Chapter 6: Reactions of Alkenes: Addition Reactions

Markovnikov's Rule

Markovnikov's rule (1870s), based on experimental observation, states that :     "when an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon that has the greater number of hydrogen substituents, and the halogen to the carbon have the fewer number of hydrogen substituents" Modern mechanistic knowledge indicates reaction occurs via protonation to give the more stable carbocation:

Propene can protonate to give two different carbocations, one 2o and the other 1o.  Formation of the more stable 2o carbocation is preferred.

The carbocation then reacts with the nucleophile to give the alkyl bromide:

Although Markovnikov's rule is specifically applied to the addition of hydrogen halides to alkenes, many additions are also described as Markovnikov or anti-Markovnikov depending on the regioselectivity of the addition reaction.

In more general terms, Markovnikov's rule can be "modernised" to cover other addition reactions by considering that the electrophile adds to the least substituted end of the alkene giving rise to the more stable intermediate. So let's rephrase our statement of Markovnokov's rule:

"when an unsymmetrical alkene undergoes addition with E-Nu, then the electrophile, E, adds to the carbon that has the greater number of hydrogen substituents, and the nucleophile, Nu, to the carbon have the fewer number of hydrogen substituents"