BENZENE

When the conjugated π-bonds form a cyclic structure, the effect of resonance is even more spectacular. The electrons are set free to do a Formula One Grand Prix. Going round and round and round, so much so that there is now no differentiation between the CC σ−bond and the CC double bond. This phenomenon is known as aromaticity.

For benzene all the six carbon−carbon bond distances are identical and the value is about mid−way between the CC σ−bond and CC double bond. The cyclic structure increases the stabilisation of the conjugated π−bonds by 9% as shown by the enthalpy of hydrogenation.

HYDROGENATION	Benzene		1,3-cyclohexadiene		Cyclohexene		Cyclohexane
∆H/kJ per π-bond		−102		−111		−119
Stabilisation / kJ	17		8		0

In the late 1880s Kekule proposed that the π-bonds switched back and forth in equilibrium. The concept was well received but the representation was clumsy. It was eventually replaced by a ring to represent a circuit of circling electrons. Some chemist preferred a dotted ring others a solid ring, either form will do fine as long as we all understand that the electrons are in resonance. This extreme resonance stabilisation prevent any attacking species to open up the π−bond. So the alkene chemistry will not work with benzene. All addition reactions are out, as they involve the breaking of the π−bond. No electrophilic addition or syn addition. The only possible addition reaction is hydrogenation under drastic condition.

+   Br2

FeBr3 �

+   HBr

Instead benzene permits the replacement of protons by attacking electrophiles. For example benzene can be brominated to give bromobenzene. This is known as an Electrophilic Substitution. Of course the reaction must be assisted by catalysts. In this case it is ferric bromide.

The mechanism proposed;

Br2	+	FeBr3	�	FeBr4‾	+   Br+
	+	Br+	�		Carbonium ion
	+	FeBr4	�		+ HBr + FeBr3

The first step is the attack of the π-bond by an electrophile to give the carbonium ion, very similar to an alkene reaction. The only difference is that we need a more powerful electrophile to do this. For example the proton of hydrogen chloride gas is not able to do this.

Once the carbonium ion is formed the benzene has to decide whether to lose its π−bond by doing an addition reaction like the alkenes. This would mean dismantling the circuit for the electron to circle round and round. Benzene decided to keep its Formula One racing circuit and also the electrophile. So it kicked out the proton instead.

Because benzene aimed to maintain its resonance stabilisation the π−bond lost its personality. We decided that benzene is in a class of its own and so we named it an aromatic hydrocarbon. Benzene−like compounds have been known in history as aromatic oils.

AROMATICITY

Benzene

Since the p−electrons are no longer localised between two atoms we can no longer use the Valence Bond Theory. The Molecular Orbital Theory (MOT) would be more appropriate. The treatment is complex, so as an approximation we only consider the MOT on the porbitals. Benzene has six p−orbitals capable of forming π-bonds so the MOT produces six molecular π−orbitals. According to Pauli's Exclusion Principle each of the orbitals can accommodate only two electrons. So starting from the lowest energy level the electrons should occupy the three π-bonding orbitals. A very stable molecule.

Let us now move to the next member of the monocyclic conjugated alkenes by increasing the π−bonds to eight to give cyclooctatetraene. The eight molecular π−orbitals from the MOT are shown. Again the electrons would occupy orbitals starting from the lowest energy and observing Pauli's Exclusion Principle. The last two p−electrons would prefer to occupy an orbital by itself (Hund's First Rule), since both are of the same energy. Because these p−orbitals do not have the tendency to pair they act as a circuit breaker and the compound loses its aromaticity. Data from experiments showed that the carbon−carbon distances for alternating bonds are different. The molecule buckles (that means not planar) to accommodate the stress of the groups and it undergoes the usual diene reactions.

Toluene o-Xylene p-Xylene

↓   ortho meta ortho meta   ↑ para

Cumene Mesitylene Styrene

Although the IUPAC nomenclature is more systematic it tends to be non−user friendly at times. For example the IUPAC name for benzene would be cyclohexatriene. So it is not uncommon to find chemists using a mixed system; applying IUPAC convention to widely used names. Ortho−xylene (or o−xylene) is also known as 1,2−dimethylbenzene and para−xylene (p−xylene) is also known as 1,4−dimethylbenzene. The ortha, meta and para designation is relative to a specific carbon chosen (shown by ↓ above). We can refer to 4−chlorotoluene as meta-chlorotoluene (or m−chlorotoluene).

Some other commercially wellknown benzene derivatives.

Benzoic Acid	Benzaldehyde	Phenol	Aniline	Anisole