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 minimum 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

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