Q1(e). 4-Nitrobenzaldehyde: LiAlH₄ against NaBH₄

Introduction. 4-Nitrobenzaldehyde has two reducible groups: the aldehyde and an aromatic nitro group. The two hydride reagents differ in strength and in how they treat ArNO₂.

Scheme: NaBH₄ reduces only the aldehyde.

Scheme: LiAlH₄ reduces CHO to CH₂OH and couples two aromatic nitro groups to an azo link.

1. Y. NaBH₄ is a mild hydride donor. It transfers H⁻ to the carbonyl carbon (nucleophilic addition) but does not reduce -NO₂: chemoselective. Y = (4-nitrophenyl)methanol (4-nitrobenzyl alcohol), O₂N-C₆H₄-CH₂OH.
2. X. LiAlH₄ reduces the CHO group to CH₂OH as well. Aliphatic nitro compounds are reduced by LiAlH₄ to amines, but aromatic nitro compounds give azo compounds: ArNO₂ → ArNO → ArNHOH, and the nitroso and hydroxylamine intermediates condense (via the azoxy compound) to Ar-N=N-Ar.
3. X = 4,4'-bis(hydroxymethyl)azobenzene, HOCH₂C₆H₄-N=N-C₆H₄CH₂OH (E), C₁₄H₁₄N₂O₂; azo compounds are not reduced further by LiAlH₄.

Conclusion. X = 4,4'-bis(hydroxymethyl)azobenzene (CHO reduced, nitro groups coupled to azo); Y = 4-nitrobenzyl alcohol (aldehyde only).

Application. The choice of hydride sets chemoselectivity in synthesis: NaBH₄ reduces aldehydes and ketones beside nitro, ester and amide groups; reduction of nitroarenes to azoarenes is a route to azo dyes and to azobenzene photoswitches.