Question

Why is tertiary carbon atom more reactive towards the $sn1$ reaction even though it is the most stable one?

Answer 1

The ${S_N}1$ reaction is a nucleophilic substitution reaction where the rate determining step is unimolecular. It is a type of organic substitution reaction. ${S_N}1$ stands for substitution nucleophilic unimolecular. Tertiary carbons have the largest number of adjacent $C - C$ bonds, the largest inductive effect, the most stable carbocation intermediate, and are thus favored in ${S_N}1$ .

Complete answer:
The ${S_N}1$ reaction is a substitution nucleophilic unimolecular reaction. It is a two-step reaction. In the first step, the carbon-halogen bond breaks heterolytically with the halogen retaining the previously shared pair of electrons. In the second step, the nucleophile reacts rapidly with the carbocation formed in the first step. This reaction is carried out in polar protic solvents such as water, alcohol, acetic acid etc. This reaction follows first-order kinetics.
Reactivity of alkyl halides:
For different substrates, the rate of ${S_N}1$ depends linearly on the stability of carbocation formed. Therefore, the order of reactivity of ${1^ \circ },{2^ \circ },{3^ \circ }$ alkyl halides will be ${3^ \circ } > {2^ \circ } > {1^ \circ }$ (stability of carbocation).
Rate is increased when a polar protic solvent is used since it increases the ionization to form carbocations. In the ${S_N}1$ mechanism, tertiary alkyl halides are more reactive.
A tertiary carbocation is more stable than a secondary carbocation which is more stable than a primary carbocation. Greater the stability of the carbocation, greater will be the ease of formation of carbocation, and hence faster will be the rate of the reaction.

Note:
Since the ${S_N}1$ reaction involves formation of an unstable carbocation intermediate in the rate-determining step, anything that can facilitate this will speed up the reaction. The normal solvents choice are both polar and protic solvents. Typical polar protic solvents include water and alcohols, which will also act as nucleophiles and the process is known as solvolysis.