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Cyclohexane (Molecule of the Month for October 2000)

Cyclohexane is a colourless and volitile liquid with a slightly pungent odor resembling that of chloroform or benzene. Air odor thresholds ranging from 25 to 300 parts per million (ppm) parts of air have been reported.

Key facts about cyclohexane:
Molecular weight: 84.18 g mol-1
Boiling point 80.7 degrees C (353.85 degrees Kelvin)
Specific gravity: 0.78 at 20 degrees C (293.15 degrees Kelvin)
Vapor density: 2.90
Melting point: 6.47 degrees C (280.62 degrees Kelvin)
Vapor pressure at 20 degrees C (293.15 degrees Kelvin): 95 mm Hg
Solubility: Insoluble in water; soluble in alcohol, ether, acetone, benzene, and ligroin.

There are several ways in which cyclohexane can be produced. Below are the two most common ways: Catalytic Hydrogenation of Benzene Unlike double bonds in alkenes, benzene cannot be reduced using the usual catalysts such as Platinum or Palladium. Only under conditions such as several hundred atmospheres can this be achieved using the usual catalysts. Therefore, a more powerful catalyst is required; Rhodium on Carbon is a suitable option. Under these conditions, benzene is reduced readily to cyclohexane. Fractional Distillation of Petroleum Petroleum is a complex mixture of hydrocarbons and must be refined before the different constituents can be used. Cyclohexane is one of these molecules of hydrocarbons. The desired molecule will be in the straight-run gasoline section of the fractional distillatory (30 200 degrees Celsius) Cyclohexane has many uses in industry - a few are given below: Cyclohexane use is tied almost exclusively to nylon.Over 90% of cyclohexane is used in the manufacture of nylon fibre and nylon molding resin. The remaining 10% of cyclohexane ends up as solvents for paint, resins, varnish and oils, or in plasticisers. It is also used as an intermediate in the manufacture of other industrial chemicals.

Chair Conformation In this conformation, all the hydrogens (white) are staggered. You can see this by rotating the model (right-clicking to turn off rotation first) to look down any one of the C-C bonds. This is the lowest energy conformation. Boat Conformation In this conformation, some of the hydrogens on neighbouring carbons are eclipsed (on top of each other). You can see this by rotating the model and looking down the C-C bonds making up the 'body' of the boat. Additionally, two hydrogens on the apex of the boat (called 'flagpole' hydrogens) are pointed towards each other. (Change to a space-filling model to see the interaction between these two hydrogens.). Both these cases lead to physical interactions which give this conformation a higher energy than the chair. Twisted Boat Conformation In fact, a slightly lower energy can be attained if the boat twists slightly, a conformation usually called the 'twist boat' conformation, shown below. See how the non-bonding conflict between hydrogens has been relieved a little.

Picture of Cyclohexane

click on the picture above to interact
with the 3D model of the
Cyclohexane structure
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Picture of Cyclohexane

Update by Karl Harrison
(Molecule of the Month for October 2000 )