Question

Write salient features of S_N1 mechanism.

Answer 1

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