Abstract

Ethyl (2E,4Z)-deca-2,4-dienoate, commonly known as pear ester, is an unsaturated fatty acid ester with molecular formula C₁₂H₂₀O₂ and molecular weight of 196.29 g·mol⁻¹. This organic compound exists as a colorless liquid at room temperature with a characteristic intense pear-like aroma. The compound exhibits a boiling point range of 70–72 °C at 0.05 mmHg and 81–88 °C at 0.1 mmHg, with a flash point of 113 °C. Ethyl decadienoate demonstrates limited water solubility of approximately 8.6 mg·L⁻¹ but shows good solubility in organic solvents. Its chemical structure features a conjugated diene system with specific (2E,4Z) stereochemistry that contributes to its distinctive organoleptic properties. The compound finds extensive application in flavor and fragrance industries due to its potent fruity characteristics and is classified as generally recognized as safe (GRAS) for use in food products.

Introduction

Ethyl decadienoate represents a significant compound in flavor chemistry, belonging to the class of unsaturated fatty acid esters. This organic molecule, systematically named ethyl (2E,4Z)-deca-2,4-dienoate, is characterized by its ten-carbon chain containing a conjugated diene system with specific E,Z stereochemistry. The compound occurs naturally in various fruits including apples, Bartlett pears, Concord grapes, and quince, as well as in fermented products such as beer and pear brandy. Its discovery in natural sources and subsequent structural elucidation established its role as a key aroma compound contributing to the characteristic scent of ripe pears. The compound's commercial importance stems from its intense fruity flavor profile, which has led to widespread use in food flavoring and fragrance applications. Structural analysis confirms the molecular formula C₁₂H₂₀O₂ with CAS registry number 3025-30-7.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Ethyl decadienoate possesses a well-defined molecular geometry characterized by a conjugated diene system within a ten-carbon aliphatic chain terminated by an ethyl ester functionality. The compound exhibits specific stereochemistry with E configuration at the C2-C3 double bond and Z configuration at the C4-C5 double bond, creating a (2E,4Z)-deca-2,4-dienoate system. This geometric arrangement results in a partially planar conjugated system extending from C2 to C5, with bond angles of approximately 120° at each sp² hybridized carbon atom. The ester carbonyl carbon demonstrates sp² hybridization with bond angles of 120°, while the remaining carbon atoms in the alkyl chain exhibit sp³ hybridization with tetrahedral geometry and bond angles of approximately 109.5°. The electronic structure features π molecular orbitals delocalized across the conjugated diene system, with highest occupied molecular orbital (HOMO) density concentrated across the C2-C5 region and lowest unoccupied molecular orbital (LUMO) density primarily on the carbonyl group and conjugated system.

Chemical Bonding and Intermolecular Forces

The covalent bonding pattern in ethyl decadienoate consists of carbon-carbon single bonds with typical lengths of 1.54 Å in the alkyl chain, carbon-carbon double bonds of 1.34 Å in the diene system, and carbon-oxygen bonds of 1.20 Å for the carbonyl group and 1.34 Å for the ester C-O bond. The conjugated diene system exhibits bond length alternation with C2-C3 and C4-C5 double bonds slightly elongated compared to isolated double bonds due to conjugation effects. Intermolecular forces are dominated by London dispersion forces arising from the extended hydrocarbon chain, with additional dipole-dipole interactions resulting from the polar ester functional group. The molecular dipole moment measures approximately 1.8–2.0 Debye, oriented along the carbonyl bond axis with partial charge separation between oxygen and carbon atoms. Van der Waals forces between hydrocarbon chains contribute significantly to the compound's physical properties, while the absence of hydrogen bond donors limits hydrogen bonding interactions to acceptor capabilities of the carbonyl oxygen.

Physical Properties

Phase Behavior and Thermodynamic Properties

Ethyl decadienoate exists as a colorless liquid at standard temperature and pressure conditions. The compound demonstrates a boiling point range dependent on pressure conditions: 70–72 °C at 0.05 mmHg, 81–82 °C at 0.1 mmHg, and 83–88 °C at 0.1 mmHg in various reported measurements. The flash point is established at 113 °C, indicating moderate flammability characteristics. Density measurements report values of approximately 0.89–0.91 g·cm⁻³ at 20 °C, consistent with typical fatty acid esters. The refractive index ranges from 1.47 to 1.49 at 20 °C, reflecting the compound's polar character and molecular structure. Thermodynamic parameters include an estimated heat of vaporization of 45–50 kJ·mol⁻¹ and heat of combustion of approximately 6500 kJ·mol⁻¹. The compound exhibits limited water solubility of 8.588 mg·L⁻¹ but demonstrates complete miscibility with most common organic solvents including ethanol, diethyl ether, and hexane.

Spectroscopic Characteristics

Infrared spectroscopy of ethyl decadienoate reveals characteristic absorption bands at 2950–2850 cm⁻¹ for C-H stretching vibrations, 1745 cm⁻¹ for ester carbonyl stretching, 1650 cm⁻¹ and 1600 cm⁻¹ for conjugated diene C=C stretching vibrations, and 1250–1150 cm⁻¹ for C-O stretching vibrations. Proton nuclear magnetic resonance spectroscopy displays distinctive signals: δ 0.88 ppm (t, 3H, CH₃), δ 1.28 ppm (t, 3H, OCH₂CH₃), δ 1.30–1.45 ppm (m, 4H, CH₂), δ 2.15–2.30 ppm (m, 2H, CH₂C=C), δ 4.12 ppm (q, 2H, OCH₂), δ 5.80–6.40 ppm (m, 4H, CH=CH), and δ 7.00 ppm (dd, 1H, CH=CHCO). Carbon-13 NMR spectroscopy shows signals at δ 14.1 ppm (CH₃), δ 14.3 ppm (OCH₂CH₃), δ 22.6 ppm, δ 28.9 ppm, δ 31.6 ppm (CH₂), δ 60.2 ppm (OCH₂), δ 121.5 ppm, δ 128.7 ppm, δ 130.2 ppm, δ 144.5 ppm (CH=CH), and δ 166.8 ppm (C=O). Mass spectrometry exhibits a molecular ion peak at m/z 196 with characteristic fragmentation patterns including m/z 151 [M-OCH₂CH₃]⁺, m/z 123 [M-CH₂CH₂CH₂CH₃]⁺, and m/z 81 [C₆H₉]⁺.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Ethyl decadienoate demonstrates reactivity patterns characteristic of α,β-unsaturated esters with extended conjugation. The compound undergoes electrophilic addition reactions across the diene system, with preferential attack at the terminal carbon of the conjugated system. Diels-Alder reactions proceed readily with suitable dienophiles, exploiting the conjugated diene system as an electron-rich diene component. Hydrogenation reactions occur catalytically with palladium or platinum catalysts, selectively reducing the double bonds to yield ethyl decanoate under mild conditions or fully saturated ethyl decanoate under vigorous conditions. Hydrolysis reactions proceed under both acidic and basic conditions, with alkaline hydrolysis demonstrating second-order kinetics with rate constants of approximately 10⁻³–10⁻⁴ L·mol⁻¹·s⁻¹ at room temperature. The compound exhibits stability under neutral conditions but undergoes autoxidation upon prolonged exposure to air, with oxidation primarily occurring at the allylic positions within the diene system. Photochemical reactivity includes [2+2] cycloadditions and E-Z isomerization upon exposure to ultraviolet radiation.

Acid-Base and Redox Properties

Ethyl decadienoate exhibits no significant acid-base character in aqueous solution, with the ester functionality demonstrating extremely weak acidity (estimated pKa > 25) and no basic properties. The compound remains stable across the pH range of 3–9, with hydrolysis becoming significant outside this range. Redox properties include susceptibility to reduction by hydride reagents such as lithium aluminum hydride, which reduces both the ester functionality and double bonds to yield decan-1-ol. Oxidation reactions occur selectively at the allylic positions using reagents such as selenium dioxide or tert-butyl hydroperoxide, yielding corresponding allylic alcohols or oxidation products. The compound demonstrates electrochemical reduction at approximately -1.8 V versus standard calomel electrode, corresponding to reduction of the conjugated system. No significant oxidation waves are observed within the accessible potential range of common solvents, indicating stability toward anodic oxidation.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory synthesis of ethyl decadienoate typically proceeds through several established routes with emphasis on stereochemical control of the (2E,4Z) diene system. The most common synthetic approach involves Wittig-type reactions between appropriately functionalized phosphonium salts and aldehydes. One efficient method utilizes ethyl 4-phosphonocrotonate with hexanal under Horner-Wadsworth-Emmons conditions, yielding the target compound with high stereoselectivity for the (2E,4Z) configuration. Reaction conditions typically employ sodium hydride or potassium tert-butoxide as base in anhydrous tetrahydrofuran at 0 °C to room temperature, providing yields of 70–85%. Alternative synthetic routes include condensation reactions between ethyl propiolate and octanal derivatives, though these methods often produce mixtures of stereoisomers requiring chromatographic separation. A more recent methodology employs cross-metathesis reactions between ethyl sorbate and appropriate alkene partners using Grubbs-type catalysts, offering improved atom economy and stereocontrol. Purification typically involves fractional distillation under reduced pressure or column chromatography on silica gel, with final product characterization by gas chromatography-mass spectrometry and nuclear magnetic resonance spectroscopy.

Analytical Methods and Characterization

Identification and Quantification

Analytical identification of ethyl decadienoate primarily employs gas chromatography coupled with mass spectrometry (GC-MS) due to the compound's volatility and thermal stability. Capillary GC columns with non-polar stationary phases such as DB-5 or equivalent provide excellent separation with retention indices of approximately 1450–1500 under standard conditions. Mass spectrometric detection utilizes electron impact ionization at 70 eV, with characteristic fragment ions at m/z 196 (M⁺, 15%), 151 (35%), 123 (20%), 108 (45%), 93 (60%), 81 (100%), and 67 (40%). Quantitative analysis typically employs internal standard methodology with compounds such as ethyl nonanoate or ethyl undecanoate as references, with detection limits of approximately 0.1 mg·L⁻¹ using flame ionization detection and 0.01 mg·L⁻¹ using mass spectrometric detection in selected ion monitoring mode. High-performance liquid chromatography with reverse-phase C18 columns and UV detection at 210 nm provides alternative quantification methods, though with lower sensitivity compared to GC-based methods.

Purity Assessment and Quality Control

Purity assessment of ethyl decadienoate focuses primarily on stereochemical purity and absence of oxidation products. Chiral gas chromatography on cyclodextrin-based stationary phases confirms the (2E,4Z) stereochemical configuration and detects potential stereoisomers including (2E,4E), (2Z,4E), and (2Z,4Z) isomers, which typically constitute less than 1% in high-quality samples. Impurity profiling identifies common contaminants including ethyl decanoate (saturation product), ethyl 4-oxo-deca-2-enoate (oxidation product), and various positional isomers. Quality control specifications for food-grade material typically require minimum 98% chemical purity by GC-FID, with individual impurities limited to less than 0.5% and total impurities less than 2.0%. Organoleptic evaluation confirms the characteristic pear-like aroma without off-notes indicative of degradation products. Stability testing under accelerated conditions (40 °C, 75% relative humidity) demonstrates shelf life exceeding 24 months when stored in sealed containers under inert atmosphere with protection from light.

Applications and Uses

Industrial and Commercial Applications

Ethyl decadienoate finds extensive application in flavor and fragrance industries due to its intense, natural pear-like aroma characteristics. The compound serves as a key ingredient in fruit flavor formulations, particularly for pear, apple, and tropical fruit profiles, with typical usage levels of 5–50 ppm in finished food products. In fragrance applications, it contributes to top notes in perfumery compositions, providing fresh fruity accents in floral and fantasy fragrance types. The compound's classification as generally recognized as safe (GRAS) by regulatory authorities enables widespread use in food products including beverages, confectionery, dairy products, and baked goods. Commercial production meets annual demand estimated at 10–20 metric tons worldwide, with primary manufacturing facilities located in Europe, United States, and Asia. The compound's stability in various food matrices and compatibility with other flavor ingredients makes it particularly valuable for creating complex flavor profiles in processed foods.

Historical Development and Discovery

The identification of ethyl decadienoate as a significant aroma compound emerged from systematic studies of fruit volatiles in the mid-20th century. Initial investigations of pear aroma composition in the 1960s identified this compound as a major contributor to the characteristic scent of Bartlett pears. Structural elucidation efforts employing classical degradation methods and emerging spectroscopic techniques established the molecular structure as ethyl (2E,4Z)-deca-2,4-dienoate. The compound's common name "pear ester" reflects its organoleptic properties and natural occurrence. Synthetic methodologies developed throughout the 1970s enabled commercial production, with early routes focusing on condensation reactions between acetylenic precursors and carbonyl compounds. Advances in stereoselective synthesis during the 1980s and 1990s provided efficient routes to the specific (2E,4Z) stereoisomer, ensuring production of material with organoleptic properties identical to the natural compound. The establishment of analytical standards and sensory thresholds in the 1990s facilitated quality control and standardized application in flavor formulations.

Conclusion

Ethyl decadienoate represents a chemically interesting and commercially valuable compound that exemplifies the intersection of organic chemistry and sensory science. Its well-defined molecular structure featuring a conjugated diene system with specific E,Z configuration governs both its chemical reactivity and organoleptic properties. The compound's physical characteristics, including volatility and limited water solubility, make it ideally suited for flavor and fragrance applications. Established synthetic methodologies provide efficient access to material meeting stringent quality requirements for food use. Analytical techniques reliably characterize the compound and detect potential impurities that might affect sensory performance. Ongoing research continues to refine production methods and explore potential applications in areas such as insect attractants and green chemistry. The compound's status as a naturally occurring flavor compound with GRAS classification ensures its continued importance in food and fragrance industries, while its well-studied chemistry provides a model system for understanding structure-property relationships in conjugated unsaturated esters.