Abstract

Ethylene (systematic name: ethene) is an unsaturated hydrocarbon with the molecular formula C₂H₄ and represents the simplest alkene. This colorless, flammable gas possesses a faint sweet odor at high concentrations and serves as the most produced organic compound globally with annual production exceeding 150 million metric tons. Ethylene exhibits planar molecular geometry with D_2h symmetry and a carbon-carbon double bond length of 1.337 Å. The compound demonstrates significant industrial importance as a precursor to polyethylene, ethylene oxide, and various other chemicals. Its physical properties include a melting point of -169.2 °C, boiling point of -103.7 °C, and density of 1.178 kg/m³ at 15 °C. The π-bond system confers high reactivity toward electrophilic addition reactions, making ethylene a fundamental building block in petrochemical processes.

Introduction

Ethylene stands as the most significant industrial organic chemical by production volume, with global capacity exceeding 190 million metric tons annually. This simplest alkene represents a cornerstone of modern petrochemical industry, serving as the primary feedstock for polyethylene production and numerous derivative chemicals. Classified as an unsaturated hydrocarbon, ethylene contains a carbon-carbon double bond that confers distinctive chemical reactivity patterns. The compound was first identified in 1669 by Johann Joachim Becher through ethanol dehydration with sulfuric acid, though systematic characterization occurred much later. Industrial ethylene production primarily occurs through steam cracking of hydrocarbons, with ethane and naphtha serving as principal feedstocks. The economic significance of ethylene drives continuous technological innovation in production methods and catalyst development.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Ethylene exhibits planar molecular geometry with D_2h point group symmetry. All six atoms lie in the same plane, with carbon-carbon bond length measuring 1.337 Å and carbon-hydrogen bond lengths of 1.086 Å. The H-C-H bond angle is 117.4°, while the H-C-C angles measure 121.3°, consistent with sp² hybridization of carbon atoms. The carbon-carbon double bond consists of one σ-bond and one π-bond, with the π-electron cloud distributed above and below the molecular plane. Molecular orbital theory describes the highest occupied molecular orbital (HOMO) as the π-bonding orbital, while the lowest unoccupied molecular orbital (LUMO) corresponds to the π* antibonding orbital. This electronic configuration results in ionization energy of 10.51 eV and electron affinity of -1.78 eV. The molecular structure demonstrates zero dipole moment due to its centrosymmetric arrangement.

Chemical Bonding and Intermolecular Forces

The carbon-carbon double bond in ethylene has bond dissociation energy of 610 kJ/mol, significantly higher than typical single bonds but weaker than carbon-carbon triple bonds. The π-bond component contributes approximately 270 kJ/mol to the total bond energy. Ethylene molecules experience weak intermolecular interactions dominated by London dispersion forces, with van der Waals radius of 4.23 Å. The relatively low polarizability results in weak intermolecular attractions, explaining the compound's low boiling point. The ethylene molecule lacks hydrogen bonding capability due to the absence of hydrogen atoms bonded to electronegative elements. The quadrupole moment measures 1.43 × 10^-26 esu, influencing molecular packing in solid phase. Crystal structure analysis reveals monoclinic packing with space group P2₁/n at temperatures below -169.2 °C.

Physical Properties

Phase Behavior and Thermodynamic Properties

Ethylene exists as a colorless gas at standard temperature and pressure with density of 1.178 kg/m³ at 15 °C. The compound undergoes phase transition to liquid at -103.7 °C (boiling point) and solidifies at -169.2 °C (melting point) under atmospheric pressure. The critical temperature measures 9.2 °C, with critical pressure of 50.5 bar and critical density of 214 kg/m³. The triple point occurs at -169.4 °C and 1.07 × 10^-4 bar. Ethylene demonstrates enthalpy of formation (ΔH_f°) of +52.47 kJ/mol and standard entropy (S°) of 219.32 J·K^-1·mol^-1. The heat capacity (C_p) measures 42.9 J·K^-1·mol^-1 at 25 °C, while enthalpy of vaporization is 13.53 kJ/mol at boiling point. The compound exhibits viscosity of 10.28 μPa·s at 25 °C and thermal conductivity of 0.0172 W·m^-1·K^-1.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrational modes including the =C-H asymmetric stretch at 3105 cm^-1, symmetric stretch at 2989 cm^-1, and C=C stretch at 1623 cm^-1. The =C-H bending vibrations appear at 1342 cm^-1 (scissoring), 943 cm^-1 (rocking), and 810 cm^-1 (wagging). Proton NMR spectroscopy shows a singlet at δ 5.28 ppm in deuterated chloroform, while carbon-13 NMR displays a signal at δ 123.3 ppm. UV-Vis spectroscopy indicates π→π* transition with maximum absorption at 170 nm (ε = 10,000 L·mol^-1·cm^-1). Mass spectrometry exhibits molecular ion peak at m/z 28 with major fragmentation patterns including loss of hydrogen (m/z 27) and formation of C₂H₂⁺ (m/z 26). Raman spectroscopy shows strong band at 1623 cm^-1 corresponding to the C=C stretching vibration.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Ethylene undergoes electrophilic addition reactions characteristic of alkenes, with reaction rates governed by π-electron availability. Halogenation occurs rapidly at room temperature, with chlorine addition proceeding through cyclic chloronium ion intermediate with second-order rate constant of 1.2 × 10⁸ L·mol^-1·s^-1. Hydrohalogenation follows Markovnikov's rule, with HCl addition exhibiting rate constant of 4.3 × 10⁶ L·mol^-1·s^-1 at 25 °C. Hydration catalyzed by sulfuric acid proceeds via carbocation mechanism with activation energy of 75 kJ/mol. Oxidation reactions include epoxidation with peracids forming ethylene oxide with rate constant of 2.5 × 10^-3 L·mol^-1·s^-1 at 25 °C, and combustion with activation energy of 210 kJ/mol. Polymerization reactions occur via radical, cationic, or coordination mechanisms, with Ziegler-Natta catalysts achieving activities exceeding 1000 kg polyethylene per gram titanium per hour.

Acid-Base and Redox Properties

Ethylene demonstrates very weak acidity with pK_a of 44 in dimethyl sulfoxide, reflecting the high energy required to remove a proton from the sp² hybridized carbon. The conjugate base, vinyl anion, exhibits high basicity and nucleophilicity. Redox properties include standard reduction potential of -1.87 V versus standard hydrogen electrode for one-electron reduction to ethylene radical anion. Oxidation potential measures +1.88 V for one-electron oxidation to ethylene radical cation. The compound resists strong bases but undergoes reaction with powerful reducing agents like lithium aluminum hydride at elevated temperatures. Electrochemical studies show irreversible reduction wave at -2.3 V and oxidation wave at +1.5 V in acetonitrile using platinum electrode. Stability in aqueous solutions ranges from pH 2 to 12, with decomposition occurring under strongly acidic or basic conditions at elevated temperatures.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory-scale ethylene production typically employs dehydration of ethanol using concentrated sulfuric acid at 160-170 °C. This method achieves yields of 80-85% with phosphoric acid on silica gel support providing superior selectivity at 300-400 °C. Alternative laboratory methods include dehalogenation of 1,2-dichloroethane with zinc dust in ethanol (95% yield) and Hofmann elimination of trimethylamine oxide from choline chloride. The Wittig reaction using methylenetriphenylphosphorane with formaldehyde represents a specialized synthetic route for labeled ethylene compounds. Purification typically involves fractional distillation at -100 °C or passage through activated alumina to remove oxygenated impurities. Small quantities of high-purity ethylene for spectroscopic studies may be obtained by cracking diethyl ether over heated alumina at 500 °C.

Industrial Production Methods

Industrial ethylene production predominantly utilizes steam cracking of hydrocarbon feedstocks, with operating temperatures of 750-950 °C and residence times of 0.1-0.5 seconds. Ethane cracking achieves ethylene yields of 75-80%, while naphtha cracking produces 25-30% ethylene with significant co-production of propylene and C4 hydrocarbons. Modern cracking furnaces employ advanced coil materials allowing outlet temperatures up to 1100 °C with improved selectivity. Separation and purification involve multistage compression to 35 bar followed by low-temperature distillation in cascaded columns, including demethanizer (-100 °C), deethanizer, and C2 splitter (-30 °C) producing polymer-grade ethylene (99.9% purity). Alternative production technologies include methanol-to-olefins (MTO) processes using SAPO-34 catalysts achieving 75% ethylene selectivity, and oxidative dehydrogenation of ethane using molten salt catalysts at 850-900 °C.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides the primary method for ethylene quantification, achieving detection limits of 0.1 ppm using porous polymer packed columns or alumina capillary columns. Fourier transform infrared spectroscopy offers specific detection through characteristic absorption bands at 950-975 cm^-1 and 3100 cm^-1 with detection limit of 2 ppm. Photoacoustic spectroscopy enables real-time monitoring with sensitivity of 5 ppb using quantum cascade lasers tuned to the 10.5 μm absorption band. Mass spectrometric methods provide definitive identification through molecular ion at m/z 28 and characteristic fragmentation pattern, with selected ion monitoring achieving detection limits below 1 ppb. Chemical detection methods employ bromine water decolorization or potassium permanganate oxidation for qualitative analysis. Electrochemical sensors based on metal oxide semiconductors offer portable detection with sensitivity of 0.5 ppm.

Purity Assessment and Quality Control

Polymer-grade ethylene specifications require minimum purity of 99.9%, with acetylene content below 5 ppm, oxygen below 10 ppm, and water below 5 ppm. Hydrogen and methane impurities are controlled below 100 ppm each, while carbon dioxide and sulfur compounds must not exceed 1 ppm. Analytical methods for purity assessment include gas chromatography with thermal conductivity detection for permanent gases, and flame ionization detection for hydrocarbon impurities. Moisture analysis employs piezoelectric quartz crystal microbalances or cavity ring-down spectroscopy with detection limits of 0.1 ppm. Acetylene determination utilizes gas chromatography with argon ionization detection or infrared spectroscopy at 730 cm^-1. Oxygen contamination is monitored using galvanic sensors or paramagnetic analyzers with sensitivity of 0.5 ppm. Quality control protocols include periodic verification using certified reference materials traceable to national standards.

Applications and Uses

Industrial and Commercial Applications

Ethylene serves as the primary feedstock for polyethylene production, accounting for approximately 60% of global consumption. High-density polyethylene (HDPE) and low-density polyethylene (LDPE) production utilizes coordination polymerization and free-radical polymerization processes, respectively. Ethylene oxide production through catalytic oxidation consumes about 15% of ethylene output, with subsequent conversion to ethylene glycol for antifreeze and polyester fiber production. Ethylene dichloride synthesis for vinyl chloride monomer production accounts for approximately 12% of ethylene use. Styrene production via ethylbenzene dehydrogenation utilizes 8% of ethylene supply. Minor applications include linear alpha-olefins production through oligomerization (5%), vinyl acetate synthesis (2%), and ethanol production through direct hydration (1%). Specialty applications include use as refrigerant (R-1150) in cryogenic systems and as anesthetic agent in medical applications.

Research Applications and Emerging Uses

Ethylene functions as a fundamental ligand in organometallic chemistry, forming complexes with transition metals including Zeise's salt (K[PtCl₃(C₂H₄)]) and chlorobis(ethylene)rhodium dimer. Research applications include studies of π-backbonding in metal-olefin complexes and mechanistic investigations of insertion reactions in coordination polymerization. Emerging applications encompass chemical vapor deposition processes for carbon nanotube growth using ethylene carbon source, and plasma-enhanced catalytic conversion to higher hydrocarbons. Electrochemical reduction of ethylene to ethane using proton-exchange membrane reactors represents developing technology for energy storage. Photocatalytic conversion of ethylene to ethylene oxide using titanium dioxide catalysts under ultraviolet irradiation offers potential for selective oxidation processes. Metathesis reactions with ethylene serve as chain transfer agents in olefin conversion processes, enabling precise control of molecular weight distributions in polyolefin synthesis.

Historical Development and Discovery

Ethylene was first documented in 1669 by German alchemist Johann Joachim Becher, who observed gas evolution during ethanol treatment with sulfuric acid. Dutch chemists Johann Rudolph Deimann, Adrien Paets van Troostwyck, Anthoni Lauwerenburgh, and Nicolas Bondt conducted systematic investigations in 1795, establishing the hydrocarbon nature of ethylene and its differentiation from hydrogen gas. The name "olefiant gas" (oil-making gas) originated from the 1795 discovery that ethylene combined with chlorine produced oil-like 1,2-dichloroethane, leading to the modern term "olefin." August Wilhelm von Hofmann introduced systematic nomenclature in 1866, proposing "ethene" according to hydrocarbon naming conventions. The compound found anesthetic application in the 1920s following clinical investigations by Luckhardt, Crocker, and Carter at the University of Chicago. Industrial significance emerged in the 1930s with development of polymerization processes, culminating in the discovery of Ziegler-Natta catalysts in 1953 that revolutionized polyolefin production. IUPAC formally adopted "ethene" as the systematic name in 1993, though "ethylene" remains prevalent in industrial and North American usage.

Conclusion

Ethylene represents the most fundamental alkene and most produced organic compound globally, with profound importance in petrochemical industry and chemical research. The compound's planar structure with carbon-carbon double bond confers distinctive reactivity patterns that enable diverse transformation pathways including polymerization, oxidation, and addition reactions. Industrial production through steam cracking continues to evolve with advanced materials and process intensification techniques improving energy efficiency and selectivity. Emerging applications in materials synthesis and energy conversion demonstrate the continuing relevance of ethylene chemistry. Future research directions include development of alternative production methods from renewable resources, catalytic processes for direct conversion to higher value chemicals, and advanced polymerization catalysts with enhanced activity and stereocontrol. The fundamental understanding of ethylene reactivity continues to inform broader concepts in chemical bonding and reaction mechanisms across organic and organometallic chemistry.