Aromatics (Organic) (complete)

Question 1

Describe the structure of benzene

Answer 1

6 Carbon atoms in a planar hexagonal ring, with one H atom bonded to each C atom. Each of the six covalent bonds are the same length (between single and double bonds). There is a cloud of delocalised electrons surrounding the molecule as one electron unused from each C atom. The orbitals with the unused electron overlap forming the pi bond.

Question 2

Name and describe the three pieces of evidence of why the structure of benzene is not the triene example but the delocalised version.

Answer 2

• C-C bond length - in the triene structure, there are three of each single and double bonds between C atoms, while in benzene all the bonds are the same length, between single and double length.
• Addition reactions - Benzene doesn’t readily undergo addition reactions, while triene does.
• Enthalpy of hydrogenation - Benzenes enthalpy of hydrogenation is -208 kJ/mol while trienes is -360 kJ/mol. As benzenes is higher, this shows that benzene is more stable than triene, as it has delocalisation stability from the delocalised electrons.