Chemical Properties

Alkynes are unsaturated hydrocarbons. Like alkenes characteristic reactions of alkynes are electrophilic addition reactions. However, alkynes are less reactive than alkenes towards electrophilic addition reactions. Alkynes show acidic nature, addition reactions and polymerisation reactions as follows:

Acidic character of alkyne:

Sodium metal and sodamide (NaNH₂) are strong bases. They react with ethyne to form sodium acetylide with the liberation of dihydrogen gas.

Addition reactions:

Alkynes contain a triple bond, so they add up, two molecules of dihydrogen, halogen, hydrogen halides etc. Formation of the addition product takes place according to the following steps.

The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule.

Addition of dihydrogen:

Alkynes react readily with hydrogen in the presence of finely-divided nickel, platinum or palladium as catalysts. The process is known as catalytic hydrogenation. The ultimate products of this reaction are alkanes.

The hydrogenation of alkynes can be restricted to alkenes by employing suitable reaction conditions. For example, use of Lindlar’s can restrict the hydrogenation to alkene stage. Reduction of alkynes to alkene stage can also be carried out with sodium or lithium in liquid ammonia.

Addition of Halogens:

Halogens, especially chlorine and bromine add on alkynes readily producing a tetra-halogen derivatives. The reaction is carried out in inert solvent like carbon tetrachloride. Due to the presence of two n-bonds, each molecule of the alkyne can react with two molecules of the halogen. For example, the addition of bromine to ethyne can be controlled to give 1, 2-dibromoethene or 1, 1, 2, 2-tetrabromoethane.

Addition of hydrogen halides:

Addition of water (Hydration):

Alkynes undergo hydration in the presence of 60% H₂SO₄ and mercury (m tetraoxosulphate (VI) as catalyst at about 330 K. The products are carbonyl compounds. In case of ethyne, ethanal is produced. The initially formed vinyl alcohol is unstable and rearranges to more stable ethanol.

Reaction	Result
Acidic character of alkyne	liberation of dihydrogen gas
Addition of dihydrogen	Alkyne to alkene to alkanes
Addition of Halogens	tetra-halogen derivatives
Addition of hydrogen halides	tetra-halogen derivatives
Addition of water (Hydration)	Carbonyl compounds