The diol shown is warmed with dilute acid. Water leaves from the carbon that gives the more stable cation, and the only group that can move from the neighbouring carbon then shifts. Which structure is formed?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: A

The diol is 1,1-diphenylethane-1,2-diol. The diphenyl-substituted carbon gives a very stable tertiary cation, so its hydroxyl is lost; the neighbouring

CH X_{2} OH

carbon can only pass on a hydride, which turns it into

CHO

. The product is diphenylacetaldehyde; Structure B (deoxybenzoin) would need a primary cation and phenyl migration.

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