C₄H₈ — Butene

Butene (C₄H₈) is an unsaturated hydrocarbon belonging to the alkene family. It contains four carbon atoms and eight hydrogen atoms, making it the second member of the alkene series after propene. Butene exists as a colorless, flammable gas with a mild gasoline-like odor and is used extensively in the petrochemical industry as a precursor for polymers, fuels, and synthetic lubricants.

Butene occurs in multiple isomeric forms: 1-butene, cis-2-butene, trans-2-butene, and isobutene (2-methylpropene). Each isomer differs in the position of the double bond and the arrangement of carbon atoms, leading to variations in their physical and chemical properties. Butene is commonly produced as a byproduct in the cracking of petroleum and natural gas and is used in the production of polybutene, butadiene, and other petrochemicals.

Butene exhibits both structural isomerism and geometric (cis-trans) isomerism due to the presence of a double bond. The four major isomers are:

• 1-Butene: \(CH_2=CH–CH_2–CH_3\) — the double bond is between the first and second carbon atoms.
• cis-2-Butene: \(CH_3–CH=CH–CH_3\) with both methyl groups on the same side of the double bond.
• trans-2-Butene: \(CH_3–CH=CH–CH_3\) with methyl groups on opposite sides of the double bond.
• Isobutene (2-Methylpropene): \((CH_3)_2C=CH_2\) — a branched-chain isomer with the double bond at the terminal position.

Each carbon atom involved in the double bond is sp² hybridized, forming a trigonal planar structure with bond angles of approximately 120°. The double bond consists of one sigma (σ) bond and one pi (π) bond. The π bond restricts rotation around the double bond, giving rise to geometric isomerism in 2-butene.

Butene is mainly produced through industrial processes involving the cracking of hydrocarbons and the dehydration of alcohols:

1. Steam Cracking of Hydrocarbons

Large quantities of butene are produced as a byproduct during the steam cracking of hydrocarbons like butane, naphtha, or propane at high temperatures (800–900°C):

This process also yields other alkenes such as ethylene and propylene.

2. Dehydration of Butanol

Butene can be prepared in the laboratory by dehydrating butanol in the presence of concentrated sulfuric acid or alumina catalyst:

3. Catalytic Dehydrogenation of Butane

In the petrochemical industry, butane is dehydrogenated at high temperature using metal oxide catalysts such as chromium oxide to yield butene:

Physical Properties:

• Butene is a colorless, flammable gas with a faint gasoline-like odor.
• It is lighter than air and forms explosive mixtures with oxygen.
• Its boiling point ranges between -6°C (1-butene) and 0.9°C (isobutene).
• It is slightly soluble in water but dissolves in organic solvents like ether, alcohol, and benzene.

Chemical Properties:

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

\(C_4H_8 + 6O_2 \rightarrow 4CO_2 + 4H_2O\)

• 2. Addition of Hydrogen: Butene adds hydrogen in the presence of a nickel catalyst to form butane:

\(C_4H_8 + H_2 \xrightarrow{Ni} C_4H_{10}\)

• 3. Addition of Halogens: Butene reacts with bromine to form dibromobutane, decolorizing bromine water — a test for unsaturation:

\(C_4H_8 + Br_2 \rightarrow C_4H_8Br_2\)

• 4. Hydrohalogenation: Butene reacts with hydrogen halides (like HBr) according to Markovnikov’s rule to yield secondary alkyl halides:

\(CH_2=CHCH_2CH_3 + HBr \rightarrow CH_3CHBrCH_2CH_3\)

• 5. Oxidation: Mild oxidation forms 1,2-butanediol, while strong oxidation produces carbon dioxide and water:

\(C_4H_8 + 2[O] \xrightarrow{} CH_3CH(OH)CH(OH)CH_3\)

• 6. Polymerization: Butene undergoes polymerization to form polybutene, a useful plastic material:

\(nC_4H_8 \xrightarrow{catalyst} [-CH_2CH(CH_2CH_3)-]_n\)

• 1. Polymer Production: Butene is used in the manufacture of polybutene and as a comonomer in producing linear low-density polyethylene (LLDPE).
• 2. Synthetic Rubber: Used in the production of butadiene, which is further polymerized to form synthetic rubber.
• 3. Fuel Industry: Used as a blending component in gasoline and as a precursor for synthetic fuels.
• 4. Chemical Synthesis: Butene serves as a feedstock for producing secondary butyl alcohol, methyl ethyl ketone, and other organic compounds.
• 5. Refrigeration: Used as a refrigerant and propellant in some aerosol applications.

Butene is considered a low-toxicity gas under normal exposure levels. However, it can act as an asphyxiant by displacing oxygen in confined spaces. Prolonged inhalation of high concentrations may cause dizziness or headaches.

Environmentally, butene is not considered a greenhouse gas, but it contributes to the formation of ground-level ozone and smog when released into the atmosphere. Therefore, industrial emissions must be monitored and minimized through proper safety controls.