Carboxylic acids, represented by the general formula RCOOH, stand as the most acidic class of common organic functional groups. The unique acidity of carboxylic acids arises from the exceptional stability of their conjugate base, the carboxylate ion (RCOO–). When a carboxylic acid donates a proton, the resulting carboxylate ion is stabilized by resonance: the negative charge is delocalized equally over two equivalent C–O bonds, each acquiring a bond order of 1.5. This delocalization over two electronegative oxygen atoms provides far greater stabilization than is available to the conjugate bases of alcohols (RO–, with charge localized on one oxygen) or phenols (ArO–, where charge is distributed into less electronegative ring carbons).
The quantitative hierarchy of acidity — the pKa ladder — is a fundamental NEET concept: carboxylic acids (pKa 4–5) are the strongest, followed by phenols (~10), water (15.7), alcohols (16–18), and terminal alkynes (~25). Each step down this ladder corresponds to a less stable conjugate base.
Substituent Effects profoundly alter carboxylic acid acidity. Electron-withdrawing groups (–I effect) — particularly halogens — stabilize the carboxylate ion by withdrawing electron density, reducing the charge density on the carboxylate oxygens. The cumulative effect is dramatic: monochloroacetic acid (ClCH2COOH, pKa 2.87) is more acidic than acetic acid (CH3COOH, pKa 4.76), dichloroacetic acid (Cl2CHCOOH, pKa 1.26) is stronger still, and trichloroacetic acid (Cl3CCOOH, pKa 0.65) approaches the strength of mineral acids. The –I effect decreases with distance: 2-chlorobutanoic acid is more acidic than 3-chlorobutanoic acid because the chlorine at the alpha-position exerts a stronger stabilizing influence on the adjacent carboxylate.
Alkyl groups exert the opposite effect. They possess a +I (electron-donating) inductive effect, pushing electron density toward the carboxylate and destabilizing it. This is why formic acid (HCOOH, pKa 3.75) is stronger than acetic acid (CH3COOH, pKa 4.76): the H atom in formic acid has no inductive effect, whereas the methyl group in acetic acid donates electrons and weakens the carboxylate stability. The trend continues: propanoic acid < butanoic acid < trimethylacetic acid (pivalic acid, pKa 5.05).
Preparation of Carboxylic Acids proceeds by four principal routes. (1) Oxidation of primary alcohols or aldehydes with KMnO4 or K2Cr2O7/H+ converts the –CH2OH or –CHO group to –COOH via strong oxidizing conditions. (2) Grignard carboxylation: RMgX + CO2 (dry ether) → RCOOMgX → (H3O+) → RCOOH. This reaction extends the carbon chain by exactly one carbon, with the carboxyl carbon coming from CO2. Strictly anhydrous conditions are essential because any protic solvent destroys the Grignard reagent by proton transfer. (3) Nitrile hydrolysis: RCN + H3O+ (reflux) → RCOOH + NH4+. Alkaline hydrolysis gives the sodium carboxylate salt. (4) Saponification of esters: RCOOR' + NaOH (reflux) → RCOONa + R'OH. This irreversible base-catalyzed hydrolysis gives the carboxylate salt directly.
Key Reactions of carboxylic acids encompass six reaction types of high NEET importance. The HVZ reaction (Hell-Volhard-Zelinsky) uses X2 and red phosphorus to alpha-halogenate carboxylic acids: RCOOH + X2/Red P → alpha-halo acid. The red phosphorus reacts with X2 to form PX3 in situ, which converts –COOH to –COX (acyl halide). This acyl halide intermediate tautomerizes to an enol, which is then halogenated at the alpha-carbon. The critical limitation: HVZ requires an alpha-hydrogen. Formic acid (no alpha-C) and benzoic acid (sp2 aromatic alpha-C) cannot undergo HVZ.
Fischer esterification is the reversible, acid-catalyzed condensation of RCOOH with R'OH: RCOOH + R'OH ⇌ RCOOR' + H2O (H2SO4 catalyst, heat). Since K ≈ 1, yield is maximized by using excess alcohol or by continuously removing water.
Acyl chloride synthesis via SOCl2 is the preferred method: RCOOH + SOCl2 → RCOCl + SO2↑ + HCl↑. Both byproducts are gases, driving the reaction to completion and simplifying purification. PCl5 (which gives liquid POCl3) is less preferred.
Reduction with LiAlH4 converts carboxylic acids to primary alcohols (RCOOH → RCH2OH). This requires strong hydride — NaBH4 is completely ineffective because the carboxyl carbonyl carbon is less electrophilic (due to resonance donation from the –OH oxygen). Decarboxylation with soda lime (NaOH + CaO, heat): RCOONa → R–H + Na2CO3, giving an alkane with one fewer carbon. For sodium formate, the "alkyl" group is H, so the product is H2 gas.
Kolbe electrolysis electrochemically oxidizes carboxylate anions at the anode: 2 RCOO– → R–R + 2 CO2. Each carboxylate radical loses CO2 to give an alkyl radical; two alkyl radicals couple to form the symmetrical alkane.
The three NEET traps that demand mastery: (1) NaBH4 does not reduce RCOOH — always "no reaction"; (2) HCOOH and ArCOOH cannot undergo HVZ — no alpha-H; (3) Fischer esterification is reversible; saponification is irreversible and gives the salt, not the free acid.