Introducing Benzene

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Benzene

It is the simplest aromatic hydrocarbon

It is colourless, sweet smelling and highly flammable point

Found in crude oil

The Kekule and delocalised models of benzene:

Kekule suggested benzene was made up a 6 carbon ring with alternating double and single bonds

Evidence to DISPROVE his theory:

1. The lack of reactivity of benzene - if it had a double bond it should decolourise bromine during electrophilic addition, however...

- Benzene doesn't undergo electrophilic reactions

-It doesn't decolourise bromine under normal conditions

This means that benzene cannot have a C=C bond.

2. The length of the C-C bonds in benzene - ALL the bonds in benzene are 0.139nm in length, but the length of a normal C-C bond is 0.153nm and C=C is 0.159nm.

3. Hydrogenation enthalpies - If benzene ad Kekule's structure, the enthalpy change of hydrogenation should be three times that of cyclohexene, BUT the hydrogenation of cyclohexene is -120kJmol, so for Kekule's model the hydrogenation of benzene should be -360kJmol but it is -208kJmol, a 152kJmol difference - this shows the actual structure of benzene consists of a delocalised cloud of electron which is a lot more stable than Kekule's model.

The delocalised model of benzene:

It is planar, cyclic, hexagonal hydrocarbon containing 6 carbon atoms and 6 hydrogen atoms

Each carbon atom uses 3 electron for bonds, and has 1 delocalised electrons

p-orbitals overlap sideways, to form a ring of electron density which created a pi-bond

Naming aromatic compounds:

Alkyl groups, halogens and nitro groups are considered prefixes to benzene.

When benzene is attached to an alkyl chain with a functional group or to an alkyl chain with 7 or more carbon atoms, phenyl is used instead of benzene.

EXCEPTIONS: Benzoic acid (benzenecarboxylic acid), phenylamine and benzaldehyde (benzenecarbaldehyde)