Question

What is the product formed when acetone heated in presence of Mg/Hg followed by hydrolysis?
A.

B.

C.

D.

Answer 1

Hint: In pinacol coupling reaction, there is a formation of carbon-carbon between the carbonyl group of ketone or an aldehyde in the presence of electron donor such as Mg or Hg in a free radical process. The product of this reaction is vicinal diol.

Complete step by step answer: When acetone is heated in the presence of Mg/Hg followed by hydrolysis reduces to gives pinacol which is named as \[{\rm{2,3 - dimethyl - 2,3 - diol}}\].
The mechanism of the formation of pinacol is as follows:
The first step is the one-electron reduction of the carbonyl group of the acetone by the reducing agent such as magnesium or mercury to a ketyl radical anion species.
In the next step, the ketyl radical anion, which is formed in the first step couples with another ketyl radical to form vicinal diol with deprotonated hydroxyl group.
In the third step, the addition of water (a proton donor) gives the diol.

So, the correct answer is “Option C”.

Note:
There is a formation of 5-membered cyclic compound with two oxygen atoms coordinated with the oxidized \[{\rm{M}}{{\rm{g}}^{{\rm{2 + }}}}\;{\rm{or}}\;{\rm{H}}{{\rm{g}}^{{\rm{2 + }}}}\] which broken up by adding water. McMurry reaction is a related reaction in which titanium(III) chloride or titanium(IV) chloride is used in place of Mg/Hg in conjunction with a reducing agent and gives metal-diol complex.