ELECTROPHILIC ADDITION TO UNSYMMETRICAL ALKENES
The addition of a hydrogen halide to an unsymmetrical alkene can result in two different products. These products are not formed in equal amounts. Markovnikov’s rule states that ‘in the addition of HX to an alkene, the hydrogen atom adds to the carbon atom that already has the greater num-ber of hydrogen atoms’. This produces the more substituted alkyl halide.
The favored product arising from addition of a hydrogen halide to an unsymmetrical alkene will be formed from the more stable of the two pos-sible carbocations. The more stable carbocation will have more alkyl groups attached to the positive center.
Different products are not possible from the reaction of a halogen with an unsymmetrical alkene unless water is used as a solvent, in which case a hydroxyl group ends up on the more substituted carbon. This demonstrates that the bromonium ion does not share the positive charge equally amongst the bromine and the two carbons.
The more substituted alcohol is the preferred product from the acidic hydrolysis of an alkene as well as from the organomercuric synthesis of alcohols.
The reaction of a symmetrical alkene with hydrogen bromide produces the same
product regardless of whether the hydrogen of HBr is added to one end of the double bond or the other. However, this is not the case with unsymmetrical alkenes (Fig. 1). In this case, two different products are possible. These are not formed to an equal extent and the more substituted alkyl halide (II) is preferred.
The reaction proceeds in a Markovnikov sense with hydrogen ending up on the least substituted position and the halogen ending up on the most substituted posi-tion. Markovnikov’s rule states that ‘in the addition of HX to an alkene, the hydro-gen atom adds to the carbon atom that already has the greater number of hydrogen atoms’. This produces the more substituted alkyl halide.
This reaction can be rationalized by proposing that the carbocation intermediate leading to product II is more stable than the carbocation intermediate leading to product I (Fig. 2). It is possible to predict the more stable carbocation by counting the number of alkyl groups attached to the positive center. The more stable carbocation on the right has three alkyl substituents attached to the positively charged carbon whereas the less stable carbocation on the left only has one such alkyl substituent. The reasons for this difference in stability, but the result is summarized by Markovnikov’s rule.
However, Markovnikov’s rule does not always hold true. For example, the reac-tion of CF3CH=CH2 with HBr gives CF3CH2CH2Br rather than CF3CHBrCH3. Here, the presence of electron-withdrawing fluorine substituents has a destabiliz-ing influence on the two possible intermediate carbocations (Fig. 3). The destabil-izing effect will be greater for the more substituted carbocation since the carbocation is closer to the fluorine substituents and so the favoured carbocation is the least substituted one in this case.
There is no possibility of different products when a halogen such as bromine or chlorine is added to an unsymmetrical alkene. However, this is not the case if water is used as a solvent. In such cases, the halogen is attached to the least sub-stituted carbon and the hydroxyl group is attached to the more substituted carbon (Fig. 4). This result can be explained by proposing that the bromonium ion is not symmetrical and that although the positive charge is shared between the bromine and the two carbon atoms, the positive charge is greater on the more substituted carbon compared with the less substituted carbon.
With unsymmetrical alkenes, the more substituted alcohol is the preferred product (Fig. 5).
The same holds true for the organomercuric synthesis of alcohols (Fig. 6).