According to me as the key atoms are different so when the h+ is released and each of them gets a negative charge as sulphur is a bigger atom compared to oxygen charge is more dispersed , hence the conjugate base formed by thiol is more stable.(I am not considering the equivalent resonance structure of carboxylic acid as i have learnt that only if the atom on which negative charge is present is same then only can we use effects like mesomeric inductive etc.)

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    $\begingroup$ The clue is in the name - carboxylic ACID $\endgroup$
    – Waylander
    Aug 4, 2023 at 13:55

1 Answer 1


Some $pK_a$ values:

$\ce{H3C–SH}$: 10.4

$\ce{H3C–C(O)–OH}$: 4.8

This comparison clearly indicates that the carboxylic acid group imparts stronger acidity than the thiol group.

The determinimg factor is not the greater electronegativity of oxygen versus sulfur. Rather it's the stabilization of the conjugate-base anion by pi-bond delocalization. If we equalize the pi-bond delocalization factor, the $\ce{S–H}$ hydrogen is actually more acidic than the the $\ce{O–H}$ hydrogen due to the weaker hydrogen-sulfur bonding versus hydrogen-oxygen bonding. Thioacetic acid, $\ce{H3C–C(O)–SH}$, with delocalized pi bonding and hydrogen bonded to sulfur, beats acetic acid with a $pK_a$ of $3.4$.


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