In addition to this, benzene has another abnormal trait which is not associated with alkenes. The bond lengths in benzene possess a special quality. In another alkene such as cyclohexene, the length of the C-C bonds differ to the length of the C=C bond. The location of the C-C and C=C bonds also alternate. If benzene was to be labelled as an alkene, then surely it would follow the same pattern. This is not the case however. X-ray crystal structures have revealed that the bonds within benzene do not actually alternate in length, allowing benzene to have its hexagonal, planar structure. This is due to a property known as aromaticity.
This property also affects the heat of hydrogenation. When a hydrocarbon is unsaturated, it can be hydrogenated by adding hydrogen into the double bond. This requires energy and a catalyst. Returning to the example of cyclohexene, this contains one double bond. The energy required to break the double bond and add hydrogen to it is -28.6 kcal mol-1. One would expect that as benzene has three times the amount of double bonds that it would need 3 times the energy. Therefore the ΔH or benzene should be 3(-28.6 kcal mol-1) = -85.6 kcal mol-1. The actual ΔH for the hydrogenation of benzene is -49.8 kcal mol-1. This is 36 kcal mol-1 less than 3 times ΔH for cyclohexene. Benzene is more stable than expected, suggesting that the structure of benzene may not be as straightforward as what was first thought.
The aforementioned chemical property “aromaticity” describes a case of delocalised bonding and is demonstrated when a conjugated hydrocarbon ring containing unsaturated bonds, lone pairs or empty orbitals displays a stabilization that would not have been expected from the stabilization of a conjugation alone. This is usually considered to be because electrons are free to cycle around circular arrangements of atoms, which are alternately single and double-bonded to one another. This is due to a theory which states that the electrons within the structure are not restricted to one bond. This is due to the types of bonds within benzene. Benzene contains two types of bonds. The single bonds are formed with electrons in line between the carbon nuclei and are called σ-bonds. Double bonds consist of a σ-bond and a π-bond. The π-bonds are formed from overlap of atomic p-orbitals above and below the plane of the ring. Since the bonds are out of the plane of the atoms (as benzene is planar) the orbitals can interact with each other freely an become delocalised. his means that instead of being tied to one atom of carbon, each electron is shared by all six in the ring. This creates a “bagel shape” and adds stability to the molecule as the electrons strengthen all the bonds within the ring equally.
To conclude, it would be incorrect to label benzene as an alkene for a number of different reasons. Firstly, it reacts more like an alkane than an alkene, reacting by substitution rather than addition if at all. We could not label it as an alkane however as it doesn’t follow in the same homologous series that alkanes follow (CnH2n+1). Secondly its bond lengths do not differ, all the C-C bonds in benzene are the same, and can therefore form a perfect, planar, hexagonal structure. Whilst the formula C6H6 would suggest double bonds, in the case of benzene there are neither single bonds nor double bonds. The bonds present are intermediate bonds, which all have the same length as one another. This is due to the property aromaticity. It is this property which also affects the heat of hydrogenation, and further alienates benzene from the alkene group.
In actual fact, benzene is neither alkane nor alkene, but instead falls under a separate category labelled arenes. Arenes are aromatic hydrocarbons that are based on the benzene molecule, all with properties similar to benzene. Their properties are unique, and therefore are placed in this separate group.
