Ethylidene Chloride vs. Ethylene Dichloride: What’s the Difference?

Ethylidene chloride, also known as 1,1-dichloroethane, and ethylene dichloride, known as 1,2-dichloroethane, are both chlorinated ethane compounds widely used in industrial chemistry.  These chemicals are often confused due to their similar names, molecular formulas (both C₂H₄Cl₂), and roles as solvents or intermediates in chemical processes. However, they differ significantly in structure, properties, production methods, applications, and health risks. The purpose of this article is to clarify these differences, highlighting their unique characteristics and uses while emphasizing safety considerations.

Chemical Overview

Ethylidene Chloride (1,1-Dichloroethane)

Ethylidene chloride has the molecular formula C₂H₄Cl₂ and the structure CH₃–CHCl₂, where both chlorine atoms are attached to the same carbon (geminal dihalide). Its CAS number is 75-34-3.  It appears as a clear, colorless liquid with a sweet, chloroform-like odor. Key physical properties include a boiling point of 57.2°C, melting point of -97°C, density of 1.2 g/cm³ at 20°C, and high volatility (vapor pressure 182 mmHg at 20°C).  It has low solubility in water (0.6% at 20°C) but is miscible with organic solvents.

Ethylene Dichloride (1,2-Dichloroethane)

Ethylene dichloride shares the molecular formula C₂H₄Cl₂ but has the structure Cl–CH₂–CH₂–Cl, with chlorine atoms on adjacent carbons (vicinal dihalide). Its CAS number is 107-06-2.  It is a colorless, oily liquid with a sweet odor. Physical properties include a boiling point of 83.5°C, melting point of -35°C, density of 1.25 g/cm³ at 20°C, and significant volatility (vapor pressure 79 mmHg at 20°C).  Like its isomer, it is sparingly soluble in water but miscible with organics.

(Structural diagrams: For ethylidene chloride, imagine a carbon chain with the second carbon bonded to two Cl atoms and one H; for ethylene dichloride, each terminal carbon bonded to one Cl and two H atoms.)

Synthesis & Production

Ethylidene chloride is primarily produced via the catalytic addition of hydrogen chloride (HCl) to vinyl chloride, using catalysts like aluminum chloride or ferric chloride at 20–55°C. It can also form as a co-product in the chlorination of ethane or ethylene, separated by distillation. Production is on a smaller scale, often as an intermediate, with annual U.S. volumes exceeding 1 million pounds but not in the millions of tons.

In contrast, ethylene dichloride is mass-produced through the direct chlorination of ethylene with chlorine gas, often in the presence of catalysts, at large scales (millions of tons annually worldwide) primarily as a precursor for vinyl chloride monomer (VCM) used in PVC production.  This difference in scale reflects ethylene dichloride's broader industrial demand.

Applications & Uses

Ethylidene chloride is used as a solvent in cleaning, degreasing, and extraction processes, particularly for temperature-sensitive substances.  It serves as an intermediate in synthesizing 1,1,1-trichloroethane, adhesives, plastic wrap, synthetic fibers, and high-vacuum resistant rubber.  Historically, it was used as a fumigant and insecticide, but these applications have declined.

Ethylene dichloride's primary use is as a precursor to vinyl chloride for PVC production, accounting for the majority of its output.  It also functions as a solvent in degreasing, a lead scavenger in fuels (historical), and in ore flotation or fabric spreading.  Its large-scale production underscores its significance in the plastics industry, far outpacing the niche applications of ethylidene chloride.

Toxicity & Safety

Both compounds are toxic, but ethylene dichloride poses greater risks. Ethylidene chloride is an irritant to eyes and respiratory tract, with liver as the primary target organ; it causes CNS effects like dizziness and is a possible human carcinogen.  Acute inhalation exposure can lead to kidney damage, with OSHA PEL at 100 ppm TWA.

Ethylene dichloride is highly toxic, flammable, and classified as a probable human carcinogen (IARC Group 2B), affecting CNS, gastrointestinal tract, and causing nausea, blurred vision, and organ damage.  It has slight aquatic toxicity and is regulated under OSHA (PEL 50 ppm TWA), REACH, and EPA for emissions.  Both require storage in cool, ventilated areas away from ignition sources, with PPE for handling.

Key Differences at a Glance

Ethylidene chloride and ethylene dichloride, while isomers, differ markedly in structure (geminal vs. vicinal), boiling points, production scales, and applications—ethylidene for niche solvents and intermediates, ethylene for massive PVC feedstock.  Ethylene dichloride's industrial dominance contrasts with ethylidene's specialty roles, but both demand caution due to toxicity and carcinogenicity risks.  Always prioritize safe handling, ventilation, and compliance with regulations to mitigate hazards.