Cyclohexane

Cyclohexane is a cycloalkane with the molecular formula C₆H₁₂. Cyclohexane is mainly used for the industrial production of adipic acid and caprolactam, which are precursors to nylon. Cyclohexane is a colourless, flammable liquid with a distinctive detergent-like odor, reminiscent of cleaning products (in which it is sometimes used).^[3]

Production

Modern production

On an industrial scale, cyclohexane is produced by hydrogenation of benzene.^[4] Producers of cyclohexane accounts for approximately 11.4% of global demand for benzene.^[5] 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.^[6]

Historical methods

Unlike benzene, cyclohexane is not easily obtained from natural resources such as coal. For this reason, early investigators synthesized their cyclohexane samples.^[7]

Early failures

In 1867 Marcellin Berthelot reduced benzene with hydroiodic acid at elevated temperatures.^[8]
In 1870, Adolf von Baeyer repeated the reaction^[9] and pronounced the same reaction product "hexahydrobenzene"
in 1890 Vladimir Markovnikov believed he was able to distill the same compound from Caucasus petroleum, calling his concoction "hexanaphtene".

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 reassigned "hexahydrobenzene" and "hexanaphtene" as methylcyclopentane, the result of an unexpected rearrangement reaction.

Success

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

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

Reactions

Cyclohexane is rather unreactive, being a non-polar, hydrophobic hydrocarbon. It reacts with superacids, such as HF + SbF₅, which will lead to cracking. Substituted cyclohexanes, however, may be reactive under a variety of conditions, many of which are important in organic chemistry.

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.^[6]

Laboratory uses

Cyclohexane is sometimes used as an organic solvent.

Cyclohexane is also used for calibration of differential scanning calorimetry (DSC) instruments, because of a convenient crystal-crystal transition at −87.1 °C.^[10]

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

Conformation

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 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.

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).^[11]

Cyclohexane phases^[11]
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.

References

1 2 3 4 5 "NIOSH Pocket Guide to Chemical Hazards #0163". National Institute for Occupational Safety and Health (NIOSH).
1 2 "Cyclohexane". Immediately Dangerous to Life and Health. National Institute for Occupational Safety and Health (NIOSH).
↑ Campbell, M. Larry (2011). "Cyclohexane". doi:10.1002/14356007.a08_209.pub2.
↑ 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
↑ Market Study Benzene, Ceresana, July 2011
1 2 Michael Tuttle Musser "Cyclohexanol and Cyclohexanone" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
↑ Warnhoff, E. W. (1996). "The Curiously Intertwined Histories of Benzene and Cyclohexane". J. Chem. Ed. 73 (6): 494. doi:10.1021/ed073p494.
↑
See:
- Bertholet (1867) "Nouvelles applications des méthodes de réduction en chimie organique" (New applications of reduction methods in organic chemistry), Bulletin de la Société chimique de Paris, series 2, 7 : 53-65.
- Bertholet (1868) "Méthode universelle pour réduire et saturer d'hydrog'ene les composés organiques" (Universelle method for reducing and saturating with hydrogen organic compounds), Bulletin de la Société chimique de Paris, series 2, 9 : 8-31. From page 17: "En effet, la benzine, chauffée à 280° pendant 24 heures avec 80 fois son poids d'une solution aqueuse saturée à froid d'acide iodhydrique, se change à peu près entièrement en hydrure d'hexylène, C₁₂H₁₄, en fixant 4 fois son volume d'hydrog'ene: C₁₂H₆ + 4H₂ = C₁₂H₁₄ … Le neuveau carbure formé par la bezine est un corps unique et défini: il bout à 69°, et offre toutes les propriétés et la composition de l'hydrure d'hexylène extrait des pétroles." (In effect, benzene, heated to 280° for 24 hours with 80 times its weight of an aqueous solution of cold saturated hydroiodic acid, is changed almost entirely into hydride of hexylene, C₁₂H₁₄, [Note: this formula for hexane (C₆H₁₄) is wrong because chemists at that time used the incorrect atomic mass for carbon.] by fixing [i.e., combining with] 4 times its volume of hydrogen: C₁₂H₆ + 4H₂ = C₁₂H₁₄ … The new carbon compound formed by benzene is a unique and well-defined substance: it boils at 69° and presents all the properties and the composition of hydride of hexylene extracted from oil.)
↑ Adolf Baeyer (1870) "Ueber die Reduction aromatischer Kohlenwasserstoffe durch Jodphosphonium" (On the reduction of aromatic compound by phosphonium iodide [H₄IP]), Annalen der Chemie und Pharmacie, 155 : 266-281. From page 279: "Bei der Reduction mit Natriumamalgam oder Jodphosphonium addiren sich im höchsten Falle sechs Atome Wasserstoff, und es entstehen Abkömmlinge, die sich von einem Kohlenwasserstoff C₆H₁₂ ableiten. Dieser Kohlenwasserstoff ist aller Wahrscheinlichkeit nach ein geschlossener Ring, da seine Derivate, das Hexahydromesitylen und Hexahydromellithsäure, mit Leichtigkeit wieder in Benzolabkömmlinge übergeführt werden können." (During the reduction [of benzene] with sodium amalgam or phosphonium iodide, six atoms of hydrogen are added in the extreme case, and there arise derivatives, which derive from a hydrocarbon C₆H₁₂. This hydrocarbon is in all probability a closed ring, since its derivatives — hexahydromesitylene [1,3,5 - trimethyl cyclohexane] and hexahydromellithic acid [cyclohexane-1,2,3,4,5,6-hexacarboxylic acid] — can be converted with ease again into benzene derivatives.)
↑ Price, D. M. (1995). "Temperature Calibration of Differential Scanning Calorimeters". Journal of Thermal Analysis 45 (6): 1285–1296. doi:10.1007/BF02547423.
1 2 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.

External links

Wikimedia Commons has media related to Cyclohexane.

International Chemical Safety Card 0242
National Pollutant Inventory – Cyclohexane fact sheet
NIOSH Pocket Guide to Chemical Hazards
Cyclohexane@3Dchem
Hermann Sachse and the first suggestion of a chair conformation.
NLM Hazardous Substances Databank – Cyclohexane
Methanol Discovered in Space
Calculation of vapor pressure, liquid density, dynamic liquid viscosity, surface tension of cyclohexane
Cyclohexane production process flowsheet, benzene hydrogenation technique

Cycloalkanes

Cyclopropane Cyclobutane Cyclopentane Cyclohexane Cycloheptane Cyclooctane Cyclononane Cyclodecane Cycloundecane Cyclododecane Cyclotridecane Cyclotetradecane

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