Cyclohexane: A Chemical Chair

Cyclohexane belongs to the category of hydrocarbons called naphthenes. It has a chemical formula of C6H12. Naphthenes are found in crude oil. Most abundant are mono ring compounds of which cyclohexane is the perfect example. In fact, the poly ring naphthenes are found in the high boiling fractions of naphtha and the name seems to have been derived from naphtha. The term Naphthene has become almost redundant except in petroleum industry.The other fractions present in the crude oil are paraffins which are long chain alkanes  with formula CnH2n+1. For example, when n=6, then we have hexane with formula C6H14. High molecular weight paraffins are the main constituents of waxes. Another important category of compounds present in the crude oil is called aromatics. The most celebrated and simplest molecule of the category is Benzene with chemical formula C6H6. The overall properties of crude oil depend on the relative amounts of these fractions.

So, we observe that normal hexane, cyclohexane and benzene have six carbons but number of hydrogen atoms go on decreasing while going from paraffins to aromatics. This indicates that we are progressing towards graphite with the loss of hydrogen atoms. Crude oils derived from the marine sources are lighter in nature meaning they have lesser amounts of waxy paraffins. This helps them to be lighter and survive in under sea hydrostatic pressure. On the other hand, plants in the arid climates have wax on their leaves which helps in the prevention of moisture evaporation and is absolutely necessary for the survival of plants under harsh conditions. Thus the crude derived from terrestrial plants have higher wax content.

Anyway, we come to cyclohexane. The carbon atoms try to have tetrahedral geometry around them when combining to 4 atoms. But this is not possible here. So they settle for a compromise for stability. Cyclohexane twists to have a shape like a chair with alternate carbons in two parallel planes and hydrogen atoms arranged in two directions. They are in positions called axial and equatorial positions with axial hydrogen atoms almost perpendicular to the carbon atoms plane with alternating up and down positions.

How was this structure confirmed? It was confirmed by using the tool called Nuclear Magnetic Resonance (NMR) which is used these days in the medical science under the name of MRI. The protons  (hydrogen atoms) behave like tiny magnets and spin around their axis. As we know from the magnets, when we place a small magnet near a strong magnet, smaller one aligns itself along the axis of stronger magnet. Energy is required to flip it back to its original position. Thus the principle of NMR is that molecules are placed in a powerful magnetic field and swept with electromagnetic energy. This will be of no useful tool if all the hydrogen atoms absorb the energy at same wavelength. Fortunately, the magnetic moments of hydrogen atoms are modified by the electronic environment around it a molecule which may be due to the presence of hetero atoms and groups or different number of groups. Thus due to the different environment, the hydrogen atoms absorb energy at different wavelengths. From this we come to know the chemical environment of different hydrogen atoms and thus elucidate its structure. In our brain, we have so many molecules which contain hydrogen atoms and these are affected by the state of our mind.

When cyclohexane is placed in a NMR cell, we get only a single signal indicating that all the hydrogen atoms are equivalent. But this is true only at normal temperature. The reason is that axial and equatorial hydrogen atoms flip up and down because energy required for this is very small and room temperature provides it. But as we cool the sample, less energy is available and a temperature reaches 40K, the atoms sort of freeze in their locations and two peaks begin to appear. These peaks are indicative of up and down orientation hydrogen atoms. This confirms a chair configuration of cyclohexane.


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