An aromatic compound contains a benzene ring, and possibly other elements An arene is a hydrocarbon that contains a benzene ring. Benzene is a liquid that is planar and hexagonally shaped. It is very chemically stable. Primary analysis revealed that benzene had an empirical formula of CH and a molecular formula of C6H6. Kekule predicted this structure for benzene , however:
- benzene did notreadily undergo electrophilic addition unlike other alkenes or cycloalkenes (no true C=C bonds)
- all six C-C bond lengths were identical (double bonds are shorter than single bonds)
- the ring was thermodynamically more stable than expected (so it was suggested that the structure oscillated between the two Kekule forms shown above, forming a resonance hybrid) the enthalpy change of hydrogenation is less exothermic than expected because of the extra stabilisation of benzene
- carbon-carbon bond length between that of single bond and double bond
- benzene is much less reactive than an alkene because of extra stability of the delocalised π system
- instead of three localised double bonds, the six π electrons were delocalised (not in any one particular position) around the ring
- the six electrons came about by the overlap of carbon p-orbitals from each carbon.
- a ring of delocalised electrons form below and above the plane of the carbon ring, as shown below: The model on the left shows the overlap of p-orbitals above and below each carbon in the ring, whilst the model to the right shows the six electrons shared in the delocalised system (3 above the ring, 3 below)
- all bond lengths were equal as there were no double bonds. The length of a C-C bond in benzene (0.139 nm) is between the length of a C-C (0.153 nm) and C=C bond (0.134 nm).
When cyclohexene (one C=C) is reduced by hydrogenation (addition of hydrogen, reduction), 120kJ of energy is released per mole.
Therefore, theoretically benzene would release 360kJmol-1 when reduced to cyclohexane, as it contains 3 double bonds (according to kekule’s structure).
- decolourising bromine water
- reacting with strong acids such as HCl
- react with halogens.
- However, benzene undergoes electrophilic substitution (the only compound in RPA to do so).