Ethene, commonly known as ethylene, is the simplest organic molecule in the alkene family, characterized by a double bond between its two carbon atoms. This small, colorless hydrocarbon gas is one of the most important organic chemicals globally due to its unique reactivity. The vast majority of commercially produced ethene is obtained through steam cracking, where hydrocarbons from petroleum and natural gas are heated to extreme temperatures. Ethene serves as a foundational building block for the petrochemical industry and as a powerful regulator in the natural world.
Manufacturing Polyethylene
The single largest application for ethene is its use as a monomer in the production of polyethylene (PE), a versatile plastic that accounts for approximately 60% of the world’s ethene consumption. This transformation occurs through polymerization, a process where thousands of individual ethene molecules are chemically linked together to form long, repeating polymer chains. The specific reaction conditions, including pressure, temperature, and the choice of catalyst, determine the final structure and properties of the resulting polyethylene.
High-Density Polyethylene (HDPE) is manufactured using low-pressure processes, often involving Ziegler-Natta or metal oxide catalysts. This method results in polymer chains that are mostly linear and pack tightly together, yielding a material that is rigid, strong, and opaque. HDPE is widely used for durable goods like pipes, detergent bottles, and industrial drums due to its high strength-to-density ratio.
Low-Density Polyethylene (LDPE), by contrast, is produced under very high pressures and temperatures using a free-radical initiation mechanism. This process introduces numerous short and long branches along the polymer chain, preventing the chains from packing closely. The resulting structure makes LDPE flexible, translucent, and ideal for applications like plastic bags, squeeze bottles, and various types of film packaging.
Linear Low-Density Polyethylene (LLDPE) is a third major type, structurally similar to HDPE but including co-monomers to create short, uniform branches. LLDPE offers superior tensile strength and puncture resistance compared to traditional LDPE. This makes it the preferred choice for stretch film, agricultural films, and heavy-duty trash liners.
Creating Key Chemical Intermediates
Ethene serves as a fundamental intermediate for synthesizing a wide variety of non-polymeric industrial chemicals. These derivatives are used across diverse sectors, including textiles, automotive, and construction. The manufacturing process often involves chemically modifying the ethene molecule’s double bond to attach other functional groups.
One significant derivative is ethylene oxide (EO), produced by the direct oxidation of ethene over a silver catalyst, accounting for about 16% of ethene demand. EO is primarily converted into ethylene glycol, which serves as the main ingredient in automotive antifreeze and coolants. Ethylene glycol is also a component in the production of polyethylene terephthalate (PET), used to make polyester fibers and plastic bottles.
Ethene is also a precursor for vinyl chloride monomer (VCM), which is then polymerized to create Polyvinyl Chloride (PVC). The initial step involves reacting ethene with chlorine to form ethylene dichloride, which is subsequently converted into VCM. PVC is a durable, flame-resistant plastic used extensively for water and sewage pipes, window frames, vinyl siding, and medical devices, utilizing approximately 11% of the world’s ethene output.
Industrial ethanol, the solvent found in many cleaning products and chemical processes, can also be synthesized from ethene. This production route involves the hydration of ethene, where water is added across the double bond, typically in the presence of an acid catalyst.
Ethene’s Role in Plant Biology
Ethene functions as a gaseous phytohormone, a naturally occurring signaling molecule that regulates growth and development in plants. This role is associated with fruit ripening, where its production triggers biochemical changes that soften the flesh, alter color, and convert starches into sugars. Many climacteric fruits, such as bananas, apples, and tomatoes, rely on an increase in ethene to initiate their final maturation stage.
Commercial growers use this knowledge by harvesting produce when it is still hard and green, then treating it with controlled amounts of ethene gas before shipping. This post-harvest application allows for precise control over the ripening timeline, minimizing spoilage during transportation. The hormone also plays a role in senescence, promoting the wilting of flowers and the shedding of leaves and fruit from the plant stem.
Ethene also mediates various plant stress responses, including the “triple response” observed in seedlings growing underground, which causes the shoot to grow shorter and thicker to push through soil obstacles. The plant’s internal regulation of ethene levels is a finely tuned system. This allows it to adapt its growth pattern in response to mechanical wounding, flooding, or other environmental changes.