Which of the following reagents will not convert Toluene into Benzald…

The correct answer is Option (4) → \(CO/HCl\) in presence of \(AlCl_3\)

Toluene can be turned into benzaldehyde only by oxidising its methyl group to –CHO, either directly with a chromium(VI) reagent that stops at the aldehyde stage, or through a dichloride that is then hydrolysed. The Gattermann-Koch reaction does something different: carbon monoxide and HCl with anhydrous \( \ce{AlCl3} \) put a new –CHO group on the benzene ring:

    \( \displaystyle \ce{C6H6} \xrightarrow[\text{anhyd. }\ce{AlCl3}/\ce{CuCl}]{\ce{CO},\ \ce{HCl}} \ce{C6H5CHO} \)

With toluene the methyl group stays and the ring is formylated, mainly at the para position, so the product is 4-methylbenzaldehyde, not benzaldehyde:

    \( \displaystyle \ce{CH3C6H5} \xrightarrow[\text{anhyd. }\ce{AlCl3}]{\ce{CO},\ \ce{HCl}} p\text{-}\ce{CH3C6H4CHO} \)

• Option (1) \(CrO_2Cl_2\) — Converts toluene into benzaldehyde. In the Etard reaction chromyl chloride oxidises the methyl group to a chromium complex, which is hydrolysed to benzaldehyde:

      \( \displaystyle \ce{C6H5CH3} \xrightarrow[\ce{H3O+}]{\ce{CrO2Cl2},\ \ce{CS2}} \ce{C6H5CHO} \)

• Option (2) \(CrO_3\) — Converts toluene into benzaldehyde. Chromic oxide in acetic anhydride gives benzylidene diacetate, which aqueous acid hydrolyses to benzaldehyde; the diacetate protects the aldehyde from further oxidation:

      \( \displaystyle \ce{C6H5CH3} \xrightarrow[\text{273-283 K}]{\ce{CrO3},\ \ce{(CH3CO)2O}} \ce{C6H5CH(OCOCH3)2} \xrightarrow{\ce{H3O+}} \ce{C6H5CHO} \)

• Option (3) \(Cl_2/\text{light}, H_2O\) — Converts toluene into benzaldehyde. Side-chain chlorination gives benzal chloride, which on hydrolysis gives benzaldehyde (the commercial method):

      \( \displaystyle \ce{C6H5CH3} \xrightarrow{\ce{Cl2}/h\nu} \ce{C6H5CHCl2} \xrightarrow[\text{373 K}]{\ce{H2O}} \ce{C6H5CHO} \)

• Option (4) \( CO/HCl \) in presence of \( AlCl_3 \) — Does not convert toluene into benzaldehyde (gives 4-methylbenzaldehyde). The Gattermann-Koch reaction formylates the ring and leaves the methyl group in place.

NCERT (2026-27 edition):
By Gattermann–Koch Reaction

By Oxidation of Methylbenzene – “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. The following methods are used for this purpose. (a) Use of chromyl chloride (\( \ce{CrO2Cl2} \)): Chromyl chloride oxidises methyl group to a chromium complex, which on hydrolysis gives corresponding benzaldehyde. … (b) Use of chromic oxide (\( \ce{CrO3} \)): 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.”