RPA 2 – Arenes

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
Scientists decided on the following formula for benzene: The circle in the middle shows the delocalised system of electrons in a benzene ring. 
  • 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). 
Thermodynamic evidence for stability of benzene
When unsaturated hydrocarbons are reduced to their corresponding saturated compound, energy is released. This energy can be calculated per molecule by doing the reaction in a bomb calorimeter.
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).
However, benzene only releases 208kJ per mole when reduced. This means that it is 152kJ per mole more stable than expected. This value is known as the resonance energy or delocalisation energy
This stability is due to the delocalised π-electrons.
Benzene has a high electron density (due to pi bonding) like alkenes, and was thought to attract electrophiles (acceptors of an electron pair). 
The stability of benzene means that under normal conditions, it cannot undergo typical alkene reactions such as:
  • 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).
Benzene is unable to react with bromine

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