Answer to Question #165363 in Organic Chemistry for SAmanta Ariel

Question

This question concerns the aromatic hydrocarbon benzene, which contains a delocalised system of eelectrons.

• Cite 3 pieces of evidence that benzene does in fact contain such a system, rather than a system of alternating single and double bonds.
• What do you understand by the term "delocalised system"?

Answer 1

1. All bonds are of the same length. A single bond is 0.153nm and a double bond is 0.134nm while all bonds in benzene are 0.139nm.
2. Benzene hydrogenation enthalpy change is -208kJ/mol while triple cyclohexene hydrogenation enthalpy change is  -3*120 = -360kJ/mol.
3. Because of the relatively large delocalisation energy it is suggested that benzene is a stable molecule. It does not react like an alkene: it doesn't react with halogens or strong acids, or take part in addition reactions.

In a delocalised system electrons are not associated with a single atom. The delocalised electrons in benzene are electrons moving in the “pi” bonds of covalently bonded carbon atoms.