C₂H₄ — Ethylene

Ethylene (C₂H₄) is the simplest member of the alkene family and one of the most important organic chemicals in the world. It is a colorless, flammable gas with a faintly sweet odor and serves as a major feedstock in the petrochemical industry. Ethylene is produced in large quantities for the manufacture of plastics like polyethylene, ethylene oxide, and other industrial chemicals.

Interestingly, ethylene also occurs naturally in plants as a plant hormone that regulates processes like fruit ripening, leaf abscission, and flower wilting. Due to its dual significance in both chemistry and biology, ethylene is studied across industrial chemistry, environmental science, and botany.

Ethylene has the chemical formula \(C_2H_4\), indicating two carbon and four hydrogen atoms. Each carbon atom in ethylene forms three sigma (σ) bonds — two with hydrogen atoms and one with the other carbon atom. The remaining unhybridized p orbitals on each carbon overlap to form a pi (π) bond, giving rise to a double bond (one σ and one π bond) between the carbon atoms.

This double bond is responsible for the molecule’s planar geometry and reactivity. Each carbon atom is sp² hybridized and forms a trigonal planar structure with bond angles of approximately 120°. The π bond restricts rotation around the double bond, making ethylene rigid compared to alkanes like ethane.

Ethylene can be prepared in laboratories and industries through several methods:

1. Laboratory Preparation by Dehydration of Ethanol

In the laboratory, ethylene is prepared by heating ethanol with concentrated sulfuric acid at about 170°C. The sulfuric acid acts as a dehydrating agent, removing water from ethanol:

2. Industrial Production by Cracking of Hydrocarbons

In industry, ethylene is mainly obtained by steam cracking of hydrocarbons like ethane, propane, or naphtha at high temperatures (750–900°C):

This method is highly efficient and forms the basis of modern petrochemical industries.

3. From Ethyl Halides

Ethylene can also be prepared by dehydrohalogenation of ethyl halides using alcoholic KOH:

4. From Natural Gas

Ethylene is also obtained from natural gas through thermal or catalytic cracking processes used in large-scale manufacturing.

Physical Properties:

• Ethylene is a colorless gas with a faint sweet odor.
• Boiling point: -103.7°C; Melting point: -169.2°C.
• It is lighter than air and burns with a luminous flame.
• Insoluble in water but soluble in organic solvents like ether and benzene.

Chemical Properties:

• 1. Combustion: Ethylene burns in oxygen to produce carbon dioxide and water with the release of heat.

\(C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O\)

• 2. Addition of Hydrogen: In the presence of a nickel catalyst, ethylene adds hydrogen to form ethane (hydrogenation reaction).

\(C_2H_4 + H_2 \xrightarrow{Ni} C_2H_6\)

• 3. Addition of Halogens: Ethylene reacts with bromine to form 1,2-dibromoethane, decolorizing bromine water — a common test for unsaturation.

\(C_2H_4 + Br_2 \rightarrow C_2H_4Br_2\)

• 4. Addition of Hydrogen Halides: When hydrogen halides like HCl or HBr are added to ethylene, haloalkanes (ethyl halides) are formed.

\(C_2H_4 + HBr \rightarrow C_2H_5Br\)

• 5. Oxidation: Ethylene undergoes mild oxidation with oxygen to produce ethylene oxide, and under strong oxidation, it forms carbon dioxide and water.

\(C_2H_4 + O_2 \xrightarrow{Ag} C_2H_4O\)

• 6. Polymerization: Ethylene polymerizes under pressure and heat to form polyethylene, one of the most widely used plastics.

\(nC_2H_4 \xrightarrow{heat,pressure,catalyst} [-CH_2-CH_2-]_n\)

• 1. Manufacture of Plastics: Ethylene is the main raw material for producing polyethylene, used in packaging, films, and pipes.
• 2. Production of Chemicals: It is used in synthesizing ethylene oxide, ethylene glycol, and ethanol.
• 3. Ripening of Fruits: Ethylene acts as a natural plant hormone that accelerates fruit ripening and is used commercially for ripening bananas, tomatoes, and citrus fruits.
• 4. Welding and Cutting: Ethylene mixed with oxygen forms oxy-ethylene flame, used for metal cutting and welding.
• 5. Anesthetic Use: Ethylene was historically used as a mild anesthetic gas before safer alternatives became available.
• 6. Laboratory and Industrial Synthesis: It is used in the preparation of ethanol, styrene, and vinyl chloride.

Ethylene is considered a low-toxicity gas but can act as an asphyxiant at high concentrations by displacing oxygen. It is not considered carcinogenic but should be handled with care since it is highly flammable.

Environmentally, ethylene plays a dual role — it naturally regulates plant growth but, in large concentrations, can contribute to tropospheric ozone formation, leading to air pollution. Controlled use in agriculture and safe handling in industrial settings are therefore essential.