Can anyone explain concisely why Trifluoroacetic Acid is more acidic than Trichloroacetic Acid, but HCl is more acidic than HF?
Well, let's start with HF and HCl. We know that HCl is stronger because Cl is more electronegative, making the H+ ion more likely to leave.
In the case of halogenated acids, you have to look at the partial charges. The halogens, because they have a strong partial negative charge, put a partial positive charge on the carbon they're attached to. That, in turn, puts a partial negative charge charge on the carbon in the carboxylic acid functional group. That partial negative makes the oxygen in the OH group slightly positive.
Since Cl has a stronger partial negative than F, the oxygen on the trichloroacetic acid will have a stronger positive charge than the oxygen in the trifluoroacetic acid, making it a weaker base, because of the same reasoning used with HCl and HF.
Does that make sense? It's all about keeping track of the partial charges and knowing how electronegativity of the atom attached to the acidic H affects the strength of the acid.
Flourine is the most electronegative atom on the periodic table. If it was all about electronegativity than HF would be the strongest haloacid and HI the weakest (which is not the case at all).
why Trifluoroacetic Acid is more acidic than Trichloroacetic Acid