Question

Which alkyl halide from the following pairs would you expect to react more rapidly by an $S_{N}2$ mechanism? Explain your answer.(I) $C{H}_{2}C{H}_{2}C{H}_{2}C{H}_{2}Br$ or $C{H}_{2}C{H}_2\underset{Br}{\underset{|}{C}}HC{H}_3$(II) $C{H}_{2}C{H}_2\underset{Br}{\underset{|}{CH}}C{H}_3$ OR $H_{3}C-\underset{C{H}_2}{\underset{|}{\stackrel{C{H}_3}{\stackrel{|}{C}}}}-Br$(III) $C{H}_2\underset{C{H}_3}{\underset{|}{C}}HC{H}_{2}C{H}_{3}Br$ OR $C{H}_{3}C{H}_2\underset{C{H}_3}{\underset{|}{C}}HC{H}_{2}Br$

Answer 1

Since $S_{N}2$ reaction, has transition state as intermediate, no carbocation formations are there and nucleophile attack on less hindered carbon from the back side.

The minimum hindrance is provided by the primary carbon and maximum hindrance provided by the tertiary carbocation.

Thus Primary alkyl halide will give $S_{N}2$ mechanism faster and tertiary alkyl halide will give slowest $S_{N}2$ mechanism.