I am being asked to compare the acidities of various compounds for an assignment, but I am confused about why butyric acid is a stronger acid than thiophenol/benzenethiol.

From what I was taught, I assumed the element effect (for elements in the same group on the periodic table) would take effect. Since sulfur is larger than oxygen and has a larger electron cloud to stabilize a negative charge, it has a more stable conjugate base and thus should be the stronger acid.

However, according to PubChem, $\mathrm{p}K_\mathrm{a} = 4.82$ for butyric acid, and $\mathrm{p}K_\mathrm{a} = 6.62$ for thiophenol.

What contributes to increased acidity of butyric acid? And why does it overrule the element effect?

  • 5
    $\begingroup$ Resonance, resonance. It matters a lot! If you want to compare apples with apples, then you should look at thiophenol (6.6) and phenol (~10), in which case your argument works. $\endgroup$
    – orthocresol
    Nov 9 '20 at 21:54
  • $\begingroup$ I'd hate to be in the lab unless there's a very effective hood. ;-) Then there's selenophenol... blogs.sciencemag.org/pipeline/archives/2012/05/15/… $\endgroup$ Nov 10 '20 at 1:31
  • $\begingroup$ “The biggest stinker I have run across. . .Imagine 6 skunks wrapped in rubber innertubes and the whole thing is set ablaze. That might approach the metaphysical stench of this material.” $\endgroup$
    – Poutnik
    Nov 10 '20 at 6:58
  • $\begingroup$ Butyric ac. is a real carboxylic one. $\endgroup$
    – Alchimista
    Nov 10 '20 at 8:31
  • $\begingroup$ Similarly, to compare pears with pears, compare butyric acid pKa=4.82 with acetic acid pKa=4.75 or formic acid pKa=3.75. Where is the increased acidity ? Carboxyls are acidic, as its carbonyl carbon atom has partial positive charge . It polarizes the $\ce{O-H}$ bond, making its braking to ions easier, and stabilizes the formed anion by charge delocalization over the whole carboxylate group. That is all. $\endgroup$
    – Poutnik
    Nov 10 '20 at 10:26

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