Structures of benzenesulfonic acid and cyclohexanesulfonic acid

Which of the two sulfonic acids above is a stronger acid?

I think that cyclohexanesulfonic acid 2 should be more acidic as there is no competitive resonance as in 1. Is that correct?


Draw out the full line-bond or Lewis structures for each and you will see that there is no way for the benzene ring to contribute to resonance structures that involve the sulfonic acid group, or the sulfonate conjugate base. Instead, the important contribution is the inductive effect. Benzene rings are more electron-withdrawing than alkyl groups, so benzenesulfonic acid should be more acidic than cyclohexanesulfonic acid. A comparison of pKa values for $\ce{PhSO3H}$ ($-2.8$) vs $\ce{MeSO3H}$ ($-1.9$) supports this assertion (Can. J. Chem. 1978, 56 (17), 2342–2354).

  • $\begingroup$ Doesn't the reason why Ortho substituted benzoic acids are more acidic than benzoic acids due to SIR, support that 2 is more acidic? $\endgroup$
    – user137644
    Nov 21 '18 at 13:37
  • $\begingroup$ @user137644 No, ortho effect is much different then inhibition of resonance and has nothing to do with this question. chemistry.stackexchange.com/questions/7683/… $\endgroup$
    – Mithoron
    Nov 21 '18 at 16:07

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