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Why is it so that azo coupling with phenol needs basic pH (9–10) and with aniline it needs acidic pH (4–5). How does pH participate in the mechanism? I have searched the internet for quite a while nowhere does it explain the actual ‘role’ of the pH. Any light on this matter would be helpful.

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Deprotonation of the phenol and protonation of aniline result in species that easily react with a diazonium cation in the intended manner.

Let's have a look at the species involved. On the one hand, there is the diazonium cation:

diazonium cation

Deprotonation of phenol yields phenolate, for which a resonance structure with a negative charge in para position to the substituent can be written:

phenolate

We conclude: Deprotonation of the phenol means activation!

phenolate reaction with diazonium cation

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

aniline reaction with diazonium

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

We conclude: Protonation of aniline means "masking" the wrong centre!

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    $\begingroup$ In the case of phenol, Why lone pairs of oxygen are not the preferred center for reaction with diazonium cation?? $\endgroup$ – Display name Jan 21 '18 at 6:55
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    $\begingroup$ If aniline is protonated, then won't the benzene ring be deactivated in aniline? NH3+ is supposed to be a strong deactivating group. This deactivation should then prevent azo coupling because azo coupling reactions need highly activated groups. $\endgroup$ – Pranoy De Mar 9 at 20:50

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