Markovnikov's Rule

Markovnikov's Rule

Markovnikov's rule (1870) This is an empirical rule based on Markovnikov's experimental observations on the addition of hydrogen halides to alkenes. The rule states that : "when an unsymmetrical alkene reacts with a hydrogen halide to give an alkyl halide, the hydrogen adds to the carbon of the alkene that has the greater number of hydrogen substituents, and the halogen to the carbon of the alkene with the fewer number of hydrogen substituents" This is illustrated by the following example: Look at the position of the H and the Br in relation to the statement of Markovnikovs rule given above. Note that the major product, which is often referred to as the Markovnikov product, is the more highly substituted alkyl halide. In the example above, the secondary bromide is formed in preference to the primary bromide.

Modern mechanistic knowledge indicates reaction occurs via protonation of the alkene to give the more stable carbocation intermediate: Here we see that in principle, 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 (e.g. the halide ion) to give the alkyl bromide and hence 2-bromopropane is the major 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 Markovnikov or anti-Markovnikov depending on the regioselectivity of the addition reaction, e.g. the acid catalysed hydration of alkenes (Markovnikov) and the hydroboration / oxidation of alkenes (anti-Markovnikov).
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 Markovnikov's rule:

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

eg. 1:

To afford the observed product, the net reaction is addition of the hydrogen atom in HBr to the doubly bonded carbon atom in the alkene, bearing the greater number of hydrogen atoms.

eg. 2:

To afford the observed major product, the net reaction is addition of the hydrogen atom in HCl to the doubly bonded carbon atom in the alkene, bearing the greater number of hydrogen atoms.

Although originally stated in relation to hydrohalogenation of unsymmetrical alkenes, Markovnikov’s rule applies to some other electrophilic addition reactions of unsymmetrical alkenes (eg. 3) and to some electrophilic addition reactions of unsymmetrical alkynes (eg. 4).
eg. 3:

eg. 4: