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Addition of an unsymmetrical substance such as HX to an unsymmetrical alkene theoretically can give two products:
and
Both products are seldom formed in equal amounts; in fact, one isomer usually is formed to the exclusion of the other. For example, the hydration of propene gives 2-propanol (not 1-propanol), and hydrogen chloride adds to 2-methylpropene to give tert-butyl chloride (not isobutyl chloride):
To understand the reason for the pronounced selectivity in the orientation of addition of electrophiles, it will help to consider one example, hydrogen bromide addition to 2-methylpropene. Two different carbocation intermediates could be formed by attachment of a proton to one or the other of the double bond carbons:
Subsequent reactions of the cations with bromide ion give tert-butyl bromide and isobutyl bromide. In the usual way of running these additions, the product is very pure tert-butyl bromide.
How could be have predicted which product would be favored? The first step is to decide whether the prediction is to be based on (1) which of the two products is the more stable, or (2) which of the two products if formed more rapidly. If we make a decision on the basis of product stabilities, we take into account ΔH0 values, entropy effects, and so on, to estimate the equilibrium constants Keq for the reactants and each product. When the ratio of the products is determined by the ratio of their equilibrium constants, we say the overall reaction is subject to equilibrium (or thermodynamic) control. Equilibrium control requires that the reaction be reversible.
When a reaction is carried out under conditions in which it is not reversible, the ratio of the products is determined by the relative rates of formation of the various products. Such reactions are said to be under kinetic control.
The products obtained in a reaction subject to kinetic control are not necessarily the same as those obtained under equilibrium control. Indeed, the equilibrium constant for interconversion of tert-butyl bromide and isobutyl bromide at 25o is 4.5, and if the addition of hydrogen bromide to 2-methylpropene were under equilibrium control, the products would be formed in this ratio:
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.
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