91Ó°ÊÓ

2 methyl-1,3-pentadiene(with conjugated double bonds) with the addition HBr is faster than 2-methyl-1,4-pentadiene with the addition HBr (with isolated double bonds). the rate limiting step of the reaction is a formation of carbocation intermediate. 2-Methyl-1,3-pentadine forms a more stable carbocation intermediate than does 2-methyl -1,4-pentadiene.

Since the more stable reactants forms the more stable carbocation intermediate.the relative free energies of activation for the rate limiting steps of the two reaction depend on whether the difference in the stabilities of the reactant is greater or less than the difference in the stabilities of the transition States leading to formation of the carbocation intermediates (which depend on the difference in stabilities of the carbocation intermediates.)There is a significant difference in the stabilities of the carbocation intermediates because one is stabilized by electron delocalisation and one is not. The transitions States look more like the carbocation intermediates than line the alkenes. therefore the difference in the stabilities of the reactants is less than the difference in the stabilities of the transition States. So the rate of reaction of HBr with 2-methyl-1,3- pentadiene is the faster reaction. (If the difference in the stability of the reactants had been greater than the difference in the stabilities of the transition States, the rate of reaction of HBr with 2-methyl-1,4-pentadiene would have been the faster reaction.)

2 methyl-1,3-pentadiene with the addition HBr is faster than 2-methyl-1,4-pentadiene with the addition HBr.