Abstract

Benzyl cinnamate, systematically named benzyl (2E)-3-phenylprop-2-enoate, is an organic ester compound with molecular formula C₁₆H₁₄O₂ and molecular weight of 238.29 g·mol⁻¹. This crystalline solid exhibits a melting point range of 34–37 °C and boiling point of 195–200 °C at 5 mmHg. The compound demonstrates limited aqueous solubility but dissolves readily in ethanol at concentrations up to 125 g·L⁻¹. Benzyl cinnamate occurs naturally in balsam resins and finds extensive application as a flavoring agent and fragrance component. Its chemical structure features conjugated π-systems that contribute to distinctive spectroscopic properties, including strong UV absorption maxima between 270–290 nm. The ester functionality renders the compound susceptible to both acid- and base-catalyzed hydrolysis, yielding cinnamic acid and benzyl alcohol as hydrolysis products.

Introduction

Benzyl cinnamate represents a significant member of the cinnamate ester family, characterized by the combination of cinnamic acid and benzyl alcohol moieties. This organic compound belongs to the class of unsaturated aromatic esters that exhibit both fragrance characteristics and chemical reactivity patterns typical of conjugated systems. The compound's dual aromatic systems connected through an ester linkage create a molecular architecture that demonstrates interesting electronic properties and reactivity patterns. Industrial production of benzyl cinnamate serves multiple sectors including flavor and fragrance industries, where it functions as a key component in various formulations. The compound's stability under normal storage conditions and compatibility with numerous organic matrices contributes to its widespread utilization across chemical applications.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Benzyl cinnamate adopts an extended molecular geometry with approximate dimensions of 1.5 nm in length and 0.7 nm in width. The cinnamate moiety exhibits planarity due to conjugation between the carbonyl group and the vinylbenzene system, with the C=C-C=O dihedral angle measuring approximately 0° indicating complete conjugation. The benzyl group rotates freely around the C(sp³)-O bond with a rotational barrier of approximately 12 kJ·mol⁻¹. Molecular orbital calculations indicate highest occupied molecular orbital (HOMO) localization primarily on the cinnamate π-system while the lowest unoccupied molecular orbital (LUMO) shows significant carbonyl character. The compound crystallizes in the monoclinic crystal system with space group P2₁/c and unit cell parameters a = 14.23 Å, b = 5.67 Å, c = 16.89 Å, and β = 115.7°.

Chemical Bonding and Intermolecular Forces

The molecular structure features typical ester bonding with C-O bond lengths of 1.36 Å for the C(sp²)-O bond and 1.43 Å for the O-C(sp³) bond. The carbonyl bond length measures 1.21 Å, consistent with conjugated ester systems. The vinyl bond in the cinnamate moiety measures 1.34 Å, indicating significant double bond character with partial conjugation. Intermolecular forces in crystalline benzyl cinnamate include van der Waals interactions with minimum contact distances of 3.5–4.2 Å between aromatic rings. The calculated dipole moment measures 2.1 Debye with orientation along the carbonyl bond axis. London dispersion forces dominate intermolecular interactions in the liquid phase, with calculated Hamaker constant of 6.5 × 10⁻²⁰ J for benzyl cinnamate-benzyl cinnamate interactions.

Physical Properties

Phase Behavior and Thermodynamic Properties

Benzyl cinnamate presents as white to pale yellow crystalline solid at room temperature with density of 1.12 g·cm⁻³ in solid form and 1.08 g·cm⁻³ in liquid form at 40 °C. The compound melts at 34–37 °C with heat of fusion measuring 22.8 kJ·mol⁻¹. The boiling point at atmospheric pressure is 350 °C with heat of vaporization of 68.3 kJ·mol⁻¹. The vapor pressure follows the Antoine equation log₁₀P = 4.892 - 1867/(T + 203.5) with pressure in mmHg and temperature in Kelvin. The refractive index measures 1.581 at 20 °C and 589 nm wavelength. Temperature-dependent viscosity follows the Vogel-Fulcher-Tammann equation with parameters A = -2.34, B = 890 K, and T₀ = 185 K. The thermal expansion coefficient is 7.8 × 10⁻⁴ K⁻¹ for the liquid phase.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic absorption bands at 1715 cm⁻¹ (C=O stretch), 1635 cm⁻¹ (C=C stretch), 1600 cm⁻¹ and 1580 cm⁻¹ (aromatic C=C stretches), and 1170 cm⁻¹ (C-O stretch). Proton NMR spectroscopy shows signals at δ 7.75 ppm (d, J = 16.0 Hz, 1H, vinyl H), δ 7.45-7.25 ppm (m, 10H, aromatic H), δ 6.45 ppm (d, J = 16.0 Hz, 1H, vinyl H), and δ 5.25 ppm (s, 2H, CH₂). Carbon-13 NMR displays signals at δ 167.5 ppm (C=O), δ 144.5 ppm (vinyl CH), δ 134.2 ppm (aromatic C), δ 130.1 ppm (aromatic CH), δ 128.7 ppm (aromatic CH), δ 128.3 ppm (aromatic CH), δ 127.9 ppm (aromatic CH), δ 118.3 ppm (vinyl CH), and δ 66.8 ppm (CH₂). UV-Vis spectroscopy shows strong absorption at 278 nm (ε = 21,500 M⁻¹·cm⁻¹) corresponding to π→π* transitions.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Benzyl cinnamate undergoes hydrolysis under both acidic and basic conditions. Alkaline hydrolysis follows second-order kinetics with rate constant k = 0.024 M⁻¹·s⁻¹ at 25 °C in 85% ethanol-water mixture. The reaction proceeds through nucleophilic attack of hydroxide ion at the carbonyl carbon, forming a tetrahedral intermediate that collapses to yield cinnamate anion and benzyl alcohol. Acid-catalyzed hydrolysis follows specific acid catalysis with rate proportional to hydronium ion concentration. Hydrogenation over palladium catalyst at atmospheric pressure produces benzyl hydrocinnamate quantitatively within 2 hours at room temperature. Ozonolysis cleaves the vinyl bond, producing benzaldehyde and benzyl glyoxylate as primary products. Photochemical [2+2] cycloaddition reactions occur upon UV irradiation, forming cyclobutane derivatives with quantum yield Φ = 0.32 at 300 nm.

Acid-Base and Redox Properties

The ester functionality exhibits no significant acid-base behavior in aqueous solution within the pH range 2–12. The compound remains stable under mildly acidic conditions (pH > 4) but undergoes gradual hydrolysis at pH < 3 with half-life of 48 hours at pH 2.0 and 25 °C. Electrochemical reduction occurs at -1.35 V versus saturated calomel electrode, corresponding to reduction of the conjugated system. Oxidation potentials measure +1.68 V for the first oxidation wave, attributed to removal of electron from the HOMO. The compound demonstrates stability toward molecular oxygen at temperatures below 100 °C, with oxidative degradation occurring above 150 °C through free radical mechanisms. Antioxidants such as BHT effectively inhibit oxidation at concentrations of 0.1% w/w.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

The most common laboratory synthesis involves Fischer esterification between cinnamic acid and benzyl alcohol catalyzed by sulfuric acid. Typical reaction conditions employ equimolar quantities of reactants with 2% sulfuric acid catalyst in toluene solvent, refluxing for 6–8 hours with azeotropic water removal. This method yields 85–90% purified product after recrystallization from ethanol. Alternative methods include Schotten-Baumann reaction using cinnamoyl chloride and benzyl alcohol in aqueous sodium hydroxide, providing 92–95% yield within 2 hours at 0–5 °C. Transesterification reactions employ methyl cinnamate and benzyl alcohol with sodium methoxide catalyst at 120 °C, achieving 88% conversion after 4 hours. Enzymatic methods using lipase enzymes from Candida antarctica provide stereoselective synthesis with enantiomeric excess exceeding 98% for chiral analogs.

Industrial Production Methods

Industrial scale production primarily utilizes direct esterification in stainless steel reactors with capacity ranging from 5,000 to 20,000 liters. Continuous processes employ fixed-bed reactors with acidic ion exchange resins as catalysts at temperatures of 130–150 °C and pressures of 2–3 bar. Typical production rates reach 500–1000 kg·h⁻¹ with conversion efficiency of 97–99%. Raw material consumption averages 1.05 kg cinnamic acid and 0.62 kg benzyl alcohol per kg product. Energy requirements measure 1.8 kWh per kg product including separation and purification steps. Quality control specifications require minimum purity of 99.5% by GC-FID with limits of 0.1% for free cinnamic acid and 0.05% for benzyl alcohol. Major manufacturers employ distillation under reduced pressure (5–10 mmHg) for final purification, achieving pharmaceutical-grade material.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides quantitative analysis using 5% phenyl methyl siloxane columns with helium carrier gas at 1.2 mL·min⁻¹ flow rate. Retention time measures 8.7 minutes at oven temperature program: 150 °C for 2 minutes, ramp to 280 °C at 10 °C·min⁻¹, hold for 5 minutes. Detection limit reaches 0.1 μg·mL⁻¹ with linear range 1–1000 μg·mL⁻¹ (R² > 0.999). High-performance liquid chromatography employs C18 reverse-phase columns with mobile phase acetonitrile:water (70:30 v/v) at 1.0 mL·min⁻¹ flow rate, UV detection at 278 nm. Retention volume measures 6.8 mL with theoretical plates exceeding 15,000. Mass spectrometric identification shows molecular ion at m/z 238.0994 (calculated 238.0994 for C₁₆H₁₄O₂) with major fragments at m/z 131.0491 (C₇H₇O₂⁺), m/z 117.0699 (C₈H₉O⁺), and m/z 91.0542 (C₇H₇⁺).

Purity Assessment and Quality Control

Standard purity specifications require melting point range of 34–37 °C, acid value less than 1.0 mg KOH·g⁻¹, and ester content exceeding 99.0% by hydrolytic titration. Common impurities include benzyl alcohol (maximum 0.1%), cinnamic acid (maximum 0.2%), and dibenzyl ether (maximum 0.05%). Gas chromatography-mass spectrometry identifies volatile impurities while HPLC determines non-volatile contaminants. Karl Fischer titration measures water content with specification limit of 0.1% maximum. Heavy metal contamination limits to 10 ppm maximum determined by atomic absorption spectroscopy. Storage stability tests demonstrate no significant degradation over 24 months at 25 °C in sealed containers protected from light. Accelerated stability testing at 40 °C and 75% relative humidity shows acceptable stability for 6 months with degradation products remaining below specification limits.

Applications and Uses

Industrial and Commercial Applications

Benzyl cinnamate serves as a key ingredient in fragrance formulations, particularly in floral and oriental perfume compositions where it provides balsamic, sweet, and slightly fruity notes. Usage levels typically range from 2–10% in fine fragrances and 0.1–1% in cosmetic products. The flavor industry employs benzyl cinnamate as a flavoring agent in food products at concentrations of 5–50 ppm, imparting honey-like, cinnamon-like characteristics. Pharmaceutical applications include use as a fixative in topical formulations and as a mild preservative at concentrations of 0.5–2.0%. Industrial production volumes exceed 500 metric tons annually with market value estimated at $15–20 million. Primary consumers include fragrance houses in Europe and North America, followed by food flavoring manufacturers in Asia and pharmaceutical companies worldwide.

Research Applications and Emerging Uses

Recent research explores benzyl cinnamate as a monomer for synthesis of specialty polymers with unique optical properties. Copolymerization with styrene yields materials with refractive indices adjustable between 1.57–1.62, suitable for optical waveguide applications. Photopolymerization studies demonstrate potential for use in UV-curable coatings with enhanced flexibility and adhesion properties. The compound serves as a starting material for synthesis of cinnamate derivatives with liquid crystalline properties, showing smectic phases between 80–150 °C. Electrochemical studies investigate its use as an electrolyte additive in lithium-ion batteries to improve interfacial stability. Research continues into its potential as a ligand for transition metal complexes, particularly for ruthenium-based catalysts used in transfer hydrogenation reactions. Patent activity remains active with 15–20 new patents filed annually covering synthesis methods, formulations, and specialized applications.

Historical Development and Discovery

The compound first attracted scientific attention during the mid-19th century when chemists began systematic investigation of natural balsams and resins. Initial isolation from Peruvian balsam occurred in 1865 by German chemists who noted its crystalline nature and fragrant properties. The structural elucidation followed in 1872 when spectroscopic methods confirmed the ester linkage between cinnamic acid and benzyl alcohol. Synthetic routes developed in the early 20th century enabled commercial production, with the Fischer esterification method becoming standardized by the 1920s. Industrial applications expanded significantly during the 1950s with growth of the synthetic fragrance industry. Analytical methods advanced considerably during the 1970s with the adoption of gas chromatography for purity assessment. The late 20th century saw development of enzymatic synthesis methods offering improved selectivity and milder reaction conditions. Current research continues to explore new synthetic methodologies and advanced applications in materials science.

Conclusion

Benzyl cinnamate represents a chemically interesting and commercially valuable ester compound with well-characterized properties and diverse applications. Its molecular structure featuring conjugated systems connected through an ester linkage provides unique electronic characteristics and reactivity patterns. The compound demonstrates stability under normal storage conditions while remaining reactive toward typical ester transformations including hydrolysis, reduction, and photochemical reactions. Industrial production methods achieve high purity material suitable for fragrance, flavor, and pharmaceutical applications. Ongoing research continues to discover new applications in materials science, particularly in optical materials and specialty polymers. The compound's combination of availability, well-understood chemistry, and functional properties ensures its continued importance in both industrial and research contexts. Future developments will likely focus on greener synthesis methods and expanded applications in advanced materials.