Markovnikov's Rule

Markovnikov’s Rule is an empirical rule used to predict the regioselectivity of the electrophilic addition reactions of asymmetric alkenes and alkynes. It states that with the addition of a protic acid (H-X, e.g H-Br) that the hydrogen atom is added to the carbon with the greatest number of hydrogens whilst the X component is added to the carbon with the least number of hydrogen atoms.

The chemical basis for Markovnikov’s Rule is the formation of the most stable carbocation during the addition process. The initial addition of hydrogen to one carbon atom in the alkene creates a positive charge on the other carbon atom creating a carbocation intermediate. The more substituted a carbocation, the more stable it is due to the effects of induction and hyperconjugation. The eventual major product of the addition reaction will be the one formed via the more stable intermediate. However, the less stable carbocation will still be formed in a low concentration will so the reaction will also produce a small amount of the minor product.

Figure 1. Markovnikov’s rule: Prediction of propene and HBr addition products.

Note that the major product is often referred to as the Markovnikov product. Although Markovnikov’s Rule was developed for - and is specifically applied to - the addition of hydrogen halides to alkenes, many other additions are also described as being Markovnikov or anti-Markovnikov depending on the regioselectivity of the addition reaction.