Alkynes

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 C_nH_2n-2.

Triple bonded carbons are sp hybridized and greater s - character is associated with it. Alkynes exhibit chain, position and functional isomerism. The minimum number of carbons required to exhibit chain isomerism by an alkyne is five.

Preparation

Industrial method: By the hydrolysis of calcium carbide.

By the dehydrohalogenation of gem or vicinal dihalides. 

By the dehalogenation of acetylene tetrabromide.

Iodoform reacts with silver powder to give acetylene.

Kolbe’s electrolysis: Sodium or potassium salts of unsaturated dicarboxylic acids, on electrolysis will give alkynes.

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.

Oxidation

Acetylene is oxidised to oxalic acid with alkaline KMnO₄. Pink colour of KMnO₄ is decolorised.

Acetylene is oxidised to acetic acid in presence of chromic acid.

Combustion

Acetylene burns in air to give oxy- acetylene flame, which is used in welding and cutting.

Polymerisation

Acetylene when passed through red hot iron tubes, it trimerises to give benzene.

Ozonolysis

Acetylene on addition of ozone, followed by reductive hydrolysis gives glyoxal.

Acidic nature of alkynes

In alkynes, triple bonded carbons are sp hybridised. Due to greater s - character (50%), the hydrogens bonded to triple bonded carbons are slightly acidic.

The hydrogens of acetylene or hydrogens bonded to triple bonded carbon of any other alkyne are acidic hydrogens and they can be easily replaced by metal atoms like Ag, Na, Cu to give corresponding metal acetylides.