Chemistry Reference
In-Depth Information
Scheme 5.9 Rearrangements via 1,2-hydride shifts generate more stable carbocations from less
stable carbocations.
Scheme 5.10 Pinacol rearrangement.
Mechanistically, the pinacol rearrangement is explained by initial carbocation formation
through solvolysis. This step, illustrated in Scheme 5.11, involves protonation of an alcohol
followed by water leaving and generating a tertiary carbocation. In looking at this cation,
one may imagine that a 1,2-hydride shift is possible. However, the only sources of hydro-
gens for such a shift are the methyl groups adjacent to the cationic center. If a hydride
migrates from one of these methyl groups, as illustrated in Scheme 5.12, the result
would be generation of a primary carbocation. Since primary carbocations are less stable
than tertiary carbocations, this migration will not occur.
While the hydride shift illustrated in Scheme 5.12 cannot occur as a part of the pinacol
rearrangement, the intermediate carbocation is subject to alkyl migrations. As shown in
Scheme 5.13, a 1,2-alkyl shift results in transfer of the cation from a tertiary center to a
center adjacent to a heteroatom. As the oxygen heteroatom possesses lone electron pairs,
these lone pairs serve to stabilize the cation. Thus, the illustrated 1,2-alkyl shift transforms
a carbocation into a more stable carbocation.
Scheme 5.11 Pinacol rearrangement proceeds through solvolysis-mediated cation formation.
