: 230
Carboxylic acids (pKa 4-5) are acidic because the carboxylate anion has two equivalent resonance structures sharing the charge over two oxygens. Substituent effects on acidity: -I groups (F, Cl, NO2, CN) stabilize the carboxylate → increase acidity. +I groups (alkyl) destabilize → decrease acidity. Acidity order for haloacetic acids: FCH2COOH > ClCH2COOH > BrCH2COOH > ICH2COOH (electronegativity order). Multiple substituents are cumulative: Cl3CCOOH (pKa 0.65) >> CH3COOH (pKa 4.76). Distance matters: alpha > beta > gamma for substituent effects. For aromatic acids: -I/-M groups (especially para-NO2) increase acidity; +I/+M groups (-OCH3, -OH) decrease it. The ortho effect is a special case — ortho-substituted benzoic acids are almost always more acidic than para isomers regardless of the substituent's electronic nature, due to steric inhibition of resonance between COO- and ring, intramolecular H-bonding in the conjugate base, and through-space field effects. Key comparison series: Cl3CCOOH > Cl2CHCOOH > ClCH2COOH > HCOOH > CH3COOH > (CH3)3CCOOH. For JEE, always compare conjugate base stability — the more stabilized carboxylate wins.