Arenes Hydrocarbons based on benzene
Arenes are hydrocarbons based on the benzene ring, a very stable molecule. Benzene is a planar
molecule, which is unsaturated but does not readily undergo addition reactions. All carbon atoms are
also equivalent, implying that all each bond is identical. The C-C bond lengths are an intermediate
between the single and double bond length.
The symbol represents benzene, which has delocalised electrons from overlapping p orbitals. This
is the cause of aromatic stability, as the electrons are spread over more than two atoms.
Benzene is a colourless liquid at rtp. Aromatic compounds are named with the root benzene, as they
are all derivatives of this compound e.g. benzoic acid, nitrobenzene
Thermochemical Evidence for Stability
The enthalpy of hydrogenation of the hypothetical non-delocalised benzene would be -360 kJ mol-1
However, in actuality, the enthalpy of hydrogenation is -208 kJ mol-1
. This value is less exothermic, as
more energy is required to overcome the enthalpy of delocalisation, 152 kJ mol-1
, which makes the
benzene more stable.
Reactions of Benzene
Arenes burn with a smoky flame, dye to the high C:H ratio. Unburnt carbon produces soot.
The aromatic ring is very stable, requiring energy for the system to be destroyed. As a result, addition
reactions are not favoured. However, the electron ring is an area of high negative charge density,
meaning it attracts electrophiles. As a result, arenes undergo electrophilic substitution, which keeps
the system intact.
Concentrated sulfuric acid and nitric acid are mixed to produce a nitronium NO2
+ electrophile: HNO3 +
H2SO4 → HSO4
– + H2O + NO2
. The arene is then added, with the reaction kept below 50°C to prevent
multiple substitution. Nitration is a useful step in producing amines, and is used to produce TNT.
Friedel-Crafts Acylation Reactions
Aluminium chloride is used as a catalyst to substitute RCO for a H on the aromatic ring. The reaction
CH3COCl + AlCl3 → CH3CO+ + AlCl4
– occurs as the aluminium atom in AlCl3 readily accepts a lone pair of
electrons into its empty orbital. The aluminium chloride is reformed when it reacts with the H+
the arene, producing HCl.