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Cyclohexane
Cyclohexane
Cyclohexane
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Cyclohexane
Cyclohexane
Cyclohexane
3D structure of a cyclohexane molecule
3D structure of a cyclohexane molecule
Skeletal formula of cyclohexane in its chair conformation
Skeletal formula of cyclohexane in its chair conformation
Ball-and-stick model of cyclohexane in its chair conformation
Ball-and-stick model of cyclohexane in its chair conformation
Names
Preferred IUPAC name
Cyclohexane[2]
Other names
Hexanaphthene (archaic)[1]
Identifiers
3D model (JSmol)
1900225
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.003.461 Edit this at Wikidata
1662
KEGG
RTECS number
  • GU6300000
UNII
UN number 1145
  • InChI=1S/C6H12/c1-2-4-6-5-3-1/h1-6H2 checkY
    Key: XDTMQSROBMDMFD-UHFFFAOYSA-N checkY
  • InChI=1/C6H12/c1-2-4-6-5-3-1/h1-6H2
    Key: XDTMQSROBMDMFD-UHFFFAOYAZ
  • C1CCCCC1
Properties
C6H12
Molar mass 84.162 g·mol−1
Appearance Colourless liquid
Odor Sweet, gasoline-like
Density 0.7739 g/ml (liquid); 0.996 g/ml (solid)
Melting point 6.47 °C (43.65 °F; 279.62 K)
Boiling point 80.74 °C (177.33 °F; 353.89 K)
Immiscible
Solubility Soluble in ether, alcohol, acetone
Vapor pressure 78 mmHg (20 °C)[3]
−68.13·10−6 cm3/mol
1.42662
Viscosity 1.02 cP at 17 °C
Hazards
GHS labelling:
GHS02: Flammable GHS08: Health hazard GHS07: Exclamation mark GHS09: Environmental hazard
Danger
H225, H304, H315, H336
P210, P233, P240, P241, P242, P243, P261, P264, P271, P273, P280, P301+P310, P302+P352, P303+P361+P353, P304+P340, P312, P321, P331, P332+P313, P362, P370+P378, P391, P403+P233, P403+P235, P405, P501
NFPA 704 (fire diamond)
NFPA 704 four-colored diamondHealth 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineFlammability 3: Liquids and solids that can be ignited under almost all ambient temperature conditions. Flash point between 23 and 38 °C (73 and 100 °F). E.g. gasolineInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
1
3
0
Flash point −20 °C (−4 °F; 253 K)
245 °C (473 °F; 518 K)
Explosive limits 1.3–8%[3]
Lethal dose or concentration (LD, LC):
12705 mg/kg (rat, oral)
813 mg/kg (mouse, oral)[4]
17,142 ppm (mouse, 2 h)
26,600 ppm (rabbit, 1 h)[4]
NIOSH (US health exposure limits):
PEL (Permissible)
 TWA 300 ppm (1050 mg/m3)[3]
REL (Recommended)
 TWA 300 ppm (1050 mg/m3)[3]
IDLH (Immediate danger)
 1300 ppm[3]
Thermochemistry
−156 kJ/mol
−3920 kJ/mol
Related compounds
Related cycloalkanes
 Cyclopentane
Cycloheptane
Related compounds
 Cyclohexene
Benzene
Supplementary data page
Cyclohexane (data page)
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
checkY verify (what is checkY☒N ?)

Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is non-polar. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used). Cyclohexane is mainly used for the industrial production of adipic acid and caprolactam, which are precursors to nylon.[5]

Cyclohexyl (C6H11) is the alkyl substituent of cyclohexane and is abbreviated Cy.[6]

Production

[edit]

Cyclohexane is one of the components of naphtha, from which it can be extracted by advanced distillation methods. Distillation is usually combined with isomerization of methylcyclopentane, a similar component extracted from naphtha by similar methods. Together these processes cover only a minority (15-20%) of the modern industrial demand and are complemented by synthesis.[7]

Modern industrial synthesis

[edit]

On an industrial scale, cyclohexane is produced by hydrogenation of benzene in the presence of a Raney nickel catalyst.[citation needed] Producers of cyclohexane account for approximately 11.4% of global demand for benzene.[8] The reaction is highly exothermic, with ΔH(500 K) = -216.37 kJ/mol. Dehydrogenation commenced noticeably above 300 °C, reflecting the favorable entropy for dehydrogenation.[9]

Catalytic hydrogenation of benzene to cyclohexane with a raney-nickel catalyst

History of synthesis

[edit]

Unlike benzene, cyclohexane is not found in natural resources such as coal. For this reason, early investigators synthesized their cyclohexane samples.[10]

Failure

[edit]

Surprisingly, their cyclohexanes boiled higher by 10 °C than either hexahydrobenzene or hexanaphthene, but this riddle was solved in 1895 by Markovnikov, N.M. Kishner, and Nikolay Zelinsky when they reassigned "hexahydrobenzene" and "hexanaphtene" as methylcyclopentane, the result of an unexpected rearrangement reaction.

reduction of benzene to methylcyclopentane

Success

[edit]

In 1894, Baeyer synthesized cyclohexane starting with a ketonic decarboxylation of pimelic acid followed by multiple reductions:

1894 cyclohexane synthesis Baeyer

In the same year, E. Haworth and W.H. Perkin Jr. (1860–1929) prepared it via a Wurtz reaction of 1,6-dibromohexane.

1894 cyclohexane synthesis Perkin / haworth

Reactions and uses

[edit]

Although rather unreactive, cyclohexane undergoes autoxidation to give a mixture of cyclohexanone and cyclohexanol. The cyclohexanone–cyclohexanol mixture, called "KA oil", is a raw material for adipic acid and caprolactam, precursors to nylon. Several million kilograms of cyclohexanone and cyclohexanol are produced annually.[9]

It is used as a solvent in some brands of correction fluid. Cyclohexane is sometimes used as a non-polar organic solvent, although n-hexane is more widely used for this purpose. It is frequently used as a recrystallization solvent, as many organic compounds exhibit good solubility in hot cyclohexane and poor solubility at low temperatures.

Cyclohexane is also used for calibration of differential scanning calorimetry (DSC) instruments, because of a convenient crystal-crystal transition at −87.1 °C.[14]

Cyclohexane vapour is used in vacuum carburizing furnaces, in heat treating equipment manufacture.

Conformation

[edit]
Main article: Cyclohexane conformation

The 6-vertex edge ring does not conform to the shape of a perfect hexagon. The conformation of a flat 2D planar hexagon has considerable strain because the C-H bonds would be eclipsed. Therefore, to reduce torsional strain, cyclohexane adopts a three-dimensional structure known as the chair conformation, which rapidly interconvert at room temperature via a process known as a chair flip. During the chair flip, there are three other intermediate conformations that are encountered: the half-chair, which is the most unstable conformation, the more stable boat conformation, and the twist-boat, which is more stable than the boat but still much less stable than the chair. The chair and twist-boat are energy minima and are therefore conformers, while the half-chair and the boat are transition states and represent energy maxima. The idea that the chair conformation is the most stable structure for cyclohexane was first proposed as early as 1890 by Hermann Sachse, but only gained widespread acceptance much later. The new conformation puts the carbons at an angle of 109.5°. Half of the hydrogens are in the plane of the ring (equatorial) while the other half are perpendicular to the plane (axial). This conformation allows for the most stable structure of cyclohexane. Another conformation of cyclohexane exists, known as boat conformation, but it interconverts to the slightly more stable chair formation. If cyclohexane is mono-substituted with a large substituent, then the substituent will most likely be found attached in an equatorial position, as this is the slightly more stable conformation.

Cyclohexane has the lowest angle and torsional strain of all the cycloalkanes; as a result cyclohexane has been deemed a 0 in total ring strain.

Solid phases

[edit]

Cyclohexane has two crystalline phases. The high-temperature phase I, stable between 186 K and the melting point 280 K, is a plastic crystal, which means the molecules retain some rotational degree of freedom. The low-temperature (below 186 K) phase II is ordered. Two other low-temperature (metastable) phases III and IV have been obtained by application of moderate pressures above 30 MPa, where phase IV appears exclusively in deuterated cyclohexane (application of pressure increases the values of all transition temperatures).[15]

Cyclohexane phases[15]
No Symmetry Space group a (Å) b (Å) c (Å) Z T (K) P (MPa)
I Cubic Fm3m 8.61 4 195 0.1
II Monoclinic C2/c 11.23 6.44 8.20 4 115 0.1
III Orthorhombic Pmnn 6.54 7.95 5.29 2 235 30
IV Monoclinic P12(1)/n1 6.50 7.64 5.51 4 160 37

Here Z is the number structure units per unit cell; the unit cell constants a, b and c were measured at the given temperature T and pressure P.

See also

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Cyclohexane

View on Grokipedia
from Grokipedia
Cyclohexane is a with the molecular formula C₆H₁₂, consisting of a six-carbon ring that predominantly adopts a stable conformation to minimize and torsional strain. This saturated appears as a clear, colorless with a mild, petroleum-like , exhibiting non-polar characteristics that render it insoluble in but miscible with organic solvents. At standard conditions, it has a of 80.7 °C, a of 6.5 °C, and a of 0.779 g/mL at 20 °C, making it a volatile and flammable substance with a of -20 °C. Cyclohexane is primarily produced on an industrial scale through the catalytic of , utilizing catalysts such as , , or in either liquid or vapor phase processes, often derived from feedstocks. Alternative methods include separation from liquids, though accounts for the majority of global production, approximately 9.6 million metric tons in 2024. Its under normal conditions allows it to serve as a versatile non-polar in applications like lacquers, resins, adhesives, and paint removers, as well as an analytical reagent in laboratories. A key industrial role of cyclohexane lies in its oxidation to and , which are precursors for and , essential monomers in nylon-6 and nylon-6,6 production, accounting for over 90% of its consumption. It also finds use as a co-solvent in formulations and in the extraction of various compounds. However, cyclohexane poses significant hazards: it is highly flammable, can cause to the skin, eyes, and upon exposure, and may lead to central nervous system depression or if ingested or inhaled in large amounts. Proper handling requires ventilation, protective equipment, and adherence to regulations classifying it as a hazardous substance under frameworks like the U.S. Toxic Substances Control Act.

Structure and conformation

Molecular structure

Cyclohexane has the molecular formula C₆H₁₂ and a molecular weight of 84.16 g/mol. It is a saturated composed of a six-membered carbon ring in which adjacent carbon atoms are connected by single bonds, with each carbon atom exhibiting sp³ hybridization and bonded to two hydrogen atoms. The C–C–C bond angles in the ring are approximately 109.5°, aligning closely with the ideal tetrahedral geometry and resulting in minimal angle strain relative to smaller cycloalkanes such as (60°) or cyclobutane (90°). Due to its highly symmetric structure and absence of polar functional groups, cyclohexane is a non-polar with no net dipole moment. The is commonly represented in Kekulé form, which explicitly shows all six carbon atoms, twelve atoms, and the single bonds forming the ring, or in , where carbon atoms are implied at the vertices of a and atoms are omitted for simplicity (equivalent to the SMILES notation C1CCCCC1).

Conformational analysis

Cyclohexane adopts a variety of conformations due to the flexibility of its six-membered ring, with the chair conformation representing the global energy minimum. In this form, all carbon-carbon bonds are staggered, and the atoms are positioned either axially ( to the ring plane) or equatorially (roughly in the ring plane), minimizing torsional strain and avoiding eclipsed interactions./12%3A_Cycloalkanes_Cycloalkenes_and_Cycloalkynes/12.03%3A_Conformations_of_Cycloalkanes) Alternative conformations include the and twist-boat forms, which are higher in energy relative to the . The conformation features eclipsed bonds along four carbons and hydrogens that introduce steric repulsion, resulting in an energy approximately 6.9 kcal/mol above the . The twist-boat, a distorted version that relieves some of these interactions, lies about 5.5 kcal/mol higher than the but serves as a local minimum. Ring inversion in cyclohexane interconverts equivalent forms through a half-chair , where one bond becomes nearly planar, leading to significant torsional strain. This has an activation barrier of approximately 10-12 kcal/mol, allowing rapid equilibration at on the order of 10^5 times per second. In the conformation, axial and equatorial positions differ in their steric environment; axial substituents experience 1,3-diaxial interactions with syn-axial hydrogens, which contribute to higher energy for axial orientations compared to equatorial. These gauche-like interactions, each worth about 0.9 kcal/mol for hydrogen pairs, are absent in the unsubstituted ring but explain substituent preferences./12%3A_Cycloalkanes_Cycloalkenes_and_Cycloalkynes/12.03%3A_Conformations_of_Cycloalkanes) The low ring strain in cyclohexane, totaling 0.0-1.0 kcal/mol, arises from near-zero angle strain (bond angles close to the ideal 109.5°) and minimized torsional strain in the chair due to staggered arrangements, making it a strain-free model for cycloalkanes./Alkanes/Properties_of_Alkanes/Cycloalkanes/Ring_Strain_and_the_Structure_of_Cycloalkanes) The conformational equilibrium between chair and twist-boat is governed by chair⇌twist-boat\text{chair} \rightleftharpoons \text{twist-boat} with a free energy difference of about 5.5 kcal/mol favoring the chair, resulting in population ratios at room temperature of approximately 99.9% chair and 0.1% twist-boat.

Solid phases

Cyclohexane displays two primary solid phases, distinguished by their molecular ordering and thermodynamic properties. The high-temperature phase I, known as the phase, exists between approximately 186 K and the of 6.5 °C (279.7 K). In this phase, the molecules exhibit significant orientational freedom, rotating nearly isotropically about their centers of mass while maintaining positional order in a face-centered cubic lattice with Fm3ˉmFm\bar{3}m. This disorder contributes to the plastic-like mechanical behavior observed in such crystals. The low-temperature phase II forms below 186 K and represents a fully ordered crystalline state with a monoclinic structure in the space group C2/cC2/c. Here, the cyclohexane molecules adopt fixed chair conformations, aligned in the lattice without the rotational mobility seen in phase I; as noted in conformational analyses, the chair form is the predominant low-energy structure for cyclohexane. The intermolecular interactions in this phase are primarily van der Waals forces, stabilizing the ordered arrangement. The transition between phase II and phase I at 186.09 is a phase change, marked by a significant increase of 35.93 J/mol· due to the onset of orientational disordering in the higher-temperature phase. This change reflects the gain in rotational as the system moves from the rigid ordered lattice of phase II to the more dynamic phase I. The of 6.5 °C further delineates the upper limit of the solid state, with the liquid density at 25 °C measured at 0.7785 g/cm³, providing context for the material's behavior near the solid-liquid boundary.

Properties

Physical properties

Cyclohexane is a colorless at , exhibiting a mild, characteristic odor reminiscent of or . Its temperatures include a of 80.7 °C at standard and a of 6.5 °C. The of liquid cyclohexane is 0.7785 g/cm³ at 20 °C. Cyclohexane is practically insoluble in , with a solubility of 0.0058 g/100 mL at 25 °C, due to its non-polar . It is miscible with common organic solvents such as , , acetone, and . The is 1.426 at 20 °C, and the dynamic is 0.98 cP at the same temperature. Thermodynamic include a heat of vaporization of 30.0 kJ/mol at the and a of 155.5 J/mol·K for the liquid phase at 25 °C. Regarding flammability, cyclohexane has a of -20 °C and an of 245 °C.
PropertyValueConditionsSource
80.7 °C760 mm HgNIST WebBook
6.5 °C-PubChem
0.7785 g/cm³20 °CLSU Solvents
Solubility in 0.0058 g/100 mL25 °CICSC
1.42620 °CPubChem
Viscosity0.98 cP20 °CPubChem
Heat of vaporization30.0 kJ/molNIST WebBook
(liquid)155.5 J/mol·K25 °CNIST WebBook
-20 °CClosed cupPubChem
245 °C-PubChem

Chemical properties

Cyclohexane, as a saturated cyclic , demonstrates significant chemical inertness under ambient conditions. It resists reactions with strong acids and bases due to the absence of functional groups that would facilitate nucleophilic or . This stability arises from its fully saturated carbon framework, where all bonds are strong C-C and C-H bonds, rendering it unreactive toward most common without initiation by external energy sources. The molecule is nonpolar, possessing a dipole moment of 0 D, which accounts for its negligible in polar solvents like . Despite this inertness, cyclohexane participates in free radical reactions. Under irradiation, it undergoes with (Cl₂) or (Br₂), substituting one or more atoms to form chlorocyclohexane or bromocyclohexane, respectively; multiple substitutions can yield positional isomers. Cyclohexane maintains thermal stability up to moderate temperatures but undergoes at elevated conditions, decomposing to and gas. Studies indicate significant occurs above 700°C, primarily through sequential dehydrogenation pathways. The complete of cyclohexane in oxygen proceeds according to the balanced equation: \ceC6H12+9O2−>6CO2+6H2O\ce{C6H12 + 9O2 -> 6CO2 + 6H2O} with a standard of of ΔH° = -3920 kJ/mol, highlighting its high energy content as a .

Production

Industrial synthesis

The primary method for industrial production of cyclohexane is the catalytic of , which accounts for over 90% of global output. This process involves the reaction of (C₆H₆) with hydrogen gas (H₂) in the presence of catalysts such as , , or to yield cyclohexane (C₆H₁₂). The reaction proceeds as follows: C6H6+3H2→C6H12\text{C}_6\text{H}_6 + 3\text{H}_2 \rightarrow \text{C}_6\text{H}_{12} This transformation is highly exothermic, with a standard enthalpy change (ΔH) of -208 kJ/mol, necessitating careful temperature control to prevent side reactions and ensure efficient heat removal, often achieved through vaporization and heat exchangers. Typical operating conditions include temperatures of 120–150°C and pressures of 10–50 bar, depending on the specific catalyst and reactor design, such as fixed-bed or slurry reactors. These conditions enable high conversion rates, with selectivity exceeding 99%, resulting in cyclohexane purity above 99.9% after purification steps like distillation to remove residual benzene (typically <100 ppm). An alternative, though minor, production route involves fractional distillation of petroleum naphtha or crude oil fractions, where cyclohexane constitutes 10–30% of the C₆ components in certain boiling ranges (e.g., C₅–200°F naphtha). This method exploits the natural occurrence of cyclohexane in fossil feedstocks but is less common due to lower yields and the need for advanced superfractionation to separate it from isomers like methylcyclopentane. Global cyclohexane production reached approximately 9.6 million metric tons per year as of 2024, driven primarily by demand for nylon precursors, with major manufacturing hubs in Asia (e.g., China and Japan) and North America (e.g., the United States). Recent capacity expansions in Asia, such as those by Sinopec and Reliance in 2023–2024 adding nearly 1.5 million tonnes annually, have supported growth. Recent developments post-2020 emphasize sustainability in cyclohexane synthesis, including the integration of bio-based from renewable sources like biomass-derived platform chemicals and the use of green produced via electrolysis powered by renewables. These shifts aim to reduce the carbon footprint of the traditional process, which relies on fossil-derived and from steam methane reforming, aligning with broader industry goals for low-emission chemical production.

Historical development

Cyclohexane was first isolated in 1890 by Russian chemist from Caucasian petroleum, where it was found in the naphtha fraction and named "hexanaphthene" due to its origin and properties. This discovery marked the initial recognition of cyclohexane as a naturally occurring , though its structure remained unclear at the time. The compound's cyclic nature was later confirmed through chemical analysis, distinguishing it from linear hydrocarbons like . The structural elucidation of cyclohexane was achieved by Adolf von Baeyer in 1894, who synthesized it via ketonic decarboxylation of pimelic acid followed by a series of reductions using sodium amalgam and hydriodic acid. To verify the six-membered ring structure, Baeyer oxidized the synthetic product with nitric acid, yielding (hexanedioic acid), which provided key evidence for the ring size and connectivity. This work resolved earlier misidentifications, such as the product of benzene reduction with hydrogen iodide, which was actually methylcyclopentane due to ring contraction. An earlier attempt by Baeyer in 1893 to synthesize cyclohexane by reducing also failed, resulting in over-reduction to acyclic products rather than the desired cycloalkane. A breakthrough in synthesis came in 1898 when Nikolai Zelinsky reported the catalytic hydrogenation of benzene to using finely divided nickel as the catalyst at elevated temperatures (around 180–200°C) and atmospheric pressure. This method represented the first viable laboratory route from an aromatic precursor, demonstrating the feasibility of selective ring saturation without skeletal rearrangement. During the 1920s, further laboratory optimizations refined this hydrogenation process, including the development of in 1926, which improved catalyst efficiency and yield for benzene conversion. The mid-20th century brought commercialization of cyclohexane production, driven by the rising demand for nylon 6,6 following its invention in the 1930s. Nylon production required adipic acid, obtained by air oxidation of cyclohexane to a mixture of cyclohexanol and cyclohexanone (KA oil), followed by nitric acid oxidation. Initial U.S. production of adipic acid used phenol as a feedstock, but by the early 1940s, DuPont shifted to cyclohexane-derived routes for cost efficiency. Post-World War II, process improvements in the 1940s–1950s, such as optimized fixed-bed reactors and cobalt-accelerated air oxidation, enabled large-scale industrial implementation by the 1950s, with global capacity expanding rapidly to support the postwar boom in textiles and engineering plastics.

Reactions and applications

Chemical reactions

One of the primary chemical transformations of cyclohexane is its autoxidation to KA oil, a mixture of cyclohexanone and cyclohexanol. This reaction occurs via liquid-phase air oxidation at temperatures of 150–160 °C under 8–15 bar pressure, catalyzed by cobalt salts such as cobalt naphthenate or cobalt stearate, with conversions limited to 5–12% to minimize over-oxidation. The products form precursors for nylon-6,6 production through subsequent oxidation to adipic acid and for nylon-6 via cyclohexanone to caprolactam. The mechanism of autoxidation proceeds via a free radical chain reaction. Initiation involves the formation of alkyl radicals (R•) from cyclohexane, often promoted by trace peroxides or heat. Propagation steps include the rapid reaction of R• with O₂ to form peroxy radicals (ROO•), followed by hydrogen abstraction from another cyclohexane molecule to yield cyclohexyl hydroperoxide (ROOH) and regenerate ROO•. Termination occurs through radical recombination. The hydroperoxides decompose thermally or catalytically to the ketone and alcohol, with cobalt facilitating this step by promoting homolytic cleavage. Dehydrogenation of cyclohexane to benzene is an endothermic, reversible reaction conducted at 450–550 °C over platinum-based catalysts, such as Pt-Re/Al₂O₃, to achieve high selectivity toward benzene while suppressing coke formation. The process follows: C6H12⇌C6H6+3H2\mathrm{C_6H_{12} \rightleftharpoons C_6H_6 + 3H_2} Equilibrium favors benzene at elevated temperatures, with hydrogen removal enhancing conversion. Nitration of cyclohexane is challenging and typically achieved through vapor-phase processes or specialized reagents, as attempts with mixed nitric and sulfuric acids at 125–140 °C lead primarily to oxidation products like cyclohexanone rather than nitrocyclohexane, with low yields (typically <10%) due to side reactions including radical decomposition and polynitration. Alkylation of cyclohexane with alkenes under conditions using Lewis acids like AlCl₃ to generate carbocations from the alkene is feasible but severely limited by steric hindrance from the cyclic structure, resulting in poor regioselectivity and low yields of monoalkylated products. This contrasts with acyclic alkanes, where linear access facilitates better addition.

Industrial and laboratory uses

Cyclohexane serves as a primary precursor in the industrial production of and through oxidation to KA oil, which are essential intermediates for manufacturing nylon-6 and nylon-6,6 polymers. Approximately 90% of global cyclohexane demand is devoted to this application, underscoring its critical role in the textiles and plastics industries. Beyond nylon synthesis, cyclohexane functions as a non-polar solvent in various industrial processes, including the formulation of paints, varnishes, and synthetic rubber, as well as an extraction solvent for fats, oils, and essential oils. Its low polarity and volatility make it suitable for these roles, often substituting for more hazardous solvents like . Global demand for cyclohexane stands at around 7.5 million tons per year in the 2020s, with a compound annual growth rate (CAGR) of 4-5% driven by expanding applications in plastics and textiles. In laboratory settings, cyclohexane is widely employed as a non-polar solvent for recrystallizations, particularly of non-polar compounds such as steroids, due to its ability to dissolve solutes selectively while promoting crystal formation upon cooling. It also serves as an NMR reference standard, with the residual proton peak appearing at 1.38 ppm in deuterated solvents like CDCl₃. Additionally, high-purity cyclohexane is used as a calibration standard for differential scanning calorimetry (DSC) instruments, leveraging its well-defined solid-solid phase transition at approximately -87°C and melting point at 6.5°C to verify temperature accuracy. Minor industrial applications include its use as a blowing agent in the production of polyurethane foams, where it expands the polymer matrix to create insulating materials, and as a calibration standard in gas chromatography for analyzing hydrocarbon mixtures.

Safety and environmental impact

Health and safety

Cyclohexane exhibits low acute toxicity, with an oral LD50 greater than 5000 mg/kg in rats. It acts as an irritant to the skin and eyes upon contact, potentially causing redness, dryness, and discomfort. Inhalation of vapors can lead to dizziness, headache, nausea, and respiratory tract irritation, with the American Conference of Governmental Industrial Hygienists (ACGIH) establishing a threshold limit value (TLV) of 100 ppm as an 8-hour time-weighted average to prevent such effects. Chronic exposure to cyclohexane may result in nervous system depression, including symptoms such as sleepiness, limb weakness, and verbal memory impairment in humans, as observed in occupational settings with prolonged inhalation. There is no evidence of carcinogenicity in humans or animals, and the International Agency for Research on Cancer (IARC) has not classified it as a carcinogen. As a highly flammable liquid classified as NFPA Class IB, cyclohexane poses significant fire and explosion hazards due to its low flash point of -20°C and the ability of its vapors to form explosive mixtures with air at concentrations between 1.3% and 8%. Ignition sources should be avoided, as vapors can travel to distant points of ignition and flashback. Safe handling requires use in well-ventilated areas to minimize inhalation risks, with personal protective equipment (PPE) including chemical-resistant gloves, safety goggles, and respirators where exposure limits may be exceeded. Storage should occur in cool, well-ventilated places away from ignition sources and incompatible materials like strong oxidizers, using approved flammable liquid containers. Under the Globally Harmonized System (GHS), cyclohexane is classified with hazard statements H225 (highly flammable liquid and vapor), H304 (may be fatal if swallowed and enters airways, due to aspiration risk), and H315 (causes skin irritation). Regulatory limits include an OSHA permissible exposure limit (PEL) of 300 ppm as an 8-hour time-weighted average. In the European Union, cyclohexane is registered under REACH and subject to restrictions under Annex XVII, particularly regarding its use in consumer products to limit exposure from flammable and irritant properties.

Environmental considerations

Cyclohexane is biodegradable under aerobic conditions, with microbial degradation pathways identified in bacteria such as Rhodococcus sp. EC1, which metabolize it through sequential oxidation to intermediates like cyclohexanol and adipic acid. However, its persistence in water is influenced by volatility, characterized by a Henry's law constant of 0.15 atm·m³/mol at 25 °C, promoting rapid evasion to the atmosphere rather than prolonged aquatic residence. In the atmosphere, its half-life is approximately 52 hours, primarily due to photodegradation and reaction with hydroxyl radicals. Ecotoxicity assessments indicate high acute toxicity to aquatic life, with an EC50 of 0.9 mg/L for Daphnia magna (water flea) over 48 hours (static test, OECD 202) and LC50 values for fish such as 4.53 mg/L (flow-through test, OECD 203) to 32–93 mg/L (static test) for fathead minnow (Pimephales promelas) over 96 hours, and 34.7 mg/L for bluegill sunfish (Lepomis macrochirus). These concentrations are near or below its water solubility limit of about 52 mg/L, leading to GHS classifications of Aquatic Acute 1 (very toxic to aquatic life) and Aquatic Chronic 1 (very toxic to aquatic life with long lasting effects). Bioaccumulation potential remains low, supported by an octanol-water partition coefficient (log Kow) of 3.44, below the threshold (log Kow > 4) for significant biomagnification in food chains. As a (VOC), cyclohexane contributes to atmospheric emissions from , including storage tanks and in petrochemical facilities, where it can comprise up to 5.4% of total VOC profiles in some operations. These emissions are regulated under the U.S. Clean Air Act, which mandates controls on VOC releases from stationary sources to mitigate formation and air quality degradation. Mitigation strategies for cyclohexane emissions include advanced technologies like non-thermal plasma dielectric barrier discharge reactors, which achieve nearly 100% decomposition of cyclohexane vapors into smaller, less harmful molecules. Post-2020 research has also advanced bio-based alternatives, such as dihydropinene (derived from ) and Cyrene (dihydrolevoglucosenone from ), offering sustainable, low-toxicity substitutes for cyclohexane in solvent and extraction applications while reducing reliance on petroleum-derived feedstocks. Globally, cyclohexane's is largely indirect, stemming from its use in nylon-6,6 production, where non-recycled plastics contribute to marine and terrestrial through microplastic accumulation. However, nylon's nature enables effective mechanical and chemical , with industry investments aiming to close the loop and minimize waste, thereby limiting long-term environmental persistence.

References

  1. https://pubchem.ncbi.nlm.nih.gov/compound/Cyclohexane
  2. https://webbook.nist.gov/cgi/cbook.cgi?ID=C110827&Mask=4EF
  3. https://www.indexbox.io/blog/cyclohexane-world-market-overview-2024-4/
  4. https://www.nj.gov/health/eoh/rtkweb/documents/fs/0565.pdf
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