Question

What is the reaction of ammoniacal cuprous chloride with acetylene?

Answer 1

The Glaser coupling reported by Carl Andreas is a type of acetylenic coupling which involves cuprous salts and an additional antioxidant. Glaser coupling is modified into the Eglinton reaction where terminal alkynes are coupled in the presence of copper salts, also this reaction is used to synthesize a number of fungal antibiotics.

Complete answer:
Ammoniacal cuprous chloride solution is a blue coloured solution prepared by dissolving cuprous chloride in water and ammonia. While acetylene or ethyne is a simple hydrocarbon with molecular formula ${C_2}{H_2}$ also acetylene is an alkyne with triple bond between the carbon atoms. Moreover, acetylene is unstable in pure form hence it is usually handled as a solution.
When acetylene at room temperature is passed through ammoniacal cuprous chloride, a red colour precipitate called cuprous acetylide with molecular formula $C{u_2}{C_2}$ which is explosive when dry is formed.
$HCCH + {\text{ 2CuCl + 2N}}{{\text{H}}_3} \to {\text{ C}}{{\text{u}}_2}{C_2}{\text{ + 2N}}{{\text{H}}_4}Cl$
But, in the presence of an additional antioxidant such as oxygen the reaction proceeds further to dimerize the acetylene which results in the formation of but-1,3-yne. Hence, the reaction of acetylene with ammoniacal cuprous chloride is a type of homo-coupling reaction of terminal alkynes. Therefore, the reaction of ammoniacal cuprous chloride with acetylene gives But-1, 3-yne.
$2Cu-C\equiv C-Cu\xrightarrow[{{O}_{2}}]{N{{H}_{4}}Cl}{{H}_{2}}C=CH-C\equiv CH$

Note:
Dimerization is the process where two molecules of similar chemical composition come together to form a dimer, while the reverse process of dimerization is called dissociation. Dimers are oligomers containing two monomers joined by either strong, weak or covalent or intermolecular bonds. Dimers are of two types i.e. Homodimer, used when two molecules are identical and heterodimer, when the molecules are non-identical.