Cyclohexane is a cycloalkane with the molecular formula6. 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.
Production and reactions On an industrial scale, cyclohexane is produced
by hydrogenation of benzene. Producers of cyclohexane accounts 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. Historical methods
Unlike benzene, cyclohexane is not easily obtained from natural resources such as coal.
For this reason, early investigators synthesized their cyclohexane samples. 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, but also because he did not believe benzene was a cyclic molecule 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:
In the same year E. Haworth and W.H. Perkin Jr. prepared it via 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
reassigned "hexahydrobenzene" and "hexanaphtene" as methylcyclopentane, the result of an unexpected
rearrangement reaction.
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 by catalytic oxidation. KA oil is
then used as a raw material for adipic acid and caprolactam.
Laboratory uses Cyclohexane is sometimes used as a repelent
organic solvent. Cyclohexane is also used for calibration of
differential scanning calorimetry instruments, because of a convenient crystal-crystal transition
at −87.1 °C. Heat treating equipment manufacturer Surface
Combustion uses cyclohexane as a gas in their high purity vacuum carburizing furnaces.
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 while the other half are
perpendicular to the plane. 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 when burned compared to the other cycloalkanes.
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
phase II is ordered. Two other low-temperature phases III and IV have been obtained by application
of moderate pressures above 30 MPa, where phase IV appears exclusively in deuterated
cyclohexane. 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
The Flixborough disaster, a major industrial accident caused by an explosion of cyclohexane.
Hexane Cyclohexane
References ^ a b c M. Larry Campbell. "Cyclohexane" in
Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2012. 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]
^ Warnhoff, E. W.. "The Curiously Intertwined Histories of Benzene and Cyclohexane". J.
Chem. Ed. 73: 494. doi:10.1021/ed073p494. ^ See:
Bertholet "Nouvelles applications des méthodes de réduction en chimie organique", Bulletin
de la Société chimique de Paris, series 2, 7 : 53-65.
Bertholet "Méthode universelle pour réduire et saturer d'hydrog'ene les composés organiques",
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." 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 "Ueber die Reduction aromatischer Kohlenwasserstoffe durch Jodphosphonium",
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."
^ Price, D. M.. "Temperature Calibration of Differential Scanning Calorimeters". Journal
of Thermal Analysis 45: 1285–1296. doi:10.1007/BF02547423. ^ a b Mayer, J.; Urban, S.; Habrylo, S.; Holderna,
K.; Natkaniec, I.; Würflinger, A.; Zajac, W.. "Neutron Scattering Studies of C6H12 and
C6D12 Cyclohexane under High Pressure". Physica status solidi 166: 381. doi:10.1002/pssb.2221660207.
External links 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