6.1.1 (a) What is Benzene?

Syllabus

(a) the comparison of the Kekulé model of Benzene with the subsequent delocalised models for Benzene in terms of p-orbital overlap forming a delocalised pi-system

(b) the experimental evidence for a delocalised, rather than Kekulé, model for benzene in terms of bond lengths, enthalpy change of hydrogenation and resistance to reaction

What does this mean?

What is Kekule Benzene?

It had been long known that the empirical formula of Benzene was CH, and that its molecular formula was C₆H₆.

There were many possible structures, most of them rather unlikely.

In 1865 August Kekule suggested a six-membered Carbon ring of alternating double and single bonds.

He claimed to be inspired by a dream about an ancient symbol showing a snake swallowing its own tail.

In 1872 he suggested that Benzene oscillated between two possible structures.

This meant that every Carbon was effectively equivalent.

Problems with Kekule Benzene.

Kekule's structure implies that there are C=C double bonds present.

So, you would assume that Benzene would behave like any other Alkene, such as Ethene.

But it doesn't.

Addition reactions

You really should know by now that the test for un-saturation (presence of one or more C=C double bonds) is the addition of Bromine water.

Anything with C=C bonds should decolourise Bromine water

Benzene doesn't do this and it doesn't undergo Addition reactions with HCl or HBr in the way that an Alkene would.

Benzene nearly always undergoes Substitution reactions - a Hydrogen is replaced.

Stability

It is (relatively) easy to measure the enthalpy change when Cyclohexene is Hydrogenated.

This involves adding one mole of Hydrogen to one mole of Cyclohexene.

Image result for cyclohexene hydrogenation

Doing the same with cyclohexadiene releases almost exactly twice as much energy because it involves adding two moles of Hydrogen.

If Kekule was correct then completely Hydrogenating Benzene - (adding 3 moles of Hydrogen) should release around three times as much energy - approximately 360 kJ/mol.

But, as we see, it releases 152 kJ/mol less energy than expected for Kekule Benzene.

We can say that actual Benzene is more stable than Kekule's Cyclohexatriene would have been.

And we can interpret this as the bonding in actual Benzene being stronger than expected.

Bond lengths.

If Benzene had alternating single and double bonds then the bond lengths would alternate between around 0.154 nanometres for C-C bonds and 0.134 nanometres for C=C bonds.

X-ray crystallography demonstrates that this is not the case.

In fact, all the bond lengths are equal at 139 nm.

And all the bond angles are the same in a perfect hexagon.

Examiners don't expect you to understand X-ray crystallography or what this sort of diagram means.

But it shows the position of the Carbon Atoms as equally separated spikes.

Proving Kekule wrong

Delocalised Electron Model

A C=C double bond consists of a sigma bond (overlapping s orbitals) and pi bond (overlapping p orbitals).

If all the Carbons are equally spaced then all the p-orbitals overlap above and below the plain of the molecule.

This means that the electrons are free to move around in a ring of de-localised pi-electrons

This is not particularly easy to draw in exams but that won't stop examiners asking you to do exactly that.

But in general we content ourselves with either:

Generally we draw the ring structure, although the alternating bond structure is sometimes more convenient.

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