Consider the following statements about the preparation of aldehydes.
(A) Nitriles are selectively reduced by diisobutylaluminium hydride to imines, which on hydrolysis give aldehydes.
(B) In the Stephen reaction, nitriles are reduced to the corresponding imines with stannous chloride in the presence of hydrochloric acid.
(C) In the Etard reaction, chromyl chloride oxidises the methyl group of toluene to a chromium complex, which on hydrolysis gives benzoic acid.
(D) Side chain chlorination of toluene followed by hydrolysis is a commercial method of manufacture of benzaldehyde.
(E) In the Gatterman-Koch reaction, benzene is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous ferric chloride.
Which of the statements given above are incorrect? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (C) and (E) only
Each of these methods stops at the aldehyde stage: nitriles are partly reduced, and the methyl group of toluene is changed into an intermediate that is hydrolysed to –CHO.
(A) Nitriles are selectively reduced by diisobutylaluminium hydride to imines, which on hydrolysis give aldehydes. — Correct statement. DIBAL-H reduces only the –CN group, so a C=C double bond in the same molecule is left untouched:
\( \displaystyle \ce{CH3-CH=CH-CH2CH2-CN} \xrightarrow[\text{2. }\ce{H2O}]{\text{1. }\ce{AlH(i-Bu)2}} \ce{CH3-CH=CH-CH2CH2-CHO} \)
(B) In the Stephen reaction, nitriles are reduced to the corresponding imines with stannous chloride in the presence of hydrochloric acid. — Correct statement. The imine is then hydrolysed to the aldehyde:
\( \displaystyle \ce{RCN + SnCl2 + HCl -> RCH=NH} \xrightarrow{\ce{H3O+}} \ce{RCHO} \)
(C) In the Etard reaction, chromyl chloride oxidises the methyl group of toluene to a chromium complex, which on hydrolysis gives benzoic acid. — Incorrect statement. Hydrolysis of the chromium complex gives benzaldehyde; the complex is not oxidised further to the acid:
\( \displaystyle \ce{C6H5CH3 + CrO2Cl2} \xrightarrow{\ce{CS2}} \ce{C6H5CH(OCrOHCl2)2} \xrightarrow{\ce{H3O+}} \ce{C6H5CHO} \)
(D) Side chain chlorination of toluene followed by hydrolysis is a commercial method of manufacture of benzaldehyde. — Correct statement.
\( \displaystyle \ce{C6H5CH3} \xrightarrow{\ce{Cl2}/h\nu} \ce{C6H5CHCl2} \xrightarrow[\text{373 K}]{\ce{H2O}} \ce{C6H5CHO} \)
(E) In the Gatterman-Koch reaction, benzene is treated with carbon monoxide and hydrogen chloride in the presence of anhydrous ferric chloride. — Incorrect statement. The catalyst is anhydrous aluminium chloride or cuprous chloride:
\( \displaystyle \ce{C6H6 + CO} \xrightarrow{\ce{HCl},\ \text{anhyd. }\ce{AlCl3}/\ce{CuCl}} \ce{C6H5CHO} \)
NCERT (2026-27 edition):
8.2.2 Preparation of Aldehydes – “Nitriles are reduced to corresponding imine with stannous chloride in the presence of hydrochloric acid, which on hydrolysis give corresponding aldehyde. This reaction is called Stephen reaction.”
“Alternatively, nitriles are selectively reduced by diisobutylaluminium hydride, (DIBAL-H) to imines followed by hydrolysis to aldehydes”
“Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde. This reaction is called Etard reaction.”
“Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde.”
“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.”