Discussion
The structures of the major products W and X in the following synthetic scheme are
- A.
- B.
- C.
- D.
Answer: D
The products are (D): W is pyridine N-oxide and X is benzaldehyde together with pyridine.
Peracetic acid oxidises the pyridine nitrogen to give pyridine N-oxide, W. The N-oxide oxygen is nucleophilic and is benzylated by benzyl bromide, giving an N-benzyloxypyridinium salt. Sodium hydroxide then removes a benzylic proton; the electrons move to form a C=O bond while the weak N-O bond breaks, releasing pyridine and leaving benzaldehyde. The N-oxide has acted as an oxidant, converting the benzyl halide to the aldehyde.
A peracid does not hydroxylate the ring, so W is not 4-hydroxypyridine, which rules out (A) and (B). (C) would need benzylation at a ring carbon, but the benzyl group goes onto oxygen, and the base then fragments that intermediate.
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