question about rearrangements/shifts

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prolixity29

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Hydrides and alkyl groups often shift in E1/SN1 reactions. Can you have a concerted shift in an SN2/E2 reaction?

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No, that would complicate the interaction between atoms during the concerted reaction too much and make it unlikely to happen.
 
Hydrides and alkyl groups often shift in E1/SN1 reactions. Can you have a concerted shift in an SN2/E2 reaction?

E1 and SN1 reactions proceed via a carbocation intermediate. The reason you get these shifts is because after the formation of the carbocation a lower energy configuration can exists without the input of extra energy (a 2 prime carbocation will become a more stable 3 prime carbocation). E2 and SN2 reactions occur via a one-step mechanism so there is no need for a shift. The molecule never exists as a less stable carbocation so there is no driving force for rearrangement.
 
no, that would complicate the interaction between atoms during the concerted reaction too much and make it unlikely to happen.

e1 and sn1 reactions proceed via a carbocation intermediate. The reason you get these shifts is because after the formation of the carbocation a lower energy configuration can exists without the input of extra energy (a 2 prime carbocation will become a more stable 3 prime carbocation). E2 and sn2 reactions occur via a one-step mechanism so there is no need for a shift. The molecule never exists as a less stable carbocation so there is no driving force for rearrangement.

+1.
 
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