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Cyclohexane: structure, uses, conformations

It has a density of 0.779 g / cm 3 ; boils at 80.7 ° C; and frozen at 6.4 ° C. It is considered insoluble in water, as 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.

3D model of the cyclohexane molecule. Jynto and Ben Mills / Public domain

Ring systems of cyclohexane are more common among organic molecules in nature than those of other cycloalkanes. This may be due both to their stability and to the selectivity offered by their well-established conformations.

Structure

Cyclohexane is a six-membered alicyclic hydrocarbon. It exists primarily 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 cycloalkanes, cyclohexane is considered to have zero relative to 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 equatorial CH 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. 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 with a molecular modeling program. When using any of these models, it is possible to easily observe the torsional relationships and orientation of the equatorial and axial hydrogen atoms.

Newman projection of cyclohexane. Durfo / CC0

Conformations

Cyclohexane can come in two conformations that are interconvertible: boat and chair. However, the latter is the most stable conformation, as there is no angle or torsional stress in the cyclohexane structure; more than 99% of the molecules are in a chair conformation at any given time.

Chair conformation of cyclohexane. Chem Sim 2001 / Public domain

Chair conformation

In a saddle conformation, all CC bond angles are 109.5 °, which relieves them of angular stress. Because the DC links are perfectly staggered, the saddle conformation is also free from torsional stress. Also, the hydrogen atoms at opposite corners of the cyclohexane ring are spaced as far apart.

Boat conformation

The chair shape can take another shape called the can shape. This occurs as a result of partial rotations on the CC single bonds of the ring. Such a conformation also does not present angular stress, but it does have torsional stress.

 

Boat conformation of cyclohexane. Keministi / Public domain

When you look at a model of the boat conformation, at the CC bond axes along each side, you find that the C − H bonds in those carbon atoms are eclipsed, producing torsional stress.

Also, two of the hydrogen atoms are close enough to each other to generate Van Der Waals repulsive forces.

Twisted boat conformation

If the boat conformation flexes, you get the twisted boat conformation that can relieve some of the torsional stress and also reduce the interactions between the hydrogen atoms.

However, the stability obtained by bending is insufficient to make the twisted boat conformation more stable than the saddle conformation.

Twisted boat conformation of cyclohexane. Keministi / CC0

Applications

Nylon Manufacturing

Almost all the commercially produced cyclohexane (more than 98%) is widely used as a raw material in the industrial production of nylon precursors: adipic acid (60%), caprolactam and hexamethylenediamine. 75% of the caprolactam produced worldwide is used for the manufacture of nylon 6.

Guitar with nylon strings. Source: pexels.com

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 non-polar solvent for the chemical industry and as a diluent in polymer 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 fractional distillation of naphtha in which an 85% cyclohexane concentrate was obtained by super-fractionation.

Fractional distillation of petroleum. Crude_Oil_Distillation-fr.svg: Image originale: Psarianos, Theresa knott; image vectorielle: Rogilbertderivative work: Utain () / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/)

This concentrate was sold as such, as further purification required carrying out a process of isomerization of pentanes, heat cracking to remove open-chain hydrocarbons, and treatment with sulfuric acid to remove aromatic compounds.

Much of the difficulty in obtaining cyclohexane with higher purity 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 catalytic fixed bed. Several processes have been developed in which nickel, platinum or palladium is used as a catalyst.

Most cyclohexane plants use benzene-producing reformer gas and large amounts of hydrogen by-products 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 feedstock are available.

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