carboxylic acid acidity

Why is vinegar sour but cooking oil (full of -OH and C=O groups too) is not? The answer is acidity. A carboxylic acid will give up its O-H proton to water far more readily than almost any other common O-H compound, and chemists capture exactly how willing with a single number, the pKa. The smaller the pKa, the stronger the acid.

Typical carboxylic acids cluster around pKa 4 to 5 (acetic acid is 4.76). That is millions of times more acidic than an alcohol (pKa about 16) or water (pKa 15.7). The reason, as covered under the carboxylate ion, is that losing the proton produces a resonance-stabilized anion whose negative charge is shared over two oxygens. Crucially, the substituent attached to the carboxyl can tune this. Electron-withdrawing groups pull electron density away through the bonds (the inductive effect), helping to spread and stabilize the negative charge, so they make the acid stronger and the pKa smaller. Electron-donating groups do the opposite.

You can see the tuning clearly. Acetic acid (CH3COOH) is pKa 4.76; replace the methyl hydrogens with chlorines and the pKa plummets — chloroacetic acid is 2.86, dichloroacetic 1.29, trichloroacetic 0.65 — because each electronegative chlorine drains charge away and props up the carboxylate. The effect also fades with distance: a chlorine two carbons further down the chain barely moves the pKa, because the inductive pull weakens sharply over each additional bond. This substituent logic is one of the cleanest demonstrations of how structure controls reactivity.