CBSE Class 11-science Answered

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.

• Modern mechanistic knowledge indicates reaction occurs via protonation to give the more stable carbocation:

Here we see that in principle, propene can protonate to give two different carbocations, one 2^o and the other 1^o.
Formation of the more stable 2^o carbocation is preferred. 

• The carbocation then reacts with the nucleophile to give the alkyl bromide and hence 2-bromopropane is the major product.