Q.(a)

(a) Malachite green is built by acid-catalysed condensation of benzaldehyde with two dimethylaniline units followed by oxidation; Thorpe condensation self-condenses a nitrile via its alpha-carbanion. (b) Compound A giving a purple $FeCl_3$ colour is phenol, which converts to aniline (B) and benzene (C).

(a)(i) Preparation of malachite green from benzaldehyde: Benzaldehyde is condensed with two molecules of N,N-dimethylaniline in the presence of a condensing/dehydrating agent (conc. $H_2SO_4$ or anhydrous $ZnCl_2$), which brings about electrophilic substitution at the para position of each dimethylaniline ring onto the aldehyde carbon, with loss of water, giving a colourless triarylmethane compound called the 'leuco base': $C_6H_5CHO + 2\,C_6H_5N(CH_3)_2 \rightarrow \text{Leuco base} + H_2O$ This leuco base is then oxidised (traditionally using $PbO_2$ or $MnO_2$ in dilute acid) to generate the resonance-stabilised triarylmethyl cation, which is the intensely coloured dye, malachite green.

(a)(ii) Thorpe nitrile condensation: Two molecules of an aliphatic nitrile possessing an alpha-hydrogen undergo base-catalysed self-condensation (analogous to the Claisen ester condensation, but for nitriles): a strong base such as sodium ethoxide removes an alpha-hydrogen to generate a stabilised carbanion, which attacks the nitrile carbon of a second molecule. For example, with acetonitrile: $2\,CH_3CN \xrightarrow{NaOC_2H_5} CH_3-C(=NH)-CH_2-CN$ giving a beta-iminonitrile, which on subsequent hydrolysis yields a beta-ketonitrile. This named reaction (the Thorpe, or Thorpe-Ziegler, condensation) is an important route to beta-ketonitriles and cyclic ketones.

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