Cyclohexane

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Cyclohexane
Identifiers
CAS number 110-82-7 YesY
PubChem 8078
ChemSpider 7787 YesY
UNII 48K5MKG32S YesY
DrugBank DB03561
KEGG C11249 YesY
ChEBI CHEBI:29005 YesY
ChEMBL CHEMBL15980 YesY
Jmol-3D images Image 1
Properties
Molecular formula C6H12
Molar mass 84.16 g/mol
Appearance colorless liquid
Odor sweet, gasoline-like
Density 0.7781 g/mL, liquid
Melting point 6.47 °C; 43.65 °F; 279.62 K
Boiling point 80.74 °C; 177.33 °F; 353.89 K
Solubility in water Immiscible
Solubility soluble in ether, alcohol, acetone
miscible with olive oil
Refractive index (nD) 1.42662
Viscosity 1.02 cP at 17 °C
Thermochemistry
Std enthalpy of
formation ΔfHo298		 -156 kJ/mol
Std enthalpy of
combustion ΔcHo298		 -3920 kJ/mol
Hazards
MSDS External MSDS
EU classification Flammable ('F)
Harmful (')
Dangerous for
the environment (N)
Severe eye irritant, may cause corneal clouding
R-phrases R11, R38, R65, R67, R50/53
S-phrases (S2), S9, S16, S25, S33, S60, S61, S62
NFPA 704
3
1
0
Flash point −20 °C; −4 °F; 253 K
Autoignition temperature 245 °C; 473 °F; 518 K
Related compounds
Related cycloalkanes Cyclopentane
Cycloheptane
Related compounds Cyclohexene
Benzene
Supplementary data page
Structure and
properties 		n, εr, etc.
Thermodynamic
data 		Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Cyclohexane is a cycloalkane with the molecular formula C6H12. Cyclohexane is used as a nonpolar solvent for the chemical industry, and also as a raw material for the industrial production of adipic acid and caprolactam, both of which being intermediates used in the production of nylon. On an industrial scale, cyclohexane is produced by reacting benzene with hydrogen. Producers of cyclohexane account for approximately 11.4% of global demand for benzene.[1] Because of its unique chemical and conformational properties, cyclohexane is also used in labs in analysis and as a standard. Cyclohexane has a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used).

Structure and physical properties

Conformation

Main article: Cyclohexane conformation

The 6-vertexed 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.There are also two other intermediate conformers; half chair, which is the most unstable conformer, and twist boat, which is more stable than the boat conformer. This 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, a combination of angle and torsional strain. This also makes cyclohexane the most stable of the cycloalkanes and therefore will produce the least amount of heat (per CH2 unit) when burned compared to the other cycloalkanes.

A cyclohexane molecule in chair conformation. Hydrogen atoms in axial positions are shown in red, while those in equatorial positions are in blue.

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 (note that application of pressure increases the values of all transition temperatures).[2]

Cyclohexane phases[2]
NoSymmetry 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.

Chemistry

Reactions

Pure cyclohexane in itself is rather unreactive, being a non-polar, hydrophobic hydrocarbon. It can react with very strong acids such as the superacid system HF + SbF5, which will cause forced protonation and "hydrocarbon cracking". Substituted cyclohexanes, however, may be reactive under a variety of conditions, many of which being important to organic chemistry. Cyclohexane is highly flammable.

Derivatives

The specific arrangement of functional groups in cyclohexane derivatives, and indeed in most cycloalkane molecules, is extremely important in chemical reactions, especially reactions involving nucleophiles. Substituents on the ring must be in the axial formation to react with other molecules. For example, the reaction of bromocyclohexane and a common nucleophile, a hydroxide anion (OH−), would result in cyclohexene:

C6H11Br + OH− → C6H10 + H2O + Br−

This reaction, commonly known as an elimination reaction or dehalogenation (specifically E2), requires that the bromine substituent be in the axial formation, opposing another axial H atom to react. Assuming that the bromocyclohexane was in the appropriate formation to react, the E2 reaction would commence as such:

  1. The electron pair bond between the C-Br moves to the Br, forming Br− and setting it free from cyclohexane
  2. The nucleophile (-OH) gives an electron pair to the adjacent axial H, setting H free and bonding to it to create H2O
  3. The electron pair bond between the adjacent axial H moves to the bond between the two C-C making it C=C

Note: All three steps happen simultaneously, characteristic of all E2 reactions.

The reaction above will generate mostly E2 reactions and as a result the product will be mostly (~70%) cyclohexene. However, the percentage varies with conditions, and generally, two different reactions (E2 and SN2) compete. In the above reaction, an SN2 reaction would substitute the bromine for a hydroxyl (OH−) group instead, but once again, the Br must be in axial to react. Once the SN2 substitution is complete, the newly substituted OH group would flip back to the more stable equatorial position quickly (~1 millisecond).

Uses

Commercially most of cyclohexane produced is converted into cyclohexanone–cyclohexanol mixture (or "KA oil") by catalytic oxidation. KA oil is then used as a raw material for adipic acid and caprolactam. Practically, if the cyclohexanol content of KA oil is higher than cyclohexanone, it is more likely (economical) to be converted into adipic acid, and the reverse case, caprolactam production is more likely. Such ratio in KA oil can be controlled by selecting suitable oxidation catalyst. Cyclohexane is sometimes used as a nonpolar organic solvent.

Although much is already known about this cyclic hydrocarbon, research is still being done on cyclohexane and benzene mixtures and solid phase cyclohexane to determine hydrogen yields of the mix when irradiated at −195 °C.

Cyclohexane is also used for calibration of differential scanning calorimetry (DSC) instruments, because of a convenient crystal-crystal transition at −87.1 C.[3]

Heat treating equipment manufacturer Surface Combustion uses cyclohexane as a carbon carrying gas in their high purity vacuum carburizing furnaces.

History

Unlike compounds like benzene, cyclohexane cannot easily be obtained from natural resources such as coal. Toward the end of the nineteenth century early chemical investigators had to depend on organic synthesis. It took them 30 years to flesh out the details.[4] In 1867 Marcellin Berthelot reduced benzene with hydroiodic acid at elevated temperatures. He incorrectly identified the reaction product as n-hexane not only because of the convenient match in boiling point (69 °C) but also because he did not believe benzene was a cyclic molecule (like his contemporary August Kekulé) but rather some sort of association of acetylene. In 1870 one of his sceptics Adolf von Baeyer repeated the reaction and pronounced the same reaction product hexahydrobenzene and in 1890 Vladimir Markovnikov believed he was able to distill the same compound from Caucasus petroleum calling his concoction hexanaphtene.

In 1894 Baeyer synthesized cyclohexane starting with a Dieckmann condensation of pimelic acid followed by multiple reductions:

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

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

Today, cyclohexane can be synthesized from benzene through more advanced reduction reactions.[5]

See also

References

  1. ↑ Market Study Benzene, Ceresana, July 2011
  2. ↑ 2.0 2.1 Mayer, J.; Urban, S.; Habrylo, S.; Holderna, K.; Natkaniec, I.; Würflinger, A.; Zajac, W. (1991). "Neutron Scattering Studies of C6H12 and C6D12 Cyclohexane under High Pressure". Physica status solidi (b) 166 (2): 381. doi:10.1002/pssb.2221660207. 
  3. ↑ Price, D. M. (1995). "Temperature Calibration of Differential Scanning Calorimeters". Journal of Thermal Analysis 45 (6): 1285–1296. doi:10.1007/BF02547423. 
  4. ↑ Warnhoff, E. W. (1996). "The Curiously Intertwined Histories of Benzene and Cyclohexane". J. Chem. Ed. 73 (6): 494. doi:10.1021/ed073p494. 
  5. ↑ Fred Fan Zhang, Thomas van Rijnman, Ji Soo Kim, Allen Cheng. On Present Methods of Hydrogenation of Aromatic Compounds, 1945 to Present Day. Lunds Tekniska Högskola 2008

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