# Is benzenesulfonic acid or cyclohexanesulfonic acid more acidic?

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).