Question

Arrange the compounds of the following set in order of reactivity towards S_N2 displacement:

1-Bromobutane, 1-Bromo-2, 2-dimethylpropane, 1-Bromo-2-methylbutane, 1-Bromo-3-methylbutane

Answer 1

The S_N2 reaction involves the formation of a transition state with the carbon atom surrounded by 5 additional atoms (groups). A transition state requires minimal steric interactions. The most suitable substrates for S_N2 reactions are 1° alkyl halides, followed by 2° and 3° alkyl halides. The order of reactivity towards S_N2 is 1° > 2° > 3° > aryl halide. Based on this, the order will be:

\[\begin{array}{cc}
\phantom{................................................................................................}\ce{CH3}\phantom{..............................}\ce{CH3}\\
\phantom{............................................................................................}|\phantom{....................................}|\\
\ce{\underset{1-Bromobutane}{CH3(CH2)CH2Br} > \ce{\underset{1-Bromo-3-methylbutane}{(CH3)2 - CH - CH2 - CH2Br} > \ce{\underset{1-Bromo-2-methylbutane}{CH3 - CH2 - CH - CH2Br} > \ce{CH3 - C - CH2Br}}}}\\
\phantom{.................................................................................................................................}|\\
\phantom{.....................................................................................................................................}\ce{\underset{1-Bromo-2, 2-dimethylpropane}{CH3}}\
\end{array}\]

Although all alkyl halides are 1°, the order of reactivity depends on the steric barrier around the carbon bearing the –Br atom. The more bulky groups around a carbon, the lower its reactivity towards S_N2.