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The American Chemical Society is a Washington-based scientific society. In 1988, two-thirds of the specimens on their comprehensive list of chemicals had one particular thing in common: they all contained a benzene ring. This made them aromatic compounds.
Aromatic compounds are organic molecules that contain rings with delocalised pi electrons, such as a benzene ring. They are also known as arenes.
Our focus today is on molecules containing the benzene ring.
Benzene is an aromatic compound with six carbon atoms and six hydrogen atoms arranged in a planar 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!
Benzene is the most widely known aromatic compound, but you can get aromatic rings of other sizes. For example, the molecule cyclotetradecaheptane, also known as [14]annulene, contains 14 carbon atoms and 14 hydrogen atoms. In fact, there is a rule for determining if a cyclic molecule shows aromaticity. It must have 4n+2 pi electrons, where n is a positive integer. This is known as Hückel's rule.
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.
Possible structures for benzene. StudySmarter Originals
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:
Benzene is often represented by a hexagon with a circle inside of it. commons.wikimedia.org
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:
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.
The three bonding electrons are actually found in special orbitals called 
orbitals. This is a little complicated and goes beyond what you’ll be tested on in an exam, but is interesting to know.
Carbon has the electronic structure of 
. In terms of orbitals, its valence shell has a pair of electrons in the 
orbital and one electron each in two of the 
orbitals. However, to form three bonds, carbon needs three unpaired electrons. To do this, it enters an ‘excited’ state - it promotes one of the electrons from 
into the empty 
orbital.
An electron is promoted from 2s to 2pz. The carbon atom is now excited. Anna Brewer, StudySmarter Originals
We know that in benzene, carbon wants to form three bonds. These bonds are all equal. To make three equal bonds, carbon needs three electrons in equal orbitals. The easiest way for it to do this is to hybridise three of its orbitals: 
and 
. These form three identical orbitals known as 
orbitals, because - you guessed it - they are made from one s orbital and 2 p orbitals.
The orbitals in benzene. Anna Brewer, StudySmarter Originals
If you aren’t sure about orbitals, see Electron Shells, Subshells and Orbitals.
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.
We’ll look more closely at the properties of benzene in Structure and Bonding, but there are a few things you should know now.
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.
If you need a quick reminder before we begin, look at Organic Compounds.
Can you name this unknown molecule? Anna Brewer, StudySmarter Originals
This molecule has a methyl group and a chlorine atom attached to the benzene ring. It needs the prefixes methyl- and chloro-. Remember, we use numbers to show the positions of other functional groups on the carbon chain. However, with other organic molecules such as alkanes, we start numbering the carbons from either end of the carbon chain. With benzene, there is no end of the chain, so we number any of the carbons as 1. We just need to make sure that we follow the ‘lowest number’ rule. If we count up the numbers showing the positions of the functional groups, we should get the lowest total possible.
Here we can call the carbon atom attached to the methyl group number 1, and so the carbon atom attached to the chlorine atom is number 3. When we list functional groups, we list them in alphabetical order. This molecule is therefore called 3-chloro-1-methylbenzene.
Our mystery molecule, with the carbons numbered, and the functional groups circled. Anna Brewer, StudySmarter Originals
Here’s another example.
Can you name this molecule? Anna Brewer, StudySmarter Originals
It is actually just a ketone, where one of the R groups is a benzene ring. We have to use the prefix phenyl-. The remaining carbon chain is 2 atoms long, taking the root name -eth-, so this molecule is known as phenylethanone.
Try this one?
Another unknown molecule. Anna Brewer, StudySmarter Originals
This next molecule has a carboxyl group and a hydroxyl group attached to its benzene ring. We’ll need to use the suffix -oic acid and the prefix hydroxy-. Counting the carbon atom attached to the carboxyl group as carbon 1, the carbon atom containing the hydroxyl group takes position 2. We therefore call this molecule 2-hydroxybenzoic acid.
2-hydroxybenzoic acid. Anna Brewer, StudySmarter Originals
A benzene ring with just a hydroxyl group attached has its own special name: phenol.
Phenol.Anna Brewer, StudySmarter Originals
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.
Catalytic reforming. Anna Brewer, StudySmarter Originals
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 doesn’t contain the full possible number of hydrogen atoms. Each carbon atom can potentially bond to two other carbon atoms and two hydrogen atoms, which would make the saturated cyclic hydrocarbon called cyclohexane, 
.
Cyclohexane, a saturated hydrocarbon. commons.wikimedia.org
However, unlike other unsaturated compounds such as alkenes, benzene doesn’t like taking part in addition reactions. This is because an addition reaction would use 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 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:
group. This produces nitrobenzene which is used in dyes and pharmaceuticals. 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.
Aromatic compounds are also called arenes and contain a ring with delocalised pi electrons. The most common ring is benzene, 
.
Benzene contains six carbon atoms bonded in a hexagon shape. The bonds between each carbon atom are identical and halfway between a single and double bond in length.
Each carbon atom in benzene contains one unbonded electron which is found in a pi orbital. These orbitals overlap above and below the benzene ring to form an area of delocalisation. The electrons can move freely within this region, which is known as a ring of aromaticity.
Benzene is made from crude oil fractions using an aluminium oxide and platinum catalyst, under conditions of 500 °C and 20 atm.
Benzene is relatively stable and has high melting and boiling points compared to alkanes.
We name benzene derivatives using the suffix -benzene or the prefix phenyl-.
Benzene often takes part in electrophilic substitution reactions such as nitration and Friedel-Crafts acylation reactions.
Aromatic compounds are compounds that contain a ring with delocalised pi electrons. The most common aromatic compound is benzene, a ring made from six carbon atoms and six hydrogen atoms.
An aromatic ring is a ring of carbon atoms with delocalised pi electrons. Each carbon atom forms three bonds: two C-H bonds and one C-H bond. Carbon’s fourth electron is found in a pi orbital. This electron delocalises, and all of the delocalised electrons move into a region above and below the ring. This makes benzene more stable.
All benzene derivatives are aromatic compounds. Examples include chlorobenzene and nitrobenzene. Other examples of aromatic compounds are vanillin and cinnamaldehyde, the main constituents of vanilla and cinnamon respectively.
Aromatic compounds all contain rings with delocalised pi electrons, such as the benzene ring.
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