Question

For a nucleophilic substitution reaction, the rate was found in the order $RI > RBr > RCl$ the reaction could be:
A) ${S_{{N^1}}}$
B) ${S_{{N^2}}}$
C) Either ${S_{{N^1}}}$ or ${S_{{N^2}}}$
D) Neither ${S_{{N^1}}}$ or ${S_{{N^2}}}$

Answer 1

${S_{{N^1}}}$ is a unimolecular reaction while ${S_{{N^2}}}$ is a bimolecular reaction. In the ${S_{{N^1}}}$ and ${S_{{N^2}}}$ reaction the alkyl halide nature plays an important role in rate as a primary alkyl halide and tertiary alkyl halide. One can apply for the substrate alkyl halide order in both nucleophilic reactions and choose the correct choice.

Complete step by step answer:

1. First of all we will learn about the nucleophilic substitution reaction. In this reaction, the leaving group in the reactant is replaced by the electron-rich nucleophilic species on the carbon atom.
2. Now there are two types of nucleophilic reactions such as ${S_{{N^1}}}$ and ${S_{{N^2}}}$. Now let's discuss these reactions in detail with the rate of leaving the group.
3. ${S_{{N^1}}}$ reaction: This is a nucleophilic substitution reaction which is a unimolecular and two-step reaction. The rate of reaction is tertiary carbon > secondary carbon > primary carbon. In this reaction the order of alkyl halide will be $RI > RBr > RCl$ as iodine will be a faster-leaving group than others due to its large size. Hence, the rate order $RI > RBr > RCl$ is applied for the ${S_{{N^1}}}$ reaction.
4. ${S_{{N^2}}}$ reaction: This is a nucleophilic substitution reaction which is a bimolecular and one-step reaction. The rate of reaction is tertiary carbon < secondary carbon < primary carbon. In this reaction the order of alkyl halide will be $RI > RBr > RCl$ as iodine will be a faster-leaving group than others due to its large size. Hence, the rate order $RI > RBr > RCl$ is applying for the ${S_{{N^2}}}$ reaction.
   As the rate given $RI > RBr > RCl$ is leaving a group of alkyl halide we can say that the reaction could be Either ${S_{{N^1}}}$ or ${S_{{N^2}}}$ which shows option C as the correct choice.

Note: The ${S_{{N^1}}}$ is a first-order reaction as the rate of reaction depends upon only one reactant. In the case of ${S_{{N^2}}}$ a reaction, it is a second-order reaction as the rate depends upon both the reactant and substrate present in the reaction.