Hexene
Hexene is an organic compound with the molecular formula C₆H₁₂, belonging to the class of alkenes characterized by a six-carbon chain and one carbon-carbon double bond.[1] These hydrocarbons exist in multiple isomeric forms, including positional isomers such as 1-hexene, 2-hexene, and 3-hexene, as well as stereoisomers (E and Z configurations for 2-hexene and 3-hexene), with additional branched and cyclic variants possible under the same formula.[1] The term "hexene" most commonly refers to these straight-chain linear alkenes, which are colorless liquids at room temperature, insoluble in water, and highly flammable due to their unsaturated structure.[1] Among the isomers, 1-hexene (also known as hex-1-ene) is the most industrially prominent, produced on a large scale as a linear alpha-olefin through the selective oligomerization of ethylene using catalysts like triethylaluminum, followed by distillation.[2] It serves primarily as a comonomer in the polymerization of ethylene to produce linear low-density polyethylene (LLDPE) and high-density polyethylene (HDPE), enhancing the flexibility, strength, and clarity of plastic films, pipes, and packaging materials.[3] Other applications include its use as a solvent, paint thinner, and intermediate in synthesizing flavors, perfumes, dyes, and resins.[2] Hexenes exhibit typical alkene reactivity, undergoing addition reactions with halogens, hydrogen, and oxidizers, and they pose safety risks as flammable liquids with vapors heavier than air, potentially causing eye and skin irritation or central nervous system depression upon exposure.[2] In the United States, a major producer, annual production of 1-hexene exceeds one billion pounds (as of 2024), driven by demand in the petrochemical sector.[4]Overview
Definition and general characteristics
Hexene refers to a class of organic compounds that are alkenes with the molecular formula C₆H₁₂, characterized by a carbon chain of six atoms containing exactly one carbon-carbon double bond.[5] This structure distinguishes hexenes from their saturated counterparts, the hexanes (C₆H₁₄), as the double bond introduces unsaturation into the hydrocarbon skeleton. The general representation includes linear and branched chain variants where the double bond can occupy different positions along the chain, but all share the fundamental feature of a single C=C linkage amid the six-carbon framework.[6] Hexenes are classified as olefins, a term historically used for alkenes due to their oily appearance and reactivity, and they belong to the broader family of unsaturated hydrocarbons.[7] The degree of unsaturation for these compounds is calculated using the formula (2C + 2 - H)/2, where C is the number of carbon atoms and H is the number of hydrogen atoms; for C₆H₁₂, this yields (2 \times 6 + 2 - 12)/2 = 1, confirming one unit of unsaturation consistent with a single double bond or equivalent ring structure, though hexenes specifically feature the former.[8] Hexenes were first synthesized in the 19th century through methods such as dehydration of alcohols or elimination reactions, reflecting early advances in organic synthesis during that era.[9] Their industrial relevance, however, expanded significantly in the 20th century with the development of polymerization techniques, particularly following the discovery of Ziegler-Natta catalysts in the 1950s, which enabled the production of polyolefins like linear low-density polyethylene.[10] In total, there are 13 constitutional isomers of hexene, encompassing various linear and branched configurations without including cyclic forms.[11]Industrial importance
Hexenes, particularly 1-hexene, play a pivotal role in the polymer industry as comonomers in the production of linear low-density polyethylene (LLDPE), where they enhance the polymer's flexibility, tensile strength, and overall mechanical properties compared to traditional ethylene homopolymers.[12] This incorporation allows for tailored material characteristics essential for films, bags, and containers, making LLDPE a cornerstone of modern plastics manufacturing.[13] Global production of hexenes reached approximately 2.16 million metric tons in 2024, with 1-hexene accounting for the vast majority due to its dominance in LLDPE applications.[14] This output reflects the compound's strategic importance, supported by major producers expanding capacities to meet rising demand.[15] Beyond polymers, hexenes serve as key intermediates in the synthesis of specialty chemicals, including surfactants derived from hydroformylation processes and synthetic lubricants that provide superior viscosity and thermal stability.[16] These applications contribute to sectors like personal care and industrial cleaning, where hexene-based products offer enhanced performance.[17] Market demand for hexenes is propelled by the packaging and automotive industries, which rely on LLDPE for lightweight components and durable wraps, with projections indicating sustained growth through 2025 amid initiatives for recyclable and bio-based plastics.[18] This trend underscores hexenes' alignment with sustainability goals, as advanced copolymers reduce material usage while maintaining functionality.[19]Nomenclature and isomers
Naming conventions
Hexenes are named according to the systematic nomenclature rules for alkenes developed by the International Union of Pure and Applied Chemistry (IUPAC). The parent chain is the longest continuous carbon chain containing the double bond, denoted by replacing the "-ane" ending of the corresponding alkane with "-ene". The chain is numbered starting from the end that gives the double bond carbons the lowest possible locants, prioritizing the first carbon of the double bond. For terminal alkenes, this yields names such as hex-1-ene for the structure CH₂=CHCH₂CH₂CH₂CH₃.[20] When the double bond is internal, the position is specified similarly, and geometric (stereoisomeric) configurations are indicated using either the cis/trans or E/Z descriptors. The cis/trans system applies to disubstituted alkenes where the substituents are identical in type, with "cis" denoting substituents on the same side of the double bond and "trans" on opposite sides; for more complex cases, the E/Z system uses Cahn-Ingold-Prelog priority rules to assign (E) for opposite high-priority groups and (Z) for same-side. Examples include cis-2-hexene and trans-2-hexene for CH₃CH=CHCH₂CH₂CH₃, or equivalently (Z)-hex-2-ene and (E)-hex-2-ene.[20] For branched hexenes, the longest chain including the double bond is selected as the parent, with branches (substituents like methyl) named and assigned the lowest possible locants after ensuring the double bond receives the lowest number. If choices exist, the chain is chosen to minimize substituent locants overall. A representative example is 2-methylpent-1-ene, where the parent is pent-1-ene with a methyl substituent at carbon 2.[20] These conventions apply across the 13 constitutional isomers of hexene. Historically, alkenes were often designated by trivial names based on natural sources or simple descriptors, such as "amylene" for pentenes, leading to inconsistencies. The transition to systematic IUPAC naming accelerated in the mid-20th century, formalized by the 1957 IUPAC recommendations on organic nomenclature, which emphasized logical, reproducible naming to support global scientific collaboration.[21][22]Types of isomers
Hexene, with the molecular formula C₆H₁₂, displays a variety of constitutional isomers due to different positions of the carbon-carbon double bond and branching patterns in the hydrocarbon chain. These constitutional isomers further give rise to stereoisomers, primarily through geometric isomerism in cases where the double bond is internal and disubstituted with different groups on each carbon. The straight-chain constitutional isomers consist of three positional variants: 1-hexene, 2-hexene, and 3-hexene. While 1-hexene lacks geometric isomerism, both 2-hexene and 3-hexene exhibit E/Z (or cis/trans) configurations, resulting in a total of five stereoisomers for the linear hexenes. The branched constitutional isomers number ten, arising from various methyl or ethyl substitutions that maintain the C₆H₁₂ formula while introducing asymmetry in the carbon skeleton. Examples include 2-methyl-1-pentene, 3-methyl-1-pentene, and 2-methyl-2-pentene, among others. These branched forms contribute additional structural diversity, with some exhibiting geometric isomerism (e.g., in 3-methyl-2-pentene and 4-methyl-2-pentene) and others featuring chiral centers that allow for optical isomerism (e.g., in 3-methyl-1-pentene). However, most hexene isomers are achiral, lacking optical activity unless a stereogenic center is present.[23] Stereoisomerism in hexene primarily manifests as geometric isomerism for alkenes with trisubstituted or tetrasubstituted double bonds where rotation is restricted, leading to distinct E and Z forms based on the priority of substituents according to Cahn-Ingold-Prelog rules. Optical isomerism occurs only in those rare cases with a chiral carbon atom, such as an asymmetric carbon bearing four different groups, but the majority of hexene stereoisomers are achiral and do not exhibit enantiomerism. In total, the 13 constitutional isomers yield 18 stereoisomers when accounting for both geometric and optical variants.[23] The following table lists the 13 constitutional isomers, with text-based structural representations, corresponding CAS registry numbers (for the parent compound or representative stereoisomer where applicable), and the number of associated stereoisomers.| Constitutional Isomer | Structural Representation | CAS Number | Number of Stereoisomers |
|---|---|---|---|
| 1-Hexene | CH₂=CH-CH₂-CH₂-CH₂-CH₃ | 592-41-6 | 1 |
| 2-Hexene | CH₃-CH=CH-CH₂-CH₂-CH₃ | 592-43-8 | 2 (E/Z) |
| 3-Hexene | CH₃-CH₂-CH=CH-CH₂-CH₃ | 693-87-8 | 2 (E/Z) |
| 2-Methyl-1-pentene | CH₂=C(CH₃)-CH₂-CH₂-CH₃ | 565-47-9 | 1 |
| 3-Methyl-1-pentene | CH₂=CH-CH(CH₃)-CH₂-CH₃ | 760-20-3 | 2 (R/S) |
| 4-Methyl-1-pentene | CH₂=CH-CH₂-CH₂-CH(CH₃)₂ | 691-37-2 | 1 |
| 2-Methyl-2-pentene | CH₃-C(CH₃)=CH-CH₂-CH₃ | 625-27-4 | 1 |
| 3-Methyl-2-pentene | CH₃-CH=C(CH₃)-CH₂-CH₃ | 922-62-3 | 2 (E/Z) |
| 4-Methyl-2-pentene | CH₃-CH=CH-CH(CH₃)-CH₃ | 4461-48-7 | 2 (E/Z) |
| 2-Ethyl-1-butene | CH₂=C(CH₂CH₃)-CH₂-CH₃ | 513-35-9 | 1 |
| 3,3-Dimethyl-1-butene | CH₂=CH-C(CH₃)₂-CH₃ | 558-37-2 | 1 |
| 2,3-Dimethyl-1-butene | CH₂=C(CH₃)-CH(CH₃)-CH₃ | 563-78-0 | 1 |
| 2,3-Dimethyl-2-butene | (CH₃)₂C=C(CH₃)₂ | 563-79-1 | 1 |