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Chemical Properties of Alkynes | chemca

Hydrocarbons

Chemical Properties of Alkynes

Reactions: Acidity, Hydration, Addition, and Cyclic Polymerization.

By chemca Team • Updated Jan 2026

Alkynes are highly reactive due to the presence of two $\pi$-bonds. They undergo electrophilic addition reactions similar to alkenes, but also show unique acidic behavior in terminal alkynes.

1. Acidic Character of Alkynes

Terminal Alkynes

Hydrogen atoms attached to the triply bonded carbon (sp-hybridized) are acidic. Terminal alkynes react with strong bases ($Na$ or $NaNH_2$) to liberate hydrogen gas.

$$ HC \equiv CH + Na \xrightarrow{\Delta} \underset{\text{Sodium Acetylide}}{HC \equiv C^-Na^+} + \frac{1}{2}H_2 $$

Note: This reaction is given ONLY by terminal alkynes (like Ethyne, Propyne) and NOT by internal alkynes (like But-2-yne). This distinguishes the two.

2. Addition Reactions

A. Addition of Water (Hydration)

Alkynes are immiscible with water. However, they react with water in the presence of $HgSO_4$ and dil. $H_2SO_4$ at 333 K to form carbonyl compounds via Tautomerization.

$$ CH \equiv CH + H_2O \xrightarrow{Hg^{2+}/H^+} [CH_2=CH-OH] \xrightarrow{\text{Isomerisation}} CH_3-CHO $$

Ethyne yields Ethanal (Acetaldehyde). All other alkynes yield Ketones.

$$ CH_3-C \equiv CH + H_2O \xrightarrow{Hg^{2+}/H^+} \underset{\text{Propanone (Acetone)}}{CH_3-CO-CH_3} $$

B. Addition of $X_2$ and $HX$

Alkynes add two molecules of dihalogen or hydrogen halide.

Bromine Water Test: Reddish-orange bromine water decolorizes upon reaction with alkynes (Test for unsaturation).
$$ HC \equiv CH + 2Br_2 \rightarrow CHBr_2-CHBr_2 $$
Addition of HX: Follows Markovnikov's rule to form Gem-dihalides.
$$ CH_3-C \equiv CH + 2HBr \rightarrow CH_3-CBr_2-CH_3 $$

3. Cyclic Polymerization

Formation of Benzene

When Ethyne is passed through a Red Hot Iron Tube at 873 K, it undergoes cyclic polymerization to form Benzene.

$$ 3CH \equiv CH \xrightarrow{\text{Red Hot Fe Tube, 873K}} C_6H_6 \text{ (Benzene)} $$

This is a key method to convert Aliphatic compounds to Aromatic compounds.

4. Oxidation (Ozonolysis)

Ozonolysis of alkynes involves the cleavage of the triple bond to form diketones or carboxylic acids depending on the conditions (oxidative workup usually gives acids).

$$ CH_3-C \equiv C-CH_3 \xrightarrow{1.O_3, 2.H_2O} 2 CH_3COOH $$

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