46cE - Organic Chemistry (Mcmurry)

Question

Which reaction in each of the following pairs would you expect to be faster? 

(c) The  $S_{N}2$ displacement on 2-bromopropane by  $C{H}_{3}C{H}_2{O}^{-}$ or by $C{N}^{-}$

Step-by-Step Solution

Answer

$C{N}^{-}$$is\text{\hspace{0.33em}}showfastreactionthan$$C{H}_{3}C{H}_2{O}^{-}$

S N 2 reaction

It is a nucleophilic substitution reaction in which an old bond is broken and a new bond is formed both processes occur simultaneously. Hence, it’s a single-step reaction.

The two reacting species are involved in the rate-determining steps.

Nucleophile

Nucleophiles are the negatively charged ions or the neutral species having the lone pair of electrons that are ready for bonding.

Species react faster with 2 − bromopropane

If the species are a better leaving group then it reacts faster with the nucleophile 

The cyanide ion is more reactive than the ethoxide ion due to which cyanide ion easily replaces the bromide ion from 2-bromoprpane.

$C{N}^{-}$ is a better leaving group than $C{H}_{3}C{H}_2{O}^{-}$

 ${\text{CN}}^{\text{-}}$$\text{is\hspace{0.33em}show\hspace{0.33em}fast\hspace{0.33em}reaction\hspace{0.33em}than\hspace{0.33em}}$${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}{\text{O}}^{\text{-}}$