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In this question, sulfonation of aniline is discussed and the answer by Waylander suggests that the sulfonation occurs on the nitrogen instead first.

The above is clear, but I can't make sense of why sulfonation of nitrogen is preferred over the sulfonation of carbon?

Here is a picture of sulfonation of the carbons in benzene, it is clear that there is a high preference to carbons in the benzene ring here:

enter image description here

Picture from David R Klein's Organic Chemistry.

tryst with freedom
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    I believe Nu addition from lone pair of N would be preferred over EAS as aromaticity is not lost in the former. This is perhaps similar to why Friedel-Crafts acylation doesn't occur in aniline, instead N undergoes acylation. – Ritam_Dasgupta Jun 01 '21 at 16:41
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    In the reaction scheme shown, there neither is an aniline, nor any nitrogen atom. Did you miss a line in the drawing? In the case of benzene, you likely do not even need fuming sulfuric acid to install the group. – Buttonwood Jun 01 '21 at 19:22
  • also, to add to @Ritam_Dasgupta, pyridine is a very good nitrogen nucleophile because it has low steric bulk around the nitrogen lone pair – Cody Aldaz Jun 01 '21 at 21:59
  • That's what is said in my book, I have shown the reaciton for benzene with the sulphuric acid to show that it is indeed reactive with it. So, I can't understand why it is reactive w/ the nitrogen as well? – tryst with freedom Jun 02 '21 at 06:28

1 Answers1

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Aniline can be protonated by strong acid. However, the site of protonation was a subject of debate over decades. It has been confirmed after numerous experimentation that protonation is more favored at the nitrogen atom that the carbon atom at the para- position. See this previous chem.SE discussion to find out the key factors resulting in the N-attack more than C-attack. Alternatively, refer 1 and 2. Thus, we can conclude that direct addition of sulfonyl group at para position is not possible. The reaction starts by the nitrogen atoms getting attacked to form anilinium cation. This cation then reacts with the rest of the sulfuric acid and form anilinium hydrogensulfate which at higher temperatures decomposes to a stable isomer. But this decomposition is not a direct reaction. It goes through a number of intermediates before arranging to the stable para isomer:

enter image description here

However, this mechanism is different for different acid concentration. Using only sulfuric acid will lead to the para-isomer but usage of fuming sulfuring acid(FSA) will lead to the meta-isomer. It has been proposed that if the acid concentration is increased, then more and more aniline gets protonated to anilinium which is meta directing and hence meta-isomer prevails when FSA was used. The paper used dimethyl aniline for the demonstration:

enter image description here

You can find more details on the major products of the reactions and their yields in ref. 4.

Notes and References

  1. The Site of Protonation of Aniline Rebecca L. Smith, Leonard J. Chyall, Brandon J. Beasley, and Hilkka I. Kenttamaa Journal of the American Chemical Society 1995 117 (30), 7971-7973 DOI: 10.1021/ja00135a016

  2. Site of Protonation in Aniline and Substituted Anilines in the Gas Phase: A Study via the Local Hard and Soft Acids and Bases Concept R. K. Roy, F. de Proft, and P. Geerlings The Journal of Physical Chemistry A 1998 102 (35), 7035-7040 DOI: 10.1021/jp9815661

  3. Mechanism of the Sulfonation of Aromatic Amines by Elliot R. Alexander:

  4. Aromatic sulphonation. Part 59. Sulphonation of aniline in concentrated aqueous and fuming sulphuric acid, Paul K. Maarsen and Hans Cerfontain, J. Chem. Soc., Perkin Trans. 2, 1977, 1008-1010, DOI: 10.1039/P29770001008

  5. Why III is an intermediate?: Why does sulfonation of aniline occur para rather than ortho?

  6. Even if there are multiple amide groups present (for instance sulfanilamide), only the amide group with the carbon-nitrogen bond will be protonated: Is sulphonamide group prone to undergo diazotization?

  7. Is reverse sulfonation of aniline possible?

Nilay Ghosh
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