Question

In the mechanism for electrophilic aromatic substitution with a diazonium ion as the electrophile, why does nucleophilic attack occur on the terminal nitrogen of the diazonium ion rather than on the nitrogen that hasthe formal positive charge?

Short answer

The nucleophilic attack on the neutral nitrogen leads to a stable product. The nucleophilic attack on the positively charged nitrogen forms an unstable compound comprising two nitrogen atoms.

Step-by-step solution

Electrophilic aromatic substitution with arene diazonium ion

In the first step, the electrophile incorporates into the benzene ring. This is followed by removing a proton by a base in the solution from the carbon that created a bond with the electrophile.

Reason for the nucleophilic attack happening on the terminal nitrogen in electrophilic aromatic substitution

The products of the nucleophilic attack on both the nitrogens can be compared to understand why the nucleophilic attack happens on the terminal nitrogen. The nucleophilic attack on the neutral nitrogen leads to a stable product. The nucleophilic attack on the positively charged nitrogen forms an unstable compound comprising two nitrogen atoms.

The terminal nitrogen is electrophilic due to the electron withdrawal by the positively charged nitrogen. It is observed that the neutral nitrogen is electron deficient, drawing the resonance contributors.

Resonance structures