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Terminal alkene

A terminal alkene, also known as an α-olefin, is an unsaturated hydrocarbon featuring a carbon-carbon double bond at the end of the carbon chain, with the general formula R-CH=CH₂, where R represents a hydrogen atom or an alkyl group. In industrial contexts, α-olefins often refer to linear terminal alkenes with four or more carbon atoms, excluding ethylene and propylene which are produced separately in much larger quantities. These compounds belong to the broader class of alkenes, which have the molecular formula CₙH₂ₙ for acyclic structures, and the terminal position of the double bond distinguishes them from internal alkenes where the double bond occurs within the chain. Common examples include ethene (H₂C=CH₂), propene (CH₃-CH=CH₂), and 1-butene (CH₃CH₂-CH=CH₂), with naming following IUPAC conventions that assign the lowest number to the double bond, often resulting in a "1-" prefix for terminal cases. Terminal alkenes exhibit physical properties similar to those of alkanes but are influenced by the presence of the . They are nonpolar, colorless compounds that are insoluble in yet soluble in nonpolar solvents like , with densities typically ranging from 0.6 to 0.7 g/mL, making them less dense than . Their varies with molecular weight: smaller terminal alkenes such as ethene (boiling point -104°C) and propene (boiling point -47.6°C) are gases at , while those with 5 to 17 carbon atoms, like 1-pentene (boiling point 30°C), are liquids, and longer chains (18+ carbons) form solids. Boiling points increase with chain length due to enhanced van der Waals forces, and terminal alkenes generally have slightly lower s than their internal isomers of the same carbon number because of reduced . Chemically, terminal alkenes are more reactive than alkanes owing to the electron-rich π-bond in the , which readily undergoes reactions. Key reactions include catalytic to form alkanes, hydrohalogenation following (where hydrogen adds to the less substituted carbon), and hydroboration-oxidation yielding anti-Markovnikov alcohols. In ozonolysis with reductive workup, terminal alkenes produce (H₂C=O) from the =CH₂ group alongside a carbonyl compound from the internal carbon. They do not exhibit geometric isomerism due to the identical hydrogens on the terminal =CH₂ carbon but can polymerize via addition mechanisms to form materials like . Terminal alkenes, particularly linear α-olefins, hold significant industrial importance as versatile feedstocks in petrochemical processes. They serve as co-monomers in the production of (LLDPE), which enhances the strength and flexibility of plastic films and packaging. Additionally, they are intermediates for synthesizing detergents, plasticizers, synthetic lubricants, and epoxides; as of 2023, global production of linear alpha-olefins was approximately 7.7 million metric tons.

Definition and Classification

Structural Definition

A terminal alkene is defined as an unsaturated containing a carbon-carbon positioned at the end of the carbon chain, specifically between the first (terminal) and second carbon atoms, with the general \ce{CH2=CHR}, where R represents a or an . This structure features a terminal =CH₂ group, distinguishing it as a monosubstituted . The two carbon atoms involved in the double bond exhibit sp² hybridization, resulting in a trigonal planar around each of these carbons. This hybridization arises from the overlap of one s and two p orbitals to form three sp² hybrid orbitals, with the remaining p orbital contributing to the π bond of the . The bond angles in this arrangement are approximately 120°, ensuring that the double-bonded carbons and their attached substituents lie in a single plane. In contrast to internal alkenes, which have the double bond situated between non-terminal carbons (general formula \ce{RCH=CHR'}, where R and R' are alkyl groups or hydrogen), terminal alkenes possess the double bond at the chain terminus. This terminal positioning results in less steric hindrance around the double bond compared to more substituted internal alkenes, influencing reactivity by facilitating greater accessibility for approaching reagents in addition reactions. A simple textual representation of the prototypical terminal alkene, ethene, is \ce{H2C=CH2}, where both carbons are equivalently terminal.

Nomenclature and Examples

Terminal alkenes are named according to IUPAC rules by selecting the longest continuous carbon chain containing the , replacing the suffix "-ane" with "-ene", and numbering the chain starting from the end that includes the to assign it the lowest , which is invariably position 1. This ensures the is prioritized in the . In common or historical usage, particularly in industrial settings, the simplest terminal alkenes retain older names: ethene is called , and propene is known as , reflecting their early discovery and commercial importance. Representative examples include (H₂C=CH₂, C₂H₄, molecular weight 28.05 g/mol), the simplest and a key building block in ; propene (H₂C=CH-CH₃, C₃H₆, molecular weight 42.08 g/mol), widely used in production; (H₂C=CH-CH₂-CH₃, C₄H₈, molecular weight 56.11 g/mol), an important olefin in processes; and 1-pentene (H₂C=CH-CH₂-CH₂-CH₃, C₅H₁₀, molecular weight 70.13 g/mol), illustrating higher homologs in the series. For unbranched chains, terminal alkenes feature the fixed at carbon 1, resulting in no positional isomers of the double bond within the chain, in contrast to internal alkenes that can have the bond at multiple internal positions.

Synthesis and Production

Industrial Production

The primary industrial method for producing terminal alkenes, particularly ethene, is of feedstocks such as or . In this process, the feedstock is mixed with and heated to temperatures of 750–900°C in tubular reactors within a furnace, where thermal breaks C–C bonds to form smaller unsaturated hydrocarbons, with ethene as the predominant terminal alkene product. This method accounts for the vast majority of global terminal alkene output, with ethene production reaching over 200 million metric tons per year as of 2024. For propene, another key terminal alkene, a significant portion is produced via catalytic dehydrogenation of , often as a complementary process to . This endothermic reaction occurs at 500–650°C and low pressure using supported catalysts such as platinum-tin on alumina (e.g., in the Oleflex process) or chromium oxide on alumina (e.g., in the Catofin process), achieving selectivities above 90% under optimized conditions. Propene output from such dehydrogenation plants has grown to meet rising demand, with global capacity approximately 20 million metric tons as of 2024. Major producers include and , operating large-scale facilities integrated with refineries; for instance, ExxonMobil's complexes in the Gulf Coast and contribute significantly to ethene supply. Energy consumption for ethene production via typically ranges from 30–40 GJ per metric ton, predominantly from for furnaces and generation. Environmental impacts are substantial, with global emitting over 260 million metric tons of CO₂ annually, equivalent to approximately 0.7% of global energy-related anthropogenic CO₂ emissions in 2023. Purification of terminal alkenes involves a series of steps post-cracking: rapid to halt reactions, compression, and cryogenic in towers to isolate ethene and propene from byproducts like , , and internal alkenes, often followed by using solvents such as to achieve polymer-grade purity (>99.9%). Higher terminal alkenes, known as linear α-olefins (LAOs) with 4 or more carbon atoms, are primarily produced through the oligomerization of using catalysts, such as nickel-based systems in the Higher Olefin Process (SHOP). This process generates a distribution of LAOs, which are then isomerized and disproportionated to adjust chain lengths, yielding products used as co-monomers in and for detergents. Global LAO production exceeds 3 million metric tons annually as of 2021, with major producers including Chevron Phillips and .

Laboratory Methods

One prominent laboratory method for synthesizing terminal alkenes involves the , which converts s or ketones to alkenes using s. In this process, a non-stabilized such as methylenetriphenylphosphorane (Ph₃P=CH₂) reacts with an (RCHO) to yield a terminal alkene (R-CH=CH₂) and as a byproduct. The mechanism proceeds via of the to the carbonyl carbon, forming a betaine that cyclizes to a four-membered oxaphosphetane ring; this ring then undergoes stereospecific decomposition to the alkene and phosphine . The of the product depends on the type: non-stabilized s typically favor the Z- through a cis-oxaphosphetane , while stabilized s (with electron-withdrawing groups) produce predominantly E-s via a trans pathway. Another key approach is the , an E2-type reaction that generates terminal alkenes from quaternary ammonium salts under basic conditions. For instance, treatment of butyltrimethylammonium hydroxide (CH₃CH₂CH₂CH₂N(CH₃)₃⁺ OH⁻) with heat affords as the major product, along with and water. This method favors the less substituted (terminal) alkene due to the bulky trimethylammonium , which directs elimination toward the hydrogen on the least hindered carbon, contrasting with Zaitsev selectivity in typical E2 reactions. The reaction requires exhaustive of the parent amine followed by hydroxide exchange, making it suitable for preparing terminal alkenes from primary alkyl chains. Olefin metathesis provides a versatile route to terminal alkenes using ruthenium-based catalysts, enabling the exchange of alkylidene groups between s. A common cross-metathesis involves reacting ethene with an internal (R-CH=CH-R') in the presence of the second-generation (a ruthenium complex) to produce the terminal alkene (R-CH=CH₂) and another alkene byproduct, often at in solvents like . The follows a Chauvin pathway, where the metal initiates [2+2] cycloadditions and redistributions with the olefin substrates, allowing selective formation of terminal products when one partner is a highly reactive terminal like ethene. These laboratory methods typically achieve yields of 70-95% for terminal alkenes, depending on substrate compatibility and conditions, with the often reaching 80-90% for unhindered aldehydes. However, limitations include the formation of side products such as internal alkenes from self-metathesis in reactions, which can reduce selectivity unless excess ethene is used, and waste in the Wittig process that requires additional purification steps. In Hofmann eliminations, over-methylation or competing substitutions may lower yields below 75% for complex amines.

Properties

Physical Properties

Terminal alkenes with two to four carbon atoms, such as ethene ( -103.7 °C), propene ( -47.6 °C), and ( -6.3 °C), are colorless gases at . Higher homologues, including ( 63.4 °C), exist as colorless liquids. Boiling and melting points of terminal alkenes increase with molecular weight owing to stronger London dispersion forces between longer hydrocarbon chains. For instance, the boiling points rise from -103.7 °C for ethene to -47.6 °C for propene and -6.3 °C for , reflecting this trend. Liquid terminal alkenes possess low , ranging from 0.6 to 0.7 g/mL; , for example, has a density of 0.673 g/mL at 20 °C. They display poor solubility due to their nonpolar nature—propene dissolves to approximately 0.045 g/100 mL at 20 °C—but are fully miscible with organic solvents. Dipole moments are near zero, as exemplified by ethene at 0 D, arising from molecular symmetry. In , terminal alkenes exhibit a characteristic C=C stretching band at around cm⁻¹. Proton NMR spectra show vinylic protons in the 4.5–6.5 ppm range.

Chemical Properties

Terminal alkenes possess an electron-rich π bond in the carbon-carbon , where the two π electrons create high that renders the molecule susceptible to electrophilic attack. This nucleophilic character arises from the π electrons being less tightly held than σ electrons, facilitating interactions with electrophiles. In terminal alkenes, the terminal =CH₂ group exhibits higher reactivity toward electrophiles compared to internal alkenes due to lower substitution and reduced stability of the double bond. The vinylic hydrogens on terminal alkenes are slightly acidic, with a pKa of approximately 44 for ethene, allowing under strong base conditions to form vinyl anions, though this is rare in practice. Conversely, terminal alkenes act as weak bases through their π electrons, which can coordinate with acids or accept protons to form carbocations, but the basicity is low due to the sp² hybridization dispersing the . Terminal alkenes are less thermodynamically stable than corresponding alkanes, as evidenced by the exothermic of hydrogenation for ethene at -137 kJ/mol, reflecting the energy release upon converting the π bond to a σ bond. This relative instability makes them prone to , forming allylic hydroperoxides under ambient oxygen, especially when exposed to or , and to when exposed to or , especially in the presence of initiators that generate radicals. Compared to internal alkenes, the terminal position in alkenes like propene influences reactivity by favoring pathways that avoid primary carbocation intermediates during electrophilic additions; for instance, occurs at the terminal carbon to generate a more stable secondary , underpinning the observed in Markovnikov addition. This contrasts with symmetrical internal alkenes, where such is absent, and highlights how the terminal structure promotes directed reactivity to stabilize intermediates.

Reactions

Electrophilic Additions

reactions to terminal alkenes proceed via a two-step in which the electron-rich π of the C=C attacks an , generating a at the more substituted carbon, followed by the addition of a to this . This process is regioselective due to the stability of the formed, with the positive charge preferring the internal carbon in terminal alkenes like propene (CH₃CH=CH₂), which yields a secondary rather than a primary one. The rate-determining step is typically the formation of the , making the reaction sensitive to the 's strength and the alkene's substitution. A classic example is the addition of hydrogen halides (HX, where X = Cl, Br, or I) to alkenes, governed by , which states that the adds to the carbon atom of the bearing the greater number of substituents, while the adds to the more substituted carbon. For propene, this results in the formation of from HBr addition: \ce{CH3-CH=CH2 + HBr -> CH3-CHBr-CH3} via a secondary intermediate. However, in the presence of , HBr addition follows an anti-Markovnikov regiochemistry through a free chain mechanism: generate alkoxy radicals that abstract from HBr to form Br• radicals, which add to the terminal carbon of the to produce the more stable secondary , followed by hydrogen to yield the primary bromide, such as from propene. This peroxide effect is unique to HBr due to the bond dissociation energies involved and does not apply to HCl or . Acid-catalyzed hydration of terminal alkenes also adheres to , involving protonation of the to form the more stable , followed by nucleophilic attack by water and to give the . For propene, the reaction yields 2-propanol (isopropanol): \ce{CH3-CH=CH2 + H2O ->[H2SO4] CH3-CH(OH)-CH3} This method is reversible and often requires concentrated acid conditions to drive the toward the product. In contrast, halogenation with X₂ (X = Br or Cl) proceeds via a intermediate, where the electrophilic bridges the , leading to anti addition as the nucleophilic attacks from the opposite face, forming a vicinal dihalide. For 1-propene, Br₂ addition gives 1,2-dibromopropane, with the bromonium forming preferentially on the less substituted side to minimize steric hindrance. Hydroboration-oxidation provides a complementary anti-Markovnikov route specific to alkenes, involving addition of (BH₃) across the — attaching to the less substituted carbon—followed by oxidation with H₂O₂ and OH⁻ to replace with a hydroxyl group, yielding primary alcohols. For (CH₂=CH-CH₂CH₃), this sequence produces without rearrangements. in these additions varies: -based mechanisms (e.g., HX, ) are non-stereospecific, often leading to racemic mixtures at chiral centers; is strictly anti; and hydroboration-oxidation is , preserving stereochemical integrity.

Polymerization Reactions

Terminal alkenes, exemplified by ethene and propene, primarily undergo through free radical or coordination mechanisms to produce high-molecular-weight polyolefins with degrees of typically ranging from 10^4 to 10^6. These processes enable the formation of linear chains via repetitive addition of the across the , resulting in head-to-tail linkages that define the backbone. Free radical polymerization is commonly employed for ethene to produce (LDPE), initiated by peroxides or oxygen under high-pressure conditions of 1500–3000 bar and temperatures of 150–300°C. In this mechanism, a adds to the of ethene, generating a new that propagates the chain until termination occurs, yielding branched structures due to intramolecular transfer. The overall reaction is represented as: n \ce{CH2=CH2} \rightarrow -[\ce{CH2-CH2}]_n- This process operates at extreme conditions to overcome the low reactivity of ethene and achieve sufficient conversion. Coordination polymerization, utilizing Ziegler-Natta catalysts such as TiCl4 activated by AlEt3 or supported TiCl4/MgCl2 systems with trialkylaluminum cocatalysts, enables the production of high-density polyethylene (HDPE) from ethene and isotactic polypropylene (PP) from propene under milder conditions of 1–100 atm and 50–150°C. The mechanism involves coordination-insertion, where the monomer binds to a transition metal center (typically titanium), followed by migratory insertion into the metal-alkyl bond, favoring head-to-tail enchainment and high stereoregularity in the case of propene. For PP, metallocene catalysts, such as C2-symmetric zirconocenes activated by methylaluminoxane (MAO), enhance control over tacticity, producing highly isotactic polymers with narrow molecular weight distributions. Copolymerization of ethene with other terminal alkenes, such as , using Ziegler-Natta catalysts yields (LLDPE), incorporating 2–6 mol% comonomer to introduce short branches that reduce crystallinity to 46–58% and lower the melting temperature to 128–131°C compared to HDPE. This branching disrupts packing in the crystalline regions, enhancing flexibility and impact resistance while maintaining processability.

Applications

Polymer Production

Terminal alkenes, particularly ethene and propene, serve as essential feedstocks in the polymer industry, with ethene being the most significant due to its dominant role in production. Approximately 60% of global ethene production is directed toward manufacturing , which encompasses variants such as (HDPE) used for rigid containers like bottles and (LDPE) employed in flexible films and . In 2024, global output surpassed 100 million metric tons, reflecting its substantial economic scale and representing over one-third of worldwide demand. This utilization underscores the economic impact, as accounts for a major portion of the market, driving investments in production capacity amid growing demand for and consumer goods. Propene, another key terminal alkene, is primarily converted into , a versatile polymer applied in textiles for fibers and fabrics, as well as in automotive components such as bumpers, interior trims, and under-the-hood parts. The development of stereoregular , enabling high crystallinity and improved mechanical properties, was pioneered by in 1954 through the use of Ziegler-Natta catalysts, earning him the in 1963 shared with . This breakthrough transformed into a high-volume , with global production exceeding 80 million tons annually by the mid-2020s, supporting diverse industrial applications and contributing significantly to the market's growth. Higher terminal alkenes like 1-butene and 1-hexene are incorporated as comonomers in polyethylene copolymerization, particularly for linear low-density polyethylene (LLDPE), to enhance flexibility and processability by disrupting chain regularity and reducing crystallinity. These α-olefins, typically added in small amounts (2-10 mol%), lower the density and improve tensile properties, making LLDPE suitable for films and stretch wraps that outperform traditional LDPE in strength-to-weight ratios. About 35% of LLDPE production utilizes 1-hexene, while 1-butene features in another significant share, optimizing performance for end-use demands. Market trends in terminal alkene-based production are shifting toward , with bio-based routes gaining traction to address environmental concerns. For instance, ethene derived from via dehydration processes offers a renewable alternative, reducing dependency and potentially lowering the by up to 70% compared to conventional methods, as demonstrated in Brazilian facilities scaling up since the . However, challenges persist, including plastic waste generation—projected to reach 884 million tons annually globally by 2050—and the need for improved infrastructure to mitigate and from end-of-life disposal. These trends highlight the industry's push for models while balancing economic viability with regulatory pressures on waste management.

Organic Synthesis Uses

Terminal alkenes are essential building blocks in due to their reactivity at the terminal double bond, enabling regioselective transformations that construct complex carbon frameworks for pharmaceuticals and fine chemicals. Their abundance as feedstocks, such as and propene, ensures economic viability for scalable processes. Regioselectivity is a key advantage, as reactions often favor anti-Markovnikov or specific patterns, minimizing formation and simplifying purification. In cross-coupling reactions, terminal alkenes participate prominently in the palladium-catalyzed , coupling with aryl or vinyl halides to form β-aryl-substituted alkenes. For instance, reacts with aryl halides (ArX) in the presence of a Pd catalyst and base to yield styrene derivatives (Ar-CH=CH₂), which serve as intermediates in the synthesis of pharmaceuticals like certain anticancer agents and drugs. This reaction's tolerance for functional groups allows integration into late-stage synthetic sequences. Ozonolysis exploits the terminal double bond for selective oxidative cleavage, converting terminal alkenes to aldehydes under reductive workup or carboxylic acids under oxidative conditions. A representative example is the of , which produces hexanal and ; hexanal is widely used in fragrance synthesis for its green, leafy notes in perfumes and flavors. This method's precision in cleaving isolated double bonds makes it invaluable for degrading complex alkenes to simpler carbonyl fragments without affecting other functionalities. Terminal alkenes function as dienophiles in Diels-Alder cycloadditions with conjugated dienes, forming rings stereoselectively. Electron-deficient terminal alkenes, such as (CH₂=CH-CHO) derivatives, react efficiently due to the activating , yielding adducts that are precursors to heterocycles like pyrans or furans after further manipulation. This pericyclic reaction's concerted nature ensures high diastereoselectivity, aiding the construction of chiral scaffolds in natural product analogs. In pharmaceutical applications, terminal alkenes undergo , adding (CO/H₂) across the double bond to form with Pd or Rh catalysts. Propene, for example, is converted regioselectively to n-butanal via the oxo process, providing a linear aldehyde used as a precursor in routes to active pharmaceutical ingredients such as ibuprofen through subsequent aldol condensations and functionalizations. This transformation's high linear-to-branched selectivity (often >95%) enhances efficiency in producing chiral centers for NSAIDs.