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Have you ever wondered what happens when trinitrotoluene, more commonly known as TNT, explodes? Detonation is triggered by a wave of pressure which causes a decomposition reaction. This produces large amounts of gas in an extremely exothermic reaction; the combination of rapidly expanding gas and heat is what makes TNT so deadly. It has a low melting point of only 80 °C, meaning it can be used in liquid form, and is insoluble in water. These properties mean we can use it in a variety of situations, from mining endeavours to military tasks in wet environments.
The equation for the decomposition reaction is shown below. As you can see, nitrogen, hydrogen and carbon monoxide gas are produced:
TNT is based on a benzene ring, and this is where the hydrogen and carbon atoms come from. The nitrogen and oxygen atoms come from three nitrate groups attached to the hydrocarbon benzene ring. TNT also contains a methyl group, as shown below.
But how do we get from a planar benzene molecule to this highly branched structure? To do this, we need to look at the reactions of benzene.
Before we go any further, let’s first remind ourselves about benzene.
Benzene is an aromatic hydrocarbon with the molecular formula . Aromatic molecules are also known as arenes.
Each carbon atom within benzene is bonded to two other carbon atoms and one hydrogen atom. This means that each carbon atom has a spare valence electron. We find these electrons in an area formed by overlapping pi orbitals above and below the benzene ring. The electrons can move freely within this area - we say that they are delocalised.
As we explored above, benzene contains delocalised pi electrons found in a ring. This ring of delocalisation is relatively strong and stable because it spreads the charge of the electrons over a greater area. It takes a lot of energy to disrupt the delocalisation. Therefore, benzene doesn’t readily take part in reactions that involve breaking the ring, such as addition reactions. However, it does take part in substitution reactions. To be more precise, these tend to be electrophilic substitutions. Let’s break that term down a little.
Substitution reactions are reactions in which one atom, group of atoms or functional group is replaced by another on a molecule.
To 'substitute' just means to swap out one thing for another, which is exactly what happens in a substitution reaction. In benzene, substitution reactions involve getting rid of some of the hydrogen atoms attached to the carbon ring and replacing them with other, more useful groups of atoms. These could be nitrate groups or methyl groups.
Electrophiles are electron pair acceptors. They have a vacant electron orbital and a positive or partially positive charge on one of their atoms.
Electrophiles are attracted to areas of high electron density - this just means areas with a lot of electrons crammed together. One example is benzene’s ring of delocalisation. It contains six electrons moving about randomly, free from any positive particles like protons, and so is a highly charged area. Electrophiles love it - after all, phile does come from the Latin term philos, meaning to love!
Because of this, benzene tends to take part in electrophilic substitution reactions.
To start, let’s first look at the general mechanism of an electrophilic substitution reaction with benzene. The electrophile is represented by .
The electrophile is attracted to benzene’s ring of delocalisation. The electrophile forms a bond with one of benzene’s carbon atoms, using one of the ring’s electrons. This leaves benzene with a positive charge. The aromatic ring is now partially destroyed.
Benzene would be a lot more stable if it repaired its electron ring. To do this, it breaks one of its C-H bonds. This releases the hydrogen atom as a ion. The spare electron returns to the ring of delocalisation, restoring it back to six electrons.
The end product is a benzene derivative, where one of the hydrogen atoms has been replaced by the electrophile.
Notice the electron ring in the mechanism above. To show that it has been disrupted in the second step, we draw a broken circle. Remember to include the positive charge in the centre of the molecule.
There are three specific examples you should know that use this mechanism:
Chlorination. We'll also look at bromination.
Friedel-Crafts acylation. We'll look at this with acyl chlorides and acid anhydrides.
Each reaction has three steps:
The electrophile is generated.
The electrophile reacts with benzene.
The catalyst is regenerated.
We'll look at each of the reactions in turn.
Do you remember TNT from the start of the article? It has three nitrate groups and one methyl group attached to a benzene ring. Nitrated arenes are also important industrially as they are the first step in making aromatic amines, used in products like dyes.
To nitrate arenes, we first need to produce the nitrate ion, . This acts as our electrophile. To do this, we mix concentrated sulfuric and nitric acids. Sulfuric acid is a stronger acid than nitric acid, so nitric acid is forced to act as a base - it accepts a proton given up by sulfuric acid. This forms nitrooxonium, :
Nitrooxonium then breaks down into water and a nitronium ion, as shown in the equation below:
Not sure what bases are? Take a quick look at Acids and Bases for more information.
The nitronium ion is an electrophile. Remember, an electrophile is an electron pair acceptor with a vacant electron orbital and a positive or partially positive charge. The nitrate ion reacts with benzene because is attracted to benzene’s ring of delocalisation, just like in the general mechanism shown above. The overall reaction involves heating benzene at 50 °C with concentrated sulfuric and nitric acids, using reflux to prevent any volatile components escaping. The end product is nitrobenzene.
You’ll notice that a hydrogen ion is given off, released from benzene during the nitration reaction. This ion reacts with to reform . This means that the sulfuric acid is just a catalyst.
Here's an equation for the overall reaction:
So how do we get from one nitrate group to three, as seen in TNT? Well, if you heat the reaction up to even higher temperatures, you increase the chance of further nitration reactions happening. Another hydrogen atom is ‘kicked out’ and replaced with a nitrate group. If we count the carbon atom with the original nitrate group as carbon 1, the second nitrate group tends to be directed towards carbon 3 or 5. This is because nitrate groups are electron withdrawing. For example, nitration reactions produce a lot of 1,3-dinitrobenzene but you won’t find much 1,2-dinitrobenzene!
You might have also noticed the methyl group in TNT. A benzene ring with a methyl group attached is commonly known as toluene, and it reacts a lot faster than a benzene ring without any methyl groups. In fact, if you want to prevent further nitration reactions happening, you have to keep the temperature below 30 °C. Methyl groups are electron releasing and direct any nitrate groups towards positions 2, 4 and 6 in the benzene ring. Just watch out - if you manage to substitute three nitrate groups into the molecule, you’ll have TNT on your hands!
We can also swap hydrogen atoms on a benzene ring with chlorine atoms, using aluminium chloride as a catalyst. This is another type of electrophilic substitution reaction and uses the same general mechanism as nitration.
Before the reaction can start, we first need to generate an electrophile to react with benzene. This is where our catalyst comes in. Aluminium chloride reacts with chlorine to form a positive chlorine cation and a negative aluminium tetrachloride ion:
The chlorine cation can now react with benzene using the mechanism shown below. This forms chlorobenzene and a hydrogen ion:
In order for our catalyst to be, well, a catalyst, we need to regenerate it.
Remember, catalysts are substances that aren't used up in a reaction. They also keep the same physical and oxidation state.
Remember the aluminium tetrachloride we produced when we generated the electrophile? The hydrogen ion (released when the chlorine cation reacted with benzene) reacts with aluminium tetrachloride to produce hydrochloric acid and reform our catalyst, aluminium chloride:
We can brominate benzene in a similar way. Simply swap chlorine gas for bromine, and use the catalyst aluminium bromide instead of aluminium chloride.
You might know from Acylation that acylation reactions involve adding the acyl group, , to another molecule. To acylate benzene, we heat an acid derivative, such as an acyl chloride or acid anhydride, with aluminium chloride at 60 °C. The reaction takes place in anhydrous conditions under reflux. Let's first focus on acylation using an acyl chloride.
Like in nitration, we first need to generate an electrophile. In this case our electrophile is the positive cation; we form it by reacting our acyl chloride with aluminium chloride, our catalyst. The below equations show you the process, using R to represent an alkyl group. Notice the other product is once again a negative aluminium tetrachloride ion:
The electrophile reacts with benzene in much the same way as explored above. Again, it releases a hydrogen ion.
Our product is a ketone with a benzene ring attached. We can use the prefix phenyl- to show this.
To regenerate aluminium chloride, the positive hydrogen ion released reacts with the negative aluminium tetrachloride ion we formed earlier. This produces aluminium chloride and hydrochloric acid:
Here's the overall equation:
Let’s now look at an example - reacting ethanoyl chloride with benzene.
First, ethanoyl chloride reacts with aluminium chloride to form our negative anion, aluminium tetrachloride and our electrophile, the positive acetyl cation:
The electrophile then reacts with benzene. You should now be familiar with the mechanism. The products are phenylethanone and a hydrogen ion:
Finally, the hydrogen ion reacts with the negative aluminium tetrachloride ion we formed earlier to regenerate the catalyst, also producing hydrochloric acid:
This gives us the following overall equation:
Likewise, acid anhydrides react with benzene to produce a ketone with a benzene ring attached. This reaction is almost exactly the same as all the other ones we've explored today - making it a lot easier to remember!
Instead of memorising each reaction separately, learn the general mechanism and practice applying it to examples.
First, let's generate the electrophile. Once again, we use aluminium chloride and produce the positive cation . But this time we produce a different negative ion:
The positive cation electrophile can now react with benzene, producing a ketone and releasing a hydrogen ion:
Finally, we need to regenerate the catalyst:
This gives us the following overall equation:
Phew - you made it through all the electrophilic substitution reactions! Here's a handy table to help summarise the new material:
Although electrophilic substitution reactions are the most common type of reaction involving benzene, aromatic compounds do take part in other reactions. You don’t need to know the mechanisms for these reactions. Some examples include:
Benzene burns just like any other hydrocarbon to produce carbon dioxide and water. However, because of its high proportion of carbon, it often combusts incompletely. This produces a lot of carbon in the form of soot.
Try writing an equation for the complete combustion of benzene. You should get the following:
We can introduce the sulfonic acid group, , to benzene in another electrophilic substitution reaction. This is done, for example, by heating benzene with concentrated sulfuric acid under reflux. It forms white crystals of benzenesulfonic acid.
As the name suggests, hydrogenation involves adding hydrogen to a molecule. Hydrogenating benzene creates a cyclic alkane, cyclohexane. However, the reaction has a high activation energy as it involves breaking benzene’s stable ring of delocalised electrons. It uses a nickel catalyst and high temperatures and pressures.
For example, hydrogenating methylbenzene would produce methylcyclohexane, as shown below:
Bromination of benzene is a type of electrophilic substitution reaction.
Benzene normally undergoes electrophilic substitution reactions. This is because addition reactions would involve disrupting its stable ring of delocalised electrons.
Addition reactions of benzene are difficult because they would involve disrupting benzene's stable ring of delocalised electrons. This takes a lot of energy.
Nitration of benzene is a substitution reaction because a hydrogen atom from benzene is swapped for a nitrate group.
No, benzene doesn't normally give elimination reactions.
Name the organic family benzene belongs to.
Describe the structure and bonding within benzene.
An electron pair acceptor with a positive or partial positive charge and a vacant electron orbital.
Explain what happens in a substitution reaction.
One atom or group of atoms is replaced by another.
Explain why benzene does not readily take part in addition reactions.
It would involve breaking benzene’s strong and stable ring of delocalised electrons, which requires a lot of energy.
What sort of reaction is the nitration of benzene?
Which of the following conditions are required for the nitration of benzene?
State the conditions required for Friedel-Crafts acylation of benzene.
60 °C, anhydrous, reflux, aluminium chloride catalyst
Name the organic molecule formed when methylbenzene is fully hydrogenated.
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