por He cyclohexane it is a relatively stable cycloalkane with the molecular formula C6H12. It is a colorless, flammable liquid that has a mild solvent odor, but can be pungent in the presence of impurities.
It has a density of 0.779 g/cm3; boils at 80.7°C; and freezes at 6.4°C. It is considered insoluble in water, since its solubility can only be as low as 50 ppm (approx.) at room temperature. However, it mixes easily with alcohol, ether, chloroform, benzene, and acetone.
Ringed systems of cyclohexane are more common among organic molecules in nature than those of other cycloalkanes. This may be due to both their stability and the selectivity offered by their well-established conformations.
In fact, carbohydrates, steroids, plant products, pesticides, and many other important compounds contain rings similar to those of cyclohexane, whose conformations are of great importance for their reactivity.
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Structure
Cyclohexane is a six-membered alicyclic hydrocarbon. It exists mainly in a conformation in which all CH bonds on neighboring carbon atoms are staggered, with dihedral angles equal to 60°.
Because it has the lowest angle and torsional stress of all the cycloalkanes, cyclohexane is considered to have zero total ring stress. This also makes cyclohexane the most stable of the cycloalkanes and therefore produces the least amount of heat when burned compared to the other cycloalkanes.
Substituent positions
There are two types of positions for substituents on the cyclohexane ring: axial positions and equatorial positions. The CH equatorial bonds lie in a band around the equator of the ring.
In turn, each carbon atom has an axial hydrogen that is perpendicular to the plane of the ring and parallel to its axis. The axial hydrogens alternate up and down; each carbon atom has an axial and an equatorial position; and each side of the ring has three axial and three equatorial positions in an alternating arrangement.
study models
Cyclohexane is best studied by building a physical molecular model or using molecular modeling software. When one of these models is used, it is possible to easily observe the torsion relationships and the orientation of the equatorial and axial hydrogen atoms.
However, one can also analyze the arrangement of hydrogen atoms in a Newman projection by looking at any pair of parallel CC bonds.
conformations
Cyclohexane can occur in two conformations that are interconvertible: boat and chair. However, the latter is the most stable conformation, since there is no angle or torsional strain in the cyclohexane structure; more than 99% of the molecules are in a chair conformation at any given time.
Saddle conformation
In a chair conformation, all CC bond angles are 109.5°, which relieves them of angle strain. Because the CC links are perfectly staggered, the saddle conformation is also free of torsional stress. Also, the hydrogen atoms at opposite corners of the cyclohexane ring are as far apart as possible.
boat conformation
The chair conformation can take another form called the boat conformation. This occurs as a result of partial rotations on the CC single links of the ring. Such a conformation also has no angular stress, but it does have torsional stress.
When viewing a model of the boat conformation, in the CC bond axes along each side, the C−H bonds in those carbon atoms are found to be eclipsed, producing torsional stress.
Also, two of the hydrogen atoms are close enough to each other to generate repulsive Van Der Waals forces.
crooked boat conformation
If the boat conformation is flexed, the twisted boat conformation is obtained which can relieve some of the torsional stress and also reduce the interactions between hydrogen atoms.
However, the stability obtained by bending is insufficient to make the crooked boat conformation more stable than the chair conformation.
Applications
nylon fabrication
Almost all of the cyclohexane that is produced commercially (over 98%) is widely used as a feedstock in the industrial production of the precursors to nylon: adipic acid (60%), caprolactam, and hexamethylenediamine. 75% of the caprolactam produced worldwide is used to make nylon 6.
Manufacture of other compounds
However, cyclohexane is also used in the manufacture of benzene, cyclohexyl chloride, nitrocyclohexane, cyclohexanol, and cyclohexanone; in the manufacture of solid fuel; in fungicidal formulations; and in the industrial recrystallization of steroids.
minority applications
A very small fraction of the cyclohexane produced is used as a nonpolar solvent for the chemical industry and as a diluent in polymeric reactions. It can also be used as a paint and varnish remover; in the extraction of essential oils; and glass substitutes.
Due to its unique chemical and conformational properties, cyclohexane is also used in analytical chemistry laboratories for molecular weight determinations and as a standard.
Manufacture
traditional process
Cyclohexane is present in crude oil in concentrations that vary between 0.1 and 1.0%. Therefore, it used to be traditionally produced by the fractional distillation of naphtha in which a concentrate of 85% cyclohexane was obtained by superfractionation.
This concentrate was sold as such, since further purification required carrying out a pentanes isomerization process, heat cracking to remove open-chain hydrocarbons, and treatment with sulfuric acid to remove aromatic compounds.
Much of the difficulty in obtaining higher purity cyclohexane was due to the large number of petroleum components with similar boiling points.
high efficiency process
Today, cyclohexane is produced on an industrial scale by reacting benzene with hydrogen (catalytic hydrogenation) due to the simplicity of the process and its high efficiency.
This reaction can be carried out using liquid or vapor phase methods in the presence of a highly dispersed catalyst or in a fixed bed catalyst. Various processes have been developed using nickel, platinum, or palladium as a catalyst.
Most cyclohexane plants use reformer gas that produces benzene and large amounts of hydrogen byproducts as feedstock for cyclohexane production.
Because hydrogen and benzene costs are critical to profitably manufacturing cyclohexane, plants are often located near large refineries where low-cost feedstocks are available.
References
Campbell, M. L. (2014). Cyclohexane. Ullmann’s Encyclopedia of Industrial Chemistry (7th ed.). New York: John Wiley & Sons.
McMurry, J. (2011). Fundamentals of Organic Chemistry (7th ed.). Belmont: Brooks/Cole.
National Center for Biotechnology Information. (2020) PubChem Database. Cyclohexane, CID=8078. Bethesda: National Library of Medicine. Retrieved from: pubchem.ncbi.nlm.nih.gov
Ouellette, RJ, & Rawn, JD (2014). Organic Chemistry – Structure, Mechanism, and Synthesis. San Diego: Elsevier.
Petrucci, RH, Herring, FG, Bissonnette, C., & Madura, JD (2017). General Chemistry: Principles and Modern Applications (11th ed.). New York: Pearson.
Solomons, TW, Fryhle, CB, & Snyder, SA (2016). Organic Chemistry (12th ed.). Hoboken: John Wiley & Sons.
Wade, LG (2013). Organic Chemistry (8th ed.). New York. pearson.