[Nemesisof99]

Which of the three is favoured in the rearrangement of carbocations(tert. from sec. etc)?
Hydride shift, Methyl shift or Phenyl shift? I've read in a book that hydride shift is favored followed by phenyl but I'm not sure..
Any help is appreciated

[nj_bartel]

I'd been told it was generally smallest first.  I know for sure hydride shifts prior to alkyl groups, but I don't know about phenyl, never come across it.  How is this a high school question?

[Nemesisof99]

Hm..I'm preparing for an olympiad so, just wanted to get the concepts clear. 

[nj_bartel]

Damn you smart high school kids    Good luck

[orgoclear]

we were taught that phenyl shift is easiest followed by hydride and then alkyl. It is because in phenyl shift the resultant carbocation would be benzylic in addition to being secondary or tertiary (according to the specific case)

[Nemesisof99]

Thanks a lot! 

[Schrödinger]

we were taught that phenyl shift is easiest followed by hydride and then alkyl. It is because in phenyl shift the resultant carbocation would be benzylic in addition to being secondary or tertiary (according to the specific case)

But then during a phenyl shift the adjacent carbon is the one that gets the positive charge, not the benzylic. (refer to diagram below).


So, I think there is another explanation as well:

Well, consider a phenyl shift and an alkyl shift.

During an alkyl shift, if you observe the transition state carefully, you will find that the alkyl carbon has 5 bonds . Although some are not full bonds, that carbon has a temporary pentavalency. This is not favourable for carbon, as its maximum covalency is 4.

When you observe the transition state for a phenyl shift, there is no problem.

Well, a hydride shift is easy to understand, because it is extremely small, and thus easy to migrate as well.

So, i think the order is phenyl > hydride > alkyl

Please correct me if i am wrong.