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.
- This article is an introduction to aromatic compounds in organic chemistry.
- We’ll begin by looking at benzene and its structure.
- We’ll then practice naming benzene derivatives.
- Finally, we’ll look at how aromatic compounds are formed and briefly explore some of the reactions they take part in.
What is benzene?
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.
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
The three bonding electrons are actually found in special orbitals called sp2 orbitals. Not all exam boards expect you to know about orbitals, but even if yours doesn't, orbitals are interesting to learn about.
Carbon has an electronic structure of 1s2 2s2 2p2. The two s subshells each have one orbital, whilst the p subshell has three orbitals which we call 2px, 2py, and 2pz. Carbon's valence shell has a pair of electrons in the 2s orbital and one electron each in the 2px and 2py orbitals. However, to form the three bonds that we see in benzene, carbon needs three unpaired electrons. To do this, it enters an ‘excited’ state - it promotes one of the electrons from 2s into the third empty 2p orbital, 2pz.
Fig. 5 - An electron is promoted from 2s to 2pz. The carbon atom is now excited
We know that in benzene, each carbon atom has one delocalised electron from a p orbital. This electron comes from 2pz. Carbon's three other electrons are used to form three equal bonds. But to make three equal bonds, carbon needs an electron in each of three equal orbitals. The easiest way for it to do this is to hybridise its three remaining orbitals: 2s, 2px and 2py. These form three identical orbitals known as sp2 orbitals, because - you guessed it - they are made from one s orbital and two p orbitals.
Fig. 6 - The orbitals in benzene
If you aren’t sure about orbitals, see Electron Shells, Subshells and Orbitals.
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.
If you need a quick reminder of naming molecules before we begin, look at IUPAC Nomenclature.
Name the following benzene derivative.
Fig. 7 - Can you name this molecule?
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 known as locants to show the positions of other functional groups on the carbon chain. With other organic molecules such as alkanes, we can start numbering the carbons from either end of the carbon chain. With benzene, there is no end to 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 locants showing the positions of all of the functional groups, we want to get the lowest total possible.
Here we can see that the methyl group is either attached to carbon 1 and the chlorine atom to carbon 3, or vice versa. Both numbering possibilities give us a total sum of 1 + 3 = 4. We must look to the next part of the lowest number rule: we give the lowest-numbered locant to the functional group with the prefix that comes first in the alphabet. chloro- alphabetically precedes methyl-, and so this molecule is known as 1-chloro-3-methylbenzene.
Fig. 8 - Our unknown molecule, with its carbon atoms numbered correctly (in larger font) and incorrectly (in brackets)
Here’s another example.
Fig. 9 - Can you name this molecule?
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?
Fig. 10 - Another unknown molecule
This next molecule has a carboxyl (-COOH) group and a hydroxyl (-OH) group attached to its benzene ring. The carboxyl group takes priority, so we 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 call this molecule 2-hydroxybenzoic acid.
What would you name a benzene ring with just a hydroxyl (-OH) group attached? Systematically, it's called hydroxybenzene, but it has its own special name: phenol.
Fig. 11 - Phenol
Find out more about this molecule in Phenol.
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 - 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.
Aromatic Chemistry - Key takeaways
Aromatic compounds are also called arenes and contain a ring of delocalised pi electrons. The most common ring is benzene (C6H6).
Benzene contains six carbon atoms bonded in a hexagon shape. The bonds between each carbon atom are identical intermediates, 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.
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