Discussion
The major product of the following reaction is :
- A.
- B.
- C.
- D.
Answer: C
The major product is (C), indole-3-carbaldehyde.
DMF and POCl3 form the Vilsmeier reagent, a chloroiminium ion. Indole is attacked by electrophiles at C-3, the most nucleophilic position, because the cation formed keeps the benzene ring intact and is stabilised by the nitrogen lone pair. The iminium intermediate is hydrolysed by aqueous NaOH to the aldehyde.
(A) and (B) are intermediates or side forms before hydrolysis, not the product after aqueous base. (D) puts the formyl group on the benzene ring at C-4, which is far less reactive than C-3.
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