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 (2o) carbocation to a (more stable) tertiary (3o) 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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