Question

Elimination reactions (especially β-elimination) are as common as the nucleophilic substitution reaction in case of alkyl halides. Specify the reagents used in both cases.

Answer 1

Alkyl halides undergo nucleophilic substitution as well as elimination (Beta-elimination) reaction. However, by proper choice of reagents and reaction conditions, a particular product can be obtained. Usually strong and bulky bases and high temperature favour elimination reactions while weaker and smaller bases and lower temperature favour substitution reactions. For example, ethyl bromide on heating with alcoholic KOH (which contain stronger base, $$\mathrm{C}{\phantom{A}}_2\mathrm{H}{\phantom{A}}_5\mathrm{O}$$ ion) at about 473-523 K undergoes elimination to give ethene. But with aqueous $$\mathrm{KOH}$$ at about 373 K, it gives ethanol.

$$\mathrm{CH}{\phantom{A}}_3\mathrm{CH}{\phantom{A}}_2\mathrm{Br}\underset{473-523\mathrm{K}}{\overset{\mathrm{alc}\cdot \mathrm{KOH}}{\to }}\mathrm{CH}{\phantom{A}}_2=\mathrm{CH}{\phantom{A}}_2(\mathrm{Elimination})$$

$$\mathrm{CH}{\phantom{A}}_3\mathrm{CH}{\phantom{A}}_2\mathrm{Br}\underset{373\mathrm{K}}{\overset{\mathrm{aq}\cdot \mathrm{KOH}}{\to }}\mathrm{CH}{\phantom{A}}_3\mathrm{CH}{\phantom{A}}_2\mathrm{OH}(\mathrm{Substitution})$$

Nucleophilic substitution: Reagents used nucleophilies like $$-\stackrel{-}{\mathrm{O}}\mathrm{H},\mathrm{NH}{\phantom{A}}_3,\stackrel{-}{\mathrm{C}}\equiv \mathrm{N};,\mathrm{AgCN}:,\mathrm{O}=\mathrm{N}-\mathrm{O},\stackrel{-}{\mathrm{O}}\mathrm{R}$$' etc. also, alc. $$\mathrm{KOH}$$ at lower temperature (373 K) undergoes substitution reaction.