Each method in List-I gives an intermediate, which is hydrolysed to an aldehyde. Match List-I (method) with List-II (intermediate).
| List-I (Method) | List-II (Intermediate) |
|---|---|
| (A) Toluene treated with CrO2Cl2 in CS2 | (I) Benzal chloride |
| (B) Toluene treated with CrO3 and (CH3CO)2O at 273–283 K | (II) Imine |
| (C) A nitrile treated with SnCl2 and HCl | (III) Chromium complex |
| (D) Toluene treated with Cl2 in the presence of light | (IV) Benzylidene diacetate |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → A-III, B-IV, C-II, D-I
Strong oxidising agents would take toluene all the way to benzoic acid. To stop at the aldehyde, these methods first turn the starting compound into an intermediate that is not oxidised further; the intermediate is then hydrolysed to the aldehyde.
(A) Toluene treated with CrO2Cl2 in CS2 — This matches (III) Chromium complex. This is the Etard reaction; chromyl chloride oxidises the methyl group to a chromium complex:
\( \displaystyle \ce{C6H5CH3 + CrO2Cl2} \xrightarrow{\ce{CS2}} \ce{C6H5CH(OCrOHCl2)2} \xrightarrow{\ce{H3O+}} \ce{C6H5CHO} \)
(B) Toluene treated with CrO3 and (CH3CO)2O at 273–283 K — This matches (IV) Benzylidene diacetate. Chromic oxide in acetic anhydride converts the methyl group into –CH(OCOCH3)2, giving benzylidene diacetate; heating this with aqueous acid gives benzaldehyde:
\( \displaystyle \ce{C6H5CH3 + CrO3 + (CH3CO)2O} \xrightarrow{\text{273--283 K}} \ce{C6H5CH(OCOCH3)2} \xrightarrow[\Delta]{\ce{H3O+}} \ce{C6H5CHO} \)
(C) A nitrile treated with SnCl2 and HCl — This matches (II) Imine. This is the Stephen reaction; the nitrile is reduced to an imine, which is hydrolysed:
\( \displaystyle \ce{RCN + SnCl2 + HCl -> RCH=NH} \xrightarrow{\ce{H3O+}} \ce{RCHO} \)
(D) Toluene treated with Cl2 in the presence of light — This matches (I) Benzal chloride. In this side chain chlorination, two hydrogens of the methyl group are replaced by chlorine; hydrolysis of benzal chloride at 373 K gives benzaldehyde. This is a commercial method of making benzaldehyde:
\( \displaystyle \ce{C6H5CH3} \xrightarrow{\ce{Cl2}/h\nu} \ce{C6H5CHCl2} \xrightarrow[\text{373 K}]{\ce{H2O}} \ce{C6H5CHO} \)
NCERT (2026-27 edition):
8.2.2 Preparation of Aldehydes – “(a) Use of chromyl chloride (CrO2Cl2): Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde.”
“(b) Use of chromic oxide (CrO3): Toluene or substituted toluene is converted to benzylidene diacetate on treating with chromic oxide in acetic anhydride. The benzylidene diacetate can be hydrolysed to corresponding benzaldehyde with aqueous acid.”
“Nitriles are reduced to corresponding imine with stannous chloride in the presence of hydrochloric acid, which on hydrolysis give corresponding aldehyde.”
“Side chain chlorination of toluene gives benzal chloride, which on hydrolysis gives benzaldehyde. This is a commercial method of manufacture of benzaldehyde.”