Carbonyl compounds dissolving metals, mechanism

The third method is the simplest to do, but has the most complicated mechanism. The Clemmensen reduction is also rather violent, and really reasonable only for compounds with just the one functional group. It uses zinc metal dissolvmg in hydrochloric acid. As the metal dissolves, it gives up two electrons—in the absence of something else to do, these electrons would reduce the H+ in the acid to H2, and give ZnCl2 and H2. But in the presence of a carbonyl compound, the electrons go to reduce the C=0 bond. 

The vast majority of the dissolving metal reductions of carbonyl compounds which have been carried out synthetically have used either alcohols or liquid NH3 as the solvent. " However, a variety of other solvents have been employed, frequently in connection with studies of the mechanism of the reductions or in exploratory synthetic studies. 

Keeping in mind the mechanism for the dissolving metal reduction of alkynes to trans alkenes in Chapter 12, write a stepwise mechanism for the following reaction, which involves the conversion of an a,p-unsaturated carbonyl compound to a carbonyl compound with a new alkyl group on the a carbon. 

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