43aE - Organic Chemistry (Mcmurry)

Question

What effect would you expect the following changes to have on the reaction rate of 1-iodo-2-methylbutane with cyanide ion? 

(a) The  $C{N}^{-}$concentration is halved, and the 1-iodo-2-methylbutane concentration is doubled. 

(b) Both the  $C{N}^{-}$and the 1-iodo-2-methylbutane concentrations are tripled.

Step-by-Step Solution

Answer

S N 2 reaction

It is a nucleophilic substitution reaction in which an old bond breaks, is a break, and a new bond is formed. Both processes co-occur. Hence, it’s a single-step reaction.

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

 

Rate of the reaction

In  $S_{N}2$ rate of the reaction depend on the reacting species and nucleophile.

Hence, on half the concentration of cyanide ion and double the concentration of 1iodo2methyl butane, the reaction’s net rate becomes unaffected because both effects cancel each other.

$\begin{array}{c}R\text{\hspace{0.33em}}=K\left(R.S\right)\left(N\right)\\ =K\left(\frac{1}{2}\right)\left(2\right)\\ =K\end{array}$