Aromatic chemistry

• Aromatic compounds are hydrocarbons containing sigma bonds and delocalized pi electrons between carbon atoms in a ring.
• Aromatic compounds are divided into two categories: benzenoids and non - benzenoids.
• Benzene is an aromatic compound consisting of a ring of six carbon atoms with six hydrogen atoms and a ring of delocalised electrons. Each bond has an intermediate length between that of a double and single bond.
• The ring structure of benzene makes it very stable in comparison to other molecules of a similar size.
• When benzene was first discovered, its structure was not known and it was thought to be of cyclohexatriene, with double bonds between each carbon. However the enthalpy of hydrogenation was found to be -208 kJ/mol instead of the predicted -360 kJ/mol.
• Aromatic compounds have high melting points due to the high stability of the delocalised ring, but low boiling points as they are non - polar molecules which usually can't be dissolved in water.
• The delocalised ring is an area of high electron density, making it susceptible to attack from electrophiles, and the ability to act as a nucelophile.
• Aromatic compounds undergo electrophilic substitution, in which the electron ring is partially destroyed and then reformed, removing a positive hydrogen ion.
• To form nitrobenzenes using electrophilic substitution, the electrophile is the NO2 + ion, which is a reactive intermediate formed in the reaction of concentrated sulfuric acid and concentrated nitric acid.
• For a mono-substitution of a single NO2 + electrophile, and for the production of aromatic amines, the reaction temperature must be less than 55 degrees celsius, as above this multiple substitutions can take place.
• The delocalised ring can act as a a nucleophile to attack acyl chlorides, with the reaction known as Friedel - Crafts acylation.
• For acylation, a reactive intermediate from the acyl chloride and an aluminium chloride catalyst is attacked by the delocalised ring, and the H + ion produced reacts with the AlCL4 - to reproduce the AlCl3 catalyst.
• The product of acylation with an acyl chloride is a phenylketone, which is used in dyes, pharmaceuticals and explosives.