Let's Study What Basically an Alkene is?
An alkene is a hydrocarbon with a carbon–carbon double bond in chemistry. The term is often used interchangeably with olefin, which refers to any hydrocarbon with one or more double bonds. However, the IUPAC recommends using the term "alkene" only for acyclic hydrocarbons with one double bond; alkadiene, alkatriene, etc., or polyene for acyclic hydrocarbons with two or more double bonds; cycloalkene, cyclooctadiene, etc., for cyclic hydrocarbons; and "olefin" for all cyclic or acyclic hydrocarbons with one or more double bonds.
The stability of the double bond is affected by alkyl groups bound to the sp2 hybridised carbon atoms of alkenes. The number of alkyl groups bound to the sp2 hybridised carbon atoms can also influence the chemical reactivity of alkenes. As a result, alkenes can be classified according to the number of alkyl groups attached to the C=C structural unit. The degree of substitution is the term for this function.
Monosubstituted alkenes have a single alkyl group bound to the sp2 hybridised carbon atom of the double bond. A terminal alkene is an alkene with its double bond at the end of the carbon atom chain. Disubstituted, trisubstituted, and tetrasubstituted alkenes have two, three, or four alkyl groups bound to the carbon atoms of the double bond, respectively.
This article will study butene structure, butene formula, butene structural formula and isomers of butene.
Butene Formula
Butene is an alkene with the formula C4H8 that is also known as butylene. Butene may refer to all of the compounds individually. They are colourless gases that are present in crude oil as a minor constituent in amounts that make extraction impossible. Butene is made by catalytic cracking of long-chain hydrocarbons left over from crude oil refining. Fractional distillation is used to remove butene from a mixture of products produced by cracking.
Butene Structure
Given below is the butene structure:
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Preparation of Butene
Butenes are produced commercially by catalytic dehydrogenation (elimination of hydrogen atoms from the molecule) of butanes, which occurs during the cracking (breaking down of large molecules) of petroleum to create gasoline. The majority of butenes are used in the manufacture of octanes, which are essential components of gasoline. This is accomplished by either allowing the butenes to react with isobutane or by dimerizing (combining two molecules of) butenes to produce octenes, which are then hydrogenated to produce octanes.
Isomers of Butene
Here are Some Structural Isomers of Butene
As already studied, the butene formula is C4H8. These four hydrocarbons all have four carbon atoms and one double bond in their molecules, but their chemical structures are different. These chemical compounds have the following IUPAC and common names:
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Properties of Structural Isomers of Butene
At room temperature and pressure, all four of these isomers are gases, but they can be liquefied by lowering the temperature or increasing the pressure on them, similar to pressurised butane. These gases are colourless but have distinct odours, and they are extremely flammable. While they are not found in high concentrations in petroleum, they can be made from petrochemicals or by catalytic cracking. The carbon-carbon double bonds make them more reactive than related alkanes, which are more inert compounds in different ways.
These 4-carbon alkenes can serve as monomers in the formation of polymers and have other uses as petrochemical intermediates due to their double bonds. They are used to manufacture synthetic rubber. Isobutylene is a branched alpha-olefin, while but-1-ene is a linear or regular alpha-olefin. But-1-ene, along with other alpha-olefins, is used as one of the comonomers in the manufacture of high-density polyethylene and linear low-density polyethylene in a small percentage. Butyl rubber is generated by cationic polymerization of isobutylene with 2–7% isoprene. Isobutylene is also used to make methyl tert-butyl ether (MTBE) and isooctane, all of which are good for the environment.
Did You Know?
The three sp2 hybrid orbitals of each carbon in the double bond are used to form sigma bonds with three atoms (the other carbon and two hydrogen atoms). The pi bond is formed by the unhybridized 2p atomic orbitals that lie perpendicular to the plane provided by the axes of the three sp2 hybrid orbitals. This bond is located outside of the main C–C axis, with half of the bond on one side and the other. The pi bond is slightly weaker than the sigma bond, with a strength of 65 kcal/mol.
Since it requires energy to break the alignment of the p orbitals on the two carbon atoms, rotation around the carbon–carbon double bond is limited. As a result, substituted alkenes can be divided into two types of isomers: cis and trans isomers. For molecules with three or four different substituents, the E–Z notation may be used to label more complex alkenes (side groups). For example, in (Z)-but-2-ene ( cis-2-butene), the two methyl groups are on the same side of the double bond, while in (E)-but-2-ene ( trans-2-butene), the methyl groups are on opposite sides. Butene's two isomers have different properties.
FAQs on Butene
1. What is butene and what is its chemical formula?
Butene is an alkene, which is a type of unsaturated hydrocarbon. Its general chemical formula is C₄H₈. This formula indicates that each molecule of butene contains four carbon atoms and eight hydrogen atoms, featuring one carbon-carbon double bond (C=C), which is characteristic of all alkenes.
2. What are the main structural isomers of butene?
Butene (C₄H₈) has three primary structural isomers, which have the same molecular formula but different structural arrangements:
- But-1-ene: A linear four-carbon chain with the double bond between the first and second carbon atoms.
- But-2-ene: A linear four-carbon chain with the double bond between the second and third carbon atoms.
- 2-Methylpropene (Isobutylene): A branched-chain isomer where a central carbon atom is bonded to two other carbon atoms and is double-bonded to a third.
3. Why does but-2-ene exhibit geometrical isomerism while but-1-ene does not?
Geometrical (cis-trans) isomerism occurs due to restricted rotation around the C=C double bond, and it requires each carbon atom in the double bond to be attached to two different groups.
- In but-2-ene (CH₃-CH=CH-CH₃), each carbon of the double bond is attached to a hydrogen atom (-H) and a methyl group (-CH₃). Since the groups are different, they can be arranged on the same side (cis-2-butene) or on opposite sides (trans-2-butene).
- In but-1-ene (CH₂=CH-CH₂-CH₃), the first carbon atom of the double bond is attached to two identical hydrogen atoms. Since the two groups on this carbon are identical, no distinct spatial arrangement is possible, and thus it cannot exhibit geometrical isomerism.
4. What are the most important industrial uses of butene?
Butene isomers are valuable petrochemicals used in several major industries. Their primary applications include:
- Fuel Additives: They are converted into high-octane gasoline components like isooctane and methyl tert-butyl ether (MTBE) to improve engine performance.
- Polymer Production: But-1-ene serves as a comonomer in the manufacturing of plastics like high-density polyethylene (HDPE) and linear low-density polyethylene (LLDPE).
- Synthetic Rubber: The isomer 2-methylpropene (isobutylene) is a key monomer used to produce butyl rubber, essential for manufacturing tyre inner tubes and other sealants.
5. How can you chemically distinguish between but-1-ene and but-2-ene?
A reliable chemical method to distinguish between but-1-ene and but-2-ene is ozonolysis. This reaction cleaves the double bond and produces different carbonyl compounds depending on the original structure.
- When but-1-ene reacts with ozone followed by a reductive workup (Zn/H₂O), it produces methanal (formaldehyde) and propanal.
- When but-2-ene undergoes the same reaction, it produces two molecules of ethanal (acetaldehyde).
Identifying these different products allows for a clear differentiation between the two isomers.
6. How is butene produced on a commercial scale?
Butene is primarily produced from petroleum through two main industrial processes. The most common method is the catalytic cracking of long-chain hydrocarbons. In this process, large, less useful hydrocarbon molecules are broken down at high temperatures into smaller, more valuable molecules like butene. Another method is the catalytic dehydrogenation of butanes, where hydrogen atoms are removed from butane to create the C=C double bond, forming butene.
7. Why is trans-2-butene more stable than cis-2-butene?
The greater stability of trans-2-butene compared to its cis-isomer is explained by the concept of steric hindrance.
- In cis-2-butene, the two bulky methyl (-CH₃) groups are located on the same side of the double bond. This proximity causes electronic repulsion and physical crowding, creating steric strain and raising the molecule's energy.
- In trans-2-butene, the methyl groups are on opposite sides of the double bond, keeping them far apart. This arrangement minimises steric strain, resulting in a lower energy state and therefore, greater stability.
