7.11 Evidence for the Mechanism of

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Objective

After completing this section, you should be able to explain the “unusual” products formed in certain reactions in terms of the rearrangement of an intermediate carbocation.

Key Terms

Make certain that you can define, and use in context, each of the key terms listed below.

alkyl shift
hydride shift

Study Notes

Whenever possible, carbocations will rearrange from a less stable isomer to a more stable isomer. This rearrangement can be achieved by either a hydride shift, where a hydrogen atom migrates from one carbon atom to the next, taking a pair of electrons with it; or an alkyl shift, in which an alkyl group undergoes a similar migration, again taking a bonding pair of electrons with it. These migrations usually occur between neighbouring carbon atoms, and hence are termed 1,2-hydride shifts or 1,2-alkyl shifts. A hydride ion consists of a proton and two electrons, that is, [H^:]⁻. Hydride ions exist in compounds such as sodium hydride, NaH, and calcium hydride, CaH₂.