Abstract

Cyclohexanecarboxylic acid, systematically named cyclohexanecarboxylic acid with molecular formula C₇H₁₂O₂ and CAS Registry Number 98-89-5, represents the carboxylic acid derivative of cyclohexane. This alicyclic carboxylic acid appears as a white crystalline solid at room temperature with a characteristic melting point range of 30–31°C and boiling point range of 232–234°C. The compound exhibits a density of 1.0274 g/cm³ at 20°C and magnetic susceptibility of −83.24×10⁻⁶ cm³/mol. Cyclohexanecarboxylic acid serves as a key intermediate in industrial processes, particularly in the synthesis of caprolactam for nylon-6 production. Its chemical behavior follows typical carboxylic acid reactivity patterns, including salt formation, esterification, and conversion to acid chlorides. The compound's structural features include a non-planar cyclohexane ring in chair conformation with the carboxyl group adopting equatorial orientation in the most stable conformation.

Introduction

Cyclohexanecarboxylic acid occupies a significant position in organic chemistry as the saturated counterpart to benzoic acid and as a model compound for studying alicyclic carboxylic acid behavior. This compound belongs to the class of cycloalkanecarboxylic acids and demonstrates properties intermediate between aliphatic and aromatic carboxylic acids. The hydrogenation of benzoic acid provides the primary synthetic route to cyclohexanecarboxylic acid, a transformation of considerable industrial importance. The compound's structural characteristics, particularly the chair conformation of the cyclohexane ring and the orientation of the carboxyl group, influence its physical properties and chemical reactivity. Cyclohexanecarboxylic acid serves as a fundamental building block in organic synthesis and industrial applications, with particular significance in polymer chemistry through its conversion to caprolactam.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Cyclohexanecarboxylic acid possesses a molecular structure characterized by a cyclohexane ring in the chair conformation with a carboxylic acid functional group attached to one carbon atom. The carbon atom of the carboxyl group exhibits sp² hybridization with bond angles of approximately 120° around the carbonyl carbon. The cyclohexane ring carbon atoms maintain sp³ hybridization with tetrahedral geometry and bond angles near 109.5°. The carboxyl group typically adopts an equatorial position on the cyclohexane ring to minimize steric interactions and 1,3-diaxial strain. Molecular orbital analysis reveals that the highest occupied molecular orbital (HOMO) resides primarily on the oxygen atoms of the carboxyl group, while the lowest unoccupied molecular orbital (LUMO) corresponds to the π* orbital of the carbonyl group. The electronic structure demonstrates charge polarization with electron density shifting toward the electronegative oxygen atoms, resulting in a calculated dipole moment of approximately 1.7 Debye.

Chemical Bonding and Intermolecular Forces

The bonding in cyclohexanecarboxylic acid consists of covalent sigma bonds between all atoms with a π bond between the carbonyl carbon and oxygen. The C–C bond lengths in the cyclohexane ring measure approximately 1.54 Å, while the C–O bond lengths are 1.36 Å for the C–OH bond and 1.23 Å for the C=O bond. These bond lengths are consistent with typical carboxylic acid bonding patterns. Intermolecular forces dominate the solid-state structure through extensive hydrogen bonding between carboxyl groups of adjacent molecules. The hydrogen bonding network forms cyclic dimers with O–H···O distances of approximately 2.70 Å, characteristic of carboxylic acid dimers. Additional van der Waals interactions between cyclohexyl groups contribute to the crystal packing efficiency. The compound exhibits moderate polarity with a calculated octanol-water partition coefficient (log P) of 1.32, indicating balanced hydrophobic and hydrophilic character.

Physical Properties

Phase Behavior and Thermodynamic Properties

Cyclohexanecarboxylic acid appears as a white crystalline solid at room temperature with a characteristic sharp melting point range of 30–31°C. The boiling point occurs at 232–234°C at atmospheric pressure (760 mmHg). The compound exhibits a density of 1.0274 g/cm³ at 20°C, slightly higher than water. The heat of fusion measures 18.7 kJ/mol, while the heat of vaporization is 62.3 kJ/mol at the boiling point. The specific heat capacity at 25°C is 1.89 J/g·K. The compound demonstrates limited solubility in water (approximately 4.2 g/L at 25°C) but high solubility in most organic solvents including ethanol, diethyl ether, and chloroform. The refractive index of the liquid form at 40°C is 1.460. The surface tension of the molten compound at 40°C measures 32.4 mN/m. The thermal expansion coefficient is 0.00095 K⁻¹ in the solid phase and 0.00112 K⁻¹ in the liquid phase.

Spectroscopic Characteristics

Infrared spectroscopy of cyclohexanecarboxylic acid reveals characteristic absorption bands at 3000–2500 cm⁻¹ for the O–H stretching vibration, 1695 cm⁻¹ for the C=O stretching vibration, and 1420 cm⁻¹ for the O–H in-plane bending vibration. The C–O stretching vibration appears at 1280 cm⁻¹. Proton nuclear magnetic resonance (¹H NMR) spectroscopy in CDCl₃ shows a broad singlet at δ 11.5 ppm for the carboxylic acid proton, multiplet signals between δ 1.0–2.3 ppm for the cyclohexyl protons, and a distinct multiplet at δ 2.4 ppm for the proton alpha to the carboxyl group. Carbon-13 NMR spectroscopy displays signals at δ 180.5 ppm for the carbonyl carbon, δ 43.2 ppm for the carbon bearing the carboxyl group, and signals between δ 25.0–35.0 ppm for the remaining cyclohexyl carbons. Mass spectrometry exhibits a molecular ion peak at m/z 128 with characteristic fragmentation patterns including loss of OH (m/z 111), COOH (m/z 83), and formation of the acylium ion (m/z 105).

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Cyclohexanecarboxylic acid demonstrates typical carboxylic acid reactivity through nucleophilic acyl substitution mechanisms. The acid dissociation constant (pK_a) measures 4.87 in water at 25°C, slightly higher than benzoic acid (pK_a = 4.20) due to the electron-donating nature of the cyclohexyl group. Esterification reactions proceed with rate constants of approximately 2.3×10⁻⁴ L/mol·s in ethanol with acid catalysis. Conversion to the acid chloride with thionyl chloride occurs with 95% yield under reflux conditions. Decarboxylation requires harsh conditions with half-life of 45 minutes at 200°C. The compound undergoes α-halogenation at the position adjacent to the carboxyl group with bromine in the presence of phosphorus catalysts, following typical Hell–Volhard–Zelinsky reaction mechanisms. Hydrogenation of the ring requires extreme conditions due to the deactivating effect of the carboxyl group, with complete saturation achieved only at 200°C and 100 atm hydrogen pressure with ruthenium catalysts.

Acid-Base and Redox Properties

As a weak carboxylic acid, cyclohexanecarboxylic acid forms stable salts with bases, with sodium cyclohexanecarboxylate exhibiting solubility of 125 g/L in water at 25°C. The compound demonstrates buffer capacity in the pH range 3.8–5.8 with optimal buffering at pH 4.87. Redox properties include reduction to cyclohexanemethanol with lithium aluminum hydride with 90% yield and oxidation to cyclohexyl radical species under electrochemical conditions. The standard reduction potential for the carboxyl group is −0.85 V versus standard hydrogen electrode. The compound exhibits stability in acidic environments but undergoes decarboxylation under strongly basic conditions at elevated temperatures. Electrochemical studies show irreversible oxidation waves at +1.45 V and reduction waves at −1.20 V versus Ag/AgCl reference electrode in acetonitrile solution.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The most common laboratory synthesis of cyclohexanecarboxylic acid involves the catalytic hydrogenation of benzoic acid. This transformation typically employs ruthenium on carbon catalyst (5% loading) under hydrogen pressure of 50–100 atm at 150–200°C, yielding 85–95% pure product. Alternative synthetic routes include the carbonation of cyclohexylmagnesium bromide with subsequent acidification, providing yields of 70–80%. Hydrolysis of cyclohexanecarbonitrile under acidic conditions (20% sulfuric acid, reflux, 6 hours) affords the carboxylic acid in 90% yield. Oxidation of cyclohexylmethanol with potassium permanganate in acetone-water mixture at 0–5°C provides moderate yields of 65–75%. Grignard reaction of cyclohexyl bromide with carbon dioxide followed by acid workup represents another viable route with typical yields of 60–70%. Purification typically involves recrystallization from petroleum ether or vacuum distillation.

Industrial Production Methods

Industrial production of cyclohexanecarboxylic acid primarily occurs through the catalytic hydrogenation of benzoic acid on large scale. Continuous hydrogenation processes employ fixed-bed reactors with ruthenium-based catalysts at temperatures of 180–220°C and pressures of 80–120 atm. The reaction proceeds with conversion rates exceeding 98% and selectivity of 95% toward the desired product. Process optimization includes careful control of hydrogen flow rates, temperature gradients, and catalyst regeneration cycles. Annual global production exceeds 50,000 metric tons, with major manufacturing facilities located in Europe, North America, and Asia. Economic factors favor the benzoic acid hydrogenation route due to feedstock availability and established infrastructure. Environmental considerations include hydrogen recycling systems and wastewater treatment for catalyst residues. The production cost analysis indicates raw material costs comprising 65% of total production expenses, with catalyst consumption accounting for 15%.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides reliable identification and quantification of cyclohexanecarboxylic acid using polar stationary phases such as polyethylene glycol. Retention indices typically range from 1350–1400 on DB-WAX columns at 180°C isothermal conditions. High-performance liquid chromatography with UV detection at 210 nm offers alternative quantification methods using C18 reverse-phase columns with mobile phases of acetonitrile-water mixtures acidified with 0.1% phosphoric acid. Titrimetric methods employing standard sodium hydroxide solution with phenolphthalein indicator allow quantitative determination with precision of ±0.5%. Spectrophotometric methods based on complex formation with copper(II) ions enable detection limits of 0.1 mg/L in aqueous solutions. Mass spectrometric detection provides definitive identification through molecular ion recognition and characteristic fragmentation patterns.

Purity Assessment and Quality Control

Purity assessment typically involves determination of acid value, which should exceed 99.5% for reagent grade material, corresponding to an acid number of 435–437 mg KOH/g. Common impurities include traces of benzoic acid (typically <0.1%), cyclohexane, and water. Karl Fischer titration determines water content with specification limits of <0.2% for anhydrous grade. Residual solvent analysis by gas chromatography monitors levels of production solvents such as toluene and hexane with limits typically below 50 ppm. Metal impurity analysis by atomic absorption spectroscopy specifies limits for catalyst residues including ruthenium (<5 ppm) and other transition metals (<10 ppm total). Crystallinity assessment by X-ray powder diffraction confirms the proper crystalline form with characteristic peaks at diffraction angles of 12.4°, 16.8°, and 21.3° (2θ values). Stability testing indicates shelf life of 2 years when stored in airtight containers protected from light and moisture.

Applications and Uses

Industrial and Commercial Applications

Cyclohexanecarboxylic acid serves primarily as a chemical intermediate in the production of caprolactam, the monomer for nylon-6 synthesis. This application consumes approximately 85% of global production through the reaction with nitrosylsulfuric acid to form the corresponding oxime, which undergoes Beckmann rearrangement to caprolactam. The compound finds use in the manufacture of plasticizers, with esters such as cyclohexylmethyl phthalate providing improved flexibility and low-temperature performance in polyvinyl chloride formulations. Additional applications include use as a corrosion inhibitor in metalworking fluids at concentrations of 0.5–2.0%, where it forms protective films on metal surfaces. The acid chloride derivative, cyclohexanecarbonyl chloride, serves as an intermediate in pharmaceutical synthesis and agrochemical production. Market analysis indicates steady demand growth of 3–4% annually, driven primarily by nylon production requirements in emerging economies.

Research Applications and Emerging Uses

In research settings, cyclohexanecarboxylic acid functions as a model compound for studying conformational effects on carboxylic acid reactivity and hydrogen bonding patterns. Recent investigations explore its potential as a building block for metal-organic frameworks (MOFs) due to its ability to form stable coordination compounds with transition metals. Emerging applications include use as a phase change material for thermal energy storage, with latent heat of fusion of 187 J/g. Research continues on its derivatization to liquid crystalline compounds with mesomorphic properties. The compound serves as a precursor to novel ionic liquids with low melting points and tailored solubility properties. Patent analysis reveals increasing activity in applications related to polymer modification, with several patents issued for its use as a comonomer in polyester and polyamide resins to enhance mechanical properties and chemical resistance.

Historical Development and Discovery

The historical development of cyclohexanecarboxylic acid parallels advances in hydrogenation technology and understanding of alicyclic chemistry. Initial reports of its preparation appeared in the early 20th century through the hydrogenation of benzoic acid using nascent hydrogen methods. The development of catalytic hydrogenation processes in the 1920s enabled practical synthesis routes, with particular contributions from researchers at IG Farben investigating saturated analogues of aromatic compounds. The compound gained industrial significance in the 1940s with the commercialization of nylon-6 production, which required efficient conversion of benzoic acid to cyclohexanecarboxylic acid as a key step. Methodological advances in the 1960s improved catalytic systems for selective hydrogenation, reducing byproduct formation and increasing process efficiency. Recent historical developments include the implementation of continuous flow hydrogenation processes and the development of heterogeneous catalyst systems with enhanced stability and recyclability.

Conclusion

Cyclohexanecarboxylic acid represents a structurally interesting and industrially important alicyclic carboxylic acid with well-characterized physical and chemical properties. Its conformational behavior, hydrogen bonding capability, and typical carboxylic acid reactivity make it a valuable compound for both industrial applications and fundamental research. The compound's primary significance lies in its role as an intermediate in nylon-6 production, though emerging applications in materials science and specialty chemicals continue to expand its utility. Future research directions include development of more sustainable production methods, exploration of novel derivatives with tailored properties, and investigation of its behavior in advanced material systems. The compound continues to serve as a model system for understanding the effects of alicyclic structure on carboxylic acid properties and reactivity patterns.