CBSE Class 12 Chemistry 2023: question 31

Nucleophilic Substitution
Nucleophilic Substitution reaction of haloalkane can be conducted according to both $S_{N}1$ and $S_{N}2$ mechanisms. $S_{N}1$ is a two step reaction while $S_{N}2$ is a single step reaction. For any haloalkane which mechanism is followed depends on factors such as structure of haloalkane, properties of leaving group, nucleophilic reagent and solvent.
Influences of solvent polarity : In $S_{N}1$ reaction, the polarity of the system increases from the reactant to the transition state, because a polar solvent has a greater effect on the transition state than the reactant, thereby reducing activation energy and accelerating the reaction. In $S_{N}2$ reaction, the polarity of the system generally does not change from the reactant to the transition state and only charge dispersion occurs. At this time, polar solvent has a great stabilizing effect on Nu${}^{-}$ than the transition state, thereby increasing activation energy and slow down the reaction rate. For example, the decomposition rate ($S_{N}1$) of tertiary chlorobutane at 25 °C in water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate ($S_{N}2$) of 2-Bromopropane and NaOH in ethanol containing 40% water is twice slower than in absolute ethanol. Hence the level of solvent polarity has influence on both $S_{N}1$ and $S_{N}2$ reaction, but with different results. Generally speaking weak polar solvent is favourable for $S_{N}2$ reaction, while strong polar solvent is favourable for $S_{N}1$. Generally speaking the substitution reaction of tertiary haloalkane is based on $S_{N}1$ mechanism in solvents with a strong polarity (for example ethanol containing water).