Abstract

Hexatriacontanoic acid, systematically named hexatriacontanoic acid and designated by the lipid number C36:0, represents a saturated very long-chain fatty acid with a linear hydrocarbon backbone of thirty-six carbon atoms. This high molecular weight carboxylic acid possesses the chemical formula C₃₆H₇₂O₂ and a molar mass of 537.0 grams per mole. The compound exhibits characteristic properties of long-chain aliphatic acids, including limited solubility in aqueous media, high melting point behavior, and typical carboxylic acid reactivity. Hexatriacontanoic acid occurs naturally in various biological systems and synthetic materials, serving as a structural component in wax esters and complex lipids. Its extended hydrocarbon chain confers significant hydrophobic character and influences its physical state, with the compound typically existing as a waxy solid at ambient temperatures.

Introduction

Hexatriacontanoic acid belongs to the homologous series of saturated straight-chain fatty acids, classified within the subgroup of very long-chain fatty acids containing more than twenty-two carbon atoms. As a C₃₆ straight-chain carboxylic acid, this compound represents an important member of the alkanoic acid family with both fundamental chemical significance and practical applications in materials science. The systematic IUPAC name hexatriacontanoic acid derives from the hydrocarbon prefix "hexatriacontane" indicating thirty-six carbon atoms, combined with the "-oic acid" suffix denoting the carboxylic acid functional group. The compound's CAS registry number 4299-38-1 provides unambiguous identification in chemical databases and regulatory contexts.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of hexatriacontanoic acid consists of an extended alkyl chain of thirty-five methylene groups terminated by a carboxylic acid functional group. The carbon atoms adopt sp³ hybridization throughout the hydrocarbon chain, with bond angles approximating the tetrahedral value of 109.5 degrees. The carboxylic acid moiety displays planar geometry with sp² hybridization at the carbonyl carbon, resulting in bond angles of approximately 120 degrees around this center. The extended conformation of the alkyl chain exhibits rotational isomerism around carbon-carbon bonds, with the all-anti conformation representing the lowest energy state in the solid phase. Molecular orbital analysis reveals typical carboxylic acid electronic structure with the highest occupied molecular orbital localized on the oxygen atoms of the carboxyl group and the lowest unoccupied molecular orbital associated with the carbonyl π* orbital.

Chemical Bonding and Intermolecular Forces

Covalent bonding in hexatriacontanoic acid follows established patterns for saturated hydrocarbons and carboxylic acids. Carbon-carbon bond lengths measure 1.54 Å throughout the alkyl chain, while carbon-oxygen bonds in the carboxyl group measure 1.36 Å for the carbonyl bond and 1.43 Å for the hydroxyl bond. The extensive hydrocarbon chain dominates intermolecular interactions through London dispersion forces, with interaction strength increasing proportionally with molecular surface area. The carboxylic acid functionality engages in strong hydrogen bonding, forming characteristic dimeric structures in the solid state and in non-polar solvents. These dimers exhibit hydrogen bond lengths of approximately 1.75 Å between carboxylic oxygen atoms. The molecular dipole moment measures approximately 1.7 Debye, primarily oriented along the carbonyl bond axis.

Physical Properties

Phase Behavior and Thermodynamic Properties

Hexatriacontanoic acid presents as a white, waxy solid at room temperature with a characteristic greasy texture. The compound exhibits polymorphism, with at least two crystalline forms identified depending on crystallization conditions. The melting point of the stable crystalline form occurs at 102.5°C, reflecting the significant van der Waals interactions between extended alkyl chains. The boiling point under reduced pressure of 1 mmHg occurs at 285°C, though decomposition may precede vaporization at atmospheric pressure. The heat of fusion measures 58.2 kJ/mol, consistent with values for long-chain fatty acids. Density of the solid phase measures 0.85 g/cm³ at 20°C, decreasing with temperature according to typical expansion coefficients for organic solids. The refractive index measures 1.43 at the sodium D-line and 20°C.

Spectroscopic Characteristics

Infrared spectroscopy of hexatriacontanoic acid reveals characteristic absorption bands at 2950 cm^-1 and 2850 cm^-1 for asymmetric and symmetric C-H stretching vibrations, respectively. The carbonyl stretching vibration appears at 1710 cm^-1 for the dimeric carboxylic acid form, while the O-H stretching vibration produces a broad band centered at 3000 cm^-1. Proton nuclear magnetic resonance spectroscopy in CDCl₃ solution displays a triplet at δ 2.35 ppm for the α-methylene protons, a multiplet at δ 1.63 ppm for the β-methylene protons, a strong singlet at δ 1.26 ppm for the internal methylene protons, and a triplet at δ 0.88 ppm for the terminal methyl group. The carboxylic acid proton appears as a broad singlet at δ 11.5 ppm. Carbon-13 NMR spectroscopy shows signals at δ 180.5 ppm for the carbonyl carbon, δ 34.5 ppm for the α-carbon, δ 25.0 ppm for the β-carbon, δ 29.7-29.3 ppm for internal methylene carbons, and δ 14.2 ppm for the terminal methyl carbon.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Hexatriacontanoic acid exhibits typical carboxylic acid reactivity, participating in nucleophilic acyl substitution reactions with rate constants comparable to other aliphatic acids. Esterification reactions with alcohols proceed with second-order kinetics and activation energies of approximately 65 kJ/mol under acid catalysis conditions. The compound undergoes decarboxylation at elevated temperatures above 300°C with first-order kinetics and an activation energy of 145 kJ/mol. Reduction with lithium aluminum hydride proceeds quantitatively to yield hexatriacontan-1-ol with complete conversion within two hours at reflux temperature. Salt formation with bases occurs rapidly with neutralization equivalents consistent with the molecular weight.

Acid-Base and Redox Properties

The acid dissociation constant pK_a of hexatriacontanoic acid measures 4.85 in aqueous solution at 25°C, typical for aliphatic carboxylic acids. The limited water solubility influences apparent acidity in aqueous media, with micelle formation affecting proton dissociation behavior. The compound exhibits no significant redox activity under standard conditions, with oxidation requiring strong oxidizing agents such as potassium permanganate or chromic acid. Electrochemical reduction occurs at potentials more negative than -2.0 V versus the standard hydrogen electrode, involving one-electron transfer to the carbonyl group.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of hexatriacontanoic acid typically proceeds through malonic ester synthesis or Kolbe electrolysis of appropriate precursors. The malonic ester approach involves alkylation of diethyl malonate with 1-chlorohexatriacontane followed by hydrolysis and decarboxylation, yielding the target acid with overall yields of 45-55%. Kolbe electrolysis of octadecanoic acid provides a direct route to the C₃₆ acid through radical coupling, though this method produces statistical mixtures of homologous compounds. Alternative synthetic routes include oxidation of hexatriacontan-1-ol with Jones reagent or potassium permanganate, providing yields exceeding 80% under optimized conditions. Purification typically involves recrystallization from acetone or ethanol, followed by chromatography on silica gel with hexane-ethyl acetate mobile phases.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides primary identification and quantification of hexatriacontanoic acid, with separation achieved using non-polar stationary phases such as dimethyl polysiloxane and temperature programming from 150°C to 350°C at 10°C per minute. Retention indices relative to n-alkane standards provide characteristic identification parameters. High-performance liquid chromatography with evaporative light scattering detection offers alternative analysis using C₁₈ reversed-phase columns with methanol-water mobile phases containing 0.1% acetic acid. Mass spectrometric analysis exhibits characteristic fragmentation patterns with molecular ion m/z 536.5 and prominent fragments at m/z 480.5 corresponding to loss of t-butyl group, m/z 339.3 corresponding to cleavage at the carbonyl group, and m/z 73.0 corresponding to the trimethylsilyl group when derivatized.

Purity Assessment and Quality Control

Purity assessment of hexatriacontanoic acid typically employs differential scanning calorimetry to determine melting point range and enthalpy of fusion, with high-purity material exhibiting sharp melting endotherms with less than 0.5°C breadth. Thin-layer chromatography on silica gel with petroleum ether-diethyl ether-acetic acid (80:20:1) mobile phase provides rapid purity evaluation, with the compound exhibiting R_f value of 0.45. Elemental analysis confirms composition within 0.3% of theoretical values for carbon (80.52%), hydrogen (13.51%), and oxygen (5.97%). Common impurities include even-numbered homologues with chain lengths from C₃₂ to C₄₀, detectable by gas chromatography-mass spectrometry.

Applications and Uses

Industrial and Commercial Applications

Hexatriacontanoic acid serves as a component in synthetic wax formulations, contributing to hardness, gloss, and melting point characteristics. The compound finds application in lubricant additives, where its long alkyl chain provides surface activity and film-forming properties. In cosmetic formulations, hexatriacontanoic acid functions as a consistency regulator and viscosity modifier in creams and ointments. The compound serves as a precursor for metallic stearate-like compounds with applications in plastics processing as acid scavengers and lubricants. Industrial production occurs on a limited scale, with annual global production estimated at 5-10 metric tons primarily for specialty chemical applications.

Research Applications and Emerging Uses

Research applications of hexatriacontanoic acid include its use as a model compound for studying self-assembly phenomena of long-chain amphiphiles at interfaces. The compound serves as a building block for synthesizing novel liquid crystalline materials with extended mesogenic units. Emerging applications investigate its incorporation into organic semiconductor layers in electronic devices, where its insulating properties provide dielectric functionality. The compound functions as a template for nanostructured materials synthesis through Langmuir-Blodgett techniques, creating ordered thin films with molecular-level control.

Historical Development and Discovery

The identification of hexatriacontanoic acid emerged during systematic investigations of natural wax composition in the early twentieth century. Initial isolation from plant waxes, particularly carnauba wax, provided the first samples for characterization. Development of synthetic methods in the 1930s enabled preparation of pure material for structure elucidation and property determination. Advances in chromatography during the 1950s facilitated separation and purification from complex natural mixtures. The establishment of the Lipid Maps classification system in the twenty-first century provided systematic identification and categorization within the very long-chain fatty acid group.

Conclusion

Hexatriacontanoic acid represents a structurally well-defined very long-chain saturated fatty acid with characteristic physical and chemical properties dictated by its extended hydrocarbon chain and carboxylic acid functionality. The compound exhibits typical carboxylic acid reactivity while demonstrating physical properties influenced significantly by intermolecular interactions between long alkyl chains. Synthetic accessibility enables laboratory preparation and commercial production for specialized applications in materials science and industrial chemistry. Ongoing research continues to explore novel applications in nanotechnology and advanced materials, particularly in self-assembling systems and organic electronic devices. The compound serves as a reference material for studying structure-property relationships in long-chain amphiphilic molecules.