Chemistry Reference
In-Depth Information
Figure 5.11 Tertiary carbocations are more stable than allylic carbocations.
5.4 CARBOCATION REARRANGEMENTS
Having addressed the structure and stability of carbocations, discussions will now be directed
to the specific side reactions to which carbocations are subject. Specifically, this section
focuses on rearrangements of carbocations known as hydride shifts and alkyl shifts.
5.4.1 1,2-Hydride Shifts
Recalling the role played by hyperconjugation in the stabilization of carbocations, a more
detailed examination of this phenomenon is warranted. Looking at Figure 5.6, we note that
carbocations are planar with an unoccupied p orbital extending both above and below the
plane of the ion. Furthermore, looking at Figure 5.8, the electrons in a carbon-hydrogen
bond adjacent to a carbocation can conjugate toward the positive charge residing in the
vacant p orbital. This donation of electron density can only occur if the carbon-hydrogen
bond is aligned with the vacant p orbital, as shown in Figure 5.12 using several perspective
views. Specifically, the carbon-hydrogen bond must lie in the same plane as the vacant
p orbital.
Figure 5.12 Hyperconjugation occurs when a carbon-hydrogen bond lies in the same plane as a
carbocation's vacant p orbital.
