SN2 reaction: faster with allylic and benyzlic halides?

[suhshi]

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can anybody explain??

thanks

 

[Shinpe]

Hmmm where does it say that?

I know that allylic and benzylic halides react faster in SN1 because the rate-limiting step is the formation of the carbocation, and allylic and benzylic carbocations are more stable than even tertiary carbocations.

But since no carbocation is formed in SN2 and its mechanism is concerted, I thought that steric hindranced would be more important than all that. Can anyone else confirm this?

 

[BTtheman777]

Allylic Species would definitely undergo SN1. This would be the case because the carbocation would be stabilized by the delocalized pi electrons. As far as benzylic carbocations, I am not sure can somebody elaborate on this?

 

[saDDS]

maybe the because the alkyl group donates electron density to the benzene it would be a better electrophile in a nucleophlic sN2...just taking a shot in the dark here

 

[Shinpe]

Allylic Species would definitely undergo SN1. This would be the case because the carbocation would be stabilized by the delocalized pi electrons. As far as benzylic carbocations, I am not sure can somebody elaborate on this?

A benzylic carbocation is also highly stabilized due to resonance with the ring. But then again I don't see how these would contributed to an SN2 rxn.

 

[Sugafoot79]

• Benzylic halides undergo nucleophilic substitution reactions very readily (review)
• Primary benzylic halides typically react via an SN2 pathway (review), and there is no competition from elimination.
• Secondary or tertiary benzylic halides typically react via an SN1 pathway (review), via the resonance stabilised carbocation.
• Unlike allylic systems, there is no "benzylic rearrangement" since that would result in loss of aromaticity.