Question

Reactivity of alkyl halides towards ${S_N}1$ nucleophilic substitution reaction is:

Answer 1

${S_N}1$ reaction is a two-step nucleophilic substitution reaction. It is generally carried out in polar protic solvents (like water, ethanol, acetic acid, etc.). It follows the first order of kinetics.

Complete answer:
In the first step of ${S_N}1$ reaction, the carbon – halogen bond undergoes a slow cleavage to produce a carbocation. The carbon – halogen bond breaks due to the solvation energy of halide ion with the proton of the protic solvent. The carbocation thus formed is immediately attacked by a nucleophile in the second step. The first step is the slowest step and is reversible. It is also called rate determining step as the rate of reaction depends on the concentration of alkyl halide and not on the concentration of hydroxide ion.
Reactivity of alkyl halides for ${S_N}1$ reaction:
In ${S_N}1$ reaction, there is a carbocation formation. The stability of carbocation determines the ease of bond formation with the nucleophile and it is in the order: $3^\circ > 2^\circ > 1^\circ$. Hence, a $3^\circ$ alkyl halide will undergo ${S_N}1$ reaction very fast because of the high stability of $3^\circ$ carbocation. The reactivity will be followed by $2^\circ$ carbocation and then $1^\circ$ carbocation.

Note:
Remember that the nucleophile in step two is free to attack from either side of the carbocation. However, in case of tertiary carbocation, the nucleophile does not attack from the backward side due to the presence of three methyl groups which causes steric hindrance.