Ethylene

Ethylene, also known by its IUPAC name ethene, is one of the simplest and most significant organic compounds in modern science and industry. Represented by the molecular formula C₂H₄, it is the simplest alkene and a colourless, flammable gas. Its characteristic carbon–carbon double bond makes it highly reactive, enabling it to serve as a cornerstone in petrochemical industries and as a naturally occurring plant hormone that regulates various physiological processes.

Background and Historical Development

The discovery of ethylene dates back to the late eighteenth century, when it was first identified as “olefiant gas,” a name derived from its ability to form oily products upon reaction with halogens. In the nineteenth century, its structural identity as a hydrocarbon with a double bond was recognised, and the name “ethylene” became standard in chemical nomenclature.
The industrial significance of ethylene grew immensely during the twentieth century with the rise of the petrochemical industry. It became a key feedstock for synthetic materials such as plastics, solvents, and fibres. Ethylene production marked the beginning of modern polymer chemistry, and today, it remains one of the most produced organic chemicals globally, with millions of tonnes manufactured annually.
Interestingly, the natural role of ethylene in plants was identified much later. It was found to function as a gaseous hormone that influences growth, ripening, and ageing processes, bridging the disciplines of chemistry, agriculture, and biology.

Structure, Properties, and Characteristics

Ethylene is a hydrocarbon composed of two carbon atoms joined by a double bond, with each carbon also bonded to two hydrogen atoms. The molecule is planar, with sp² hybridised carbon atoms and a bond angle of approximately 117°. The double bond gives the molecule its reactivity and ability to participate in addition reactions.
Key physical properties include:

• Molecular weight: 28.05 g mol⁻¹
• State: Colourless gas at room temperature
• Odour: Faintly sweet or musky
• Boiling point: –103.9 °C
• Melting point: –169.4 °C
• Density: Lighter than air
• Flammability: Highly flammable with a wide explosive range in air

The π-bond of ethylene is relatively weak compared to σ-bonds, making the molecule reactive in electrophilic addition and polymerisation reactions.

Chemical Reactivity and Important Reactions

Ethylene’s double bond allows it to engage in numerous reactions fundamental to organic and industrial chemistry.

• Polymerisation: Ethylene can polymerise under suitable conditions to form polyethylene, one of the most widely used plastics. Variations in catalysts and conditions produce different forms such as low-density polyethylene (LDPE) and high-density polyethylene (HDPE).
• Oxidation and Epoxidation: Controlled oxidation of ethylene produces ethylene oxide, which is a precursor for ethylene glycol, an essential component of antifreeze and polyester fibres.
• Hydration: In the presence of catalysts, ethylene reacts with water to form ethanol.
• Halogenation and Hydrohalogenation: Reaction with halogens or hydrogen halides yields compounds such as ethylene dichloride and ethyl halides, important intermediates in chemical manufacture.
• Alkylation and Oligomerisation: Ethylene can undergo catalytic reactions to form higher olefins and other hydrocarbons used in fuels and lubricants.

Ethylene also forms coordination complexes with transition metals, as seen in Zeise’s salt, which was among the earliest organometallic compounds discovered.

Production and Industrial Manufacture

Ethylene is produced predominantly through the steam cracking of hydrocarbons. In this process, light alkanes such as ethane, propane, or naphtha are heated with steam to temperatures above 800 °C, breaking larger molecules into smaller ones, including ethylene. The process is energy-intensive and carried out in large industrial furnaces.
After cracking, the gas mixture is rapidly quenched to prevent further reactions, and the components are separated by a combination of compression, cooling, and cryogenic distillation. Ethylene is then purified to high standards before being used as a feedstock for other processes.
Some ethylene is also recovered from refinery gases and coke-oven gas, though these are minor sources compared to steam cracking. With global production exceeding 200 million tonnes per year, ethylene is often regarded as the foundation of the petrochemical industry.

Industrial and Commercial Applications

Ethylene’s versatility as a starting material makes it invaluable across multiple industries.
Major uses include:

• Polyethylene Production: The largest single use, accounting for over half of all ethylene produced. Polyethylene is used in packaging, films, containers, and countless plastic products.
• Ethylene Oxide and Glycol Manufacture: Ethylene oxide derived from ethylene is converted into ethylene glycol, used in polyester fibres, resins, and antifreeze.
• Vinyl Chloride Production: Ethylene is chlorinated to form ethylene dichloride, a precursor to vinyl chloride monomer (VCM), which is then polymerised into polyvinyl chloride (PVC).
• Styrene and Synthetic Rubber: Ethylene is used in producing ethylbenzene, which leads to styrene, the basis for polystyrene and other plastics.
• Agriculture and Ripening Agent: Ethylene gas is used to ripen fruits artificially, synchronising harvests and improving market presentation.

Ethylene’s wide range of derivatives makes it the most economically significant organic compound in global trade.

Role in Plant Biology

Beyond industry, ethylene serves as a plant hormone regulating growth and development. It is synthesised naturally from the amino acid methionine through a series of enzymatic steps.
In plants, ethylene influences:

• Fruit Ripening: It promotes the conversion of starches to sugars, softening of tissue, and development of aroma and colour.
• Leaf Abscission and Senescence: Ethylene accelerates leaf and flower shedding, an adaptive process during stress or seasonal changes.
• Response to Stress: It mediates responses to mechanical injury, flooding, and drought by altering plant growth patterns.
• Flowering Induction: In crops like pineapple, controlled exposure to ethylene can stimulate flowering for uniform fruiting.

Because of its gaseous nature, even trace amounts of ethylene can trigger significant physiological changes. This has made its control and measurement vital in post-harvest handling and storage of horticultural products.

Environmental and Safety Considerations

Ethylene, while not highly toxic, poses considerable safety and environmental concerns. Its high flammability and wide explosive limits require careful storage and handling, often in pressurised, temperature-controlled conditions.
Safety aspects:

• It acts as a simple asphyxiant at high concentrations by displacing oxygen.
• Combustion of ethylene produces carbon dioxide and water, but incomplete burning may release carbon monoxide.
• Industrial operations demand rigorous leak detection and fire prevention measures.

Environmental concerns:

• Ethylene production contributes significantly to carbon dioxide emissions due to the energy intensity of steam cracking.
• The compound itself is a volatile organic compound (VOC) that can participate in photochemical smog formation.
• Efforts are ongoing to develop sustainable production routes, including bio-based ethylene derived from ethanol and catalytic conversion of carbon dioxide to ethylene using renewable energy sources.

Advantages, Limitations, and Challenges

Advantages:

• High reactivity and versatility as a chemical feedstock.
• Central role in polymer, solvent, and chemical manufacturing.
• Cross-disciplinary relevance in both industry and biology.
• Well-established technologies for large-scale production and conversion.

Limitations and challenges:

• High energy requirement and reliance on fossil fuels.
• Emissions and environmental footprint of conventional processes.
• Safety hazards associated with storage and transport.
• Market fluctuations tied to oil and gas prices.

Ongoing innovation aims to address these limitations through new catalytic systems, process intensification, and circular carbon technologies.

Future Prospects and Research Directions

The future of ethylene production and utilisation is closely tied to sustainability goals and environmental responsibility. Current research focuses on green ethylene derived from bioethanol or carbon dioxide, aiming to decouple production from fossil resources. Electrochemical conversion methods and renewable-powered cracking technologies are in early stages of industrial adoption.
In agriculture and food technology, improved ethylene control through inhibitors, absorbers, and smart packaging materials is enhancing post-harvest management and reducing food waste. Meanwhile, the development of advanced catalysts and energy-efficient cracking furnaces promises to reduce industrial emissions.