Question

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

2-Bromo-2-methylbutane, 1-Bromopentane, 2-Bromopentane

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

2-Bromo-2-methylbutane, 1-Bromopentane, 2–Bromopentane.

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:

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