Consider the following statements about the preparation of aldehydes.
(A) Acyl chlorides are hydrogenated to aldehydes over a catalyst of palladium on barium sulphate.
(B) Esters are reduced to aldehydes with diisobutylaluminium hydride (DIBAL-H).
(C) Benzene gives benzaldehyde when treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride.
(D) Strong oxidising agents oxidise toluene and its derivatives to benzaldehydes.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (B) and (C) only
Aldehydes are prepared by partial reduction of acid derivatives (acyl chlorides, nitriles, esters) or by putting a –CHO group on an aromatic ring. In every method the reaction has to be stopped at the aldehyde stage.
(A) Acyl chlorides are hydrogenated to aldehydes over a catalyst of palladium on barium sulphate. — True. This is the Rosenmund reduction:
\( \displaystyle \ce{C6H5COCl + H2} \xrightarrow{\ce{Pd-BaSO4}} \ce{C6H5CHO + HCl} \)
(B) Esters are reduced to aldehydes with diisobutylaluminium hydride (DIBAL-H). — True. DIBAL-H followed by water reduces the ester group to –CHO:
\( \displaystyle \ce{CH3(CH2)9COOC2H5} \xrightarrow[\text{2. }\ce{H2O}]{\text{1. DIBAL-H}} \ce{CH3(CH2)9CHO} \)
(C) Benzene gives benzaldehyde when treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride. — True. This is the Gatterman–Koch reaction; anhydrous AlCl3 or CuCl is used:
\( \displaystyle \ce{C6H6 + CO} \xrightarrow{\ce{HCl},\ \text{anhyd. }\ce{AlCl3}/\ce{CuCl}} \ce{C6H5CHO} \)
(D) Strong oxidising agents oxidise toluene and its derivatives to benzaldehydes. — False. Strong oxidising agents take the methyl group all the way to –COOH and give benzoic acids. Benzaldehyde is obtained only with reagents such as chromyl chloride or chromic oxide in acetic anhydride, which turn the methyl group into an intermediate that is difficult to oxidise further.
NCERT (2026-27 edition):
8.2.2 Preparation of Aldehydes – “Acyl chloride (acid chloride) is hydrogenated over catalyst, palladium on barium sulphate. This reaction is called Rosenmund reduction.”
“Similarly, esters are also reduced to aldehydes with DIBAL-H.”
“Strong oxidising agents oxidise toluene and its derivatives to benzoic acids. However, it is possible to stop the oxidation at the aldehyde stage with suitable reagents that convert the methyl group to an intermediate that is difficult to oxidise further.”
“When benzene or its derivative is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous aluminium chloride or cuprous chloride, it gives benzaldehyde or substituted benzaldehyde.”