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Most of the reactions undergo ring expansion whenever favourable. But this reaction [1, p. 514] doesn't seems to do so:
Practice Problem 11.18 Provide a mechanism for the following reaction.
Could anybody explain why?
There are 2 things to consider: What would be the energy difference between before and after rearrangement from initial $2^\circ$-carbocation (I) and each of rearranged carbocations II and III:
It is obvious that the rearrangement of carbocation I to give carbocation II by ring expansion have minimal energy change since both initial and final carbocations are $2^\circ$ and ring expansion does not relieve too much energy because it is cyclopentyl to cyclohexyl. However, the rearrangement of carbocation I to give carbocation III by 1,2-hydride transfer relieves good amount of energy since initial carbocation is $2^\circ$ and final rearranged carbocation is $3^\circ$. Although the carbocation II would undergo further stabilization by 1,2-hydride transfer to relieve extra energy, I to III happens much faster than I to II transfer to give 3 as the major product.
