A-level Chemistry/AQA/Module 4/Aromatic Chemistry

Aromatic chemistry is the study of reactions involving the benzene (phenyl-) group C₆H₆. An aromatic compound containing this group - such as benzene, methylbenzene or TNT (trinitrotoluene) - is called an arene, and a functional group containing a benzene ring is called an aryl group.

Structure

From around 1865, the accepted model of benzene was the Kekulé structure - effectively cyclohexa-1,3,5-triene. It was thought that there were three double bonds in each benzene ring, with each of the 6 carbon atoms being attached to a single hydrogen nucleus. This model was later revised to suggest that the double bonds oscillated rapidly between the two possible positions (i.e. 1,3,5 and 2,4,6) in the molecule before the 1930s...

...when it was discovered through experiment that bonding in benzene was actually hybrid - neither single nor double - for all C-C links. It was found that, rather than localised π bonds between carbon atoms, a delocalised ring is formed - with each carbon atom donating one p-orbital electron. From this, the skeletal symbol for benzene (a hexagon with internal circle) is derived - and although rarely used, benzene can also be drawn out in full with each carbon atom shown and an internal ring representing the bond.

As there are three areas of electron density around each carbon atom, repulsion theory dictates that the molecule is planar with a 120° bond angle.

This structure is evidenced by:

• Bond length data - experiment has found that C-C bonds are 0.154 nm long, whereas C=C bonds are 0.134 nm. Observation of benzene showed that only one bond length of 0.139 nm was present - uniform to all carbon-carbon links in the molecule.
• thermochemical stability - hydrogenation of cyclohexene (i.e. cyclohexane with one double bond) produces an enthalpy change of -120kJ per mole. Hydrogenation of benzene would therefore be expected to produce an enthalpy change of -360kJ per mole if the structure contained three double bonds - but in reality only -208kJ pre mole is observed. The ring structure makes benzene more stable.

Halogenation

When a benzene ring is exposed to HCl in the presence of AlCl₃ or FeCl₃, halobenzenes are formed.