Discussion on AIPMT

When SN1 comes to favorable reactants.

SN1: Likes tertiary alkyl halides > Secondary > dislikes primary (almost none reactive) depends on the concentration of the reactant only .

Sn1 Mechanism:
Nucleophilic substitution reactions may occur via an Sn1 mechanism in which only the organic reactant is involved in the rate determining step. Such reactions occur when
conditions exist that favor the ionization of the organic reactant.
The following conditions are
needed for Sn1 reactions to occur.

1) The Leaving Group: The leaving group must be a weak base.

2) The Carbon Group: The rate-determining step involves production of a carbocation, and this step will occur faster for those compounds that yield the more stable carbocations.
For example, tertiary compounds react faster than secondary. Primary compounds react extremely slowly.
Carbocation intermediates are also stabilized by dispersal of the positive charge through delocalization of electrons.
Sn1 reactions that produce such resonance-stabilized carbocation intermediates are also quite fast.

3) The Solvent: Ionization processes are facilitated by polar solvents.Since the rate of an Sn1 reaction is directly dependent on such an ionization, the reaction occurs faster in more polar solvents.