The classification of organic compounds is based on the structure of the molecules. The aliphatic compounds have open chain structures such as hexane and can contain single C- C or double C = C and even a triple bond between the carbon atoms. Aromatic compounds are unsaturated cyclic molecules that possess additional stability as a result of the arrangement of the pie electrons associated with the unsaturation of the ring system.
These compounds are also known as arenes and can be carbocyclic which indicates that ring skeleton comprise of only carbon atoms. These can be heterocyclic as well where at least one atom apart from carbon is in the ring. These hetero atoms can typically be N, O or S. The term aromatic shows that many of the natural fragrances that we get to see around us are basically due to the fragrant compounds have benzene ring in their molecular structure.
The thermochemical studies showed benzene is significantly more stable than what the structure looks like and hence the aromatic compounds are better known as benzene like compounds which are characterised by stability and chemical reactivity. The presence or absence of an electric ring current when the molecule is placed in a magnetic field. In case we put a poly-ene compound in a magnetic field, the field may generate a current of electrons that will flow around the ring.
When these electrons are moved these electrons generate their own magnetic field which causes the chemical shifts of the protons attached to the ring to change very much from their normal values. While the benzene and similar compounds show large and usually very clear chemical shifts from magnetically induced ring current but these currents do not necessarily can match thermochemical stability.
The next discussion on aromaticity is based directly on the bond length in the ring. In benzene the C = C bond lengths are all of equal length and hence equivalent bond length is an indicator for the aromatic ring. So the bond length might not be equal but vary in (Ï) pie bond order. This leads to the idea that in bond length none of the bonds have order of 0.4 or 0.9 and they are all in intermediate range. Anything which is not considered from specific length but an intermediate average length is considered under the aromatic.
The term aromatic was originally applied to benzene like structures because of the distinctive aroma of these compounds but the term now means something completely different in modern chemistry. Aromatic chemistry undergo distinctive reactions which set them apart from other functional groups. These are highly saturated compounds but unlike alkenes and alkynes they are relatively unreactive and will tend to undergo reactions which involve a retention of their unsaturation.
Benzene is a six membered ring structure with three formal double bonds. The six (Ï) pie electrons involved are not localized between any two carbon atoms but are delocalized around the ring which results in an increased stability. This is the reason why a benzene molecule is written with a circle win the centre of the ring which signifies the delocalization of the six Ï electrons. Any reactions which disrupt the delocalization are not favoured since it means a loss of stability and hence benzene undergoes reactions where aromatic ring system is retained. These six carbon atoms of the benzene ring are sp2 hybridised and the molecule maintains a cyclic planar structure.
The cyclic planar sp2 hybridised atoms follows the Huckel rule. This rule states that the ring system must have 4n + 2 (Ï) pie electrons. So any ring system which has 6, 10, 14 (Ï) pie electrons are aromatic. Benzene fits in properly with Huckel rule because it has just six (Ï) pie electrons.
The aromatic substances has acquired a large number of non-systematic names. The IUPAC rules discourages the use of most of these names but allow for some of the more popular ones. This is in comparison to the existing other organic compounds.
When the mixtures of hydrocarbon from natural sources like petroleum or coal are separated, some compounds emerge with very pleasant odours and are classified as aromatic hydrocarbons and all of these one common feature of six membered ring of carbon atoms. This ring can give more than one Lewis structure where the double bonds can be located in different positions. The arrangement of double bonds in different position and their preference for the attachment with functional groups give rise to many non-systematic nomenclature. This is mainly due to the fact that actual bonding is a combination of the structures representing a stable form.
The substituted benzene have the general formula of C6H5G and has three specific class.
The substituted benzene molecules are formed replacing one or more of the H atoms on the benzene ring with other atoms or groups of atoms.
- Named by adding substituent name as prefix to benzene
- Ortho, - meta and - para used as prefixes for the substituent groups to show the position of the substituent
- Or even the number can be used to show the position
- These are named by adding substituent names as a prefix to benzene
Tri and poly substituted
ToluenePhenolAnilineBenzaldehydeBenzoic acidStyrene AcetophenoneOrtho xylene
- The atoms of the ring of benzene are numbered and these substituent are placed from carbon 1 atom
- These are named by adding substituent names as prefix to benzene
All parent aromatic compounds have IUPAC names like benzene, naphthalene and pyridine along with the numbering system. This makes the naming of substituted derivatives easy.
So the isomeric 1.2 - , 1,3 - , and 1,4 di substituted benzenes are commonly termed using the O- ortho, m- meta, and p- para nomenclature.
1, 2 - di ethyl benzene (o â diethyl benzene)
1, 2 - di chloro benzene (o - di chloro benzene)
1, 4 - dimethoxy benzene (p- dimethoxy benzene)
1, 2, 4 - tri methyl benzene
Studies involving the analysis of large polycyclic aromatic hydrocarbons have shown that the synthesizing, analysis and isolation are usually done from non-synthetic sources. Without pure materials many secondary reasons then arise that kept researchers from studying large polycyclic aromatic hydrocarbons which include the scarcity of information on biological activities, occurrence and proper methods of analysis. The extensive review of smaller polycyclic aromatic hydrocarbons helps in identifying the large molecules as the rules are almost same for the smaller molecules.
One of the simplest multi ring aromatic hydrocarbon is the 2 ring naphthalene with 10 ring carbons. The number of isomeric structures for polycyclic aromatic hydrocarbons greatly increase with increasing carbon number. The number of variation also become higher. Broadly there are two categories of polycyclic aromatic hydrocarbons, those with only six member rings and those containing at least one five member ring.
These are called alternant and non-alternant polycyclic aromatic hydrocarbons. The alternant arise due to the pie electrons and their bonds can be arranged in an alternating continuous fashion. These are evident when the kekule structures are drawn. The isomeric structural differences arise when the progression begins with benzene at one ring and only one structure can be formed by adding one ring, the two ring naphthalene. By adding another ring to this one ring, it results in two isomeric forms of 14 carbon polycyclic aromatic hydrocarbons. They are phenanthrene and anthracene.
Molecules like tetracene, chrysene, tetraphene and triphenylene have ring joined only by one shared face for each ring connection and are known as ortho – fused polycyclic aromatic hydrocarbons. As the number of fused rings increase, the important structural factor arise where the addition to a ring to phenanthrene at the C face provides the benzo [C] phenanthrene. Here the peripheral hydrogen atoms overlap in case the molecule is planar.
The steric form without spatially arranged adjacent atoms is given out by carbon skeleton of the molecule with assumed non-planar configuration.
When additional ring is added by filling the open space of phenanthrene yet another class of polycyclic aromatic hydrocarbons is created. Hence the isomeric forms keeps increasing with each derived form of polycyclic aromatic hydrocarbons.
The list of aromatic compounds systematic and non-systematic is quite big. Some of the examples are given below.
- Benzoic acid
- Benzo nitrile