Which of the following reactions are involved in preparation of aldehydes?
(A) Gatterman-Koch reaction
(B) Etard Reaction
(C) Stephen Reaction
(D) Rosenmund Reaction
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A), (B), (C) and (D)
Aldehydes are prepared by stopping at the –CHO stage: by putting a –CHO group on an aromatic ring, by mild oxidation of a methyl group on a ring, or by partial reduction of an acid chloride or a nitrile. All four named reactions do one of these.
(A) Gatterman-Koch reaction — Correct. Benzene with carbon monoxide and HCl in the presence of anhydrous \( \ce{AlCl3} \) or CuCl gives benzaldehyde.
\( \displaystyle \ce{C6H6 + CO + HCl} \xrightarrow{\text{anhyd. }\ce{AlCl3}/\ce{CuCl}} \ce{C6H5CHO} \)
(B) Etard Reaction — Correct. Chromyl chloride oxidises the methyl group of toluene to a chromium complex, which is hydrolysed to benzaldehyde; the oxidation stops at the aldehyde.
\( \displaystyle \ce{C6H5CH3} \xrightarrow[\text{(ii) }\ce{H3O+}]{\text{(i) }\ce{CrO2Cl2},\ \ce{CS2}} \ce{C6H5CHO} \)
(C) Stephen Reaction — Correct. A nitrile is reduced by stannous chloride and HCl to an imine, which is hydrolysed to the aldehyde.
\( \displaystyle \ce{RCN + SnCl2 + HCl -> RCH=NH} \xrightarrow{\ce{H3O+}} \ce{RCHO} \)
(D) Rosenmund Reaction — Correct. An acyl chloride is hydrogenated over palladium on barium sulphate; the poisoned catalyst stops the reduction at the aldehyde.
\( \displaystyle \ce{RCOCl + H2} \xrightarrow{\ce{Pd-BaSO4}} \ce{RCHO + HCl} \)