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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asked Apr 24, 2015 at 4:35
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1 Answer
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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:
Deprotonation of phenol yields phenolate, for which a resonance structure with a negative charge in para position to the substituent can be written:
We conclude: Deprotonation of the phenol means activation!
In the case of aniline, the lone pair on the nitrogen atom is the preferred centre for the reaction with the diazonium cation:
This isn't what we want! Protonation of aniline avoids this side reaction.
We conclude: Protonation of aniline means "masking" the wrong centre!
answered Apr 24, 2015 at 6:17
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8$\begingroup$ In the case of phenol, Why lone pairs of oxygen are not the preferred center for reaction with diazonium cation?? $\endgroup$– user20764Jan 21, 2018 at 6:55
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9$\begingroup$ 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. $\endgroup$ Mar 9, 2019 at 20:50
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2$\begingroup$ Klaus, could you add to your post to answer the comments? They're good questions, and making a new post for them seems unnecessary. $\endgroup$– harryMay 4, 2021 at 12:23
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$\begingroup$ what is the role of Ph in the answer given? $\endgroup$ May 15, 2021 at 6:40
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1$\begingroup$ @PranoyDe orthocresol's answer here answers why HCl is being used to form the azobenzene. $\endgroup$ Aug 14, 2021 at 4:08