Section 1: Structure and Acidity Basis
Carboxylic acids (RCOOH) contain the carboxyl functional group, a combination of carbonyl (C=O) and hydroxyl (–OH) on the same carbon. Their characteristic property — high acidity (pKa 4–5) — originates from the resonance-stabilized carboxylate ion (RCOO–). When RCOOH donates H+, the resulting RCOO– distributes the negative charge equally across two C–O bonds (bond order 1.5). This is fundamentally different from alkoxide ions (RO–, charge on one O) and phenoxide ions (ArO–, charge partially on ring carbons). The result is that carboxylic acids are 10^11 to 10^13 times more acidic than alcohols and about 10^5 times more acidic than phenols.
Section 2: Substituent Effects on Acidity
Electron-withdrawing groups (–I effect) stabilize RCOO–, increasing acidity. Halogens are the most common example: Cl3CCOOH (pKa 0.65) > Cl2CHCOOH (1.26) > ClCH2COOH (2.87) > CH3COOH (4.76). This –I effect is additive (more groups = more acidic) and distance-dependent (closer to –COOH = stronger effect). Electron-donating alkyl groups (+I effect) destabilize RCOO– and decrease acidity: HCOOH (3.75) > CH3COOH (4.76) > C2H5COOH (4.87) > (CH3)3CCOOH (5.05). Formic acid lacks any alkyl +I effect, making it the strongest simple unsubstituted carboxylic acid.
Section 3: Preparation Methods
Four preparation routes are tested in NEET: (1) Oxidation of 1° alcohols or aldehydes with KMnO4/H+ or K2Cr2O7/H+. (2) Grignard + CO2: RMgX + CO2 (dry ether) → RCOOH (one carbon added). (3) Nitrile hydrolysis: RCN + H3O+ → RCOOH + NH4+. (4) Saponification: RCOOR' + NaOH → RCOONa + R'OH (irreversible).
Section 4: Reactions of Carboxylic Acids
Six key reactions cover all NEET questions: HVZ (alpha-halogenation), Fischer esterification (reversible), decarboxylation (soda lime, –1 carbon), acyl chloride (SOCl2 preferred), LiAlH4 reduction (to 1° alcohol; NaBH4 fails), and Kolbe electrolysis (at anode → R–R + CO2).
Section 5: Critical NEET Distinctions
The three most frequently examined distinctions: (a) LiAlH4 vs. NaBH4 for RCOOH reduction — only LiAlH4 works; (b) HVZ requires alpha-H — HCOOH and ArCOOH are excluded; (c) Fischer esterification (reversible) vs. saponification (irreversible, gives salt).