Question

Which one of the following carbonyl compounds cannot be prepared by addition of water on an alkyne in the presence of HgSO₄ and H₂SO₄ ?

• A. Formaldehyde
• B. Acetaldehyde
• C. Propanal
• D. 2-Butanone

Hint:

In alkyne hydration, Ethyne is the {only} alkyne that produces an aldehyde. All other alkynes produce ketones.

Answer 1

The Correct Option is A

Step 1: Hydration of alkynes (Kucherov Reaction) follows Markovnikov's rule.
Step 2: Ethyne ($HC \equiv CH$) gives Acetaldehyde ($CH_3CHO$).
Step 3: Higher alkynes always give ketones. For example, Propyne gives Acetone ($CH_3COCH_3$).
Step 4: Since the reaction requires at least two carbons (the alkyne), Formaldehyde (1-carbon) cannot be formed. Furthermore, higher aldehydes like Propanal cannot be formed because the -OH group attaches to the more substituted carbon, yielding a ketone.