Write salient features of SN1 mechanism.

Answer: Salient features of S_N1 (unimolecular nucleophilic substitution):

1. It is a first order reaction: rate = k[RX]; the rate depends only on the concentration of the substrate.
2. It takes place in two steps. Step 1 (slow, rate-determining): the C–X bond ionises to form a planar carbocation intermediate. Step 2 (fast): the nucleophile attacks the carbocation.
3. Because the carbocation is planar, the nucleophile can attack from either side; an optically active substrate gives a racemic mixture (racemisation).
4. Reactivity order: 3° > 2° > 1° > CH₃X (stability of carbocation); polar protic solvents favour the reaction.