Question

The herbicide oxyfluorfen can be prepared by reaction between a phenol and an aryl fluoride. Propose a mechanism.

Short answer

Aryl halides comprising electron-withdrawing substituents undergo nucleophilic substitution reaction.The first step in this reaction is the nucleophilic addition of hydroxide ion to the aromatic ring resulting in a stabilized carbanion intermediate. The elimination of halide ion further leads to a substitution product.

Step-by-step solution

Nucleophilic Aromatic Substitution Reaction  

Aryl halides comprising electron-withdrawing substituents undergo nucleophilic substitution reaction.The first step in this reaction is the nucleophilic addition of hydroxide ion to the aromatic ring resulting in a stabilized carbanion intermediate. The elimination of halide ion further leads to a substitution product.

Mechanism for the preparation of oxyfluorfen 

The base KOH picks up the acidic proton of the phenol to generate a phenoxide ion. The nucleophilic attack of the phenoxide ion on the aryl fluoride generates a negatively charged Meisenheimer complex. The attack of this ion is facilitated by the electron-withdrawing nitro group present in the aryl fluoride. The fluoride ion gets eliminates to produce oxyfluorofen as the product.

Mechanism for the preparation of oxyfluorfen