The diol shown is treated with cold dilute acid. Assume the cation forms at the carbon that stabilises it better and that a group on the neighbouring carbinol carbon then migrates. Which structure is the major product?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: B

The diol is 2-methyl-1,1-diphenylpropane-1,2-diol. The diphenylcarbinyl cation is far more stable than the tertiary alkyl cation, so water leaves from the diphenyl carbon. A methyl group then migrates from the other carbon, which becomes

C = O

. The product is 3,3-diphenylbutan-2-one; Structure A comes from the wrong cation (phenyl migration).

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