Aromatic Hydrocarbons (Leaving Cert Chemistry): Revision Notes

Aromatic Hydrocarbons

What are Aromatic Hydrocarbons?

Aromatic hydrocarbons, also known as arenes, are a class of hydrocarbons characterised by a ring structure with delocalized electrons. The most well-known example is benzene, which forms the basis of many aromatic compounds.

Structure of Aromatic Compounds

Benzene ($C₆H₆$)

• Bonding: Benzene consists of six carbon atoms arranged in a hexagonal ring. Each carbon is bonded to one hydrogen atom.
• Delocalization: The six pi-electrons in benzene are shared across the ring, creating a system of delocalized electrons. This delocalization provides benzene with extra stability.
• Representation: Instead of alternating single and double bonds, benzene is often represented as a hexagon with a circle inside, symbolising the equal distribution of electrons.

Methylbenzene (Toluene, $C₆H₅CH₃$)

• Structure: Methylbenzene is a benzene ring with one hydrogen atom replaced by a methyl group ($CH₃$).
• Properties: Like benzene, methylbenzene is non-polar and has similar chemical behaviour.

Ethylbenzene ($C₆H₅C₂H₅$)

• Structure: Ethylbenzene consists of a benzene ring with an ethyl group ($C₂H₅$) attached. It is an important intermediate in the production of styrene (used to make polystyrene plastics).

Physical Properties of Aromatic Hydrocarbons

Physical State: Benzene, methylbenzene, and ethylbenzene are liquids at room temperature.

Solubility:

• Non-polar: Aromatic hydrocarbons are non-polar molecules.
• Solubility in Water: They are insoluble in water due to their non-polar nature.
• Solubility in Non-Polar Solvents: They dissolve easily in non-polar solvents such as hexane or benzene itself.

Demonstration of Solubility of Methylbenzene (Toluene)

To demonstrate the solubility properties of methylbenzene:

• When mixed with water, methylbenzene does not dissolve and forms a separate layer, indicating that it is insoluble in polar solvents.
• However, when mixed with a non-polar solvent like hexane, methylbenzene dissolves readily, demonstrating its solubility in non-polar solvents.