Question

What undergoes ${S_N}1$ reactions?

Answer 1

The ${S_N}1$ reaction is a nucleophilic substitution process with a unimolecular rate controlling step. It's an organic substitution process of some sort. ${S_N}1$ is an acronym for substitution nucleophilic unimolecular reaction.

Complete answer:
Carbocations are the intermediates in the SN1 reaction. Greater the stability of the carbocations, more easily will the product be formed and hence faster will be the rate of the reaction. The stability of the carbocations decreases in the order
${3^ \circ }$ carbocation $> {2^ \circ }$ carbocation $> {1^ \circ }$ carbocation $> CH_3^ +$
Therefore, the reactivity of alkyl halides towards SN1 reactions decrease in the same order,
${3^ \circ }$ alkyl halide $> {2^ \circ }$ alkyl halides $> {1^ \circ }$ alkyl halides $>$ methyl halides.
As tertiary alkyl halide can form a tertiary carbocation, which is stabilised by the three alkyl groups attached to it, it tends to undergo the ${S_N}1$ process. Because alkyl groups donate electrons, the induction effect allows the positive charge in the carbocation to be delocalized.
A carbocation intermediate is formed during this reaction. It is most commonly encountered in the reactions of tertiary or secondary alkyl halides with secondary or tertiary alcohols when the circumstances are strongly acidic or strongly basic. In inorganic chemistry, the ${S_N}1$ reaction is known as the dissociative mechanism.
The rate-determining stage of the ${S_N}1$ reaction can be accelerated by using a solvent that facilitates the production of the carbocation intermediate.

Note:
Both polar and protic solvents are recommended for this type of reaction. The polarity of the solvent aids in the stabilisation of ionic intermediates, whereas the protic nature of the solvent aids in the solvation of the leaving group.