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C can be converted to D with NaOH and heating at 300 °C. What is the mechanism of the reaction?

4‐acetamidobenzene‐1‐sulfonic acid to N‐(4‐hydroxyphenyl)acetamide

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  • $\begingroup$ Actually I am fairly curious. Google searching reveals sources such as Quora saying it's nucleophilic addition-elimination, but there are few details in those answers and I am not finding a primary source. Vote to reopen. $\endgroup$ Sep 19 at 10:14
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    $\begingroup$ I am frankly surprised that the N-acetyl group survives the conditions. $\endgroup$
    – Waylander
    Sep 19 at 11:17
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    $\begingroup$ @waylander that group probably deprotonates at N and hunkers down. Would not be as good a leaving group as sulfite. $\endgroup$ Sep 19 at 11:21
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    $\begingroup$ Due to apparent interest from community I voted to reopen upfront after adding chemical names and correcting units. It would be nice if OP could add their research to prevent the question from being closed again. $\endgroup$
    – andselisk
    Sep 19 at 21:36
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Finally I find a primary reference by Oae et al.[1] They react benzenesulfonic acid with fused potassium hydroxide using isotopically labelled chemicals. The isotopic labelling results favor a direct nucleophilic substitution of hydroxide for sulfite, so that the only ring position involved in the reaction is that originally holding the sulfonate function. This is called $\ce{S_N2}$ in the paper; today we use addition-elimination since with an aromatic substrate, a full-fledged addition intermediate is formed in this mechanism. A benzyne mechanism and attack on sulfur (followed by rearrangement of the oxygen atoms) are rejected. The abstract is quoted below.

Benzenesulfonic acid-1-¹⁴C was fused with potassium hydroxide at 320-350℃ for five minutes. The phenol obtained was then degraded as usual, and the distribution of carbon-14 in the phenol was determined. Practically all the ¹⁴C activity was found at the 1 position of the phenol, which means that there is no migration of the ¹⁴C originally labeled at the position 1 or the benzenesulfonic acid. Benzenesulfonic acid was also treated with potassium hydroxide enriched with ¹⁸O. The resulting phenol was found to be incorporated, with the same concentration of ¹⁸O as that of the ¹⁸O enriched water used . The results show that the mechanism of the reaction is of a simple SN2 type and not of the elimination-addition type involving the benzyne intermediate, while there was no oxygen migration in the reaction.

Reference

1. Shigeru Oae, Naomichi Furukawa, Masahiro Kise and Mitsuyoshi Kawanishi, "The Mechanism of the Alkaline Fusion of Benzenesulfonic Acid". Bulletin of the Chemical Society of Japan 39, 1212-1216 (1966).

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  • $\begingroup$ Old paper. Isn't this mechanism called addition-elimination now, because it's two step, unlike "ordinary" SN2? $\endgroup$
    – Mithoron
    Sep 20 at 15:44
  • $\begingroup$ @mithoron Rephrased my summary to reflect this. The quotation still has to use the authors' SN2 term. $\endgroup$ Sep 20 at 16:32

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