Question

Write salient features of S_N1 mechanism.

Answer 1

Salient features of S_N1 mechanism:

1. Two step mechanism.
2. Heterolyis of C-X bond in the slow and reversible first step to form planar carbocation intermediate.
3. Attack of the nucleophile on the carbocation intermediate in the fast second step to form the product.
4. When S_N1 reaction is carried out at a chiral carbon in an optically active substrate, the product formed is nearly racemic.This indicates that S_N1 reaction proceeds mainly with racemization. This means both the enantiomers of product are formed in almost equal amount.Racemization in an S_N1 reaction is the result of formation of planar carbocation intermediate. Nucleophiles can attack planar carbocation from either side which results in formation of both the enantiomers of the product.