Four carbonyl compounds are shown beside the options (the drawn compound is acetone, for reference). Which structure cannot form an enol?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

Enolisation needs a hydrogen on the carbon adjacent to the carbonyl. Benzaldehyde has none (the alpha atom is an aromatic carbon with no alpha-H), whereas acetaldehyde, cyclohexanone and butanone all have alpha hydrogens.

Practice General Organic Chemistry II: Acids and Bases More on Keto-enol tautomers and ortho effect in drawn compounds

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