Carboxylic Acids Why carboxylic acids matter Carboxylic acids (–COOH) show up everywhere: in vinegar (acetic acid), citrus fruits (citric acid), skin-care (salicylic acid), preservatives (benzoic acid) and fatty acids (soap chemistry). For NEET, you must know their structure, acidity trends, preparations, and signature reactions like Fischer esterification, HVZ α-halogenation, decarboxylation, and Kolbe electrolysis. Where you meet them: vinegar (acetic acid), lemon (citric acid), and aspirin (acetylsalicylic acid) — chemistry in daily life. Structure of –COOH and resonance of the carboxylate The carboxyl group is two parts on the same carbon: a carbonyl (C=O) and a hydroxyl (O–H). The carbonyl carbon is sp2 and planar; the –COOH group is overall planar around that carbon. On deprotonation, the carboxylate anion (–COO−) is resonance-stabilized with two equivalent contributors — the negative charge is delocalized over both oxygens. This equal sharing makes the two C–O bonds equivalent in the anion, explaining strong acidity compared to alcohols and phenols. –COOH basics: structure, resonance-stabilized –COO⁻ with two equivalent C–O bonds, and where carboxylic acids sit on the acidity scale vs alcohols and strong acids. 2026-05-26T17:05:39.931Z Dimer of acetic acid: two molecules forming a cyclic dimer via two O–H···O hydrogen bonds, shown in gas phase and broken in water. Label O, H-bonds (dashed red), and show equalized C–O bonds in carboxylate. Clean 2D vector, white background, textbook style. gpt-image-2 Carboxylic acid dimer: two molecules hydrogen-bonded via two O–H···O bonds — explains unusually high boiling points. Key structural points Planar at the –COOH carbon (sp2). Carboxylate (R–COO−) has two equivalent C–O bonds (resonance). Strong intramolecular and intermolecular hydrogen bonding; dimer formation in non-polar phases. Carboxylic acids donate a proton to water to give resonance-stabilized carboxylate. Acid dissociation Lower pK a (higher K a ) means stronger acid. Acid dissociation constant Henderson–Hasselbalch Useful for buffer problems involving RCOOH/RCOO− pairs. Physical properties you can predict Boiling points are high due to strong hydrogen bonding and dimer formation (even in vapour). Short-chain acids (up to about four carbons) are miscible with water; solubility drops with longer non-polar chains. Ethanoic acid (acetic acid; CC(=O)O) has m.p. 16.6° C; the pure liquid is called glacial acetic acid because it freezes in a cool room. remember Dimerization boosts boiling point. In water, acids prefer hydrogen bonding with water, breaking dimers — hence good solubility for small acids. Preparation of carboxylic acids Common methods (chain effects noted) Oxidation of primary alcohols (1° ROH) to acids: reagents KMnO4, K2Cr2O7 (acidic), or hot, concentrated HNO3. Example: ethanol (CH3CH2OH) → ethanoic acid (acetic acid; CC(=O)O). Oxidation of aldehydes (R–CHO) to acids: Tollens’ reagent, Fehling’s solution (for aliphatic aldehydes), or KMnO4/mild acid. Example: ethanal → ethanoic acid. Hydrolysis of nitriles (R–C≡N): acid or base hydrolysis gives carboxylic acid with one extra carbon. R–C≡N + 2H2O/H+ → R–COOH + NH4+. Chain length increases by +1. Grignard + CO2 (carboxylation): R–MgX reacts with dry CO2 to give R–COOMgX; acid workup (H3O+) gives R–COOH. Chain length increases by +1 (adds –COOH). Hydrolysis of acid derivatives: acyl halides, acid anhydrides, esters, and amides hydrolyse (acidic or basic) to give the parent carboxylic acid (or its salt under basic conditions). Strong oxidants convert 1° alcohols → aldehydes → acids; with proper conditions, step directly to acids. Carboxylation of Grignard reagents (CO2 insertion) dry ether; anhydrous; CO2 (solid CO2 or gas) Nucleophilic carbon (R−) of R–MgX attacks electrophilic carbon of CO2 to form R–C(O)O−MgX. H3O+ Acidic workup protonates the carboxylate to give R–COOH and MgX+ salts. Protonation of carboxylate oxygen R–MgX inserts into CO2 to give a magnesium carboxylate, which on acidic workup yields the carboxylic acid (chain +1). nucleophile/base alkylmagnesium halide Grignard reagent carbon dioxide electrophile carbon dioxide Acyl halides/anhydrides hydrolyse rapidly to acids; esters and amides more slowly (base gives carboxylate; acidification regenerates acid). Hell–Volhard–Zelinsky (HVZ): α-bromination of carboxylic acids If the acid has at least one α-hydrogen (on the carbon next to –COOH), HVZ introduces a halogen at that α-position using Br2 and red phosphorus. Mechanistically, the acid is converted to the acyl bromide in situ, enolizes, brominates at α, and hydrolyses back to the α-bromo acid. HVZ: α-bromination of carboxylic acids having α-H. Red P reacts with Br2 to form PBr3 in situ, converting R–CH2–COOH to R–CH2–COBr (acyl bromide). Acyl bromide enolizes; the enol (or enol bromide) undergoes electrophilic bromination at α-carbon. Aqueous workup hydrolyses the α-bromo acyl bromide back to R–CH(Br)–COOH. Temporary conversion to acyl bromide allows enolization and electrophilic α-bromination; hydrolysis regenerates the acid with Br at α-carbon. alkanoic acid substrate carboxylic acid bromine bromine electrophile activator (forms PBr3 in situ) phosphorus red phosphorus HVZ mechanism (Br2/Red P) 2026-05-26T17:05:40.286Z Four-panel mechanism diagram for HVZ on propanoic acid. Panel labels: 1) PBr3 formation and acyl bromide; 2) enolization; 3) attack of Br2 at α-carbon; 4) hydrolysis to 2-bromopropanoic acid. Curved arrows in red, atoms labeled, vector style. gpt-image-2 HVZ in one glance: acyl bromide formation → enolization → α-bromination → hydrolysis back to α-bromo acid. Named reaction reference for α-halogenation of acids. neet-alert HVZ needs an α-H. Formic acid (no α-carbon) and benzene carboxylic acids without α-H do not undergo HVZ. Acidity: stronger than alcohols and phenols Typical pK a of carboxylic acids is about 3–5, far lower (stronger acid) than phenols (≈10) and alcohols (≈16–18). Reason: the conjugate base carboxylate is stabilized by resonance over two oxygens with equal contributors; the phenoxide has resonance but the negative charge is delocalized into a ring with one oxygen, less stabilizing for charge. Inductive effects from substituents and the hybridization/attachment (sp2 vs sp3) also matter. Carboxylic acids are only slightly more acidic than alcohols because both have an –OH group. Carboxylic acids are much stronger acids ( pK a ≈ 3–5) than alcohols (≈16–18) due to strong resonance stabilization of the carboxylate (–COO−) by two equivalent oxygen atoms. 2026-05-26T17:05:40.366Z pKa comparison bar chart, y-axis pKa 0–18, bars for HCOOH, CH3COOH, C6H5COOH, ClCH2COOH, CCl3COOH, phenol, ethanol. Labels include values. Clean vector chart, neutral palette. gpt-image-2 Bar chart of pK a : show formic, acetic, benzoic, chloroacetic, trichloroacetic, phenol, ethanol to visualize acidity differences. Acidity ranking ( pK a ) with reasons Compound pK a Why Examples Formic acid (methanoic acid; O=CO) 3.75 No alkyl electron-donating group; conjugate base well stabilized. Acetic acid (ethanoic acid; CC(=O)O) 4.76 Methyl is weakly +I; slightly less acidic than formic. Propionic acid (propanoic acid) 4.87 Stronger +I from ethyl decreases acidity a bit more. Chloroacetic acid (ClCH2COOH) 2.86 –I effect of Cl stabilizes carboxylate; more acidic. Fluoroacetic acid (FCH2COOH) 2.59 Very strong –I from F; even more acidic than chloroacetic. Trichloroacetic acid (CCl3COOH) 0.66 Three Cl atoms exert powerful –I; very strong acid. Benzoic acid (benzenecarboxylic acid; c1ccccc1C(=O)O) 4.20 Carboxyl attached to sp2 carbon pulls electron density; slightly more acidic than acetic. o-Nitrobenzoic acid 2.16 –I/–M of –NO2 plus ortho-effect (steric + possible intramolecular H-bond). m-Nitrobenzoic acid 3.49 –I only (no –M at meta). p-Nitrobenzoic acid 3.43 –I and –M through resonance at para. Salicylic acid (2-hydroxybenzoic acid; Oc1ccccc1C(=O)O) 2.97 Ortho –OH enables intramolecular H-bonding in conjugate base; ortho-effect increases acidity. Oxalic acid (ethanedioic acid; O=C(O)C(=O)O) 1.25, 4.27 Diprotic: first proton is much more acidic than the second. Picric acid (2,4,6-trinitrophenol) — comparison ≈0.4 Very strong phenol due to 3 –NO2; still different class. remember Position effect on benzoic acids: Ortho-effect makes o-substituted benzoic acids more acidic irrespective of group nature (steric factors and intramolecular H-bonding). Meta and para depend on the substituent’s −I/−M effects. Substituent and position effects (quick rules) Electron-withdrawing groups (−I/−M), e.g., −NO2, −X, increase acidity by stabilizing RCOO−. Electron-donating groups (+I/+M), e.g., −CH3, −OCH3 (para/ortho via +M), decrease acidity. Distance matters: α-substituents have the strongest inductive effect; β weaker; γ negligible for aliphatic acids. Benzoic acids: Ortho-effect boosts acidity for any ortho-substituent; at meta, only −I counts; at para, both −I and −M operate if possible. Reactions of –COOH you must command With active metals (Na, Mg, etc.): salt + H2. With bases: neutralization to carboxylates. With NaHCO3/Na2CO3: effervescence of CO2 — distinguishing test from phenols. With NH3 (then heat): ammonium carboxylate → amide (R–CONH2) + H2O. With PCl5/PCl3/SOCl2: acyl (acid) chlorides form (RCOCl). Two acids + dehydrating agent (e.g., P2O5): acid anhydrides form. Fischer esterification with alcohols (H+): reversible formation of esters. HVZ α-halogenation (Br2/red P) if α-H present. Decarboxylation: soda-lime (NaOH/CaO) gives alkane (−CO2). Kolbe electrolysis (from RCOO−): radical coupling to R–R + CO2. Reduction: LiAlH4 or B2H6 reduce acids to 1° alcohols; NaBH4 does not. Core transformations Neutralization with base. Acid + alcohol ⇌ ester + water (acid-catalysed; reversible). Fischer esterification: carboxylic acid + alcohol, acid catalysis, reversible to ester + water. Fischer esterification (acid-catalysed) Protonate the carbonyl oxygen to activate the acyl group. Alcohol attacks the carbonyl carbon to form a tetrahedral intermediate. Loss of water (dehydration) Proton transfers set up leaving of water; collapse to the ester. H+ (catalytic), remove water or use excess alcohol to drive forward Deprotonation regenerates the acid catalyst and gives the ester. alkanoic acid acyl component carboxylic acid alkanol nucleophile alcohol catalyst hydronium acid catalyst Acid-catalysed nucleophilic acyl substitution converts R–COOH and R′–OH to R–COOR′ and water; reversible equilibrium. Reversible esterification; drive forward by removing water. tip Esterification is an equilibrium. Use a dehydrating agent (e.g., conc. H2SO4) or remove water as it forms, or take excess alcohol to push it right. Soda-lime decarboxylation One-carbon shorter alkane forms from a carboxylate salt. Kolbe electrolysis (anodic decarboxylation) Coupling of radicals formed at the anode; best for simple aliphatic chains. Thermal decarboxylation routes (e.g., soda-lime) to alkanes (−CO2). Electrolytic decarboxylation of carboxylates; dimerization to alkanes. Strong reduction of acids to primary alcohols. Mild reducing agents can stop at aldehydes when reducing carboxylic acids. Carboxylic acids require strong hydride donors like LiAlH4 (or B2H6) and reduce all the way to 1° alcohols. Any aldehyde formed is immediately reduced further; NaBH4 does not reduce acids. Reducing power: LiAlH4 and B2H6 reduce –COOH → 1° alcohol; NaBH4 does not. Diborane reduces acids but typically does not reduce esters under standard conditions taught at NCERT level. neet-alert Acyl (acid) chloride, RCOCl Highest Cl− is a very good leaving group; strong −I activates carbonyl. Acid anhydride, (RCO)2O High RCOO− is a good leaving group. Ester, RCOOR′ Moderate RO− is a fair leaving group (as ROH). Amide, RCONH2 Lowest NH2− is a poor leaving group; resonance donation from N reduces reactivity. Reactivity of acid derivatives (towards nucleophilic acyl substitution) Derivative Reactivity Reason (leaving group ability) Distinguishing tests: acid vs phenol vs alcohol vs ester Simple lab distinctions Test Carboxylic acid Phenol Alcohol Ester NaHCO3 effervescence (CO2 bubbles) Yes (RCOOH + NaHCO3 → RCOONa + CO2 + H2O) No No No FeCl3 (violet complex) No Yes (phenols) No No Odour Pungent/sour (short-chain) Phenolic Often mild Sweet/fruity (esters) Na metal (H2 gas) Yes Yes (slow) Yes No (neutral under these conditions) NaHCO3 fizz is the quick spot test for carboxylic acids — phenols do not liberate CO2. remember Soap chemistry: saponification and micelles Soaps are sodium or potassium salts of long-chain fatty acids (e.g., palmitate, stearate, oleate). Base hydrolysis (saponification) of triglycerides yields glycerol and these salts. In water, soap anions form micelles: ionic carboxylate heads face water; long hydrophobic tails cluster inside, trapping grease. Base hydrolysis of esters (fats) → glycerol + carboxylate salts (soaps). Cross-sectional micelle diagram with ~40 carboxylate heads (blue circles with −), hydrocarbon tails (gray lines) pointing inward; trapped oil droplet in center. Labels: hydrophilic head, hydrophobic tail, micelle. Clean vector schematic. 2026-05-26T17:05:40.985Z Soap micelle: hydrophilic carboxylate heads outside, hydrophobic tails inside capturing dirt/oil. gpt-image-2 Special cases and named links Formic acid (methanoic acid; O=CO) is the only common reducing carboxylic acid: gives positive Tollens’ and Fehling’s tests (oxidized to CO2). Industrial acetic acid: made by carbonylation of methanol (Monsanto process); also present in vinegar (5–8%). Aspirin (acetylsalicylic acid) is produced from salicylic acid and acetic anhydride. Nylon-6,6 from adipic acid + hexamethylenediamine (polyamide formation). Benzoic acid and its salts (sodium benzoate) act as food preservatives (E210). Citric acid is a key food acidulant and part of the citric acid (Krebs) cycle. Formic acid reduces [Ag(NH3)2]+ to Ag mirror while oxidizing to CO2. Formic acid reduces Cu2+ to Cu2O (brick-red) under alkaline conditions. 2026-05-26T17:05:41.363Z Photo-realistic style not required; diagrammatic illustration of a beaker labeled 'glacial acetic acid' with a thermometer at 16.6 C , showing partially solidified contents. Clean vector style. gpt-image-2 Glacial acetic acid freezing around 16.6 C — a beaker with crystalline solidifying acetic acid. remember Acetic acid (ethanoic acid) is used in food (vinegar), as a solvent, and as a chemical building block. Salicylic acid treats acne and is the precursor of aspirin. IUPAC naming essentials Parent chain numbering starts at the –COOH carbon (this is C-1). Suffix: “-oic acid” for monocarboxylic acids (e.g., ethanoic acid). Dicarboxylic acids: suffix “-dioic acid” (e.g., ethanedioic acid for oxalic acid). Common names often persist (acetic, benzoic), but write IUPAC first in answers. Compounds containing the –COOH group (R–C(=O)–OH). carboxylic acid R–COOH alkanoic acid Conjugate base of a carboxylic acid (R–COO−), resonance-stabilized. carboxylate pK a −log K a ; lower pK a means stronger acid. ortho-effect o-Substituted benzoic acids are more acidic irrespective of substituent nature due to steric and intramolecular H-bonding effects. Fischer esterification Acid-catalysed equilibrium reaction of R–COOH with R′–OH to give R–COOR′ + H2O. Base hydrolysis of esters (fats) giving glycerol and carboxylate salts (soaps). saponification Hell–Volhard–Zelinsky reaction: α-halogenation (usually bromination) of carboxylic acids having α-H. HVZ Anodic decarboxylation of carboxylate anions to give coupled alkanes (R–R) with CO2 loss. Kolbe electrolysis Loss of CO2 from a carboxyl group; soda-lime gives one-carbon-shorter alkane. decarboxylation Mixture of NaOH and CaO used to decarboxylate sodium carboxylates. soda lime fatty acid Long-chain carboxylic acids (C12–C18) found in fats and oils. Anhydrous acetic acid (≈99.5%) that freezes around 16.6 C . glacial acetic acid acyl chloride R–COCl; a highly reactive acid derivative. R–CO–O–CO–R; formed by dehydration of two carboxylic acids. anhydride ester R–COOR′; derived from acids and alcohols. R–CONH2; least reactive common acid derivative. amide Methanoic acid that can reduce Tollens’/Fehling’s reagents; oxidized to CO2. formic acid (reducing) 2-Hydroxybenzoic acid; used in acne therapy and as aspirin precursor. salicylic acid Key terms