Chapter 23, Problem 23.64P

Interpretation Introduction

Interpretation:

The complete, detailed mechanism is to be drawn for the reaction shown.

Concept introduction:

Aromatic compounds undergo electrophilic aromatic substitution reactions. The reaction is actually an addition-elimination rather than direct substitution. The mechanism consists of the addition of the incoming electrophile to a ring carbon, producing an arenium cation, with a positive charge on an adjacent carbon. The second step is the elimination of the proton from the same carbon where the electrophile is attached. When alternate sites are available, the electrophile preferentially attaches to the position that produces the most stable arenium cation.

Any substituents already present on the aromatic ring influence the position of substitution via their directing effect. Substituents that are electron-donating or weakly electron-withdrawing tend to direct the incoming electrophile to ortho-para positions. Substituents that are strongly electron-withdrawing tend to direct the incoming electrophile to meta positions.