Chemistry
Topic 10: Aromatic Chemistry
Arenes are molecules based on the hydrocarbon benzene C 6H6, which is the
simplest one. Although benzene is an unsaturated molecule, it is very stable. It
has a hexagonal ring structure with a special type of bonding.
Benzene is a colourless liquid at room temperature and pressure and is
obtained from crude oil. The structure of benzene was first suggested to be
cyclic by Kekulé in 1865. The structure proposed was a ring structure with
alternative single and double bonds.
Evidence that the Kekulé structure is incorrect is:
1. Hydrogenation
Benzene might be expected to behave like alkenes; i.e. react readily with
bromine and hydrogen to undergo addition reactions, but this is not the case.
Cyclohexene (C6H10) can be hydrogenated by breaking one C=C double bond and 1 H-H bond and
then forming one C-C single bond and 2 C-H bonds:
The enthalpy of this reaction should therefore be exactly three times
the hydrogenation of cyclohexane, i.e. 3 x -120kJmol -1 = -360kJmol-1.
However, the enthalpy change for this reaction is only -208kJmol -1. This
is less exothermic than expected, suggesting that the extra energy
(152kJmol-1 – resonance energy) is the stability that is conferred by the
π - delocalised electron system
Heat of hydrogenation of benzene is less than 3 x C=C.
The enthalpy of hydrogenation of benzene is significantly less than three times the enthalpy of
hydrogenation of a typical alkene. This is good evidence that delocalisation confers stability to the
molecule.
Topic 10: Aromatic Chemistry
Arenes are molecules based on the hydrocarbon benzene C 6H6, which is the
simplest one. Although benzene is an unsaturated molecule, it is very stable. It
has a hexagonal ring structure with a special type of bonding.
Benzene is a colourless liquid at room temperature and pressure and is
obtained from crude oil. The structure of benzene was first suggested to be
cyclic by Kekulé in 1865. The structure proposed was a ring structure with
alternative single and double bonds.
Evidence that the Kekulé structure is incorrect is:
1. Hydrogenation
Benzene might be expected to behave like alkenes; i.e. react readily with
bromine and hydrogen to undergo addition reactions, but this is not the case.
Cyclohexene (C6H10) can be hydrogenated by breaking one C=C double bond and 1 H-H bond and
then forming one C-C single bond and 2 C-H bonds:
The enthalpy of this reaction should therefore be exactly three times
the hydrogenation of cyclohexane, i.e. 3 x -120kJmol -1 = -360kJmol-1.
However, the enthalpy change for this reaction is only -208kJmol -1. This
is less exothermic than expected, suggesting that the extra energy
(152kJmol-1 – resonance energy) is the stability that is conferred by the
π - delocalised electron system
Heat of hydrogenation of benzene is less than 3 x C=C.
The enthalpy of hydrogenation of benzene is significantly less than three times the enthalpy of
hydrogenation of a typical alkene. This is good evidence that delocalisation confers stability to the
molecule.
