Consider the addition of HBr to 3-methyl-1-butene. Here the expected product according to Markovnikoff’s rule is 2-bromo-3-methylbutane but the actual major product is 2-bromo- 2-methylbutane.
This is because, the secondary carbocation formed during the reaction is rearranged to give the more stable tertiary carbocation. Attack of Br-1 on this tertiary carbocation gives the major product 2-bromo2-methylbutane
