Question

Explain the mechanism of sn1 and sn2 reaction with proper example

Answer 1

If you have studied these reactions, you would definitely know what are primary, secondary and tertiary halides.

In SN1 reaction mechanism, the rds step involves the formation of a carbocation intermediate. The more stable the intermediate, the faster is the reaction. The increased +I(inductive effect) due to alkyl groups increases the cation stability. Thus, the stability order of carbocations is:

Tertiary>secondary>primary

Therefore, for SN1 reaction, the rate of reaction will be in the order:

Tertiary>secondary>primary halide.

However, for SN2 mechanism, which is a one step mechanism, the more facile backside attack of nucleophile (on alkyl halide) leads to a faster reaction. The backside attack of nucleophile is favoured when there is less steric hindrance on central carbon. This is found to be on primary halide. As the number of alkyl group on carbon increases, the steric hindrance increases and rate is decreased.

So the reactivity order for SN2 is

primary> secondary> tertiary halide.