Why RCOOH is acidic: The carboxylate anion (RCOO-) has 2 equivalent resonance structures — charge equally shared between 2 O atoms. This massive stabilization makes the conjugate base very stable.

Substituent effects on acidity:

• -I groups INCREASE acidity (stabilize COO-): F > Cl > Br > I (by electronegativity)
• +I groups DECREASE acidity (destabilize COO-): (CH3)3C > (CH3)2CH > C2H5 > CH3
• Distance matters: alpha-substitution > beta > gamma for -I effect
• Multiple substituents are additive: Cl3CCOOH > Cl2CHCOOH > ClCH2COOH

Aromatic acids: -I/-M groups on ring increase acidity (especially para > meta for -M groups like -NO2); +I/+M groups decrease acidity. Ortho effect complicates — ortho-substituted benzoic acids are generally more acidic than para regardless of substituent nature.