Carboxylic Acids

Definition and Structure

Functional Group

Carboxylic acids contain the carboxyl group (-COOH), a carbonyl (C=O) attached to a hydroxyl (-OH). The general formula: R-COOH, where R = alkyl, aryl or H.

Molecular Geometry

Planar carboxyl carbon; sp2 hybridized. Bond angles approx. 120°. Resonance stabilizes the carboxyl group via delocalization between C=O and C–O bonds.

Resonance Structures

Two major resonance contributors: carbonyl double bond and hydroxyl oxygen lone pair delocalization. Explains acidity and reactivity.

Hydrogen Bonding

Strong intermolecular hydrogen bonding via -OH and C=O groups. Leads to higher boiling points than comparable alcohols and ketones.

R–C(=O)–OH || O | HResonance: R–C(–O⁻)=O⁺H ↔ R–C(=O)–O⁻

Nomenclature

IUPAC Naming

Suffix: -oic acid replaces terminal -e in alkane names. Example: ethane → ethanoic acid. Numbering starts at the carboxyl carbon.

Common Names

Based on source or trivial names: acetic acid (ethanoic acid), formic acid (methanoic acid), benzoic acid. Widely used in industry and literature.

Prefixes and Substituents

Substituents named as prefixes; carboxyl group assumed at C-1. Multiple carboxyl groups indicated by suffixes -dioic acid, e.g., oxalic acid (ethanedioic acid).

Derivatives Naming

Esters: alkyl alkanoates; acid chlorides: alkanoyl chlorides; anhydrides: alkanoyl anhydrides. Based on parent acid name.

Physical Properties

Boiling and Melting Points

Higher than corresponding alcohols and aldehydes due to strong hydrogen bonding. Monomers associate into dimers in non-polar solvents.

Solubility

Short-chain carboxylic acids soluble in water due to hydrogen bonding. Solubility decreases with increasing alkyl chain length.

Odor and Appearance

Lower acids are volatile with pungent odors (acetic acid vinegar scent). Higher acids are waxy solids or oily liquids with less odor.

Density and State

Density generally higher than water for lower acids. State varies from gases (formic acid vapors) to solids (benzoic acid).

Acidic Characteristics

Acidity Mechanism

Proton dissociation from -COOH group. Stabilization of carboxylate ion by resonance increases acidity over alcohols.

pKa Values

Typical pKa: 4–5. Electron-withdrawing groups lower pKa; electron-donating groups raise it. Example: trifluoroacetic acid pKa ~0.3.

Factors Affecting Acidity

Inductive effect, resonance, solvation, and hydrogen bonding. Aromatic substitution impacts acidity significantly.

Comparison with Other Acids

Stronger than phenols and alcohols, weaker than mineral acids. Moderate acidity useful in organic synthesis and catalysis.

R–COOH ⇌ R–COO⁻ + H⁺Ka = [R–COO⁻][H⁺] / [R–COOH]pKa = -log Ka

Synthesis Methods

Oxidation of Primary Alcohols and Aldehydes

Reagents: KMnO4, CrO3, or Jones reagent. Converts R–CH2OH or R–CHO to R–COOH under acidic conditions.

Hydrolysis of Nitriles

Acidic or basic hydrolysis of R–CN yields R–COOH after intermediate amide formation. Common in industrial synthesis.

Carbonation of Grignard Reagents

Reaction of R–MgX with CO2 followed by acid workup produces R–COOH. Versatile for carbon chain elongation.

Other Methods

Oxidative cleavage of alkenes, hydrolysis of esters or amides, microbial fermentation for specific acids.

Derivatives of Carboxylic Acids

Esters

Formed by reaction with alcohols under acidic catalysis. General formula R–COOR'. Used as solvents, fragrances.

Acid Chlorides

Highly reactive; prepared using SOCl2 or PCl5. Used in acylation reactions and polymer synthesis.

Anhydrides

Formed by dehydration of two acids or reaction of acid chlorides with acids. Important acylating agents.

Amides

Produced by reaction with amines or ammonia. Stable, present in proteins and synthetic polymers.

R–COOH + R'–OH ⇌ R–COOR' + H2O (esterification)R–COOH + SOCl2 → R–COCl + SO2 + HCl (acid chloride formation)2 R–COOH → (R–CO)2O + H2O (anhydride formation)

Reactions of Carboxylic Acids

Acid-Base Reactions

React with bases to form carboxylate salts. Neutralization reactions important in industrial and biological contexts.

Reduction

Reduced by LiAlH4 to primary alcohols. Milder reagents do not affect carboxylic acids.

Decarboxylation

Thermal or catalytic removal of CO2. Requires β-keto acids or special conditions.

Substitution Reactions

Conversion to acid chlorides, esters, amides via nucleophilic acyl substitution. Key step in synthetic pathways.

Other Reactions

Halogenation at α-position, formation of acid anhydrides, and formation of mixed anhydrides used in peptide synthesis.

Spectroscopic Characterization

Infrared Spectroscopy (IR)

Strong, broad O–H stretch 2500–3300 cm⁻¹. Sharp C=O stretch near 1700 cm⁻¹. Diagnostic for carboxyl group.

NMR Spectroscopy

¹H NMR: acidic proton ~10–13 ppm (broad). Alkyl protons shifted by electron-withdrawing effect. ¹³C NMR: carbonyl carbon at ~170–180 ppm.

Mass Spectrometry

Characteristic fragmentation: loss of COOH or CO2. Molecular ion often visible; useful in molecular weight determination.

UV-Vis Spectroscopy

Limited absorption; conjugated carboxylic acids absorb in UV region. Used for substituted derivatives.

Biological Importance

Role in Metabolism

Carboxylic acids form core of fatty acids, amino acids, citric acid cycle intermediates. Central in energy production and biosynthesis.

Structural Components

Part of peptides (amides), lipids (fatty acids), and secondary metabolites. Influence protein folding and membrane properties.

Signaling Molecules

Some carboxylic acids act as hormones or neurotransmitter precursors (e.g., prostaglandins, neurotransmitters).

Enzyme Substrates and Inhibitors

Carboxylic acids and derivatives are substrates for carboxylases and ligases; inhibitors mimic transition states in enzyme active sites.

Industrial Applications

Production of Polymers

Monomers like terephthalic acid used in polyester production. Acrylic and methacrylic acids for plastics and resins.

Food Industry

Acetic acid as preservative and flavoring. Citric acid as acidulant and chelating agent.

Pharmaceuticals

Precursors in drug synthesis, analgesics, anti-inflammatory agents. Salicylic acid is a key intermediate.

Cosmetics and Personal Care

Used in exfoliants, pH adjusters, and antimicrobial agents in skincare products.

Environmental Impact

Natural Occurrence

Produced by microbial degradation, plant metabolism. Present in soils, waters; part of carbon cycle.

Pollution Concerns

Excess discharge can acidify environments. Industrial waste requires treatment to prevent ecological damage.

Biodegradability

Generally biodegradable via microbial pathways. Used as substrates in wastewater treatment.

Green Chemistry Approaches

Use of bio-based feedstocks, catalytic oxidations, and solvent-free reactions to minimize environmental footprint.

Safety and Handling

Hazards

Irritant to skin, eyes, respiratory tract. Concentrated acids corrosive. Lower acids volatile and flammable.

Personal Protective Equipment (PPE)

Use gloves, goggles, lab coat. Work in fume hood to avoid inhalation.

Storage

Store in cool, well-ventilated areas away from bases, oxidizers. Use compatible containers.

Disposal

Neutralize before disposal. Follow institutional and governmental regulations.

References

• Clayden, J., Greeves, N., Warren, S., & Wothers, P. Organic Chemistry. Oxford University Press, 2012, pp. 89–115.
• Smith, M. B. Organic Chemistry: An Acid-Base Approach. CRC Press, 2016, vol. 1, pp. 320–340.
• March, J. Advanced Organic Chemistry: Reactions, Mechanisms, and Structure. Wiley, 1992, vol. 1, pp. 350–390.
• Pavia, D. L., Lampman, G. M., Kriz, G. S., & Engel, R. G. Introduction to Organic Laboratory Techniques. Cengage Learning, 2014, pp. 200–230.
• Carey, F. A., & Sundberg, R. J. Advanced Organic Chemistry Part A: Structure and Mechanisms. Springer, 2007, pp. 210–250.