Acetone is treated with magnesium amalgam in dry benzene, followed by water. The product is

A2,3-dimethylbut-2-ene
B2,3-dimethylbutane-2,3-diol
C2,3-dimethylbutane
Dpropan-2-ol

Answer: B

Mg(Hg) transfers an electron to the carbonyl to give a ketyl radical anion, and two ketyl radicals couple through carbon. After hydrolysis the 1,2-diol (pinacol) is obtained. Simple reduction to propan-2-ol would need a hydride or protic dissolving metal conditions.

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