The first structure of benzene ring or structure was proposed by Friedrich Kekule in 1872 and this structure consisted of six membered ring with alternating single and double bonds with one hydrogen atoms attached to each of the carbon atoms. Although, Kekule proposal was consistent with many of the chemical properties of benzene but carried on with the controversy for many years.
We also use the term arene to describe aromatic hydrocarbons. This is more so as we term an H removed from alkane as alkyl and hence a removing H from arene is called an aryl group.
The benzene rings or aromatic rings are common in all kinds of medications with frequent use of the structure. Benzene is the archetype aromatic compound and has the molecular formula C6H6 with the entire molecule being flat. The alternating double and single bonds in the ring is not exactly correct as the benzene ring shows resonance, which means the double bonds are actually delocalized around the entire six carbon atoms of the ring.
The term aromatic hydrocarbon derives from the fact that many of the smaller molecules in this family have strong odours. The prototype of aromatic molecule is the benzene ring with a C6H6 arrangement.
A benzene ring consists of 6 carbon atoms bonded to each other to form a regular hexagon. There is a carbon at each vertex of this hexagon and the C â€“ C â€“ C bond angle comes to about 120 degrees. There is a hydrogen atom attached to each carbon atom which finally gives the formula of C6H6. All of these benzene rings are planar.
The circle or line inside the benzene ring represent some of the electrons involved in carbon â€“ carbon bonding. These electrons are in orbitals that extend above and below the plane of the benzene ring. These electrons circulate around inside the benzene ring and are not part of any specific C â€“ C bond. These electrons are delocalised and the C â€“ C bond order in benzene rings is somewhere between the single bond of alkanes and double bonds of alkene.
Hence the bonding between carbons of benzene rings can be also called as bonds and half. This unique bonding gives the aromatic rings unique structure, the specific chemistry and infrared spectra. There are variety of molecules that consist of the benzene ring and either one hydrogen or some hydrogens or even all of these hydrogens can be replaced by any number substituents. The arrangement of these substituents are also different which finally leads to the formation of many isomeric compounds.
There are no isomers of mono substituted rings because putting together at any of these six positions on the ring gives six equivalent structures. These are formed by simply rotating the toluene molecule six times through 60 degree angle. There are three unique ways to arrange two substituents on a benzene ring. The bonds needs to be broken and then generate these isomers and cannot be generated by simply rotating the molecule. The structures of the three isomers of di-substituted benzene rings can give us the ortho isomer, meta isomer and finally the para isomer.
The simplest aromatic hydrocarbon benzene was discovered in 1825 by Michael Faraday. The structure of benzene presented an intermediate problem to chemists of that time.
Later by 1890’s Kekule came up with a more reasonable benzene ring structure which is still considered to be the benchmark for explaining all kinds of bonding and delocalised electrons of the benzene ring.
The Kekule proposals gained worldwide acceptance and were also supported by the experimental work by Baeyer by late 19th century. Though it didn’t explain the unusual stability of benzene and this is typified by its chemical reactions which are exclusively substitution rather than the expected addition. The physical properties like enthalpies of hydrogenation and combustion are significantly lower than the expected for the Kekule’s cyclo hexa triene structure.
The enthalpy of hydrogenation of the double bond in cyclohexane is – 120 KJ per mol and to that of cyclo hexa 1,3 diene with two double bonds has almost twice at -232 KJ per mol.
The cyclo hexa triene would be expected to have an enthalpy of hydrogenation of three times the value of cyclo hexane. The value of benzene is exothermic than this value provides the idea that it’s 151 KJ per mol more stable than the cyclo hexa triene. This is known as the resonance energy of benzene or its aromatic stabilization. This stabilizing feature dominates the chemistry of benzene and its derivatives.
The formulation of benzene structure as a planar cyclic arrangement of CH units is one of the classic examples in organic chemistry. Benzene is known to have equivalent carbon-carbon bond lengths at each position around the ring and so a description of benzene as cyclo hexa triene might prove wrong. If benzene existed as cyclo hexa triene then the double bonds would be shorter than the single bonds with an alternated bond length from one atom to the next in the ring.
The structure of benzene cannot be represented properly with just one single structure by using single and double bond shows a big problem in drawing structure. The bond alternation implied by the cyclo hexa triene structure can be avoided by recognizing that there are two possible arrangements for the (Ï€) pie bonds in benzene.
The double bond can either be placed between C1 and C2 or between C1 and C6. Benzene is better represented as a combination of both these structures. The kekule structures depicting benzene having localized double bonds are not an accurate representation of the benzene ring structure as these are considered to be the short cut for counting double bonds and electrons.
By convention the resonance contributors differing only with respect to formal localisation of electrons and not with respect to positions of atoms.
One resonance structure can therefore always be converted to another by moving only electrons and can be represented with arrows showing the motion of the electrons.