Chemistry Reference
In-Depth Information
Figure 5.13 Hyperconjugation can be viewed as formation of a pseudo-double-bond.
Scheme 5.8 Hyperconjugation leads to migration of hydrogen atoms through a 1,2-hydride shift.
When the alignment of a carbon-hydrogen bond with a vacant p orbital takes place
allowing for hyperconjugation, a “pseudo-double-bond” develops. As illustrated in
Figure 5.13, this can be envisioned as a double bond with a closely associated hydrogen ion.
If, as shown in Figure 5.13, hyperconjugation results in the formation of species posses-
sing both double-bond character and associated hydrogen ions, equilibrium-controlled
migration of the associated hydrogen ion can be expected. This transformation, shown
in Scheme 5.8, is known as a 1,2-hydride shift and results in the migration of a proton
from carbon 1 to carbon 2.
While the example illustrated in Scheme 5.8 shows equilibrium between two chemically
identical carbocations, there are factors influencing the direction of these transformations
when applied to more complex systems. If we consider Scheme 5.9, we notice that the posi-
tive charge migrates exclusively to the tertiary center, reflecting the increased stability of
tertiary carbocations over primary carbocations. In general, where 1,2-hydride shifts are
possible, rearrangement of less stable carbocations to more stable carbocations is expected.
5.4.2 1,2-Alkyl Shifts
Moving from discussion of 1,2-hydride shifts to 1,2-alkyl shifts, it is important to remem-
ber that hydride shifts occur much more readily than the corresponding alkyl shifts. In fact,
as a general rule, alkyl shifts will not occur unless a hydride shift cannot take place.
Among the most famous examples of a reaction involving a 1,2-alkyl shift is the pinacol
rearrangement. This reaction, shown in Scheme 5.10, results in the conversion of a vicinal
diol to a ketone.
