Hammond postulate and electrophilic addition

The regioselectivity in addition of hydrogen halides to alkenes results from the difference in the transition state energies of two competing reactions. We recall that, according to Hammond s postulate, the structure of a transition state resembles that of an intermediate with similar energy (Section 3.17). 

The transition state that leads to the less stable primary carbocation is of higher energy than the transition state that leads to the more stable secondary carbocation. Subsequent reaction of either carbocation with a nucleophile occurs rapidly. 

Write the structure of the carbocation formed in the addition reaction of HBr with each of the following alkenes. 

Rank the following carbocations in order of their stabihries. 

Reaction of either 1-butene or 2-butene with HBr gives the same product, 2-bromobutane. The reaction of 1-butene is faster than the reaction of 2-butene, even though both reactions proceed via a common carbocation intermediate. What is responsible for the difference in the rates  

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