Arenes

Arenes

• An arene is a hydrocarbon with a ring of 6 carbon atoms and a delocalised pi-electron system. Benzene is the simplest example
• A compound containing a benzene ring is often referred to as an aromatic compound

Structure and Bonding of Benzene

Kekule formula

• Triene structure proposed by a 19th century chemist called Kekule as a structure for benzene. However, there is considerable evidence to suggest that it is not entirely correct

1. Benzene does not easily undergo addition reactions like alkenes do e.g. it does not decolourise bromine in the dark

2. All the bonds between C atoms around the ring are the same length. C-C bonds are longer than C=C bonds. So if benzene had the triene structure shown above, it would have three long C-C bonds and three short C=C bonds. In fact, all carbon carbon bonds are the same length in benzene and are intermediate in length between the single and double carbon bond 

3. Hydrogenation of Benzene is less exothermic than expected by comparison with alkenes. An enthalpy profile diagram shows that less heat is released when hydrogen is added to benzene than would be predicted for addition of H2 across three double bonds. This suggests that the pi bonds in benzene are more stable than expected

A better description of Benzene's structure

• Benzene is a planar molecule (all 12 atoms are in the same plane) and all six carbon lengths are the same
• Each carbon…