But the addition product is 99+% tert-butyl bromide so the reaction clearly is kinetically controlled, tert-butyl being formed considerably faster than isobutyl bromide. The slow, or rate-determining, step in this reaction is the formation of the intermediate cation rather than the reaction of the cation with bromide ion. So to account for the formation of tert-butyl bromide we have to consider why the tert-butyl cation is formed more rapidly than the isobutyl cation:

Alkyl groups are more electron donating than hydrogen. This means that the more alkyl groups there are on the positive carbon of the cation, the more stable and the more easily formed the cation will be. The reason is that electron-donating groups can partially compensate for the electron deficiency of the positive carbon. As a result, we can predict that the tert-butyl cation with three alkyl groups attached to the positive center will be formed more readily than the primary isobutyl cation with one alkyl group attached to the positive center.

Thus the problem of predicting which of the two possible products will be favored in the addition of unsymmetrical reagents to alkenes under kinetic control reduces to predicting which of two possible carbocation intermediates will be formed most readily. With simple alkenes, we shall expect the preference of formation of the carbocations to be in the order:

tertiary > secondary > primary.

The reaction scheme can be represented conveniently in the form of an energy diagram (Figure 10-10). The activation energy, ΔH₁ tert for the formation of the tert-butyl cation is less than ΔH₁prim for the formation of the isobutyl cation because the tertiary ion is much more stable (relative to the reactants) than the primary ion, and therefore is formed at the faster rate. The second step, to form the product from the intermediate cation, is very rapid and requires little activation energy. Provided that the reaction is irreversible, it will take the lowest-energy path and form exclusively tert-butyl bromide. However, if the reaction mixture is allowed to stand for a long time, isobutyl bromide begins to form. Over a long period, the products equilibrate and, at equilibrium, the product distribution reflects the relative stabilities of the products rather than the stability of the transition states for formation of the intermediates.

Figure 10-10: Energy diagram showing the progress of addition of hydrogen bromide to 2-methylpropene.

A rather simple rule, formulated in 1870 and known as Markownikoff's rule, correlates the direction of kinetically controlled additions of HXHX to unsymmetrical alkenes. This rule, an important early generalization of organic reactions, may be stated as follows: In addition of HX to an unsymmetrical carbon-carbon double bond, the hydrogen of HX goes to that carbon of the double bond that carries the greater number of hydrogens. It should be clear that Markownikoff's rule predicts that addition of hydrogen bromide to 2-methylpropene will give tert-butyl bromide.