In the question, "What is the role of pH in azo coupling reaction of diazonium with phenol and aniline?" asked by @DhrubaBanerjee and answered by @Klaus-DieterWarzecha, I find several ends untied. I considered starting a bounty on the previous question, but since my question is looking for slightly more specificity than just looking at the whole picture, I decided to ask a separate question.

That post discusses why azo coupling of aniline is conducted in acidic medium. However, I am not convinced of a few things.

My query is twofold:

  1. The $\ce{-^+NH3}$ group is inductively deactivating (as mentioned here). That is a turn-off for attacks on electrophiles (like a diazonium cation). But why is the protonation of aniline preferred?

In the case of aniline, the lone pair on the nitrogen atom is the preferred centre for the reaction with the diazonium cation:

Diazonium and aniline

This isn't what we want! Protonation of aniline avoids this side reaction.

I do agree with Klaus-Dieter Warzecha that an alkaline $\mathrm{pH}$ would lead to azo N-coupling, but if we were concerned about the electrophilic attack occurring from the nitrogen atom, in my opinion, it should have been acylated at $\ce{N}$, leaving no room for side reactions but also keeping the ring activated (although less than before). I believe the yield would be higher.

  1. If an equimolar mixture of Aniline and Phenol was added to a reaction vessel containing benzenediazonium chloride in acidic medium, would the major product be phenol's couple product or aniline's couple product?

Major product?

This Wikipedia article states that phenol's coupling is base-catalyzed. I assume it accelerates the reaction by assisting the formation of phenolate ion. Going by that, the aforementioned conditions will be less favorable for phenol's coupling.

But I find it hard to take in that a protonated aniline could couple faster than phenol. Phenol would still be capable of coupling (only unaccelerated), while aniline should have been weakened by the deactivation.

Hence I would choose phenol to form the major product. I wish to know if my reasoning is correct or not, and if not, an explanation.

I would greatly appreciate any constructive discussion/specific answers.


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