electrophillic additions and rearrangements

electrophillic additions and rearrangements

During   electrophillic     addition ,    carbocations are the intermediates .They can rearrange by a 1,2 HYDRIDE SFIFT or 1,2 ALKYL SHIFT .What is the thermodynamic driving force for this shifting process? If  the hydride shift results in the conversion of a secondary (2^o) carbocation to a (more stable) tertiary (3^o) carbocation or from less satble carbocation to more stable carbocation then rearrangement takes place.  It is thus a thermodynamically downhill reaction, and, as it turns out, the energy barrier to the shift is relatively low, meaning that it can occur quite rapidly .

The following is an example of rearrangement reaction.

In most examples of carbocation rearrangements  ,that you are likely to encounter, the shifting species is a hydride or methyl group.  However,  any alkyl group is capable of shifting.  Sometimes, the entire side of a ring will shift over in a ring-expanding rearrangement.  Consider the following electrophilic addition of HBr to an the following alkene: 

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