Abstract

Myristoleic acid, systematically named (9Z)-tetradec-9-enoic acid, is a monounsaturated fatty acid with the molecular formula C₁₄H₂₆O₂. This fourteen-carbon carboxylic acid features a cis double bond at the Δ9 position, classifying it as an omega-5 fatty acid. The compound exhibits characteristic physical properties including a melting point of -4°C and boiling point of approximately 225°C at 15 mmHg. Myristoleic acid demonstrates typical carboxylic acid reactivity including esterification, saponification, and hydrogenation reactions. Spectroscopic characterization reveals distinctive infrared absorption bands at 1710 cm⁻¹ for the carbonyl stretch and 3005 cm⁻¹ for the cis-alkene C-H stretch. The compound serves as an important intermediate in organic synthesis and finds applications in specialty chemical manufacturing.

Introduction

Myristoleic acid represents a significant member of the monounsaturated fatty acid family, distinguished by its fourteen-carbon chain with a single cis double bond. Classified as an organic carboxylic acid, this compound belongs to the broader category of alkenoic acids. The systematic IUPAC nomenclature identifies it as (9Z)-tetradec-9-enoic acid, precisely describing both the chain length and stereochemistry of the unsaturated center. While less common than its saturated analog myristic acid, myristoleic acid maintains importance in chemical research due to its structural features that bridge the properties of saturated and polyunsaturated fatty acids.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

The molecular structure of myristoleic acid consists of a fourteen-carbon aliphatic chain with a carboxylic acid functional group at one terminus and a cis double bond between carbons 9 and 10. The carboxylic acid group exhibits planar geometry with bond angles of approximately 120° around the carbonyl carbon, consistent with sp² hybridization. The cis configuration of the double bond introduces a 30° bend in the hydrocarbon chain, significantly influencing the molecule's overall conformation and packing behavior. Molecular orbital analysis reveals that the highest occupied molecular orbital localizes primarily on the carboxylic oxygen atoms and the π-system of the double bond, while the lowest unoccupied molecular orbital demonstrates antibonding character between the carbonyl carbon and oxygen.

Chemical Bonding and Intermolecular Forces

Covalent bonding in myristoleic acid follows typical patterns for carboxylic acids with a carbonyl carbon-oxygen bond length of 1.21 Å and carbon-oxygen single bond length of 1.36 Å. The C9=C10 double bond measures 1.33 Å with a bond dissociation energy of approximately 264 kJ/mol. Intermolecular forces include strong hydrogen bonding between carboxylic acid dimers with an association energy of approximately 30 kJ/mol, as well as significant London dispersion forces along the hydrocarbon chain. The calculated dipole moment measures 1.7 Debye, oriented along the carboxylic acid group with minor contribution from the bent hydrocarbon chain. These intermolecular interactions significantly influence the compound's physical properties including its relatively low melting point compared to saturated analogs.

Physical Properties

Phase Behavior and Thermodynamic Properties

Myristoleic acid exists as a colorless to pale yellow liquid at room temperature with a characteristic fatty odor. The compound solidifies at -4°C and boils at 225°C under reduced pressure of 15 mmHg. At atmospheric pressure, decomposition precedes boiling. The density measures 0.895 g/cm³ at 20°C. Thermodynamic parameters include a heat of vaporization of 85 kJ/mol and heat of fusion of 35 kJ/mol. The specific heat capacity at constant pressure measures 2.1 J/g·K near room temperature. The refractive index is 1.451 at 20°C using the sodium D-line. These properties reflect the compound's intermediate position between fully saturated fatty acids and more highly unsaturated analogs.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorption bands at 1710 cm⁻¹ corresponding to the carbonyl stretching vibration, 3005 cm⁻¹ for the cis-alkene C-H stretch, and 1280-1320 cm⁻¹ for the C-O stretching vibrations. The broad O-H stretching absorption appears centered at 3000 cm⁻¹. Proton NMR spectroscopy shows distinctive signals: δ 0.88 ppm (t, 3H, terminal CH₃), δ 1.25 ppm (m, 16H, methylene chain), δ 2.00 ppm (m, 4H, CH₂-C=), δ 2.34 ppm (t, 2H, CH₂-COOH), δ 5.35 ppm (m, 2H, CH=CH), and δ 11.0 ppm (s, 1H, COOH). Carbon-13 NMR displays signals at δ 14.1 ppm (CH₃), δ 22.6-34.2 ppm (methylene carbons), δ 129.8 and 130.1 ppm (olefinic carbons), and δ 180.2 ppm (carbonyl carbon). Mass spectrometry exhibits a molecular ion peak at m/z 226 with characteristic fragmentation patterns including loss of water (m/z 208) and cleavage adjacent to the double bond.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Myristoleic acid undergoes characteristic carboxylic acid reactions including esterification with alcohols under acid catalysis, with second-order rate constants typically ranging from 10⁻⁴ to 10⁻³ L·mol⁻¹·s⁻¹ depending on the alcohol nucleophile. The compound demonstrates base-catalyzed saponification with a rate constant of approximately 0.1 L·mol⁻¹·s⁻¹ at 25°C. Hydrogenation of the double bond proceeds with palladium catalyst at rates of 50-100 L·mol⁻¹·s⁻¹ under mild conditions. Oxidation reactions occur readily at the double bond position with potassium permanganate or ozone, leading to cleavage products. Thermal stability extends to approximately 150°C, above which decarboxylation becomes significant with an activation energy of 120 kJ/mol.

Acid-Base and Redox Properties

As a carboxylic acid, myristoleic acid exhibits typical acid-base behavior with a pKa of 4.9 in aqueous solution at 25°C. The compound forms stable salts with alkali metals and ammonium ions. Buffer capacity is maximal in the pH range 3.9-5.9. Redox properties include a standard reduction potential of -0.5 V for the carboxylic acid group. The double bond undergoes electrophilic addition reactions with halogens and hydrogen halides, with reaction rates influenced by the electron-donating character of the alkyl chain. Stability in alkaline conditions is good, while strong oxidizing conditions lead to degradation of both the carboxylic acid and alkene functionalities.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of myristoleic acid typically proceeds through one of three primary routes: partial hydrogenation of myristoleic acid derivatives, dehydration of hydroxy derivatives, or chain extension of shorter unsaturated acids. The most efficient laboratory method involves the Wittig reaction between nonanal and the ylide generated from (carbethoxymethyl)triphenylphosphonium bromide, followed by saponification of the resulting ester. This method produces the cis isomer with 90% stereoselectivity and overall yields of 65-75%. Alternative approaches include the partial hydrogenation of tetradec-9-ynoic acid with Lindlar's catalyst, which affords the cis isomer with 95% selectivity but requires additional synthetic steps to prepare the alkyne precursor. Purification typically employs fractional distillation under reduced pressure or recrystallization from acetone at low temperatures.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides the primary method for identification and quantification of myristoleic acid, using polar stationary phases such as cyanopropyl polysiloxane. Retention indices typically range from 1650-1700 on such columns at programmed temperature conditions. High-performance liquid chromatography with UV detection at 200 nm offers an alternative method, particularly for thermally labile samples. Fourier transform infrared spectroscopy confirms identity through characteristic carbonyl and alkene absorptions. Proton NMR spectroscopy provides definitive structural confirmation through the distinctive pattern of olefinic and methylene protons. Quantitative analysis achieves detection limits of 0.1 μg/mL by GC-MS using selected ion monitoring at m/z 226.

Purity Assessment and Quality Control

Purity assessment employs differential scanning calorimetry to measure melting behavior, with pure myristoleic acid exhibiting a sharp melting endotherm at -4°C. Acid value titration determines carboxylic acid content, with pure material exhibiting an acid value of 248 mg KOH/g. Peroxide value measurements assess oxidative stability, with fresh samples typically showing values below 5 meq/kg. Common impurities include the saturated analog myristic acid, positional isomers of the double bond, and trans isomers formed during processing. Quality specifications for research-grade material typically require minimum purity of 98% by GC, acid value between 247-249 mg KOH/g, and peroxide value below 10 meq/kg.

Applications and Uses

Industrial and Commercial Applications

Myristoleic acid serves as a specialty chemical intermediate in the production of surfactants, lubricants, and cosmetic ingredients. Ester derivatives find application as emollients in personal care products due to their favorable spreading characteristics and skin feel. The compound functions as a building block for synthesizing more complex molecules including pheromones and fragrance compounds. Metal salts of myristoleic acid demonstrate utility as lubricant additives and corrosion inhibitors. The annual global production is estimated at 10-20 metric tons, primarily serving niche applications where specific chain length and unsaturation pattern provide advantageous properties compared to more common fatty acids.

Conclusion

Myristoleic acid represents a chemically interesting monounsaturated fatty acid with distinctive structural features that influence its physical properties and chemical behavior. The cis configuration at the Δ9 position differentiates it from saturated analogs and contributes to its liquid state at room temperature and modified reactivity pattern. Well-established synthetic routes enable laboratory preparation with high stereochemical control, while analytical methods provide comprehensive characterization of purity and identity. Applications leverage the compound's specific structural attributes in specialty chemical contexts. Further research opportunities exist in developing more efficient synthetic methodologies and exploring new applications that exploit the unique combination of carboxylic acid functionality and cis-alkene geometry in a fourteen-carbon framework.