Butene
Butene, also known as butylene, refers to a group of four isomeric alkenes with the molecular formula C₄H₈ that serve as fundamental building blocks in organic chemistry and the petrochemical industry. These isomers—1-butene, (2E)-but-2-ene (trans-2-butene), (2Z)-but-2-ene (cis-2-butene), and 2-methylpropene (isobutene)—are all unsaturated hydrocarbons featuring a single carbon-carbon double bond, which imparts reactivity suitable for polymerization and other synthetic processes.[1][2] Butenes are colorless gases with a slight aromatic odor under standard conditions, with low solubility in water (approximately 222–263 mg/L at 25°C) and high flammability, forming explosive mixtures with air; their molecular weight is 56.108 g/mol across all isomers. Physical properties vary slightly among the isomers, including boiling points ranging from -6.9°C for 2-methylpropene to 3.7°C for (2Z)-but-2-ene, and they exhibit log Kow values of 2.31–2.40, indicating moderate lipophilicity. They are primarily produced as byproducts from the steam cracking of naphtha or other petroleum fractions during ethylene and propylene manufacture, with additional routes including the catalytic or oxidative dehydrogenation of n-butane.[1][2][3] In industrial applications, butenes are versatile feedstocks for producing polymers, fuels, and chemicals; for instance, 1-butene is copolymerized with ethylene to form linear low-density polyethylene (LLDPE), while 2-methylpropene is used to synthesize butyl rubber via cationic polymerization with isoprene and to produce fuel oxygenates like methyl tert-butyl ether (MTBE) and high-octane gasoline components such as isooctane. Other uses include the manufacture of butadiene for synthetic rubbers like styrene-butadiene rubber and the production of butyl alcohols serving as solvents. Due to their reactivity, butenes require careful handling to mitigate risks of fire, explosion, and asphyxiation.[2][4][5]Overview
Chemical Identity
Butene refers to a class of organic compounds known as alkenes, which are unsaturated hydrocarbons characterized by the presence of at least one carbon-carbon double bond. The molecular formula for butene is C4H8, corresponding to a four-carbon chain incorporating exactly one such double bond.[1] These compounds are among the simplest olefins, following ethylene (C2H4) and propylene (C3H6), and are fundamental building blocks in organic chemistry due to their reactivity stemming from the double bond.[4] Butenes were first identified in the early 20th century during investigations into petroleum cracking processes, which revealed these gaseous alkenes as byproducts of breaking down larger hydrocarbon chains under heat.[6] The development of thermal cracking techniques enabled the systematic production and study of butenes from crude oil fractions. The degree of unsaturation for butene can be calculated using the general formula for hydrocarbons: [(2C + 2 - H)/2], where C is the number of carbon atoms and H is the number of hydrogen atoms. For C4H8, this yields [(2×4 + 2 - 8)/2] = 1, confirming the presence of one double bond or equivalent unsaturation feature characteristic of alkenes.[7] Butene exists in multiple structural and stereoisomers, each sharing this core formula and unsaturation level.[1]Nomenclature and Classification
Butene, with the molecular formula C4H8, is named according to IUPAC rules for alkenes by identifying the longest carbon chain containing the double bond, replacing the -ane suffix of the corresponding alkane (butane) with -ene, and assigning the lowest possible locant to the double bond's first carbon.[8] For example, the straight-chain isomer with the double bond between carbons 1 and 2 is but-1-ene, while the one between carbons 2 and 3 is but-2-ene; the chain is numbered from the end that gives the double bond the lowest number, prioritizing this over substituent locants if needed.[9] Butenes are classified into structural isomers based on the position and branching of the double bond. The primary structural isomers are 1-butene (a terminal alkene with the double bond at position 1), 2-butene (an internal alkene with the double bond at position 2), and 2-methylpropene (a branched isomer with the double bond at the end of a three-carbon chain attached to a methyl group).[4] These differ in connectivity: 1-butene and 2-butene share a linear four-carbon skeleton but vary in double bond placement (positional isomers), while 2-methylpropene features a branched structure. Stereoisomers arise in cases like 2-butene, where restricted rotation around the double bond allows cis and trans configurations; the cis isomer has both methyl groups on the same side, and the trans on opposite sides.[10] For more precise designation, especially with different substituents, IUPAC uses the E/Z system based on Cahn-Ingold-Prelog priority rules: higher-priority groups (ranked by atomic number) on the same side yield Z (from German zusammen, together), and on opposite sides yield E (entgegen, opposite); for 2-butene, the Z isomer corresponds to cis-2-butene, and E to trans-2-butene.[11] 1-Butene and 2-methylpropene lack such stereoisomers due to the double bond's terminal or symmetric substitution. Common names persist in industrial contexts, such as α-butylene for 1-butene, β-butylene for 2-butene (with cis and trans qualifiers), and isobutylene for 2-methylpropene, reflecting historical usage before widespread IUPAC adoption.[2]Isomers
1-Butene
1-Butene, systematically named but-1-ene, is the straight-chain terminal isomer of butene, characterized by the molecular formula C₄H₈ and the structural formula CH₂=CH-CH₂-CH₃. In this configuration, the carbon-carbon double bond is positioned between the first and second carbon atoms, making it an alpha-olefin. This terminal placement imparts distinct electronic and steric properties compared to internal butene isomers.[4] Key physical properties of 1-butene include a boiling point of -6.3 °C, a melting point of -185.3 °C, and a liquid density of 0.60 g/cm³ at 20 °C. These values reflect its volatile and low-viscosity nature as a gas under standard conditions, with a colorless appearance and mild odor. Compared to other butene isomers, 1-butene has a slightly lower boiling point, indicating higher volatility due to its linear structure.[12] As a terminal alkene, 1-butene displays heightened reactivity in polymerization processes, where the unsubstituted double bond facilitates easier coordination with catalysts like those in Ziegler-Natta systems, promoting efficient chain propagation. This reactivity enables its use in producing polybutene and as a key building block for branched polymers. Commercially, 1-butene holds significant importance as a comonomer in ethylene polymerization to manufacture linear low-density polyethylene (LLDPE), comprising over 60% of its global demand by enhancing polymer flexibility and strength.[13][14]2-Butene
2-Butene, systematically named but-2-ene, is a butene isomer characterized by the structural formula CH₃-CH=CH-CH₃, where the carbon-carbon double bond is positioned internally between the second and third carbon atoms. This internal placement allows for geometric isomerism, a feature absent in the terminal double bond of 1-butene. The restricted rotation about the double bond leads to two distinct stereoisomers: (E)-2-butene, the trans form with methyl groups on opposite sides, and (Z)-2-butene, the cis form with methyl groups on the same side.[15] The trans isomer, (E)-2-butene, is thermodynamically more stable than the cis isomer by approximately 1 kcal/mol, owing to minimized steric interactions between the adjacent methyl groups. This stability difference is evident in their respective heats of hydrogenation: cis-2-butene releases about 4 kJ/mol more energy upon hydrogenation to butane compared to the trans form. The cis isomer possesses a small net dipole moment of roughly 0.3 D due to the asymmetric arrangement of the methyl groups, rendering it weakly polar, whereas the trans isomer has a dipole moment of 0 D, making it nonpolar.[16][17] Physical properties of the isomers reflect these structural and electronic differences. Both are colorless, flammable gases under standard conditions, with densities around 0.62 g/cm³ at 20 °C—0.616 g/cm³ for cis and 0.604 g/cm³ for trans. Boiling points are 3.7 °C for (Z)-2-butene and 0.9 °C for (E)-2-butene, the higher value for the cis form arising from enhanced dipole-dipole interactions. Melting points show greater disparity: -138.9 °C for cis versus -105.5 °C for trans, as the symmetric trans structure enables more efficient molecular packing in the solid phase.[18][19] Interconversion between the cis and trans forms proceeds via thermal activation, which surmounts the high rotational barrier of about 60 kcal/mol around the double bond, or through catalytic methods at milder conditions. Acidic catalysts, such as ion-exchanged clays, or metal-based systems like Pd/Fe₃O₄, promote isomerization by facilitating protonation or surface adsorption mechanisms, often achieving equilibrium compositions favoring the more stable trans isomer.[20][21]2-Methylpropene
2-Methylpropene, also known as isobutene, is the branched structural isomer of butene with the molecular formula C₄H₈ and the structural formula (CH₃)₂C=CH₂, featuring a methyl group attached to the second carbon of the propene chain.[22] This configuration distinguishes it from the linear butene isomers, classifying it as a structural isomer within the butene family./Alkenes/Properties_of_Alkenes/Structure_of_Alkenes) Key physical properties of 2-methylpropene include a boiling point of -6.9 °C, a melting point of -140.3 °C, and a density of 0.60 g/cm³ at 20 °C, reflecting its gaseous state at standard temperature and pressure.[22] These values indicate high volatility, as evidenced by its vapor pressure of approximately 2,308 mm Hg at 25 °C.[22] The branching in its structure reduces the molecular surface area, leading to weaker van der Waals forces compared to linear isomers, which enhances its volatility and makes it suitable for applications requiring readily evaporable compounds.[23] Due to the symmetric substitution around the double bond—where one carbon bears two identical methyl groups—2-methylpropene exhibits no stereoisomers, lacking the possibility of cis-trans (geometric) isomerism present in some linear butenes.[22] This achiral nature simplifies its handling in reactions where stereochemistry is not a factor./Alkenes/Properties_of_Alkenes/Structure_of_Alkenes)Physical Properties
Thermodynamic Properties
The butene isomers exhibit distinct thermodynamic properties influenced by their molecular structures, with 1-butene and 2-methylpropene displaying lower boiling points due to their terminal or branched configurations, while cis- and trans-2-butene have slightly higher values owing to internal double bonds. Boiling points range from -7.0 °C for 2-methylpropene to 3.73 °C for cis-2-butene, and melting points vary widely from -185.3 °C for 1-butene to -105.5 °C for trans-2-butene, reflecting differences in intermolecular forces and packing efficiency in the solid state.[4][24][25][22]| Isomer | Boiling Point (°C) | Melting Point (°C) |
|---|---|---|
| 1-Butene | -6.3 | -185.3 |
| cis-2-Butene | 3.73 | -139.3 |
| trans-2-Butene | 0.88 | -105.5 |
| 2-Methylpropene | -7.0 | -140.3 |