Effect of Substituents on Acid Strength

Not all carboxylic acids are equally acidic; the identity and position of substituents on the carbon chain can shift the $\text{p}K_a$ substantially, and the direction of that shift follows directly from how the substituent affects the stability of the resulting carboxylate anion (§8.15).

Electron-withdrawing substituents increase acid strength. A substituent that pulls electron density away from its surroundings through the $\sigma$-bond framework (the inductive effect) -- most commonly a halogen, but also groups such as $\text{-NO}_2$ or $\text{-OH}$ -- helps disperse and stabilise the negative charge sitting on the carboxylate oxygens, over and above the resonance stabilisation already present in every carboxylate. A more stable conjugate base corresponds to a stronger acid, so introducing an electron-withdrawing group raises acidity. This is seen clearly across the chloroacetic acid series: acetic acid itself has $\text{p}K_a \approx 4.76$; replacing one $\alpha$-hydrogen with chlorine (chloroacetic acid, $\text{ClCH}_2\text{COOH}$) lowers the $\text{p}K_a$ sharply to about $2.86$; a second chlorine (dichloroacetic acid) lowers it further to about $1.29$; and a third (trichloroacetic acid) lowers it to about $0.65$ -- each additional electron-withdrawing chlorine makes the compound a measurably stronger acid, because each one further stabilises the developing negative charge.

Electron-donating substituents decrease acid strength. An alkyl group, by contrast, is weakly electron-donating (the same hyperconjugative/inductive push discussed for aldehyde/ketone reactivity in §8.7), and pushing extra electron density towards an already-negative carboxylate oxygen is destabilising, not stabilising. Pivalic acid, $(\text{CH}_3)_3\text{CCOOH}$, whose $\alpha$-carbon carries three electron-donating methyl groups instead of acetic acid's three simple hydrogens, is accordingly a slightly weaker acid than acetic acid ($\text{p}K_a \approx 5.03$ versus acetic acid's $4.76$). …