# More acidic - protonated carbonyl or protonated hydroxyl?

Consider two acetic acid derivatives, one with a protonated carbonyl oxygen and one with a protonated hydroxyl oxygen.

An argument for the one with the protonated hydroxyl oxygen being more acidic is as follows:

This structure has a lone pair on its protonated hydroxyl oxygen and the oxygen is adjacent to an $sp^2$ carbon. Therefore we can draw an extreme resonance structure in which the hydroxyl oxygen has a +2 formal charge. No such "extreme" structure exists for the one with the protonated carbonyl oxygen.

Now, we can definitely draw this resonance structure, but its contribution is likely very small and bordering on insignificant. I can't see how delocalization of electrons away from a highly electronegative, positively charged oxygen can help stabilize it.

Therefore: Which is more acidic? The argument that the protonated hydroxyl one should be more acidic seems to be flawed at best.

• Protonated ketones are not a good reference point, because they are not able to delocalize positive charge on oxygen as shown on the picture above for protonated carbonyl group of carboxylic acid.
– user28158
Mar 22, 2016 at 18:15