Markovnikov Rule

Markovnikov Rule

Rule : The negative part of the addendum adds on to the carbon atom that is joined to the least number of hydrogen atoms.

In the case of the halogen acids the halogen atom is the negative part, and so isopropyl halide is obtained.

Markovnikov's rule is empirical, but may be explained theoretically on the basis that the addition occurs by a polar mechanism. As with halogens, the addition of halogen acid is an electrophilic reaction, the proton adding first, followed by the halide ion. Also, the addition is predominantly trans, and this may be explained in terms of the formation of a bridged carbonium ion.

Now consider the case of propene. Since the methyl group has a +1 effect, the 𝜋–electrons are displaced towards the terminal carbon atom which, in consequence, acquires a negative charge. Thus, the proton adds on to the carbon farthest from the methyl group, and the halide ion then adds to the carbonium ion :