NEET 2021 Chemistry Question 96 Solution

When benzene or its derivative is treated with carbon monoxide ($CO$) and hydrogen chloride ($HCl$) in the presence of anhydrous aluminium chloride ($AlCl_3$) and cuprous chloride ($CuCl$), benzaldehyde is formed. This is the Gattermann-Koch reaction: $$C_6H_6 + CO + HCl \xrightarrow{\text{Anhyd. } AlCl_3 / CuCl} C_6H_5CHO + HCl \implies \mathbf{\text{(a)-(ii)}} delivery$$

Aldehydes and ketones having at least one methyl group linked to the carbonyl carbon atom (methyl ketones, $RCOCH_3$) are oxidized by sodium hypohalite ($NaOX$) to sodium salts of corresponding carboxylic acids having one carbon atom less, while the methyl group is converted to haloform ($CHX_3$). This is the Haloform reaction: $$R-CO-CH_3 + 3\,NaOX \longrightarrow RCOONa + CHX_3\downarrow + 2\,NaOH \implies \mathbf{\text{(b)-(iii)}}$$

Carboxylic acids react with alcohols in the presence of a mineral acid catalyst (such as concentrated sulfuric acid) to form esters with elimination of water. This is Esterification: $$R'COOH + R-CH_2-OH \xrightleftharpoons{\text{Conc. } H_2SO_4} R'COOCH_2R + H_2O \implies \mathbf{\text{(c)-(iv)}}$$

Carboxylic acids having an $\alpha$-hydrogen are halogenated at the $\alpha$-position on treatment with chlorine or bromine in the presence of a small amount of red phosphorus, followed by hydrolysis, yielding $\alpha$-halocarboxylic acids. This is the Hell-Volhard-Zelinsky reaction: $$R-CH_2-COOH \xrightarrow[\text{(ii) } H_2O]{\text{(i) } X_2 / \text{Red P}} R-CH(X)-COOH \implies \mathbf{\text{(d)-(i)}}$$