C₃H₆ — Propylene

Propylene (C₃H₆), also called propene, is an unsaturated hydrocarbon belonging to the alkene family. It is a colorless, flammable gas with a faint petroleum-like odor and is one of the most important intermediates in the petrochemical industry. Propylene is mainly used to produce polypropylene plastic, one of the most versatile polymers found in packaging, textiles, automotive parts, and consumer products.

Propylene is produced in large volumes as a byproduct of petroleum refining and natural gas processing. Its double bond makes it chemically reactive, allowing it to undergo addition reactions, oxidation, and polymerization. Besides its industrial significance, propylene also plays an essential role in the synthesis of various organic chemicals like isopropanol, acrylonitrile, and propylene oxide.

Propylene’s chemical formula is \(C_3H_6\), and its structural formula is \(CH_3–CH=CH_2\). It contains a carbon–carbon double bond, making it an unsaturated hydrocarbon. The first two carbon atoms form a double bond, while the third carbon forms a single bond with the second carbon and three hydrogen atoms.

Each of the two doubly bonded carbon atoms in propylene is sp² hybridized, forming a trigonal planar arrangement with bond angles of approximately 120°. The third carbon atom is sp³ hybridized. The C=C double bond consists of one sigma (σ) bond and one pi (π) bond. The π bond restricts rotation around the double bond, giving propylene its characteristic planar geometry and reactivity typical of alkenes.

Propylene can be obtained by several industrial and laboratory processes:

1. Steam Cracking of Hydrocarbons

This is the main industrial method for producing propylene. Hydrocarbons like propane, naphtha, or ethane are heated with steam at high temperatures (800–900°C), leading to thermal cracking:

Propylene is then separated from other products like ethylene and butenes through fractional distillation.

2. Catalytic Dehydrogenation of Propane

Propylene is produced by the catalytic removal of hydrogen from propane using chromium or platinum catalysts:

This method is common in refineries and provides a high-purity source of propylene.

3. Byproduct of Petroleum Refining

Propylene is also obtained as a byproduct during the catalytic cracking of crude oil fractions, especially in the production of gasoline and other fuels.

4. Laboratory Preparation from Isopropanol

Propylene can be prepared in the laboratory by dehydrating isopropanol (isopropyl alcohol) using concentrated sulfuric acid:

Physical Properties:

• Propylene is a colorless and flammable gas with a faint sweet odor.
• It has a boiling point of -47.6°C and a melting point of -185.2°C.
• It is slightly soluble in water but dissolves well in ethanol and ether.
• Propylene is lighter than air and forms explosive mixtures with oxygen.

Chemical Properties:

• 1. Combustion: Propylene burns in oxygen to produce carbon dioxide and water:

\(C_3H_6 + 4.5O_2 \rightarrow 3CO_2 + 3H_2O\)

• 2. Hydrogenation: Propylene reacts with hydrogen in the presence of a nickel catalyst to form propane:

\(C_3H_6 + H_2 \xrightarrow{Ni} C_3H_8\)

• 3. Halogenation: Propylene reacts with halogens like chlorine or bromine to form dihaloalkanes:

\(CH_3CH=CH_2 + Br_2 \rightarrow CH_3CHBrCH_2Br\)

• 4. Hydrohalogenation: It reacts with hydrogen halides like HBr or HCl to form isopropyl halides, following Markovnikov’s rule:

\(CH_3CH=CH_2 + HBr \rightarrow CH_3CHBrCH_3\)

• 5. Oxidation: Controlled oxidation of propylene produces propylene oxide, while complete oxidation forms carbon dioxide and water:

\(CH_3CH=CH_2 + 1.5O_2 \xrightarrow{Ag} CH_3CHCH_2O\)

• 6. Polymerization: Under high temperature, pressure, and catalyst conditions, propylene polymerizes to form polypropylene:

\(nCH_3CH=CH_2 \xrightarrow{catalyst} [-CH_2-CH(CH_3)-]_n\)

• 1. Polymer Production: Propylene is primarily used to produce polypropylene, a lightweight, heat-resistant plastic used in containers, automotive parts, and fibers.
• 2. Chemical Synthesis: Used as a raw material for propylene oxide, acrylonitrile, cumene, and isopropanol.
• 3. Fuel Applications: Acts as a component in liquefied petroleum gas (LPG) and as a fuel for industrial heating and cutting.
• 4. Refrigerant Gas: Propylene is used as an environmentally friendly refrigerant (R-1270) in modern cooling systems.
• 5. Organic Synthesis: It serves as a starting material for the production of alcohols, aldehydes, and acids through oxidation and hydration reactions.
• 6. Agriculture: In small quantities, propylene can be used to influence fruit ripening similar to ethylene, though less efficiently.

Propylene is considered relatively non-toxic and safe for limited exposure. However, as a flammable gas, it poses a fire and explosion hazard when mixed with air. Inhalation of high concentrations may cause dizziness, nausea, or mild respiratory irritation due to oxygen displacement.

Environmental Aspects: Propylene is not classified as a greenhouse gas, but its oxidation in the atmosphere can contribute to the formation of ground-level ozone. Hence, controlled emission management is crucial in industrial plants.