Question

Phenylacetylene on treatment with \(HgSO_4/H_2SO_4, H_2O\) produces

• A. acetophenone
• B. phenylacetaldehyde
• C. phenylacetic acid
• D. 1-phenylethanol
• E. 2-phenylethanol

Hint:

Hydration of terminal alkynes (except acetylene) always yields ketones, whereas hydration of acetylene yields acetaldehyde.

Answer 1

The Correct Option is A

Concept: This is the Kucherov reaction, which involves the hydration of alkynes in the presence of mercuric sulfate and sulfuric acid. The reaction follows Markovnikov's rule.

Step 1: Addition of water.
The \(OH^-\) group attaches to the more substituted carbon of the triple bond (the one attached to the phenyl ring): \[ Ph-C \equiv CH + H_2O \xrightarrow{Hg^{2+}, H^+} [Ph-C(OH)=CH_2] \] The intermediate formed is an enol.

Step 2: Tautomerization.
Enols are unstable and rapidly undergo keto-enol tautomerization to form the more stable carbonyl compound. The hydrogen from the oxygen moves to the terminal carbon, and the double bond moves to the oxygen: \[ [Ph-C(OH)=CH_2] \rightarrow Ph-CO-CH_3 \] The final product is Methyl Phenyl Ketone, commonly known as acetophenone.