How do you determine if a reaction will be #"S"_N1# or #"S"_N2#?

1 .Effect of the Nucleophile :- The nucleophile takes part in the slow step (the only step)
of the #S_N 2# reaction but not in the slow step of the Therefore, a strong nucleophile
promotes the #S_N 2# but not the #S_N 1# . Weak nucleophiles fail to promote the #S_N 2# reaction;
therefore, reactions with weak nucleophiles often go by the mechanism if the substrate
is secondary or tertiary.
#S_N 1# :- Nucleophile strength is unimportant (usually weak).
SN2:- Strong nucleophiles are required.
2. Effect of the Substrate :- The structure of the substrate (the alkyl halide) is an important
factor in determining which of these substitution mechanisms might operate.
Most methyl halides and primary halides are poor substrates for #S_N 1# substitutions because
they cannot easily ionize to high-energy methyl and primary carbocations. They
are relatively unhindered, however, so they make good #S_N 2# substrates.
Tertiary halides are too hindered to undergo #S_N 2# displacement, but they can ionize
to form tertiary carbocations. Tertiary halides undergo substitution exclusively through
the mechanism. Secondary halides can undergo substitution by either mechanism,
depending on the conditions.
.
3. Effect of the Solvent :-