Essay on Benzene Rings

The most commonly encountered aromatic compound (chemical compounds that contain conjugated planar ring systems with delocalized pi electron clouds instead of discrete alternating single and double bonds) is benzene. The usual structural representation for benzene is a six-carbon ring (represented by a hexagon) which includes three double bonds. This hints as to why there is a simplified name for this explanation: benzene rings. Here is a visual representation of benzene rings: Each of the carbons represented by a corner of the octagon is also bonded to one other atom. In benzene, itself, these atoms are hydrogens. Single bonds separate the double bonds; therefore, the arrangement is recognized as involving conjugated double bonds. An alternative symbol uses a circle inside the hexagon to represent the six pi electrons. Above I have displayed the three-bond symbol. However, if the hexagon contains neither the three double bonds nor the circle, the compound is not aromatic. It is simply cyclohexane and there are two hydrogens on each carbon atom. The structure with three double bonds was proposed by Kekule, a scientist, as an attempt to explain how a molecule whose molecular formula was C6H6 could be built out of carbons which make four bonds. The ring and the three double bonds fit the molecular formula, but the structure doesn't explain the chemical behavior of benzene at all well. Each of the double bonds would be expected to show the characteristic behavior of an alkene and undergo addition reactions, but this is not how benzene reacts. According to Kekule, regularly, it is usual to expect a carbon-carbon double bond to react quickly with bromine to make a dibromo compound. This is what alkenes do very readily, and in fact it is a useful test for alkenes in the laboratory. Benzene does not react with bromine unless a very bright light or a strong catalyst is used, and then the reaction is not an addition reaction. We conclude that there is something quite unusual about the double bonds in benzene. Kekule (thinking about this problem before bonds were understood as pairs of electrons) suggested that there are two forms of benzene which differ in the locations of the double bonds. His idea was that these were in rapid equilibrium, so rapid that there was never a fixed location for the double bond. One could say that an approaching bromine molecule could not "find" a double bond to react with. There were several other structures proposed for benzene, but a much more satisfactory approach became possible when we began to understand that covalent bonds consist of pairs of electrons shared between atoms. The difference between the two structures Kekule envisioned (called Kekule structures) is only the difference between the locations of three pairs of electrons. This is exactly the type of situation where resonance must be involved. The hybrid or "average" of the two Kekule structures has one sigma bond and one-half of a pi bond between each two carbon atoms. Thus, each carbon is joined to each of its neighbors by a one-and-half bond. Each bond in the benzene ring has the same number of electrons and is the same length. This picture is in complete accord with experiments which show that all carbon-carbon bonds in benzene are the same length, with no hint of shorter (double) or longer (single) bonds. It also helps explain why benzene does not undergo additional reactions: there are no simple pi bonds. Resources http://www.chemguide.co.uk/basicorg/conventions/names3.html https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/react3.htm http://public.wsu.edu/~hemeteam/Chem116/Worksheets/BENZENE%20NAMING%20EXPLAINED.pdf http://chemistry.tutorvista.com/organic-chemistry/benzene.html