Which reagent system converts propanone into pinacol (2,3-dimethylbutane-2,3-diol) by bimolecular reduction?

A $LiAlH X_{4}$ followed by $H X_{3} O X^{+}$
B $Mg - Hg$ followed by water
C $Zn - Hg$ /conc. $HCl$
D $NaBH X_{4}$ in ethanol

Answer: B

Mg-Hg amalgam transfers single electrons to the carbonyl to give ketyl radical anions, which couple to give the 1,2-diol (pinacol) after water work-up.

LiAlH X_{4}

and

NaBH X_{4}

give propan-2-ol, and Clemmensen reduction gives propane.

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