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 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.
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 benzo pyrene.
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 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.
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. Its 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.
- Structure must be cyclic containing some number of conjugated pi bonds
- Within the ring each atom must have un-hybridised p orbital and with sp2 hybridisation or sp hybridisation
- Unhybridised p orbital must overlap to form a continuous ring of parallel orbitals
- Most of the structure is planar or nearly planar for effective overlapping
- Delocalisation of the pi bond electrons over ring must lower the electronic energy.
Compared to this the antiaromatic compound is one that refers 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 hexa triene is comparatively unstable than the structure of benzene. The cyclo butadiene meets the first three criteria for a continuous ring of overlapping p orbitals but localisation of the pi electrons increase the electronic energy.
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.
A class of compounds with aromatic properties in which one or more of the ring carbons are replaced by a heteroatom, an atom other than carbon like nitrogen oxygen or sulphur. Some of these compounds preserve the six membered ring and are therefore called benzenoid aromatics. The 4n + 2 pi electrons which reside inside a five membered ring and hence are called non benzenoid aromatics.
A key difference between these two classes of aromatic compounds is that in non benzenoid hetero aromatic compounds, a formally non-bonding pair of electrons on the heteroatom becomes part of 4n + 2 pi electrons of the aromatic pi system, whereas in benzenoid aromatic compounds, the non-bonding pairs on heteroatoms are normally separate from the aromatic system.
- Benzenoid hetero aromatics:
- Pyridine, quinoline and pyrimidine
- Non benzenoid hetero aromatics:
- Pyrrole, furan and Thiophenes
The ultraviolet spectra of non benzenoid carbocyclic aromatic compounds exhibit a resemblance to those of their benzenoid analogues but there is a substantial shift to longer wavelengths. Another important class of non benzenoid aromatic molecules are azulenes, which is isoelectronic with naphthalene but their spectra are distinctly different.
Cyclo octatetraene is a cyclic compound having four alternate double bonds and thus the number of pi bonds electrons is 8 which does not fit in Huckel rule because n comes out to be 1.5 and is not considered as an integer. Hence, cyclo octatetraene is non aromatic and moreover, it is found to be tub shaped and not planar.
It is apparent from above consideration that an aromatic compound need not necessarily contain a benzene ring and actually many a compounds not having benzene ring are found to be aromatic in nature. An aromatic compound may be benzenoid or having a benzene ring or non benzenoid and all that is required for aromatically is the presence of a flat ring with pi electrons in the form of continuous electron clouds above and below the plane of the ring.
Tropolone is another non benzenoid aromatic compound which was first discovered along with many different kinds of natural products. Tropolone may be synthesised either from cyclo heptane derivative or by ring expansion of aromatic compounds.
The nonaromatic compounds are more stable than an antiaromatic compound but is considered to be less stable than aromatic compounds. As far the reactivity is concerned the order of reactivity is reverse of the stability and the nonaromatic compounds are more reactive than aromatic compounds. The reactivity is basically mainly due to the presence of delocalised electron cloud below and above the planar structure of aromatic compounds.
Difference Between Aromatic and Nonaromatic Compounds
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An aromatic compound is characteriz/ed 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 a fully filled orbits as compared to anti aromatic 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 criteria for aromatic character.
There are many examples of nonaromatic compounds. To name a few the list is as follows.
1, 3, 5-cyclo heptatriene
4-vinyl cyclo hexene
1, 5, 9-cyclo deca triene