Abstract

Erucic acid, systematically named (13Z)-docos-13-enoic acid, is a monounsaturated omega-9 fatty acid with the molecular formula C₂₂H₄₂O₂ and a molar mass of 338.57 g·mol^-1. This long-chain carboxylic acid exists as a white waxy solid at room temperature with a characteristic melting point of 33.8 °C and boiling point of 381.5 °C. The compound demonstrates limited water solubility but dissolves readily in organic solvents including methanol and ethanol. Erucic acid exhibits a density of 0.860 g·cm^-3 and a flash point of 349.9 °C. Industrially significant, this fatty acid serves as a precursor to brassylic acid through ozonolysis and finds applications in polymer manufacturing, lubricant formulations, and surfactant production. The compound occurs naturally in various Brassicaceae family plants, particularly in high-erucic acid rapeseed oil where it constitutes 20-54% of the fatty acid content.

Introduction

Erucic acid represents a significant monounsaturated long-chain fatty acid within organic chemistry and industrial applications. Classified as an omega-9 fatty acid with the lipid number designation 22:1ω9, this compound belongs to the broader category of alkenoic acids characterized by a single cis double bond at the thirteenth carbon. The systematic IUPAC nomenclature identifies the compound as (13Z)-docos-13-enoic acid, reflecting its 22-carbon chain length and specific double bond position. Historically identified in various Brassica species, erucic acid derives its name from the Eruca genus of flowering plants, particularly Eruca sativa. Industrial interest in this compound emerged during the mid-twentieth century with the development of high-erucic acid rapeseed varieties for non-food applications. The compound's structural features, particularly the extended hydrocarbon chain with strategic unsaturation, confer unique chemical properties that facilitate diverse industrial transformations and applications.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of erucic acid consists of a 22-carbon alkyl chain with a carboxylic acid functional group at the first carbon position and a cis double bond between carbons 13 and 14. The carbon atoms adopt sp³ hybridization throughout the saturated regions of the molecule, with bond angles approximating the tetrahedral value of 109.5°. The double bond region features sp² hybridized carbon atoms with bond angles of approximately 120°. The cis configuration at the double bond introduces a 30° bend in the molecular structure, significantly influencing packing efficiency and intermolecular interactions. The carboxylic acid group exhibits planar geometry with sp² hybridization at the carbonyl carbon. Electronic distribution analysis reveals polarization at the carboxylic acid functionality with calculated dipole moments ranging from 1.6-1.8 Debye. The extended hydrocarbon chain demonstrates predominantly non-polar character with minimal electron density variation along the saturated segments.

Chemical Bonding and Intermolecular Forces

Covalent bonding in erucic acid follows typical patterns for long-chain carboxylic acids. Carbon-carbon bond lengths measure 1.54 Å in saturated regions and 1.34 Å at the double bond position. Carbon-oxygen bonds in the carboxylic acid group measure 1.36 Å for the C-O bond and 1.23 Å for the C=O bond. The O-H bond length measures 0.97 Å. Bond dissociation energies approximate 368 kJ·mol^-1 for C-H bonds, 347 kJ·mol^-1 for C-C bonds, and 439 kJ·mol^-1 for O-H bonds. Intermolecular forces include strong hydrogen bonding between carboxylic acid groups with interaction energies of 25-30 kJ·mol^-1. Van der Waals interactions between hydrocarbon chains contribute significantly to the compound's physical properties with dispersion forces measuring 0.5-4.0 kJ·mol^-1 per methylene unit. The cis configuration at the double bond prevents efficient crystalline packing, resulting in lower melting points compared to saturated analogs.

Physical Properties

Phase Behavior and Thermodynamic Properties

Erucic acid presents as a white waxy solid at ambient temperature with a characteristic crystalline structure. The compound melts at 33.8 °C to form a clear, colorless liquid. Boiling occurs at 381.5 °C with concomitant decomposition observed at elevated temperatures. The density of solid erucic acid measures 0.860 g·cm^-3 at 20 °C, decreasing to 0.849 g·cm^-3 in the liquid state at 40 °C. Thermodynamic parameters include a heat of fusion of 53.2 kJ·mol^-1 and heat of vaporization of 92.5 kJ·mol^-1. The specific heat capacity measures 2.15 J·g^-1·K^-1 for the solid phase and 2.38 J·g^-1·K^-1 for the liquid phase. The refractive index of liquid erucic acid is 1.447 at 40 °C and 589 nm wavelength. The compound exhibits limited water solubility (<0.01 g·L^-1 at 25 °C) but demonstrates high solubility in polar organic solvents including methanol (125 g·L^-1), ethanol (98 g·L^-1), and chloroform (156 g·L^-1).

Spectroscopic Characteristics

Infrared spectroscopy of erucic acid reveals characteristic absorption bands at 3005 cm^-1 (=C-H stretch), 2920 cm^-1 and 2850 cm^-1 (C-H stretch), 1705 cm^-1 (C=O stretch), and 1280 cm^-1 (C-O stretch). The cis double bond produces a distinctive out-of-plane bending vibration at 720 cm^-1. Proton nuclear magnetic resonance spectroscopy shows signals at δ 0.88 ppm (t, 3H, CH₃), δ 1.25 ppm (m, 28H, CH₂), δ 1.62 ppm (m, 2H, CH₂CH₂CO), δ 2.00 ppm (m, 4H, CH₂CH=CHCH₂), δ 2.34 ppm (t, 2H, CH₂CO), δ 5.34 ppm (t, 2H, CH=CH), and δ 11.2 ppm (s, 1H, COOH). Carbon-13 NMR exhibits signals at δ 14.1 ppm (CH₃), δ 22.7-34.2 ppm (CH₂), δ 129.7 ppm and δ 130.0 ppm (CH=CH), and δ 180.2 ppm (COOH). Mass spectrometry demonstrates a molecular ion peak at m/z 338 with characteristic fragmentation patterns including m/z 339 (M+H)⁺, m/z 321 (M-OH)⁺, and m/z 265 (M-COOH)⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Erucic acid undergoes characteristic carboxylic acid reactions including esterification, amidation, and reduction. Esterification with methanol catalyzed by sulfuric acid proceeds with a rate constant of 3.2×10^-4 L·mol^-1·s^-1 at 60 °C and activation energy of 65 kJ·mol^-1. Amidation reactions with ammonia demonstrate second-order kinetics with rate constants of 2.8×10^-5 L·mol^-1·s^-1 at 25 °C. Catalytic hydrogenation of the double bond proceeds with palladium catalyst at rates of 0.15 mol·L^-1·min^-1 under 3 atm hydrogen pressure at 80 °C, yielding behenic acid. Ozonolysis cleaves the double bond selectively with reaction rates of 2.4×10^-3 s^-1 at -78 °C in dichloromethane, producing pelargonic acid and brassylic acid. The compound demonstrates stability under ambient conditions but undergoes autoxidation at the allylic positions with rate constants of 0.12 M^-1·s^-1 for oxygen uptake at 30 °C.

Acid-Base and Redox Properties

Erucic acid behaves as a typical carboxylic acid with a pK_a value of 4.78 in aqueous solution at 25 °C. The acid dissociation constant decreases slightly with increasing temperature with ΔpK_a/ΔT = -0.012 K^-1. Buffer capacity maximizes in the pH range 4.0-5.8 with optimal buffering at pH 4.78. The compound demonstrates stability across pH ranges from 2-10 with hydrolysis rates below 0.1% per year at 25 °C. Redox properties include a standard reduction potential of -0.42 V for the COOH/CHO couple in acetonitrile. Electrochemical oxidation occurs at +1.23 V versus standard hydrogen electrode with single-electron transfer mechanisms. The double bond undergoes epoxidation with peracids with rate constants of 1.8×10^-3 L·mol^-1·s^-1 using m-chloroperbenzoic acid in chloroform at 25 °C.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of erucic acid typically proceeds through elongation of oleic acid or other shorter-chain unsaturated fatty acids. The most efficient method involves malonate-based chain elongation of oleoyl chloride. The synthesis begins with conversion of oleic acid to oleoyl chloride using thionyl chloride at 60 °C for 2 hours. The resulting acid chloride undergoes reaction with malonic acid in pyridine at 0-5 °C followed by decarboxylation at 120 °C to yield erucic acid with overall yields of 75-80%. Alternative routes employ Wittig reactions between tridecyl triphenylphosphonium bromide and methyl 9-oxononanoate followed by hydrolysis. This method produces erucic acid with 65-70% yield and excellent stereoselectivity for the cis isomer. Purification typically involves recrystallization from acetone at -20 °C or fractional distillation under reduced pressure (0.1 mmHg, 200 °C). Laboratory-scale preparations achieve purities exceeding 99.5% as determined by gas chromatography.

Industrial Production Methods

Industrial production of erucic acid primarily utilizes extraction from high-erucic acid rapeseed (HEAR) oil through fractional distillation and crystallization. The process begins with mechanical pressing of rapeseed to obtain crude oil containing 45-50% erucic acid. The oil undergoes winterization at 4 °C to remove saturated triglycerides followed by transesterification with methanol using sodium methoxide catalyst at 60 °C. The resulting fatty acid methyl esters undergo fractional distillation at 180-220 °C under 0.5 mmHg pressure to separate methyl erucate from other esters. Subsequent hydrolysis with aqueous sodium hydroxide at 80 °C followed by acidification with hydrochloric acid yields crude erucic acid. Final purification employs recrystallization from hexane at -10 °C to achieve industrial grade purity of 98-99%. Global production estimates approximate 50,000 metric tons annually with major production facilities in Europe, Canada, and China. Production costs range from $2.50-3.50 per kilogram depending on rapeseed market prices and energy costs.

Analytical Methods and Characterization

Identification and Quantification

Analytical identification of erucic acid employs gas chromatography with flame ionization detection using polar stationary phases such as SP-2560 or CP-Sil 88 capillary columns. Retention indices relative to methyl esters of saturated fatty acids range from 22.0-22.5 under standard conditions (180 °C isothermal). Quantification utilizes internal standard methodology with methyl heptadecanoate as reference compound. Detection limits reach 0.01% with linear response ranges from 0.1-100%. High-performance liquid chromatography with evaporative light scattering detection provides alternative quantification methods using C18 reverse-phase columns with mobile phases of acetonitrile/water/acetic acid (80:20:0.1 v/v). Infrared spectroscopy offers complementary identification through characteristic carbonyl and double bond absorptions. Nuclear magnetic resonance spectroscopy enables structural confirmation through analysis of olefinic proton signals at δ 5.34 ppm and methylene proton integration patterns.

Purity Assessment and Quality Control

Purity assessment of erucic acid employs differential scanning calorimetry to determine melting point depression and impurity content. Pharmaceutical grade specifications require minimum purity of 99.5% with limits on related substances including brassidic acid (<0.3%), behenic acid (<0.2%), and other fatty acids (<0.5%). Industrial specifications vary by application with polymer grade requiring erucic acid content exceeding 98% and acid values of 165-175 mg KOH·g^-1. Iodine values measure 74-76 g I₂·100 g^-1 confirming the presence of one double bond. Peroxide values must not exceed 5 mEq·kg^-1 for stability considerations. Storage conditions recommend protection from light and oxygen at temperatures below 25 °C to prevent oxidation. Shelf life under nitrogen atmosphere exceeds 24 months with proper storage conditions.

Applications and Uses

Industrial and Commercial Applications

Erucic acid serves as a crucial industrial feedstock for the production of brassylic acid through ozonolysis, with annual global consumption exceeding 20,000 metric tons for this application. The dicarboxylic acid product finds extensive use in manufacturing specialty polyamides (nylon 1313) and polyesters with superior mechanical properties and chemical resistance. Amide derivatives, particularly erucamide, function as effective slip agents and lubricants in plastic films with annual consumption of approximately 15,000 metric tons worldwide. Hydrogenation produces behenyl alcohol, utilized as a pour point depressant in lubricating oils and as a thickener in personal care products. Silver behenate derivatives serve as important precursors in photographic emulsion technology. The compound's surface-active properties make it valuable in surfactant formulations, particularly as erucic acid-based imidazolines for corrosion inhibition. Market demand remains stable with growth rates of 2-3% annually driven by polymer and lubricant applications.

Research Applications and Emerging Uses

Research applications of erucic acid focus on its transformation into value-added chemicals through selective chemical modifications. Catalytic deoxygenation routes show promise for producing long-chain hydrocarbons suitable as diesel fuel additives with cetane numbers exceeding 70. Metathesis reactions with ethylene generate α,ω-diesters for polymer applications with molecular weights controllable through reaction conditions. Electrochemical reduction methods are being developed for producing erucyl alcohol with Faradaic efficiencies exceeding 85%. Emerging applications include use as a phase change material for thermal energy storage due to its favorable melting point and high latent heat of fusion. Nanotechnology applications explore erucic acid as a surface modification agent for nanoparticles to enhance dispersion in hydrophobic matrices. Patent analysis indicates growing intellectual property activity in catalytic processes for erucic acid transformation, with 45 new patents filed internationally in the past three years.

Historical Development and Discovery

The identification of erucic acid traces back to early investigations of rapeseed oil composition in the late 19th century. German chemists first isolated the compound in 1891 from rapeseed oil through fractional crystallization of the fatty acids. The structural elucidation proceeded through chemical degradation studies, with ozonolysis experiments in 1925 confirming the double bond position at carbon 13. The cis configuration was established through comparison with synthetic materials in 1934. Industrial interest emerged during World War II when erucic acid derivatives were evaluated as lubricants for military applications. The 1960s saw significant development of analytical methods for erucic acid quantification, particularly gas chromatographic techniques that enabled precise measurement in oil samples. The 1970s brought regulatory attention to erucic acid content in food products, leading to the development of low-erucic acid rapeseed varieties. Recent decades have witnessed advances in catalytic processes for erucic acid transformation, particularly metathesis and ozonolysis technologies that enable efficient production of specialty chemicals from this renewable resource.

Conclusion

Erucic acid represents a structurally distinctive monounsaturated fatty acid with significant industrial importance derived from its unique combination of long hydrocarbon chain and strategic double bond positioning. The compound's physical properties, including its relatively low melting point and high boiling point, facilitate processing and purification operations. Chemical reactivity patterns enable diverse transformations including ozonolysis to dicarboxylic acids, hydrogenation to saturated derivatives, and derivatization to amides and esters. Industrial applications span polymer precursors, lubricant additives, and surfactant formulations with established market presence. Ongoing research focuses on catalytic transformation routes to value-added products and emerging applications in materials science. The compound's renewable origin from vegetable oil sources aligns with growing emphasis on sustainable chemical feedstocks. Future developments will likely focus on process intensification for existing applications and exploration of new chemical transformations leveraging the molecule's unique structural features.