Cyclohexane

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
Other names
Hexanaphthene (archaic)
Identifiers
3D model (JSmol)
3DMet
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)
−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%
Lethal dose or concentration (LD, LC):
12705 mg/kg (rat, oral)
813 mg/kg (mouse, oral)
17,142 ppm (mouse, 2 h)
26,600 ppm (rabbit, 1 h)
NIOSH (US health exposure limits):
PEL (Permissible)
TWA 300 ppm (1050 mg/m3)
REL (Recommended)
TWA 300 ppm (1050 mg/m3)
IDLH (Immediate danger)
1300 ppm
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 ?)
Infobox references

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.

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

Production

Modern

On an industrial scale, cyclohexane is produced by hydrogenation of benzene in the presence of a Raney nickel catalyst. Producers of cyclohexane account for approximately 11.4% of global demand for benzene. 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.

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

Early

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

Failure

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

In 1894, Baeyer synthesized cyclohexane starting with a ketonization 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

Although rather unreactive, cyclohexane undergoes catalytic oxidation to produce 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.

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.

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

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 angle strain because its bonds are not 109.5 degrees; the torsional strain would also be considerable because all of the bonds would be eclipsed bonds. 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

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).

Cyclohexane phases
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


This page was last updated at 2024-01-05 13:16 UTC. Update now. View original page.

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