The diol $(C X_{6} H X_{5}) X_{2} C (OH) - C (OH) (CH X_{3}) (CH X_{2} CH X_{3})$ (1,1-diphenyl-2-methylbutane-1,2-diol) is heated with dilute acid. Take the migratory aptitude as phenyl > ethyl > methyl. The main product is

A2,2-diphenylpentanal
B2-methyl-1,2-diphenylbutan-1-one
C3,3-diphenylpentan-2-one
D4,4-diphenylpentan-3-one

Answer: C

The diphenylcarbinyl cation

Ph X_{2} C X^{+-}

is much more stable than the tertiary alkyl cation, so the hydroxyl on the C-2 carbon is retained. The groups on C-2 are ethyl and methyl, and ethyl migrates better. The cationic carbon becomes

C (Ph) X_{2} (CH X_{2} CH X_{3})

and C-2 becomes

C (= O) CH X_{3}

, giving

CH X_{3} CO - C (C X_{6} H X_{5}) X_{2} CH X_{2} CH X_{3}

, 3,3-diphenylpentan-2-one.

Practice Alcohols, Phenols and Ethers More on Pinacol-pinacolone and related rearrangements

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The diol (CX6​HX5​)X2​C(OH)−C(OH)(CHX3​)(CHX2​CHX3​) (1,1-diphenyl-2-methylbutane-1,2-diol) is heated with dilute acid. Take the migratory aptitude as phenyl > ethyl > methyl. The main product is

Answer: C

More questions on Pinacol-pinacolone and related rearrangements

The diol $(C X_{6} H X_{5}) X_{2} C (OH) - C (OH) (CH X_{3}) (CH X_{2} CH X_{3})$ (1,1-diphenyl-2-methylbutane-1,2-diol) is heated with dilute acid. Take the migratory aptitude as phenyl > ethyl > methyl. The main product is