Q34E - Organic Chemistry (Mcmurry)

Question

Methyl aryl ethers, such as anisole, are cleaved to iodomethane and a phenoxide ion by treatment with LiI in hot DMF. Propose a mechanism for this reaction.

Step-by-Step Solution

Answer

The mechanism for the given reaction is:

                   Anisole                                             Phenoxide ion               iodomethane

Step-by-Step Solution Step 1: Nucleophilic bimolecular substitution reaction S N 2

The ${\mathrm{S}}_{\mathrm{N}}2$  reaction is a nucleophilic substitution reaction where a bond is broken, and another is formed simultaneously. Two reacting species are involved in the rate-determining step of the reaction. The term ${\mathrm{S}}_{\mathrm{N}}2$  stands for substitution nucleophilic bimolecular.

Step 2: Aprotic solvent

A solvent is a substance in which a solute dissolves to produce a solution. It is of two types, i.e., protic and aprotic solvent.

Polar aprotic solvents are solvents that contain no hydrogen atoms connected directly to an electronegative atom and don’t show any hydrogen bonding. Examples of aprotic solvents are DMF, acetone, and dimethyl sulfoxide.

Step 3: Mechanism for the given reaction

                       Anisole                                           Phenoxide ion               iodomethane

This reaction is an ${\mathrm{S}}_{\mathrm{N}}2$ displacement and can’t occur at any aryl carbon. DMF is a polar aprotic solvent that increases the rate of an  ${\mathrm{S}}_{\mathrm{N}}2$ reaction by making anions more nucleophilic