Question
Attempt I believe the answer should be B,C and D. In C and D the oxyge(or Nitrogen in D) can rearange by donating lone pair to carbocation.
But the books answer says B. Where am I wrong. Please help.
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3$\begingroup$ In (C) and (D) the carbocation is quite stabilized by conjugation with the lone pairs of O and N respectively. Rearrangement here refers to one caused by a Wagner–Meerwein rearrangement , as resonance is not a form of rearrangement. Only in B does a rearrangement lead to a more stable intermediate with more alpha hydrogens $\endgroup$ – Yusuf Hasan Apr 23 '20 at 9:53
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$\begingroup$ @YusufHasan in (b) better will be ring expansion after rearrangement.. $\endgroup$ – Zenix Apr 23 '20 at 11:37
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1$\begingroup$ @Zenix I would say that ring expansion isn't absolutely necessary in this case, because 5-membered rings aren't strained a lot as compared to 3 or 4 membered ones. They will only go for 6 membered if there isn't a huge loss in hyperconjugation. Here, ring expansion after rearrangement would lead to a loss of alpha hydrogens $\endgroup$ – Yusuf Hasan Apr 23 '20 at 11:57
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