Chemistry Reference
In-Depth Information
Scheme 5.12 1,2-Hydride shifts will not occur when the product cation is less stable than the
starting cation.
Scheme 5.13 Alkyl migrations occur when the resulting carbocation is more stable than the starting
carbocation.
Scheme 5.14 Conclusion of the pinacol rearrangement involves migration of the positive charge to
the adjacent oxygen atom followed by deprotonation.
Mechanistic conclusion of the pinacol rearrangement is illustrated in Scheme 5.14 and
involves initial donation of an oxygen lone pair to the cation, thus migrating the charge
to the oxygen atom. The resulting oxygen cation then releases a proton, liberating the
illustrated neutral ketone.
As the mechanistic steps discussed for the pinacol rearrangement have been illustrated
using arrow pushing, it is important to recognize that in all cases, the arrows have been
drawn pushing electrons toward positive charges. This point has been previously discussed
and will continue to be emphasized.
5.4.3 Preventing Side Reactions
Because of 1,2-hydride and alkyl shifts, it is possible to obtain multiple products from S
N
1
reactions. Thus, to induce one product to predominate, we must find a way to stabilize the
carbocation. This is done by using highly polar solvents such as acetic acid, dimethyl
formamide, and dimethyl sulfoxide. In using this strategy, the lifetime of a carbocation
can be extended, allowing the most stable product more time to form. As a result,
