Question

Which one of the following compounds is more reactive towards $S_{N}2$ reaction?
$(C{H}_3{)}_{3}CBr,(C{H}_3{)}_{2}CHBr,C{H}_{3}Br$.

Answer 1

Since $S_{N}2$ reaction requires the approach of the nucleophile to the carbon-bearing the leaving group, the presence of bulky substituents on or near the carbon atom have a dramatic inhibiting effect. Of the simple alkyl halides, methyl halides react most rapidly in $S_{N}2$ reactions because there are only three small hydrogen atoms.

A primary alkyl halide will prefer a $S_{N}2$ reaction.

Thus the order of reactivity followed is:

Primary halide > Secondary halide > Tertiary halide.

Reactivity $\Rightarrow$ ${CH}_{3}Br>{\left({CH}_3\right)}_{2}CHBr>{\left({CH}_3\right)}_{3}CBr$

$\hookrightarrow$less bulky $\hookrightarrow$bulky $\hookrightarrow$more bulky