Abstract

Vinyl chloride (IUPAC name: chloroethene, molecular formula: C₂H₃Cl) is an organochloride compound of significant industrial importance. This colorless gas with a mildly sweet odor possesses a boiling point of −13.4 °C and melting point of −153.8 °C. The compound exhibits a density of 0.911 g/mL at standard conditions and vapor pressure of 2580 mmHg at 20 °C. Vinyl chloride serves as the primary monomer for polyvinyl chloride (PVC) production, accounting for over 40 million tonnes of global annual consumption. The molecule demonstrates characteristic chemical reactivity through its electron-deficient double bond, undergoing addition reactions and polymerization. Its industrial synthesis predominantly occurs through thermal cracking of 1,2-dichloroethane at elevated temperatures. The compound requires careful handling due to high flammability (flash point: −61 °C) and explosive limits between 3.6–33% by volume in air.

Introduction

Vinyl chloride represents a fundamental building block in modern industrial chemistry, classified as an organochloride compound within the broader category of halogenated alkenes. First synthesized in 1835 by Justus von Liebig and Henri Victor Regnault through dehydrohalogenation of 1,2-dichloroethane, the compound gained industrial significance following Fritz Klatte's 1912 patent describing its catalytic production from acetylene and hydrogen chloride. The development of ethylene-based production routes in the mid-20th century established vinyl chloride as a commodity chemical with global production exceeding 40 million tonnes annually. Structural characterization reveals a planar molecule with C_s point group symmetry, featuring a chlorine atom bonded to an sp²-hybridized carbon within an unsaturated hydrocarbon framework.

Molecular Structure and Bonding

Molecular Geometry and Electronic Structure

Vinyl chloride adopts a planar molecular geometry with bond angles consistent with VSEPR theory predictions for an alkene system. The carbon-carbon double bond length measures 1.369 Å, intermediate between typical single (1.54 Å) and double (1.34 Å) carbon-carbon bonds, indicating partial conjugation with the chlorine atom. The C-Cl bond length measures 1.689 Å, slightly shorter than typical carbon-chlorine single bonds (1.77 Å) due to hyperconjugation effects. The H-C-H bond angle at the methylene group is 117.6°, while the H-C-Cl bond angle is 122.4°. The chlorine atom exhibits sp² hybridization with a lone pair occupying the p_z orbital perpendicular to the molecular plane.

Molecular orbital analysis reveals that the highest occupied molecular orbital (HOMO) resides primarily on the chlorine atom with π-bonding character, while the lowest unoccupied molecular orbital (LUMO) demonstrates π* antibonding character across the carbon-carbon double bond. This electronic configuration renders the molecule susceptible to electrophilic addition reactions at the β-carbon position. The ionization potential measures 10.00 eV, consistent with chlorinated hydrocarbons. Photoelectron spectroscopy confirms three distinct ionization events corresponding to σ(C-Cl), π(C=C), and n(Cl) electrons at 11.2 eV, 10.5 eV, and 10.0 eV respectively.

Chemical Bonding and Intermolecular Forces

The carbon-chlorine bond dissociation energy measures 339 kJ/mol, significantly lower than typical C-Cl bonds in chloromethane (350 kJ/mol) due to stabilization of the resulting radical through resonance with the vinyl system. The carbon-carbon double bond energy is 610 kJ/mol, reduced from ethylene's 682 kJ/mol due to electron-withdrawing effects of the chlorine substituent. The molecule exhibits a dipole moment of 1.44 D with the chlorine atom bearing partial negative charge (δ⁻ = −0.17) and the β-carbon atom carrying partial positive charge (δ⁺ = +0.13).

Intermolecular interactions are dominated by van der Waals forces with negligible hydrogen bonding capacity. The polarizability measures 4.56 × 10⁻²⁴ cm³, while the Lennard-Jones parameters are σ = 4.28 Å and ε/k = 329 K. The compound demonstrates limited solubility in polar solvents (2.7 g/L in water at 25 °C) but complete miscibility with many organic solvents including ethanol, benzene, and chloroform. The Henry's law constant for air-water partitioning is 2.75 × 10⁻² atm·m³/mol.

Physical Properties

Phase Behavior and Thermodynamic Properties

Vinyl chloride exists as a colorless gas under standard conditions with a characteristic mildly sweet odor detectable at concentrations as low as 300 ppm. The compound liquefies under moderate pressure (2580 mmHg at 20 °C) to form a mobile liquid with density of 0.911 g/mL. The triple point occurs at −153.8 °C with vapor pressure following the Antoine equation: log₁₀(P) = 4.048 − (1024.5/(T − 39.5)) where P is in mmHg and T in Kelvin.

The enthalpy of formation for gaseous vinyl chloride is −94.12 kJ/mol, while the liquid phase formation enthalpy is −108.8 kJ/mol. The standard entropy (S°) measures 263.8 J/mol·K for the gas phase. The heat capacity follows the polynomial expression C_p = 10.28 + 0.01527T − 4.196×10⁻⁶T² J/mol·K for the temperature range 298–1500 K. The critical temperature measures 158.85 °C with critical pressure of 55.2 atm and critical volume of 184 cm³/mol. The Joule-Thomson coefficient is 0.121 K/atm at 25 °C.

Spectroscopic Characteristics

Infrared spectroscopy reveals characteristic vibrations including ν(C=C) at 1605 cm⁻¹, ν(C-Cl) at 820 cm⁻¹, and δ(CH₂) at 1410 cm⁻¹. The out-of-plane bending vibration appears at 960 cm⁻¹ with medium intensity. Proton NMR spectroscopy displays three distinct signals: the trans-proton at δ 5.85 ppm (J = 15.8 Hz), the cis-proton at δ 5.45 ppm (J = 8.9 Hz), and the methylene protons as a doublet of doublets at δ 6.40 ppm (J = 15.8 Hz, 8.9 Hz). Carbon-13 NMR shows signals at δ 113.5 ppm (CH₂), 127.8 ppm (CHCl), and 138.2 ppm (C=).

UV-Vis spectroscopy demonstrates weak absorption maxima at 210 nm (ε = 150 M⁻¹cm⁻¹) corresponding to n→π* transitions and stronger absorption at 185 nm (ε = 5000 M⁻¹cm⁻¹) from π→π* transitions. Mass spectrometry exhibits a molecular ion peak at m/z 62 with characteristic fragmentation patterns including loss of chlorine (m/z 27, C₂H₃⁺), loss of hydrogen chloride (m/z 26, C₂H₂⁺), and formation of Cl⁺ at m/z 35.

Chemical Properties and Reactivity

Reaction Mechanisms and Kinetics

Vinyl chloride undergoes characteristic electrophilic addition reactions with rate constants governed by the electron-withdrawing nature of the chlorine substituent. Addition of hydrogen chloride follows Markovnikov orientation with a rate constant of 1.2 × 10⁻⁴ M⁻¹s⁻¹ at 25 °C, producing 1,1-dichloroethane. The reaction exhibits an activation energy of 65 kJ/mol. Hydrochlorination proceeds through a polar mechanism with protonation at the β-carbon as the rate-determining step.

Free radical addition reactions demonstrate regioselectivity favoring anti-Markovnikov products. Addition of hydrogen bromide in the presence of peroxides occurs with rate constant 8.7 × 10⁻³ M⁻¹s⁻¹ at 80 °C, producing 1-bromo-1-chloroethane. The polymerization reaction follows radical chain mechanism with propagation rate constant k_p = 1.1 × 10⁴ M⁻¹s⁻¹ at 50 °C and activation energy of 22 kJ/mol. The Q-e values for copolymerization are Q = 0.044 and e = 0.34, indicating low reactivity and strong electron-accepting character.

Acid-Base and Redox Properties

Vinyl chloride exhibits negligible acidity with estimated pK_a > 40 for vinylic proton abstraction. The compound demonstrates resistance to hydrolysis with half-life exceeding 100 years at neutral pH and 25 °C. Under strongly basic conditions (pH > 12), dehydrochlorination occurs slowly with second-order rate constant 3.8 × 10⁻⁵ M⁻¹s⁻¹ at 80 °C.

Redox behavior includes electrochemical reduction at −2.1 V versus standard hydrogen electrode, producing chloride anion and ethylene. Oxidation potentials measure +1.8 V for one-electron transfer, leading to formation of cationic species. The compound demonstrates stability toward molecular oxygen but undergoes rapid oxidation with strong oxidizing agents such as potassium permanganate and ozone. Reaction with ozone proceeds with rate constant 1.3 × 10⁻¹⁸ cm³/molecule·s at 25 °C, producing formyl chloride and formaldehyde as primary products.

Synthesis and Preparation Methods

Laboratory Synthesis Routes

Laboratory-scale synthesis typically employs dehydrohalogenation of 1,2-dichloroethane using alcoholic potassium hydroxide. The reaction proceeds through E2 elimination mechanism with second-order kinetics (k = 2.4 × 10⁻⁴ M⁻¹s⁻¹ at 70 °C) producing vinyl chloride in 85–90% yield. Alternative routes include reaction of acetylene with hydrogen chloride over mercuric chloride catalyst at 150–200 °C, achieving conversions exceeding 95% with selectivity >98%.

Modern laboratory methods utilize gas-phase pyrolysis of 1,2-dichloroethane in quartz reactors at 500 °C with residence times of 0.5–2 seconds. This method provides vinyl chloride with 99.5% purity after fractional distillation. Small-scale electrochemical synthesis employs electrolysis of 1,2-dichloroethane in dimethylformamide solution at platinum electrodes, achieving Faradaic efficiencies of 75–80% at current densities of 100 mA/cm².

Industrial Production Methods

Industrial production predominantly utilizes thermal cracking of 1,2-dichloroethane in tubular reactors at 500–550 °C and pressures of 15–30 atm. The endothermic reaction (ΔH = 71 kJ/mol) employs direct-fired heaters with residence times optimized to 2–4 seconds for maximum conversion (50–60%) while minimizing side products including chloroprene and 1,3-butadiene. The process incorporates quench systems utilizing cold dichloroethane to rapidly cool reactor effluents to 200 °C within 0.1 seconds.

Modern integrated facilities employ balanced processes where hydrogen chloride byproduct from cracking is utilized in oxychlorination of ethylene. The oxychlorination process uses copper(II) chloride catalysts supported on alumina at 220–240 °C, converting ethylene, hydrogen chloride, and oxygen to 1,2-dichloroethane with 95% selectivity. Global production capacity exceeds 50 million tonnes annually with average plant sizes of 300,000–500,000 tonnes/year. Production economics are dominated by ethylene and chlorine costs, accounting for 70–80% of variable production expenses.

Analytical Methods and Characterization

Identification and Quantification

Gas chromatography with flame ionization detection provides primary analytical methodology with detection limits of 0.1 ppm using capillary columns with polyethylene glycol stationary phases. Retention indices measure 1.85 relative to n-alkanes on DB-624 columns. Mass spectrometric detection enhances specificity with selected ion monitoring at m/z 62, 64 achieving detection limits of 5 ppb.

Fourier-transform infrared spectroscopy offers quantitative analysis through characteristic absorption bands at 820 cm⁻¹ and 1605 cm⁻¹ with detection limits of 2 ppm using 10-meter pathlength gas cells. Photoacoustic spectroscopy provides real-time monitoring capabilities with response times under 30 seconds and detection limits of 0.5 ppm. Electrochemical sensors based on solid-state electrolytes achieve detection limits of 0.2 ppm with response times under 10 seconds.

Purity Assessment and Quality Control

Industrial grade vinyl chloride specifications require minimum purity of 99.95% with maximum acetylene content of 5 ppm, ethylene content of 10 ppm, and 1,2-dichloroethane content of 15 ppm. Moisture content is limited to 50 ppm to prevent corrosion and polymerization initiation. Analysis employs gas chromatography with thermal conductivity detection for major components and flame ionization detection with cryogenic focusing for trace impurities.

Stability testing demonstrates that inhibited grades containing 50–100 ppm of hydroquinone or phenothiazine as polymerization inhibitors remain stable for six months storage at 25 °C. Uninhibited material undergoes autopolymerization with initiation times of 48–72 hours at ambient temperature. Quality control protocols include testing for peroxides using iodometric titration with specifications limiting peroxide content to 1 ppm as hydrogen peroxide equivalent.

Applications and Uses

Industrial and Commercial Applications

Vinyl chloride serves exclusively as monomer for polyvinyl chloride production, accounting for over 99% of global consumption. PVC manufacturing employs suspension (80%), emulsion (12%), and bulk (8%) polymerization processes with annual capacity exceeding 60 million tonnes worldwide. The suspension process utilizes water as dispersion medium with peroxydicarbonate initiators at 50–70 °C, producing particles of 100–200 μm diameter.

Specialty applications include production of vinyl chloride-vinyl acetate copolymers containing 5–20% vinyl acetate for improved flexibility and processability. Chlorinated PVC with 65–70% chlorine content provides enhanced thermal stability and flame resistance. Cross-linked PVC formulations utilize multifunctional monomers including divinyl benzene for applications requiring improved mechanical properties at elevated temperatures.

Research Applications and Emerging Uses

Research applications focus on development of novel polymerization catalysts including metallocene and single-site catalysts providing improved control over molecular weight distribution and tacticity. Advanced initiation systems employing ultraviolet and radiation-induced polymerization enable production of specialty grades with tailored properties for medical and electronic applications.

Emerging applications include synthesis of block copolymers with controlled architecture for membrane separation technologies. Gradient copolymers with composition drift along the polymer chain exhibit enhanced compatibility in polymer blends. Functionalized vinyl chloride monomers incorporating reactive groups enable post-polymerization modification for specialty adhesives and coatings applications.

Historical Development and Discovery

The initial synthesis of vinyl chloride in 1835 by Liebig and Regnault employed reaction of 1,2-dichloroethane with alcoholic potassium hydroxide, establishing the fundamental dehydrohalogenation chemistry still utilized today. Industrial production commenced in the 1920s using acetylene-based processes following Klatte's catalytic methodology. The period 1930–1950 witnessed expansion of production capacity driven by growing PVC demand for electrical insulation and packaging applications.

The transition to ethylene-based production methods began in the 1950s with development of direct chlorination and oxychlorination processes. The balanced process integrating cracking with oxychlorination was commercialized in the 1960s, dramatically improving economics and enabling large-scale production. Environmental and health considerations led to implementation of closed-loop systems in the 1970s, reducing workplace exposures by orders of magnitude. Continuous process improvements have focused on energy integration, catalyst development, and waste minimization throughout the late 20th and early 21st centuries.

Conclusion

Vinyl chloride occupies a central position in industrial organic chemistry as the primary precursor to polyvinyl chloride, one of the most widely produced synthetic polymers. The compound's molecular structure featuring an electron-deficient double bond adjacent to a chlorine substituent governs its chemical behavior, enabling both electrophilic addition and free radical polymerization reactions. Industrial production relies on thermal cracking of 1,2-dichloroethane in highly optimized processes that integrate material and energy flows to achieve economic viability.

Ongoing research focuses on development of more selective catalysts for polymerization, improved process efficiency through advanced reactor design, and creation of novel copolymer architectures expanding application possibilities. Environmental considerations continue to drive innovations in recycling technologies and waste minimization strategies throughout the PVC lifecycle. The fundamental chemistry of vinyl chloride remains an active area of investigation with particular interest in reaction mechanisms under supercritical conditions and development of alternative synthesis routes from renewable feedstocks.