NaNH_2$?
The second step requires a stronger base than alcoholic KOH because the vinyl halide bond is stronger (due to partial double bond character/resonance). $NaNH_2$ in liquid ammonia provides the necessary strength for the second elimination.

3. From Terminal Alkynes (Alkylation)

Synthesis of Higher Alkynes

Terminal alkynes are acidic in nature due to the sp-hybridized carbon. They react with strong bases ($Na$ or $NaNH_2$) to form metal acetylides, which then react with primary alkyl halides to form higher alkynes.

$$ CH \equiv CH + Na \xrightarrow{liq. NH_3} CH \equiv C^- Na^+ + \frac{1}{2}H_2 $$ $$ CH \equiv C^-Na^+ + R-X \rightarrow CH \equiv C-R + NaX $$

Condition: This method is best for $1^\circ$ alkyl halides. With $2^\circ$ or $3^\circ$ halides, elimination dominates.

4. Kolbe's Electrolysis

Electrolysis of Salts

Electrolysis of aqueous solution of potassium maleate or fumarate yields Ethyne at the anode.

$$ \underset{\text{Potassium Maleate}}{CH(COOK)=CH(COOK)} + 2H_2O \xrightarrow{\text{Electrolysis}} \underset{\text{Ethyne}}{CH \equiv CH} + 2CO_2 + 2KOH + H_2 $$

At Anode: Ethyne + $CO_2$ | At Cathode: $H_2$ gas

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