Compound	pKa	Structural Note
Trifluoroacetic acid	0.2	Three highly electronegative F atoms pull electron density from the carboxylate
Trichloroacetic acid	0.7	Three inductively withdrawing Cl atoms
Chloroacetic acid	2.9	One inductively withdrawing Cl atom
Formic acid	3.8	No alkyl group donating electron density
Benzoic acid	4.2	Aromatic ring is mildly electron-withdrawing relative to alkyl
Acetic acid	4.8	Simple alkyl carboxylic acid — the typical reference value

Why carboxylic acids are acidic at all: deprotonation gives a carboxylate ion whose negative charge is delocalized equally over both oxygens by resonance. This is the same reasoning developed in Chapter 3’s Gentle Exercise (“why are carboxylic acids stronger acids than alcohols?”) — an alkoxide has nowhere to delocalize its charge, while a carboxylate does.

Why the halogenated acids are so much stronger: induction, not resonance. Electronegative halogens near the carboxyl group pull electron density through the sigma bonds, stabilizing the negative charge on the carboxylate. This effect drops off quickly with distance — a halogen on the carbon adjacent to the carboxyl (α) has a much larger effect than one further away.

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