Carboxylic acid

A carboxylic acid is an organic compound that contains a carboxyl functional group (−COOH) attached to an alkyl, aryl, or other organic substituent. The carboxyl group consists of a carbonyl (C=O) and a hydroxyl (O−H) group bonded to the same carbon atom, giving these compounds their characteristic acidic properties. Carboxylic acids are widespread in nature and serve as fundamental building blocks in biochemistry, industrial chemistry, and materials science. They occur in substances ranging from simple fatty acids to complex biomolecules like amino acids and are key intermediates in numerous chemical processes.

Structure and bonding

The carboxyl group is planar and features a trigonal carbon atom that is sp²‑hybridized. The carbon–oxygen double bond is shorter (approximately 1.23 Å) than the carbon–oxygen single bond (approximately 1.36 Å). The hydroxyl oxygen bears two lone pairs that can participate in resonance with the carbonyl π system. This delocalization stabilizes the carboxylate anion formed upon deprotonation, and it equalizes the two C−O bond lengths in the conjugate base to about 1.27 Å. In the molecular orbital picture, the carboxyl group is described by a three‑centre π system involving the two oxygens and the central carbon, lowering the energy of the anion and explaining the relatively high acidity of carboxylic acids.

Nomenclature

IUPAC systematic names for carboxylic acids are formed by replacing the terminal “‑e” of the parent alkane with “‑oic acid.” The carboxyl carbon is always designated as position 1. For example, methanoic acid (HCOOH) is derived from methane, and ethanoic acid (CH₃COOH) from ethane. When the carboxyl group is attached to a ring, the suffix “‑carboxylic acid” is used, as in cyclohexanecarboxylic acid. Many carboxylic acids retain trivial names accepted by IUPAC, such as formic acid (methanoic acid), acetic acid (ethanoic acid), butyric acid (butanoic acid), and benzoic acid (benzenecarboxylic acid). Dicarboxylic acids are named with the suffix “‑dioic acid,” e.g., ethanedioic acid (oxalic acid). Salts and esters are named by replacing “‑ic acid” with “‑ate” and preceding it with the cation or alkyl group, respectively, as in sodium acetate or ethyl acetate.

Physical properties

Lower carboxylic acids (up to about nine carbons) are colourless liquids with pungent or unpleasant odours; formic acid and acetic acid are notable examples. Higher homologues are waxy solids. The boiling points of carboxylic acids are markedly higher than those of alcohols, aldehydes, or ketones of comparable molecular mass because they form strong intermolecular hydrogen‑bonded dimers in the liquid and vapour phases. This dimerisation effectively doubles the molecular weight and leads to substantial volatility suppression. Short‑chain acids are miscible with water due to hydrogen bonding between the carboxyl group and water molecules. As the hydrocarbon chain lengthens, solubility decreases; acids with more than about ten carbon atoms are virtually insoluble in water but dissolve in organic solvents.

Acidity and chemical properties

Carboxylic acids are Brønsted‑Lowry acids, donating a proton from the hydroxyl group to a base. In aqueous solution they establish the equilibrium:

RCOOH + H₂O ⇌ RCOO⁻ + H₃O⁺

Typical pKₐ values for aliphatic carboxylic acids fall in the range of 4–5, making them weak acids but substantially stronger than alcohols (pKₐ ≈ 16) and phenols (pKₐ ≈ 10). The acidity arises from the resonance stabilisation of the carboxylate anion and the inductive electron‑withdrawing effect of the carbonyl oxygen. Electron‑withdrawing substituents near the carboxyl group enhance acidity; for instance, trichloroacetic acid (pKₐ ≈ 0.7) is a strong acid. Dicarboxylic acids are stronger than their monocarboxylic counterparts, particularly when the two carboxyl groups are in close proximity.

Beyond acidity, the carboxyl group is a versatile functional group capable of numerous transformations. The carbonyl carbon is electrophilic and can be attacked by nucleophiles, leading to substitution at the acyl carbon. The O−H bond can also be cleaved in reactions with active metals, bases, and certain reducing agents.

Synthesis

Carboxylic acids can be prepared by a variety of laboratory and industrial methods. Oxidation of primary alcohols or aldehydes with strong oxidants such as potassium permanganate, chromic acid, or Jones reagent affords the corresponding acid in good yields. Oxidative cleavage of alkenes with hot, concentrated potassium permanganate or ozone followed by an oxidative work‑up also yields carboxylic acids, sometimes with loss of a carbon. Hydrolysis of nitriles (R−C≡N) under acidic or basic conditions generates carboxylic acids, as does the carbonation of Grignard reagents with carbon dioxide. Industrial production often involves carbonylation of alcohols, e.g., the Monsanto and Cativa processes for acetic acid from methanol and carbon monoxide. Fermentation is another important route, especially for citric acid, lactic acid, and acetic acid (vinegar).

Reactions

Nucleophilic acyl substitution

The most characteristic reactions of carboxylic acids are nucleophilic acyl substitutions, in which the −OH group is replaced by another nucleophile. These include:

• Esterification (Fischer–Speier esterification): reaction with an alcohol in the presence of an acid catalyst to form an ester and water. The reaction is reversible and can be driven by removal of water or using excess alcohol.
• Amide formation: direct thermal dehydration of an ammonium salt or reaction with an amine under dehydrating conditions (e.g., DCC, EDCI) yields an amide.
• Acid chloride and anhydride formation: treatment with thionyl chloride (SOCl₂), phosphorus pentachloride (PCl₅), or phosphorus trichloride (PCl₃) converts carboxylic acids to acyl chlorides. Dehydration with strong acids or acetic anhydride gives symmetric anhydrides.
• Reduction: lithium aluminium hydride (LiAlH₄) reduces carboxylic acids to primary alcohols; borane (BH₃) is a milder alternative.

Decarboxylation

Carboxylic acids lose carbon dioxide upon heating, especially when the α‑carbon bears an electron‑withdrawing group or when the resulting carbanion is stabilised. β‑Keto acids, for example, decarboxylate readily at room temperature. The Kolbe electrolysis involves oxidative decarboxylation to form symmetrical dimers.

α‑Halogenation (Hell–Volhard–Zelinsky reaction)

In the presence of phosphorus tribromide or phosphorus trichloride, aliphatic carboxylic acids undergo selective bromination (or chlorination) at the α‑carbon. This is a key reaction for introducing a leaving group that enables further substitution or elimination.

Salt and soap formation

Reaction with alkali metal hydroxides or carbonates yields carboxylate salts. The sodium or potassium salts of long‑chain fatty acids are soaps, possessing a hydrophilic head and a hydrophobic tail that enable them to form micelles and act as surfactants.

Occurrence and biological importance

Carboxylic acids are ubiquitous in biological systems. Fatty acids are aliphatic monocarboxylic acids that serve as energy storage molecules, components of membrane lipids, and signalling molecules. Acetic acid is the metabolic intermediate acetyl‑CoA’s central unit. Amino acids, the monomers of proteins, contain both an amino group and a carboxylic acid group; their acid–base behaviour dominates protein folding and enzyme catalysis. Citric acid and other intermediates of the Krebs cycle are polycarboxylic acids that drive cellular respiration. In plants, long‑chain carboxylic acids form cutin and suberin, protective waxes. Bile acids are steroid carboxylic acids that facilitate fat digestion.

Industrial and daily‑life applications

Acetic acid is one of the most produced organic chemicals; its derivatives include vinyl acetate (paints, adhesives), cellulose acetate (fibres, films), and acetic anhydride (aspirin synthesis). Fatty acid salts (soaps) and esters (biodiesel, lubricants, cosmetics) constitute huge markets. Benzoic acid and its salts are common food preservatives. Terephthalic acid is a monomer for polyesters such as PET. Formic acid serves as a preservative, antibacterial agent, and a reducing agent in textile and leather processing. Many pharmaceuticals, from simple analgesics like aspirin (acetylsalicylic acid) to complex antibiotics, contain carboxylic acid motifs that influence solubility, binding, and metabolic stability.

Environmental and safety aspects

Short‑chain carboxylic acids are generally biodegradable and exhibit low acute toxicity, though concentrated solutions are corrosive. Formic acid and acetic acid are particularly hazardous in vapour form, causing eye, skin, and respiratory tract irritation. Some halogenated acids, such as trichloroacetic acid, are more persistent and show higher toxicity. Dicarboxylic acids (oxalic acid) can form insoluble calcium oxalate crystals, posing a risk of kidney stones upon ingestion. In the atmosphere, carboxylic acids contribute to the acidity of precipitation and are products of volatile organic compound oxidation. Industrial handling requires appropriate personal protective equipment and ventilation to prevent corrosive damage.