Aromatic Chemistry

What is benzene?

Our focus today is on molecules containing the benzene ring.

We call molecules like benzene 'aromatic compounds' because the first few were discovered in sweet-smelling oils. In fact, benzene was first isolated from benzoin, a fragrant resin made from certain Asian tree species. However, not all sweet-smelling compounds show true aromaticity, and not all aromatic compounds smell nice!

Shape

As we mentioned above, benzene is an aromatic hydrocarbon ring containing six carbon atoms and six hydrogen atoms. Try drawing it out and see what sort of structures you can come up with.

Fig. 1 - Possible structures for benzene

In actual fact, benzene has a completely different structure to all three molecules shown above. It doesn’t even contain a single double bond! Instead, each of benzene’s carbon atoms is bonded to just one hydrogen atom and two other carbon atoms, forming a hexagon. We give benzene the following symbol:

Fig. 2 - Benzene is often represented by a hexagon with a circle inside of it

Bond length

If benzene doesn’t contain any double bonds, what sort of bonds does it have?

Each of benzene's carbon-carbon bonds is the same length, and is neither a single bond nor a double bond, but something in between. We call them intermediates. You can see this in the table below, which shows the lengths of different carbon bonds:

Fig. 3 - Bond length

Delocalisation

If we count the electrons involved in benzene, we come across a problem. Carbon has four valence electrons. In benzene, two electrons from each carbon atom form bonds with adjacent carbon atoms. One electron bonds to a hydrogen atom. These electrons are all part of sigma bonds. This leaves one electron remaining. But where is it?

This is where delocalisation comes in. The final electron in each of benzene’s carbon atoms is found in a pi orbital. You might remember from Alkenes that, while sigma orbitals and bonds stretch between adjacent atoms, pi orbitals go above and below each atom. In benzene, the pi orbitals of all six carbon atoms overlap and form one big region of electron density. The electrons delocalise. This means they can move about freely within the region and don’t belong to one particular carbon atom.

Fig. 4 - The pi orbitals in benzene. All six overlap, producing an area of electron density above and below the plane

Bond angle

Each of benzene’s carbon atoms has three bonds: two C-C bonds and one C-H bond. These bonds try to spread themselves out as far apart as possible. This results in an angle of 120° between each bond. Therefore, benzene forms a trigonal planar molecule.

Properties of benzene

We’ll look more closely at the properties of benzene in Structure and Bonding, but there are a few things you should know now.

• Benzene is a trigonal planar molecule. It has bond angles of 120°. Because it is so flat, molecules can pack closely together, so it has relatively high melting and boiling points.
• Benzene’s electron ring makes it stable compared to other hydrocarbons. This is known as aromatic stability.
• Although it is unsaturated, benzene resists addition reactions.

How do you name aromatic compounds?

Now that we know what benzene is, we can now look at naming different molecules containing its characteristic ring.

Benzene derivatives use the suffix -benzene. However, if there are multiple functional groups present they sometimes use the prefix phenyl- instead. Let’s look at some examples to remind ourselves of nomenclature rules.

Try this one?

How do you form aromatic compounds?

To make benzene rings and other aromatic compounds, we use a process called catalytic reforming. To do this, we take fractions from crude oil that are around six to eight carbon atoms long. We then heat them with a catalyst and hydrogen gas to 500 °C at a pressure of about 20 atm. The catalyst is a mixture of aluminium oxide and platinum. This is why the process is sometimes known as platforming. At such high temperatures, some of the hydrocarbons tend to decay into carbon, which contaminates the catalyst, but adding hydrogen stops this process. The products are benzene derivatives and more hydrogen gas.

Fig. 12 - Catalytic reforming

How do aromatic compounds react?

Take a look at benzene again. It is an unsaturated molecule. We’ve met that term before when describing alkenes with C=C double bonds. Although benzene doesn’t have any double bonds, it is unsaturated because it doesn’t contain the full possible number of hydrogen atoms. Each carbon atom in benzene is bonded to two other carbon atoms and one hydrogen atom, but it can potentially bond to two hydrogen atoms. This would make a saturated cyclic hydrocarbon called cyclohexane, C₆H₁₂. Hydrogenation to make cyclohexane is just one example of a reaction of benzene.

Fig. 13 - Cyclohexane, a saturated hydrocarbon

Unlike other unsaturated compounds such as alkenes, benzene doesn’t like taking part in addition reactions. This is because an addition reaction uses up one of the delocalised electrons in benzene’s overlapping pi orbitals, ruining the ring of delocalisation. This takes a lot of energy. Instead, benzene often takes part in substitution reactions. These are reactions that involve swapping one atom or group of atoms for another.

The ring of delocalised electrons is an area full of lots of electrons squashed into a small space. We can say that it has a high electron density. This means that it is attractive to electrophiles. You should remember that electrophiles are electron pair acceptors, with an empty orbital and positive or partial positive charge (-phile comes from the Latin word philos, meaning ‘love’ - electrophiles really love electrons!).

If we put these two ideas together, we can conclude that aromatic compounds like benzene often take part in electrophilic substitution reactions. We’ll look at these in more depth in Reactions of Benzene. Some examples include:

• Nitration reactions, swapping a hydrogen atom for the -${\mathrm{NO}}_2$ group. This produces nitrobenzene which is used in dyes and pharmaceuticals.
• Friedel-Crafts acylation reactions, where benzene reacts with an acid derivative in the presence of an aluminium chloride catalyst. The product is used for plastics and detergents.

In the article we mentioned above, we'll also look at reactions such as combustion. Because benzene has a high ratio of carbon to hydrogen atoms, it burns with a characteristically sooty flame. This is one way of identifying aromatic compounds.