Organic chemistry tricks that make carboxylic acids stick

Acyl substitution reactivity: acid chloride (most reactive) > anhydride > ester > amide (least reactive). More electronegative leaving group = more reactive. This order appears on every orgo exam.

Carboxylic acids (pKa ~5) are much more acidic than alcohols (pKa ~16) because the carboxylate anion is resonance-stabilized. EWG groups lower pKa (stronger acid). EDG groups raise pKa (weaker acid).

RCOOH + R'OH ⇌ RCOOR' + H2O. Acid catalyzed, reversible. Drive equilibrium: excess alcohol, remove water (Dean-Stark), or use dehydrating agent. Mechanism goes through tetrahedral intermediate.

Base-catalyzed ester hydrolysis. Unlike acid hydrolysis, saponification is irreversible because the carboxylate salt cannot be re-protonated under basic conditions. This is how soap is made (fat + NaOH).

Carboxylic acids are resistant to NaBH4. Only LiAlH4 reduces them — through the aldehyde intermediate to primary alcohol. Two equivalents of hydride are added overall.

All carboxylic acid derivative reactions go through the same mechanism: (1) Nucleophile attacks carbonyl C → tetrahedral intermediate (C now has 4 groups, no longer sp²). (2) Leaving group departs → reforms C=O. Different from SN2 (no inversion, no Walden). The leaving group ability determines rate: Cl⁻ > RCOO⁻ > RO⁻ > NR₂⁻. Adding good nucleophile or removing product drives equilibrium.

Acid chlorides (most reactive derivative) made from carboxylic acids: RCOOH + SOCl₂ → RCOCl + SO₂ + HCl (thionyl chloride — gaseous byproducts are easy to remove). Also: PCl₃ or PCl₅. Oxalyl chloride (COCl)₂ is mild and widely used. Acid chlorides react with: alcohols → esters, amines → amides, water → carboxylic acid, organocuprates → ketones. Keep dry — hydrolyze with water.

Anhydrides form by dehydration of two carboxylic acid molecules (heating, or P₂O₅). Cyclic anhydrides form readily from diacids with 1,4- or 1,5-dicarboxylic acids (succinic, maleic, phthalic). Anhydrides react like acid chlorides but more slowly: with alcohols → ester + carboxylic acid, with amines → amide + carboxylic acid. Acetic anhydride (Ac₂O) is the most common — used for acetylation (aspirin synthesis).

Beta-keto acids (carbonyl at beta position) readily lose CO₂ when heated. Mechanism: 6-membered cyclic transition state → CO₂ leaves, enol forms, tautomerizes to ketone. Malonic acid and beta-keto acids are classic examples: malonic ester synthesis gives carboxylic acids via decarboxylation. Requirements: carboxyl group must be beta to another carbonyl. Alpha-keto acids and gamma-keto acids do NOT decarboxylate easily.

The Hell-Volhard-Zelinsky (HVZ) reaction brominates carboxylic acids at the alpha carbon. Reagents: Br₂ + PBr₃ (or PCl₃ for chlorination). The phosphorus converts acid to acid bromide first → enol of acid bromide → bromination → hydrolysis → alpha-bromo acid. Unlike ketone halogenation (which can be mono- or poly-), HVZ gives mono-bromination. Products are precursors to alpha-amino acids (via Gabriel or azide substitution).

EWG (halogens, NO₂, CN) near the carboxyl group stabilize the carboxylate anion → increase acidity (lower pKa). Effect decreases with distance: alpha > beta > gamma. Multiple halogens: trichloroacetic acid (pKa 0.7) vs acetic acid (pKa 4.76). EDG (alkyl groups) slightly decrease acidity. Resonance effects: benzoic acid (pKa 4.2); EWG on ring increase acidity; EDG decrease acidity. Tested on every orgo exam.

Strategy for making substituted acetic acids (R-CH₂-COOH). Steps: (1) Diethyl malonate + strong base (NaOEt) → enolate at central C. (2) Alkylate with R-X (SN2). (3) Alkylate again if needed (disubstitution). (4) Saponify (NaOH) → dicarboxylic acid. (5) Heat → decarboxylation → monocarboxylic acid. Gives: R-CH₂-COOH or R-CR'-COOH. Classic way to make carboxylic acids with specific substituents.

Aspirin (acetylsalicylic acid) synthesis: salicylic acid (has both OH and COOH) + acetic anhydride → aspirin + acetic acid. The phenol OH is acetylated (more reactive than COOH toward anhydrides). Acid catalyst (H₃PO₄ or H₂SO₄) optional. This is an acylation of a phenol by an anhydride — a classic example of nucleophilic acyl substitution. Salicylate ion (from aspirin hydrolysis) is the active anti-inflammatory agent.