Consider the following statements about the preparation of aldehydes and ketones.
(A) Addition of water to ethyne in the presence of H2SO4 and HgSO4 gives acetaldehyde.
(B) Alkynes other than ethyne give aldehydes on addition of water in the presence of H2SO4 and HgSO4.
(C) Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with a Grignard reagent, gives ketones.
(D) Treating a nitrile with a Grignard reagent followed by hydrolysis yields a ketone.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A), (C) and (D) only
(A) Addition of water to ethyne in the presence of H2SO4 and HgSO4 gives acetaldehyde. — True. Water adds across the triple bond and the enol formed changes to acetaldehyde:
\( \displaystyle \ce{HC#CH + H2O} \xrightarrow{\ce{H2SO4},\ \ce{HgSO4}} \ce{CH3CHO} \)
(B) Alkynes other than ethyne give aldehydes on addition of water in the presence of H2SO4 and HgSO4. — False. Ethyne is the only alkyne that gives an aldehyde; all other alkynes give ketones. For example, propyne gives propanone:
\( \displaystyle \ce{CH3C#CH + H2O} \xrightarrow{\ce{H2SO4},\ \ce{HgSO4}} \ce{CH3COCH3} \)
(C) Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with a Grignard reagent, gives ketones. — True. The Grignard reagent first gives the dialkylcadmium, which then replaces the chlorine of the acyl chloride by an alkyl group:
\( \displaystyle \ce{2R-MgX + CdCl2 -> R2Cd + 2Mg(X)Cl} \)
\( \displaystyle \ce{2R'COCl + R2Cd -> 2R'COR + CdCl2} \)
(D) Treating a nitrile with a Grignard reagent followed by hydrolysis yields a ketone. — True. The Grignard reagent adds to C≡N and the addition product is hydrolysed to a ketone:
\( \displaystyle \ce{R'C#N + RMgX} \xrightarrow{\text{ether}} \ce{R'C(=NMgX)R} \xrightarrow{\ce{H3O+}} \ce{R'COR} \)
NCERT (2026-27 edition):
8.2.1 Preparation of Aldehydes and Ketones – “(ii) By hydration of alkynes: Addition of water to ethyne in the presence of H2SO4 and HgSO4 gives acetaldehyde. All other alkynes give ketones in this reaction (Unit 9, Class XI).”
8.2.3 Preparation of Ketones – “Treatment of acyl chlorides with dialkylcadmium, prepared by the reaction of cadmium chloride with Grignard reagent, gives ketones.”
“Treating a nitrile with Grignard reagent followed by hydrolysis yields a ketone.”