1-Butene
1-Butene, also known as but-1-ene or alpha-butylene, is a linear alpha-olefin and one of the four isomeric alkenes with the molecular formula C₄H₈ and a molecular weight of 56.11 g/mol.[1] It features a straight-chain structure with a terminal carbon-carbon double bond between the first and second carbon atoms (CH₂=CH-CH₂-CH₃), distinguishing it from other butene isomers like cis-2-butene, trans-2-butene, and isobutene.[1] As a colorless, flammable gas at standard temperature and pressure, 1-butene exhibits a slightly aromatic odor and key physical properties including a boiling point of -6.47 °C, a melting point of -185.33 °C, and a liquid density of 0.62 g/cm³.[1] Chemically, it is reactive as an alkene, capable of polymerization and reactions with oxidizing agents, and it serves as a simple asphyxiant with extreme flammability (lower explosive limit of 1.6% and upper of 9.3% in air).[1] 1-Butene is primarily produced on an industrial scale as a byproduct of petroleum refining processes, such as fluid catalytic cracking and steam cracking of hydrocarbons, where it emerges from the C4 fraction.[2] Dedicated "on-purpose" production methods include the dimerization of ethylene using nickel-based catalysts in processes like the Alphabutol process, which yield high-purity 1-butene.[1] Global production exceeded 1.35 million metric tons in 2024, driven by demand in the petrochemical sector.[3] The compound's most significant application is as a comonomer in the copolymerization of ethylene to produce linear low-density polyethylene (LLDPE) and to modify high-density polyethylene (HDPE), improving properties like tensile strength and flexibility in plastics used for packaging, films, and pipes.[4] It also functions as a monomer in synthetic rubber production and as an intermediate for chemicals like butadiene, as well as in gasoline additives to enhance octane ratings. Due to its role in polymer manufacturing, the 1-butene market is projected to grow from approximately USD 3.3 billion in 2023 to USD 4.2 billion by 2034.[5]Structure and nomenclature
Molecular structure
1-Butene possesses the molecular formula C₄H₈ and the structural formula CH₂=CH-CH₂-CH₃.[1] The defining feature of its structure is the carbon-carbon double bond between the terminal carbon atoms, designated as C1 and C2, where both carbons exhibit sp² hybridization. This hybridization leads to a trigonal planar geometry around these carbons, with bond angles of approximately 120°.[1] The double bond consists of a σ bond and a π bond, contributing to the molecule's unsaturation and reactivity.[1] Because the double bond is terminal, with two identical hydrogen atoms attached to C1, 1-butene lacks the possibility of cis-trans (geometric) isomerism, unlike internal alkenes such as 2-butene.[1] The C=C double bond length is approximately 1.34 Å, shorter than the adjacent C-C single bonds at about 1.54 Å, reflecting the higher bond order of the double bond.[1] The overall molecular geometry features planarity around the C=C double bond, while the methylene (CH₂) and methyl (CH₃) groups at C3 and C4 adopt tetrahedral arrangements due to sp³ hybridization. 1-Butene exhibits a small dipole moment of approximately 0.35 D, arising from slight asymmetry despite its largely non-polar hydrocarbon nature.[6]Names and identifiers
1-Butene, an alkene, follows the IUPAC nomenclature for unsaturated hydrocarbons by specifying the position of the carbon-carbon double bond. Its preferred IUPAC name is but-1-ene.[7] Common synonyms include ethylethylene, α-butene, α-butylene, butene-1, and 1-butylene. These trivial names, such as ethylethylene, originated in early organic chemistry to describe the structure as an ethylene unit substituted with an ethyl group.[7][1] In chemical databases and regulatory contexts, 1-butene is assigned the following identifiers:| Identifier | Value |
|---|---|
| CAS Registry Number | 106-98-9[7] |
| EC Number | 203-449-2[8] |
| UN Number | 1012[9] |
| InChI | InChI=1S/C4H8/c1-3-4-2/h3H,1,4H2,2H3[10] |
| SMILES | CCC=C[10] |
Properties
Physical properties
1-Butene is a colorless gas at room temperature and standard atmospheric pressure. Its molecular formula is C₄H₈, with a molar mass of 56.11 g/mol. Due to the presence of the carbon-carbon double bond, 1-butene is more volatile than the saturated hydrocarbon n-butane, exhibiting a lower boiling point of -6.3 °C compared to -0.5 °C for n-butane. The physical properties of 1-butene under standard conditions are summarized in the following table:| Property | Value | Conditions/Notes |
|---|---|---|
| Density (liquid) | 0.626 g/cm³ | At boiling point (-6.7 °C) |
| Vapor density (air = 1) | 1.93 | - |
| Melting point | -185.3 °C | - |
| Boiling point | -6.3 °C | At 1 atm |
| Solubility in water | 0.22 g/L | At 25 °C |
| Solubility in organics | Soluble | In ethanol, ether, benzene |
| Heat of vaporization | 20.9 kJ/mol | At boiling point |
| Critical temperature | 146.4 °C | - |
| Critical pressure | 40.2 bar | - |
| Flammable limits in air | 1.6–9.3 vol% | Lower–upper |
| Flash point | -80 °C | - |
| Autoignition temperature | 385 °C | - |
Chemical properties
1-Butene is classified as a terminal alkene, characterized by a carbon-carbon double bond between the first and second carbon atoms, where the sp²-hybridized carbons result in high electron density at the π-bond, making it nucleophilic toward electrophiles.[11][1] This compound exhibits relative stability under ambient conditions but undergoes exothermic polymerization upon heating or exposure to catalysts, and it is susceptible to oxidation, potentially forming peroxides.[12][9] The π-bond in 1-butene displays weak basicity, while the allylic hydrogens possess a pKa of approximately 43, indicating low acidity.[13] Spectroscopically, 1-butene features an infrared absorption at approximately 1640 cm⁻¹ attributed to the C=C stretching vibration. In ¹H NMR, the vinyl protons appear as signals in the range of 4.9–5.9 ppm, with the alkyl chain protons showing shifts typical of methylene and methyl groups adjacent to the unsaturated system.[14][15] Under acidic conditions, 1-butene can isomerize to 2-butene via protonation of the double bond and subsequent hydride shift, reflecting its tendency toward more stable internal alkene isomers.[16]Production
Industrial production
1-Butene is primarily produced on an industrial scale through the separation of C4 hydrocarbon streams generated as byproducts from petroleum refineries, particularly via steam cracking of naphtha. In this process, a mixed C4 raffinate stream containing butanes, butenes, and butadiene is obtained, from which 1-butene is isolated using extractive distillation with polar solvents like acetonitrile or N-methylpyrrolidone to enhance selectivity, or through superfractionation involving multiple distillation columns to separate based on boiling point differences. This method accounts for the majority of global 1-butene supply, as steam cracking is a cornerstone of olefin production in the petrochemical industry.[1] A secondary but increasingly important route involves the selective dimerization of ethylene, where two molecules of ethylene are catalytically combined to form 1-butene with high specificity. This process employs Ziegler-Natta catalysts, typically titanium-based systems activated by alkylaluminum compounds, to achieve dimerization under mild conditions. A notable example is the Alphabutol process, which uses a titanium-based catalyst to produce 1-butene with over 90% selectivity. Another example is the nickel-catalyzed Shell Higher Olefin Process (SHOP), which integrates dimerization into a broader oligomerization scheme to produce a range of linear alpha-olefins, including 1-butene with high linearity in the C4 fraction while minimizing internal olefins like 2-butene. These on-purpose production methods are gaining traction to meet rising demand independent of refinery outputs.[17] Global production capacity for 1-butene reached approximately 2.8 million metric tons in 2023 and is projected to exceed 2.9 million tons by 2032, driven by expansions in Asia-Pacific facilities. Major producers include ExxonMobil, Shell, and LyondellBasell, which together control a significant share of the market through integrated petrochemical complexes. The 1-butene market was valued at around USD 3.5 billion in 2023 and is expected to grow to USD 5.8 billion by 2033, reflecting a compound annual growth rate (CAGR) of about 5.2%, primarily fueled by increasing demand for linear low-density polyethylene (LLDPE) production.[5][18][19] Following production, 1-butene undergoes purification via cryogenic distillation to achieve polymer-grade purity exceeding 99.5%, exploiting subtle differences in boiling points among butene isomers (1-butene at -6.3°C versus 2-butene at 0.9–3.7°C). This step is energy-intensive, often requiring low temperatures around -100°C and high reflux ratios, due to the close relative volatilities, but is essential for downstream applications.[20]Laboratory synthesis
In laboratory settings, 1-butene is commonly prepared on a small scale through the dehydration of 1-butanol, a primary alcohol that undergoes elimination of water under acidic conditions to form the alkene. This reaction can be catalyzed by concentrated sulfuric acid at temperatures of 140–180°C, where the mechanism proceeds via an E1 pathway involving carbocation formation, though careful temperature control is needed to minimize isomerization to 2-butene. Alternatively, vapor-phase dehydration over γ-alumina (γ-Al₂O₃) at 350–410°C yields a mixture of linear butenes with over 90% efficiency, favoring 1-butene as the primary product due to the catalyst's selectivity for terminal alkenes.[21][22] A standard setup for this dehydration involves passing 1-butanol vapor over the catalyst in a tube furnace, followed by condensation and fractional distillation to isolate pure 1-butene, as the crude product often contains 10–30% 2-butene and trace butadiene. Yields of isolated 1-butene typically range from 70–90%, depending on the catalyst and conditions, with distillation columns or low-temperature traps essential for separating the lower-boiling 1-butene (boiling point –6.3°C) from isomers.[22] Another established route is the elimination reaction of primary butyl halides, such as 1-bromobutane or 1-chlorobutane, with alcoholic KOH, which promotes an E2 mechanism to directly afford 1-butene by abstracting a β-hydrogen and the halide. The reaction is typically conducted by refluxing the halide in ethanol with KOH at 78°C for several hours, producing 1-butene as the major product with minimal substitution due to the strong base and protic solvent. Yields are generally 70–85%, and purification again relies on distillation to remove unreacted halide, ethanol, and any 2-butene formed via minor pathways.[23] Partial hydrogenation of 1,3-butadiene represents a third laboratory method, selectively adding one equivalent of hydrogen to the diene using poisoned palladium catalysts to halt at the monoene stage and favor the 1-butene isomer. Catalysts such as Pd supported on carbon or in ionic liquids, often modified with additives like quinoline or lead to suppress over-hydrogenation, achieve high selectivity (up to 90%) for 1-butene at room temperature and moderate hydrogen pressure (1–5 atm). Overall butene yields reach 80–95%, but fractional distillation or gas chromatography is required to separate 1-butene from cis- and trans-2-butene byproducts. Historically, 1-butene was prepared via the thermal pyrolysis of esters like n-butyl acetate, which decomposes in the gas phase at 400–500°C to yield 1-butene and acetic acid through a concerted six-membered transition state elimination. This method, though less common in modern labs due to the availability of simpler precursors, provided yields of 60–80% and was valued for its clean byproduct profile, with distillation used for purification.Chemical reactions
Addition reactions
1-Butene, as a terminal alkene, undergoes electrophilic addition reactions at its carbon-carbon double bond, where the electron-rich π-bond attacks electrophiles, leading to saturated products. These reactions follow standard mechanisms for alkenes, with regioselectivity often governed by Markovnikov's rule, which predicts that the hydrogen atom adds to the carbon with more hydrogens, forming the more stable carbocation intermediate.[11] Exceptions, such as in hydroboration, provide anti-Markovnikov selectivity.[24]Hydrohalogenation
In hydrohalogenation, 1-butene reacts with hydrogen halides like HCl in an electrophilic addition mechanism. The first step involves protonation of the double bond by H⁺, forming a secondary carbocation at the internal carbon (C2), as this is more stable than the primary carbocation at C1. The chloride ion then attacks the carbocation, yielding 2-chlorobutane as the major product, consistent with Markovnikov's rule.[25][26] The reaction proceeds without stereoselectivity due to the planar carbocation intermediate, producing a racemic mixture if a chiral center forms.[11] The overall equation is: \ce{CH2=CH-CH2-CH3 + HCl -> CH3-CHCl-CH2-CH3} This addition is regioselective, with minimal formation of 1-chlorobutane under standard conditions.[25]Hydration
Acid-catalyzed hydration of 1-butene involves addition of water across the double bond, catalyzed by strong acids like H₂SO₄. The mechanism begins with protonation of the alkene to generate the secondary carbocation at C2, followed by nucleophilic attack by water on the carbocation, and deprotonation to form 2-butanol. This follows Markovnikov regiochemistry, placing the OH group on the more substituted carbon.[27][26] The reaction is reversible and requires heating to drive equilibrium toward the alcohol, often using dilute acid to minimize side reactions like elimination.[27] Unlike hydrohalogenation, a small amount of rearrangement can occur if the carbocation rearranges, but for 1-butene, the secondary carbocation is stable. The product is: \ce{CH2=CH-CH2-CH3 + H2O ->[H+] CH3-CH(OH)-CH2-CH3} This method provides a synthetic route to secondary alcohols from terminal alkenes.[26]Halogenation
Halogenation of 1-butene with bromine (Br₂) proceeds via electrophilic addition, forming a three-membered bromonium ion intermediate on the less substituted face of the double bond. The bromide ion then attacks from the opposite side, resulting in anti addition and the formation of 1,2-dibromobutane.[11][28] The reaction is stereospecific, producing a racemic mixture of enantiomers from the achiral alkene, and occurs readily in inert solvents like CCl₄ without light or peroxides to avoid radical pathways.[11] The equation is: \ce{CH2=CH-CH2-CH3 + Br2 -> BrCH2-CHBr-CH2-CH3} This vicinal dibromide is useful for further transformations, such as dehydrohalogenation to alkynes.[28]Hydrogenation
Catalytic hydrogenation of 1-butene adds hydrogen across the double bond to produce n-butane, using heterogeneous catalysts like platinum (Pt) or palladium (Pd) under mild conditions (room temperature to 100°C, 1-5 atm H₂). The mechanism involves adsorption of the alkene and H₂ onto the metal surface, followed by sequential addition of hydrogen atoms in a syn manner, though the heterogeneous nature often leads to non-stereospecific overall addition for simple alkenes.[29][26] Nickel catalysts like Raney Ni can also be employed, especially for larger scales.[26] The reaction is highly exothermic and quantitative, fully saturating the double bond without isomerization under standard conditions. The equation is: \ce{CH2=CH-CH2-CH3 + H2 ->[Pt or Pd] CH3-CH2-CH2-CH3} This process is a key step in determining unsaturation in organic compounds via hydrogen uptake.[29]Hydroboration-oxidation
Hydroboration-oxidation provides an anti-Markovnikov hydration of 1-butene, where borane (BH₃) adds across the double bond with boron attaching to the less substituted carbon (C1) due to steric and electronic factors in a concerted, syn addition. Subsequent oxidation with hydrogen peroxide (H₂O₂) and sodium hydroxide (NaOH) replaces boron with OH, yielding 1-butanol.[24] The reaction is stereospecific (syn) and regioselective, avoiding carbocation rearrangements, and proceeds at low temperatures (0-25°C) in ether solvents.[24] Unlike acid-catalyzed hydration, no secondary alcohol forms, making it complementary for primary alcohol synthesis. The two-step process is:- \ce{CH2=CH-CH2-CH3 + BH3 -> (CH3-CH2-CH2-CH2)3B}
- \ce{(CH3-CH2-CH2-CH2)3B + H2O2, NaOH -> 3 CH3-CH2-CH2-CH2-OH}