$17$Nucleophilic Substitution Nucleophilic Substitution reaction of haloalkane can be conducted according to both $\displaystyle \mathrm{S_N}1$ and $\displaystyle \mathrm{S_N}2$ mechanisms. $\displaystyle \mathrm{S_N}1$ is a two step reaction while $\displaystyle \mathrm{S_N}2$ 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 $\displaystyle \mathrm{S_N}1$ 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 $\displaystyle \mathrm{S_N}2$ 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 ($\displaystyle \mathrm{S_N}1$) of tertiary chlorobutane at 25 $\displaystyle ^\circ$C in water (dielectric constant 79) is 300000 times faster than in ethanol (dielectric constant 24). The reaction rate ($\displaystyle \mathrm{S_N}2$) 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 $\displaystyle \mathrm{S_N}1$ and $\displaystyle \mathrm{S_N}2$ reaction, but with different results. Generally speaking weak polar solvent is favourable for $\displaystyle \mathrm{S_N}2$ reaction, while strong polar solvent is favourable for $\displaystyle \mathrm{S_N}1$. Generally speaking the substitution reaction of tertiary haloalkane is based on $\displaystyle \mathrm{S_N}1$ mechanism in solvents with a strong polarity (for example ethanol containing water). Answer the following questions : (a) Why racemisation occurs in $\displaystyle \mathrm{S_N}1$ ? (b) Why is ethanol less polar than water ? (c) Which one of the following in each pair is more reactive towards $\displaystyle \mathrm{S_N}2$ reaction ?

(i)$\displaystyle \mathrm{CH_3-CH_2-I}$ or $\displaystyle \mathrm{CH_3CH_2-Cl}$(ii)chlorocyclohexane ($\displaystyle C_6H_{11}-Cl$) or (chloromethyl)cyclohexane ($\displaystyle C_6H_{11}-CH_2-Cl$) [structure images]

OR

(c) Arrange the following in the increasing order of their reactivity towards $\displaystyle \mathrm{S_N}1$ reactions :(i)2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane(ii)1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-Bromo-3-methylbutane

Chemistry · Haloalkanes and Haloarenes