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What is benzene and describe the 2 models used to explain it's structure. Provide a piece of evidence to show which of these models is incorrect.

Benzene

With a formula of C6H6, the molecule has a cyclic structure due to it's 6 carbon atoms joined in a ring. It is symmetrically planar (flat) with it's 6 hydrogen's sticking out in the same plane. There are two models used to explain benzene's structure- the Kekulé model and the delocalised model.

The Kekulé model

Proposed by the German chemist Friedrich August Kekulé in 1865, he hypothesised that carbon could exist in a ring with alternating single and double bonds between them.

(At this point, a digram would be drawn of a standard skeletal benzene structure and the molecular structure diagram showing all the carbons and hydrogens, see 'Benzene structure according to Kekule' in text books for illustration)

If this model were correct, there would be 3 bond lengths of a C-C bond (147pm) and 3 bonds with the length of a C=C bond (135pm). However, X-ray diffraction have shown that all the carbon-carbon bonds within benzene are of the same length of 140pm, suggesting that the model is incorrect. This value shows that the carbon bonds have a length between that of a single bond and a double bond.

The delocalised model

This phenomenon can be explained by the delocalised model. In this model, the p-orbitals (check AS notes) of all 6 carbon atoms overlap to create π bonds. Two ring-shaped clouds of electrons form with one above and one below the plane of the 6 carbon atoms.

(At this point, you'd draw a diagram of the formation of π bonds in benzene due to the overlap of p-orbitals)

With this, carbon-carbon bonds are the same length (140pm) as the bonds are all identical. The electrons in the rings are said to be delocalised because they don't belong to a specific carbon atom. They are then represented as a circle in the ring of carbons instead of double and single bonds in a diagram.

(You would then illustrate this as a skeletal or molecular diagram)

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