The non-aromatic compounds without ring structure are termed aliphatic whereas those with a ring structure like cyclohexane are termed alicyclic. The aromatic hydrocarbons often consist of several fused rings as in case of benzopyrene.
Both classes of hydrocarbons occur naturally and notably in all kinds of oil and coal deposits. The aromatic compounds are also products of incomplete combustion of organic compounds which are released into the environment by both human activities and by natural causes. As compared to aromatic hydrocarbons are interesting not only because of its structure but also due to their mutagenic and carcinogenic properties.
Hydrocarbons are compounds composed of carbon and hydrogen alone and they are classified into two main groups, aromatic and non-aromatic compounds. The aromatic hydrocarbons contain ring system with delocalised electron cloud while the non-aromatic hydrocarbons do not contain such a ring system. These include mainly the alkanes, which are saturated hydrocarbons, alkenes which contain one or more double bonds and finally the alkenes which contain one or more triple bonds.
A cyclic compound which doesn’t necessitate a continuous form of overlapping ring of p orbitals need not be considered as aromatic or even anti aromatic and hence these are termed as nonaromatic or aliphatic.
Non-aromatic compounds electronic energy is similar to that of its open-chain counterpart. Crude oil consists mainly of alkanes and large releases into the sea due to the wreckage of oil tankers have caused the death of many seabirds and other marine organisms because of their physical effects of oiling or smothering with these compounds.
The working knowledge of aromatic compounds has included the cyclic compounds containing conjugated double bonds with unusually large resonance energies. So aromatic compounds are those which meet the following criteria.
Compared to this the antiaromatic compound is one that refers to the first three criteria but delocalised pi electrons over ring increase the electronic energy. The aromatic structures are more stable than their open-chain counterparts.
For example, 1, 3, 5 hexatriene is comparatively unstable than the structure of benzene. The cyclobutadiene meets the first three criteria for a continuous ring of overlapping p orbitals but localisation of the pi electrons increase the electronic energy.
Some of the aromatic compounds are shown below.
Nonaromatic chiral compounds are also resolved on polysaccharide based chiral stationary phase. Several nonaromatic compounds resolved on cellulose tri benzoate. The detection by a UV detector is often difficult and hence refractive index detector is used for enantiomer peak detection.
An aromatic compound is characterized by its cyclic and planar structure and along with that conjugated system of the p orbitals which are found to be perpendicular to the molecule’s plane. This conjugation makes the molecule very stable.
In comparison, a nonaromatic is found to be either non-cyclic or not in a planar form. While an aromatic compound will always follow the Huckel rule with a 4n+2 electrons conjugation and we get to see fully filled orbits as compared to antiaromatic compound with just 4n electrons. This makes the compound very reactive.
Apart from this, aromatic compound is differentiated from similar nonaromatic compounds by the level of susceptibility. The London diamagnetism leads to larger susceptibility for aromatic compounds. This means aromatic compounds will show diamagnetic susceptibility exaltation and this becomes a criterion for the aromatic character.
There are many examples of non-aromatic compounds. A few lists is given below.