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Pentene
Pentene
Pentene
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Straight chain pentenes
1-Pentene
cis-2-Pentene
trans-2-Pentene
Names
IUPAC names
Pent-1-ene
cis-Pent-2-ene
trans-Pent-2-ene
Other names
amylene, n-amylene, n-pentene, beta-n-amylene, sym-methylethylethylene
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.042.636 Edit this at Wikidata
EC Number
  • 246-916-6 (1-pentene)

    273-308-8 (cis-2-pentene)

    271-255-5 (trans-2-pentene)
UNII
  • (1-pentene): InChI=1S/C5H10/c1-3-5-4-2/h3H,1,4-5H2,2H3
    Key: YWAKXRMUMFPDSH-UHFFFAOYSA-N
  • (cis-2-pentene): InChI=1S/C5H10/c1-3-5-4-2/h3,5H,4H2,1-2H3/b5-3-
    Key: QMMOXUPEWRXHJS-HYXAFXHYSA-N
  • (trans-2-pentene): InChI=1S/C5H10/c1-3-5-4-2/h3,5H,4H2,1-2H3/b5-3+
    Key: QMMOXUPEWRXHJS-HWKANZROSA-N
  • (1-pentene): CCCC=C
  • (cis-2-pentene): CC/C=C\C
  • (trans-2-pentene): CC/C=C/C
Properties
C5H10
Molar mass 70.135 g·mol−1
Density 0.64 g/cm3 (1-pentene)[1]
Melting point −165.2 °C (−265.4 °F; 108.0 K) (1-pentene)[1]
Boiling point 30 °C (86 °F; 303 K) (1-pentene)[1]
−53.7·10−6 cm3/mol
Hazards
Safety data sheet (SDS) MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
☒N verify (what is checkY☒N ?)

Pentenes are alkenes with the chemical formula C
5H
10. Each molecule contains one double bond within its molecular structure. Six different compounds are in this class, differing from each other by whether the carbon atoms are attached linearly or in a branched structure and whether the double bond has a cis or trans form.

Straight-chain isomers

[edit]

1-Pentene is an alpha-olefin. Most often, 1-pentene is made as a byproduct of catalytic or thermal cracking of petroleum or during the production of ethylene and propylene via thermal cracking of hydrocarbon fractions.

As of 2010s, the only commercial manufacturer of 1-pentene was Sasol Ltd., where it is separated from crude by the Fischer-Tropsch process.[2]

2-Pentene has two geometric isomers: cis-2-pentene and trans-2-pentene. Cis-2-Pentene is used in olefin metathesis.

Branched-chain isomers

[edit]

The branched isomers are 2-methylbut-1-ene, 3-methylbut-1-ene (isopentene), and 2-methylbut-2-ene (isoamylene).

Isoamylene is one of the three main byproducts of deep catalytic cracking (DCC), which is very similar to the operation of fluid catalytic cracking (FCC). The DCC uses vacuum gas oil (VGO) as a feedstock to produce primarily propylene, isobutylene, and isoamylene. The rise in demand for polypropylene has encouraged the growth of the DCC, which is operated very much like the FCC. Isobutylene and isoamylene feedstocks are necessary for the production of the much debated gasoline blending components methyl tert-butyl ether and tert-amyl methyl ether.

Production of fuels

[edit]

Propylene, isobutene, and amylenes are feedstocks in the alkylation units of refineries. Using isobutane, blendstocks are generated with high branching for good combustion characteristics. Amylenes are valued as precursors to fuels, especially aviation fuels of relatively low volatility, as required by various regulations.[3]

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Pentene

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Pentene refers to a group of unsaturated hydrocarbons with the molecular formula C₅H₁₀, characterized by a carbon-carbon and five carbon atoms arranged in various structural configurations. These compounds are alkenes, also known as olefins, and exhibit isomerism due to the position of the double bond and branching of the carbon chain. The primary acyclic isomers include 1-pentene (CH₂=CH-CH₂-CH₂-CH₃), (E)-2-pentene and (Z)-2-pentene (geometric isomers of CH₃-CH=CH-CH₂-CH₃), 2-methylbut-1-ene (CH₂=C(CH₃)-CH₂-CH₃), 3-methylbut-1-ene (CH₂=CH-CH(CH₃)₂), and 2-methylbut-2-ene ((CH₃)₂C=CH-CH₃). Pentene isomers are colorless at room temperature, with most being liquids and boiling points ranging from approximately 20°C to 40°C; for example, 3-methylbut-1-ene (a gas) boils at 20°C, 1-pentene at 30°C, and cis-2-pentene at 37°C. They possess high vapor pressures (around 500-600 mm Hg at 25°C), low solubility (140-200 mg/L), and densities near 0.64-0.66 g/cm³, making them less dense than and prone to floating on its surface. Chemically, pentenes are reactive due to the , undergoing addition reactions such as and , and they are highly flammable with flash points below -20°C. Pentenes are primarily produced as byproducts during the catalytic or cracking of fractions in refineries, particularly in the production of and . Industrially, they serve as key intermediates in , blending agents for high-octane to improve performance, and components in the manufacture of polymers, adhesives, and pesticides. For instance, 1-pentene is widely used in the synthesis of linear , while 2-pentene isomers act as polymerization inhibitors or feedstocks in reactions. Due to their volatility and potential for inhalation exposure, pentenes are regulated in environmental contexts, with effects primarily on the at high concentrations.

Overview

Definition and nomenclature

Pentenes are unsaturated hydrocarbons belonging to the class of alkenes, characterized by the presence of a single carbon-carbon and having the molecular formula C₅H₁₀. These compounds represent the five-carbon analogs of ethene and propene, exhibiting reactivity typical of alkenes due to the sp²-hybridized carbon atoms in the . The term "pentene" derives from "," the saturated with formula C₅H₁₂, with the suffix "-ene" denoting unsaturation at the . According to IUPAC rules for alkene , the parent chain is the longest continuous carbon chain that includes the , numbered from the end that assigns the lowest possible to the carbons. Substituents, if present, receive the lowest possible numbers, and the chain is named by replacing the "-ane" ending of the corresponding with "-ene." For , this results in names like pent-1-ene for the terminal where the double bond is between carbons 1 and 2. In cases of geometric isomerism, such as in 2-pentene (CH₃CH=CHCH₂CH₃), the configurations are designated as cis-2-pentene or (2Z)-pent-2-ene when the methyl and ethyl groups are on the same side of the double bond, and trans-2-pentene or (2E)-pent-2-ene when they are on opposite sides, following the Cahn-Ingold-Prelog priority rules for stereodescriptors.

Molecular formula and general structure

Pentene has the molecular formula C5H10C_5H_{10}, which represents the empirical and molecular formula for this class of alkenes. This formula aligns with the general pattern for monoalkenes, CnH2nC_nH_{2n} where n=5n=5. The for C5H10C_5H_{10} is calculated by comparing it to the saturated (C5H12C_5H_{12}), revealing two fewer atoms and thus one degree of unsaturation. This unsaturation is accounted for by a single carbon-carbon in the structure. The general structure of pentenes consists of five carbon atoms connected by single bonds except for one carbon-carbon , forming either linear or branched chains, with the remaining valences filled by atoms. The two carbon atoms in the exhibit sp2sp^2 hybridization, forming three bonds each in a trigonal planar arrangement. In terms of bonding , the C=CC=C has a length of approximately 1.34 , which is shorter than the typical C−CC-C of 1.54 due to the additional . The bond angles around the sp2sp^2-hybridized carbons are approximately 120°, reflecting the planar trigonal . Skeletal formulas and line diagrams are commonly used to represent the general structure of pentene, depicting the carbon chain as a line with the shown as two parallel lines between the relevant carbons, while atoms are omitted for simplicity. This notation emphasizes the connectivity and the position of the without specifying exact configurations.

Physical properties

Boiling and melting points

Pentene isomers exhibit points typically ranging from approximately 20°C to 38°C, reflecting their relatively low molecular weights and nonpolar nature, while points are generally very low, often below -130°C, due to weak intermolecular forces. These temperatures vary among isomers primarily due to structural differences, such as the position of the double bond, geometric isomerism, and branching. The boiling point of 1-pentene is 29.9°C, lower than that of internal alkenes like cis-2-pentene at 36.9°C and trans-2-pentene at 36.3°C, as the terminal results in a more linear with slightly stronger van der Waals interactions. Branching tends to lower points compared to linear analogs of similar because the more spherical reduces surface area and thus weakens London dispersion forces; for example, 3-methylbut-1-ene boils at 20.1°C compared to 1-pentene at 29.9°C. For geometric isomers, cis-2-pentene has a higher than trans-2-pentene due to its bent , which creates a net dipole moment and enables stronger dipole-dipole interactions, whereas the more symmetric trans isomer relies solely on dispersion forces. Melting points follow a similar trend influenced by packing efficiency; for example, 1-pentene melts at -165.2°C, cis-2-pentene at -151.4°C, and trans-2-pentene at -140.2°C, with trans isomers often showing higher values due to better crystal lattice formation. The following table compares these properties for major pentene isomers:
Isomer (°C) (°C)
3-Methylbut-1-ene20.1-168.5
1-Pentene29.9-165.2
2-Methylbut-1-ene31.2-137.5
cis-2-Pentene36.9-151.4
trans-2-Pentene36.3-140.2
2-Methylbut-2-ene38.0-133.6
Data sourced from CRC Handbook of Chemistry and Physics via .

Solubility and density

Pentenes exhibit low in due to their hydrophobic, nonpolar character, primarily resulting from the predominance of C-H and C=C bonds, which limit interactions with polar molecules. For instance, 1-pentene has a of 148 mg/L (0.0148 g/100 mL) at 25 °C, rendering it effectively immiscible. Similarly, cis-2-pentene and trans-2-pentene each display a of 203 mg/L at 25 °C, while 2-methyl-1-butene is soluble to 130 mg/L at 20 °C. In contrast, pentenes are highly soluble in nonpolar organic solvents, such as , ethyl , , and , often miscible in all proportions, which facilitates their use in and extraction processes. The densities of pentenes are characteristically low, reflecting their composition and reliance on weak van der Waals forces for intermolecular interactions, which contribute to their overall lightness compared to . Typical values range from 0.62 to 0.66 g/cm³ at 20 °C across isomers, with specific examples including 1-pentene at 0.6405 g/cm³, cis-2-pentene at 0.6556 g/cm³, trans-2-pentene at 0.6482 g/cm³, 3-methyl-1-butene at 0.627 g/cm³, 2-methyl-1-butene at 0.6504 g/cm³, and 2-methyl-2-butene at 0.66 g/cm³.
IsomerDensity (g/cm³ at 20 °C)Water Solubility (mg/L at 25 °C unless noted)
1-Pentene0.6405148
cis-2-Pentene0.6556203
trans-2-Pentene0.6482203
3-Methyl-1-butene0.627130
2-Methyl-1-butene0.6504130 (at 20 °C)
2-Methyl-2-butene0.66190
These slight variations in density among isomers arise from differences in molecular packing and branching, where more compact branched structures can sometimes yield marginally higher densities due to enhanced van der Waals interactions, though the effect is minimal within the pentene series.

Chemical properties

Reactivity of the double bond

The carbon-carbon double bond in pentenes confers high reactivity, primarily through electrophilic addition mechanisms, where the π electrons of the double bond attack electrophiles, leading to the formation of a more stable σ bond. A key example is catalytic hydrogenation, in which molecular hydrogen adds across the double bond in the presence of a platinum catalyst to produce the corresponding alkane, pentane. This syn addition reaction is highly exothermic, with the general equation for pentene given by: \ceC5H10+H2−>[Pt]C5H12\ce{C5H10 + H2 ->[Pt] C5H12} The process requires elevated temperatures and pressures but proceeds selectively without rearranging the carbon skeleton. Halogenation reactions involve the addition of such as , forming vicinal dihalides via an anti addition mechanism through a cyclic intermediate. For 1-pentene, reaction with Br₂ yields 1,2-dibromopentane, as shown in the equation: \ceC5H10+Br2−>C5H10Br2\ce{C5H10 + Br2 -> C5H10Br2} This reaction occurs readily at in inert solvents and is often used to test for unsaturation due to the distinctive color change of . Acid-catalyzed hydration adds water across the following , where the hydroxyl group attaches to the carbon with fewer hydrogen atoms, forming the more stable intermediate. In the case of 1-pentene, this produces 2-pentanol as the major product under conditions such as dilute H₂SO₄ and heat. The minimizes the formation of primary alcohols, emphasizing the electrophilic nature of the step. Polymerization represents a chain-growth where multiple pentene monomers link via the , initiated by Ziegler-Natta catalysts such as with aluminum alkyl co-catalysts. For 1-pentene, this yields atactic or isotactic poly(pentene), depending on catalyst , with the repeating unit derived from sequential 1,2-addition. These catalysts enable controlled molecular weight and , crucial for properties. Among pentene isomers, terminal alkenes such as 1-pentene exhibit greater reactivity toward than internal ones like 2-pentene, owing to the lower thermodynamic stability of terminal double bonds, as evidenced by higher heats of for terminal isomers.

Stability and isomerization

Pentenes exhibit good stability under normal conditions but decompose when heated to high temperatures, emitting acrid smoke and irritating fumes. They are also susceptible to auto-oxidation in the presence of air, particularly upon exposure to light or heat, leading to the formation of unstable peroxides and hydroperoxides that pose risks if concentrated or shocked. Isomerization of pentenes involves the migration of the position, often catalyzed by acids or bases, favoring the more stable internal s. In acid-catalyzed processes, of the generates a intermediate, which rearranges via or alkyl shifts before to form the new ; for example, 1-pentene isomerizes to 2-pentene through a secondary . This process is exemplified by the following skeletal rearrangement: \ceCH3−CH2−CH2−CH=CH2+H+−>[carbocation]CH3−CH2−CH+−CH2−CH3−>CH3−CH2−CH=CH−CH3+H+\ce{CH3-CH2-CH2-CH=CH2 + H+ ->[carbocation] CH3-CH2-CH^{+}-CH2-CH3 -> CH3-CH2-CH=CH-CH3 + H+} Internal alkenes like 2-pentene are thermodynamically more stable than terminal ones like 1-pentene due to greater hyperconjugation from additional alkyl substituents, which delocalizes electrons and lowers the overall energy. Cis-trans isomerization in disubstituted pentenes, such as 2-pentene, requires overcoming a high rotational energy barrier around the double bond, estimated at approximately 270 kJ/mol, typically achieved thermally at elevated temperatures, via UV photoexcitation to a triplet state, or with catalysts that lower the barrier.

Isomers

Linear isomers

Linear isomers of pentene refer to the straight-chain alkenes with the molecular formula C₅H₁₀, featuring an unbranched carbon skeleton and a single carbon-carbon . The two primary linear isomers are 1-pentene and 2-pentene, distinguished by the position of the double bond. 1-Pentene possesses a terminal double bond, while 2-pentene has an internal double bond and exhibits geometric isomerism due to restricted around the double bond, resulting in cis and trans configurations. 1-Pentene has the CH₂=CH–CH₂–CH₂–CH₃, characteristic of a with the between the first and second carbon atoms. This positioning imparts high reactivity at the allylic position and makes it a prototypical linear α-olefin. Its is 30 °C, reflecting its relatively low molecular weight and non-polar nature. Industrially, 1-pentene is a key product of ethylene oligomerization processes, such as the Shell Higher Olefin Process (SHOP), where it serves as a building block for detergents and lubricants due to its linear structure enabling precise control. 2-Pentene, with the formula CH₃–CH=CH–CH₂–CH₃, features the between the second and third carbon atoms, classifying it as an internal . This exists in two stereoisomers: the trans (E) form, where the methyl and ethyl groups are on opposite sides of the , and the cis (Z) form, where they are on the same side. The trans is more stable due to reduced steric hindrance, with a standard of formation (ΔfG°) of 69.96 kJ/mol in the gas phase at 298 , compared to 71.89 kJ/mol for the cis . Consequently, the cis-trans equilibrium at favors the trans form in approximately a 70:30 ratio, driven by the energy difference of about 1.93 kJ/mol. The cis-2-pentene has a of 36.9 °C, slightly higher than that of trans-2-pentene at 36.3 °C, attributable to the cis 's higher dipole moment from its bent geometry. The of 2-pentene arises from the sp² hybridization of the double-bonded carbons, enforcing planarity and preventing free rotation, which leads to distinct physical and chemical properties between the isomers. For instance, the cis form exhibits greater polarity, influencing its and intermolecular interactions, while the trans form's linearity enhances packing efficiency in the liquid state. These traits make linear pentene isomers valuable in illustrating fundamental principles of geometry and reactivity.

Branched isomers

Branched isomers of pentene feature a non-linear carbon chain with a methyl , leading to distinct physical and chemical properties compared to their linear counterparts. These isomers include 2-methyl-1-butene, 3-methyl-1-butene, and 2-methyl-2-butene, each with the molecular formula C₅H₁₀ but differing in the position of the and branch. The branching influences stability, reactivity, and applications, often enhancing ratings due to increased substitution around the . 2-Methyl-1-butene has the structure CH₂=C(CH₃)CH₂CH₃, where the methyl branch is attached directly to the sp²-hybridized carbon of the terminal . This boils at 31.2 °C and exhibits reduced stability relative to more substituted isomers, partly due to steric hindrance from the adjacent that limits hyperconjugative stabilization in certain . 3-Methyl-1-butene features the structure CH₂=CHCH(CH₃)₂, with the isopropyl branch at the beta position to the terminal . It has a lower of 20.1 °C, reflecting its more compact branched structure. This is notably used in copolymerization reactions, such as with butene-1, to introduce branched units that modify behavior and crystallinity in polymers. 2-Methyl-2-butene, with the structure (CH₃)₂C=CHCH₃, is an internal trisubstituted that serves as the most stable branched pentene due to enhanced from three alkyl substituents on the . Its is 38 °C, higher than the terminal branched isomers. Among pentene isomers, higher branching like this increases the research octane number (RON) to 97.3, making it valuable for fuel blending. Additionally, 2-methyl-2-butene is a predominant product in the catalytic cracking of higher hydrocarbons, often serving as a model compound in studies of olefin cracking over zeolites. The structural diagrams for these isomers emphasize the branch positions: in 2-methyl-1-butene, the methyl is on the internal carbon of the ; in 3-methyl-1-butene, it forms an isopropyl tail; and in 2-methyl-2-butene, two methyls flank one side of the internal , contributing to its thermodynamic preference over linear pentenes like 1-pentene.

Synthesis and production

Industrial methods

Pentenes are primarily produced on an industrial scale through processes that convert lighter or heavier hydrocarbons into mixtures of C5 olefins, with 1-pentene being a key alpha-olefin targeted via selective oligomerization. One major method is oligomerization, exemplified by the Shell Higher Olefin Process (SHOP), which employs homogeneous catalysts with bidentate phosphane ligands in a to oligomerize into a range of linear alpha-olefins, including 1-pentene. The process follows a Schulz-Flory distribution, where chain growth occurs via insertion into a nickel-alkyl bond, followed by β-hydride elimination to release the olefin; odd-chain products like 1-pentene are formed through a combination of oligomerization, , and metathesis steps. This two-phase system allows efficient catalyst recycling, contributing to over 1 million tons of alpha-olefins annually, with 1-pentene forming part of the lighter fraction. Thermal cracking of heavier feedstocks like in crackers at 700–850°C and low pressure yields a mixture of pentenes as byproducts alongside primary light olefins such as and . The process involves free-radical mechanisms that break C-C bonds in the (primarily C5–C10 alkanes), producing trans-2-pentene as the dominant in the C5 olefin fraction, with typical yields of total C5 olefins around 1–2 wt% (e.g., 1-pentene ~0.12 wt%, 2-pentene ~0.14 wt%). This method accounts for a significant portion of mixed pentene production, often separated as C5 for further processing. In petroleum refineries, (FCC) of vacuum gas oil or heavy residues at 500–650°C using catalysts like Y-type generates branched pentenes (e.g., 2-methyl-1-butene, 2-methyl-2-butene) as components of the boiling range. The mechanism on acidic sites promotes skeletal and cracking, yielding C5 olefins at 5–10 wt% in the stream, serving as byproducts for or extraction. Global production of C5 olefins from cracking processes is estimated at less than 2 million metric tons per year as of the early 2020s, with additional contributions from alpha-olefin processes.

Laboratory preparation

One common laboratory method for synthesizing pentenes involves the acid-catalyzed dehydration of pentanols. In this process, 1-pentanol is heated with concentrated sulfuric acid at around 180°C, leading to the formation of 1-pentene through an E1 mechanism. The reaction begins with protonation of the hydroxyl group, followed by loss of water to generate a primary carbocation, which then loses a proton from the adjacent carbon to form the double bond; however, due to the instability of primary carbocations, rearrangement to more stable secondary carbocations can occur, potentially yielding some 2-pentene as a byproduct. Another approach is the , which is particularly useful for obtaining terminal alkenes like 1-pentene with high . This involves treating 1-aminopentane with excess methyl iodide to form the quaternary ammonium iodide, followed by reaction with (Ag₂O) in water to generate the corresponding hydroxide. Upon heating, the hydroxide undergoes E2 elimination, favoring the less substituted alkene due to the bulky trimethylammonium . This method is suitable for educational and small-scale preparations where anti-Zaitsev orientation is desired. The offers a versatile route for preparing specific pentene isomers, especially when stereocontrol is needed. For 1-pentene, butanal (CH₃CH₂CH₂CHO) is reacted with methylenetriphenylphosphorane (Ph₃P=CH₂), a non-stabilized prepared from methyltriphenylphosphonium bromide and a strong base such as . The mechanism proceeds via nucleophilic attack of the on the carbonyl, forming a betaine intermediate that cyclizes to an oxaphosphetane and collapses to the and triphenylphosphine oxide. This reaction typically affords 1-pentene with 60-80% yield and predominant Z , though can be induced if the E isomer is required. Following synthesis, pentene mixtures are purified by under reduced pressure to separate isomers based on their points. For example, 1-pentene distills at 30°C, while cis-2-pentene boils at 37.9°C and trans-2-pentene at 36.3°C, allowing reasonable separation with an efficient column. This technique is essential in settings to isolate pure isomers for further study or use.

Applications

Fuel components

Pentenes play a significant role in gasoline formulations as olefinic components that enhance octane performance and combustion efficiency. In typical , alkenes including pentenes comprise about 2-5% by volume, with pentenes specifically contributing to the light olefin fraction derived from processes. These hydrocarbons help achieve desired properties such as volatility and energy content while supporting the overall blend's anti-knock characteristics. Branched pentene isomers, such as 2-methyl-2-butene, exhibit high ratings, with a research octane number (RON) of 97.3 and motor octane number (MON) of 84.7, enabling blends to reach RON values exceeding 90 and thereby reducing . In contrast, straight-chain isomers like 1-pentene have a lower RON of 90.9 and MON of 77.1, making them less effective for boosting but still useful in lighter fractions. The structural differences among pentene isomers influence their suitability, with branched forms preferred for superior anti-knock properties. The of pentenes proceeds via the balanced reaction C₅H₁₀ + 8 O₂ → 5 CO₂ + 5 H₂O, yielding a high of approximately 44 MJ/kg, comparable to other hydrocarbons and supporting efficient energy release in engines. Pentenes are often blended with aromatics to optimize , a practice that intensified after the 1970s phase-out of lead additives, as refiners turned to olefins and aromatics to maintain high without . Straight-chain pentenes like 1-pentene are less desirable in extended storage blends due to their reduced stability and propensity for , which can lead to gum formation and fuel degradation.

Chemical feedstocks

Pentenes, particularly 1-pentene, are key starting materials in for producing higher-value chemicals through processes like . In this reaction, 1-pentene reacts with and in the presence of a catalyst, such as or complexes, to form aldehydes, primarily hexanal and its branched 2-methylpentanal. The general equation for the hydroformylation is: C5H10+CO+H2→C6H12O\text{C}_5\text{H}_{10} + \text{CO} + \text{H}_2 \rightarrow \text{C}_6\text{H}_{12}\text{O} These aldehydes serve as intermediates for detergent production; hexanal can be oxidized to hexanoic acid or hydrogenated to 1-hexanol, which is further processed into surfactants and other detergent components. Another major application involves 1-pentene as a comonomer in the production of linear low-density polyethylene (LLDPE) copolymers with ethylene. This incorporation enhances the polymer's properties, such as flexibility and tensile strength, making it suitable for films, packaging, and other materials. Industrial producers like Sasol utilize 1-pentene in this capacity to meet demand for advanced polyolefins. For fine chemicals, 2-pentene undergoes allylic oxidation to yield pentenols, such as 2-penten-1-ol, which are valuable in the synthesis of flavors, fragrances, and pharmaceutical intermediates due to their unsaturated alcohol functionality.

Safety and environmental aspects

Health hazards

Pentenes pose low acute toxicity to humans, primarily through inhalation due to their high volatility, which facilitates vapor exposure in occupational settings. Inhalation of high concentrations can irritate the respiratory tract, leading to symptoms such as coughing, shortness of breath, headache, dizziness, and nausea. The median lethal concentration (LC50) for 1-pentene in rats is approximately 175,000 mg/m³ (about 61,000 ppm) over 4 hours, indicating relatively low acute inhalation toxicity compared to other hydrocarbons. Direct contact with pentenes may cause mild to the skin and eyes, with symptoms including redness and discomfort upon prolonged exposure, though they are not considered severe irritants. Pentenes are not classified as carcinogenic by the International Agency for Research on Cancer (IARC), with no sufficient evidence of carcinogenicity in humans or animals. For occupational exposure, while there is no specific OSHA (PEL) for pentenes, analogous aliphatic hydrocarbons like have a PEL of 1,000 ppm as an 8-hour time-weighted average, serving as a guideline to prevent and systemic effects. Chronic exposure to pentenes may result in potential neurotoxic effects, such as , due to their as unsaturated hydrocarbons that can act as weak anesthetics at elevated levels; however, data are limited and primarily derived from analog studies showing no observed levels (NOAELs) around 2,500 ppm for body weight decreases in rodents. measures include immediate removal to with ventilation for exposure, flushing eyes with for at least , and washing skin with soap and ; medical attention is advised if symptoms like or persist. Among isomers, terminal alkenes like 1-pentene exhibit higher reactivity due to the position of the , potentially leading to greater irritation compared to internal isomers like 2-pentene.

Ecological impact

Pentenes exhibit ready biodegradability in aerobic aquatic and environments due to their simple structure, which allows microbial degradation primarily through oxidation pathways. Studies indicate that linear alkenes like 1-pentene are expected to biodegrade rapidly in water, with half-lives on the order of days under favorable conditions, such as the presence of acclimated microbial populations. Given their high volatility, pentenes released into ecosystems tend to partition preferentially into the atmosphere rather than persisting in or . As volatile organic compounds (VOCs), they contribute to tropospheric formation by undergoing photooxidation in the presence of hydroxyl radicals and oxides, with alkenes demonstrating high reactivity in this process—often quantified by maximum incremental reactivity (MIR) values exceeding those of alkanes. The low of pentenes (approximately 0.16 g/L for 1-pentene) further limits their aqueous persistence, promoting evaporative loss over dissolution. In aquatic systems, pentenes pose low to moderate toxicity to and , with 96-hour LC50 values for like fathead minnows (Pimephales promelas) ranging from 10.7 to 19.0 mg/L. Their octanol-water partition coefficients (log Kow ≈ 2.7–2.8) indicate low potential, with estimated factors (BCF) below 100, reducing long-term trophic transfer risks. Under U.S. Environmental Protection Agency (EPA) regulations, pentenes are classified as VOCs subject to emission controls aimed at reducing precursors, but they are not designated as hazardous air pollutants (HAPs). In spill scenarios, such as refinery leaks involving light mixtures in the , pentenes typically undergo rapid evaporation due to their high (≈635 mm Hg at 25°C), minimizing persistence and facilitating cleanup through absorption and ventilation rather than prolonged remediation.

References

  1. https://www.csus.edu/indiv/m/mackj/chem1b/docs/ch10.pdf
  2. https://pubchem.ncbi.nlm.nih.gov/compound/1-Pentene#section=Chemical-and-Physical-Properties
  3. https://webbook.nist.gov/cgi/cbook.cgi?ID=C109671
  4. https://pubchem.ncbi.nlm.nih.gov/compound/3-Methyl-1-butene
  5. https://www.tceq.texas.gov/downloads/toxicology/dsd/final/pentene.pdf
  6. https://pubchem.ncbi.nlm.nih.gov/compound/cis-2-PENTENE#section=Chemical-and-Physical-Properties
  7. https://pubchem.ncbi.nlm.nih.gov/compound/1-Pentene#section=Use-and-Manufacturing
  8. https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_Chemistry)/Alkenes/Naming_the_Alkenes
  9. https://www.britannica.com/science/hydrocarbon/Nomenclature-of-alkenes-and-alkynes
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