Why vinyl halide and aryl halide does not give SN
1
& SN
2 reaction easily but allyl halide and benzyl
halide undergo these reactions very easily?
Answers
A SN1 / SN2 mechanism on vinyl halide would look like this.
A SN2 mechanism is not favoured for 3 reasons:
The substituents around a double bond are within the same plane, therefore an SN2 would give steric hindrance as depicted in the above scheme.
It would mean that the nucleophile would need to attack the double bond — a region with high electron density, causing electronic repulsion.
The transition state of this SN2 reaction would be a sp hybrid carbon, which is not energetically favorable.
A SN1 would need to generate a vinyl cation, which is an unstable cation hence not something you normally would observe
For ‘normal’ halides the reaction is too slow due to poor leaving group ability and instability of the resulting vinyl cation, so in practice you will not see SN1 reactions on vinyl halides.
But organic chemistry is versatile so with some adaptation a SN1 on a vinyl carbon is feasible. You have two option to make it work:
Using other, better leaving groups (e.g. triflate or nonaflate)
and/or
By stabilization of the vinyl cation. (Typical groups would be alpha-aryl , cyclopropyl, or adjacent double bonds.)
A SN1 / SN2 mechanism on vinyl halide would look like this.
A SN2 mechanism is not favoured for 3 reasons:
The substituents around a double bond are within the same plane, therefore an SN2 would give steric hindrance as depicted in the above scheme.
It would mean that the nucleophile would need to attack the double bond — a region with high electron density, causing electronic repulsion.
The transition state of this SN2 reaction would be a sp hybrid carbon, which is not energetically favorable.
A SN1 would need to generate a vinyl cation, which is an unstable cation hence not something you normally would observe
For ‘normal’ halides the reaction is too slow due to poor leaving group ability and instability of the resulting vinyl cation, so in practice you will not see SN1 reactions on vinyl halides.
But organic chemistry is versatile so with some adaptation a SN1 on a vinyl carbon is feasible. You have two option to make it work:
Using other, better leaving groups (e.g. triflate or )
and/or
By stabilization of the vinyl cation. (Typical groups would be alpha-aryl , cyclopropyl, or adjacent double bonds.)
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