The Shape of Aromatic Molecules (Cambridge (CIE) A Level Chemistry): Revision Note

Shape of Benzene & Aromatic Molecules

• Aromatic molecules consist of one or more rings with conjugated π systems

• Conjugated π systems arise from alternating double and single bonds in which the electrons are delocalised

• Aromatic compounds are called ‘aromatic’ as they often have pleasant odours

Examples of aromatic compounds table

Functional group	Example	Name
Arene		Propyl benzene
Chlorobenzene		2-methylchlorobenzene
Phenol		2,3-dimethyl phenol

Shape of benzene & aromatic compounds

• Benzene and other aromatic compounds contain sp² hybridised carbons as two of their p orbitals have mixed with an s orbital

• This means that each carbon atom in benzene and other aromatic compounds has one p orbital

 sp² hybridisation

The carbon atoms in aromatic compounds are sp² hybridised as two of their p orbitals mix with an s orbital

• Each carbon atom in the ring forms three σ bonds using the sp² orbitals

• The remaining p orbital overlaps laterally with p orbitals of neighbouring carbon atoms to form a π bond

• This extensive sideways overlap of p orbitals results in the electrons being delocalised and able to freely spread over the entire ring

• Benzene and other aromatic compounds are regular and planar compounds with bond angles of 120^o

• The delocalisation of electrons means that all of the carbon-carbon bonds in these compounds are identical and have both single and double bond character

• The bonds all being the same length is evidence for the delocalised ring structure of benzene

The planar structure of benzene

Like other aromatic compounds, benzene has a planar structure due to the sp² hybridisation of carbon atoms and the conjugated π system in the ring