Acidity comparison on R-PO3H2

Question

Compare the acidity $$(K_\mathrm{a1})$$ of the following three acids: $$\ce{H3PO4},$$ $$\ce{Ph-PO(OH)2}$$ and $$\ce{H3C-PO(OH)2}.$$

Thoughts

These three acids are of type $$\ce{R-PO3H2},$$ where $$\ce{R} = \ce{OH}, \ce{Ph}, \ce{CH3}.$$ The ionization equations of them are

$$\ce{R-PO3H2 <=> R-P(=O)(OH)O- + H+}$$

Comparing with hydrogen, benzene ring can reduce the charge of the anion because of the delocalised $$\pi$$ bond. Also, methyl group is an electron donating group, which will destabilize the anion. Hence, the $$\mathrm{p}K_\mathrm{a1}$$ I suggest is $$\ce{CH3} > \ce{OH} > \ce{Ph}.$$ This answer is analogue to $$\ce{R-SO3H}.$$

One thing I'm not sure is that this logic does not apply to $$\ce{R-CO2H}.$$ Clearly, the acidity of carbonic acid is weaker than methanoate acid. I don't know which explanation should be applied in the situation of phosphorus.

• "Clearly, the acidity of carbonic acid is weaker than methanoate acid" - not really, it's slightly stronger, actually, it's just that it's equilibrium with CO2 messes things up. en.wikipedia.org/wiki/Carbonic_acid – Mithoron Dec 15 '19 at 16:29