Question

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

1-Bromo-3-methylbutane, 2-Bromo-2-methylbutane, 2-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:

\[\begin{array}{cc}
\ce{CH3}\phantom{.........................................}\ce{CH3}\phantom{...................................}\ce{CH3}\phantom{........}\\
|\phantom{...............................................}|\phantom{.........................................}|\phantom{...........}\\
\ce{\underset{1-Bromo-3-methylbutane}{H3C - CH - CH2 - CH2Br} > \ce{CH3 - CH - CH - CH3} > \ce{CH3 - C - CH2 - CH3}}\\
\phantom{.....................................}|\phantom{.........................................}|\\
\phantom{.......................................}\ce{\underset{2-Bromo-3-methylbutane}{Br}\phantom{..............}\ce{\underset{2-Bromo-2-methylbutane}{Br}}}\
\end{array}\]