Question

The final product formed by ethyne and acetic acid in the presence of $H{g^{2 + }}$ as a catalyst is:
A.Vinyl acetate
B.Ethyl acetate
C.Acetylene acetic acid
D.Ethylidene acetic acid

Answer 1

Electrophilic additions to alkynes results in useful monomers such as ethyne on reacting with hydrochloric acid gives vinyl chloride as a product in the presence of a catalyst.

Complete step by step solution:
Acetylene or ethyne reacts with acetic acid to produce vinyl acetate in the presence of sulphuric acid and ${\text{H}}{{\text{g}}^{{\text{2 + }}}}$. Vinyl acetate is a monomer which is used to produce a polymer PVC. Vinyl is an alkenyl functional group with the formula $- CH = C{H_2}$ attached to any R group of atoms. It is basically the ethylene molecule with lesser hydrogen atoms. The single vinyl compound can be made to polymerize so as to form polyvinyl chloride.
The reaction mechanism has following steps:
A bridged mercurinium ion is formed as an intermediate between $H{g^{2 + }}$ and ethyne.
A three–membered ring can be opened by addition of acetic acid.
Proton transfer to the solvent is done to produce an organomercury vinyl ester.
Addition of proton to the alkene leads to the formation of carbocation.
$H{g^{2 + }}$ is expulsed to form the alkene.
$HC \equiv CH{\text{ + C}}{{\text{H}}_3}COOH + HgS{O_4}{\text{ + }}{{\text{H}}_2}{\text{S}}{{\text{O}}_4}{\text{ }} \to {\text{ }}{{\text{H}}_3}C - COO - CH = C{H_2}$
Therefore, the final product formed by ethyne and acetic acid in the presence of ${\text{H}}{{\text{g}}^{{\text{2 + }}}}$ as a catalyst is vinyl acetate. This is the major industrial process used for its preparation. It is an acetate ester of vinyl alcohol so we call this method of preparation as hydroesterification.

Hence, the correct option is (A).

Note:
Vinyl acetate can also be produced from acetylene and acetic acid using zinc acetate under mild conditions. One more product is formed when acetylene reacts with acetic acid, it is ethylidene diacetate.