Reactions of Benzene

What is benzene?

Before we go any further, let’s first remind ourselves about benzene.

Each carbon atom within benzene is bonded to two other carbon atoms and one hydrogen atom, forming a cyclical ring. Each carbon atom also 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.

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Electrophilic substitution reactions of benzene

As we now know, benzene contains delocalised pi electrons found in a ring. The ring of delocalisation is relatively strong and stable because it distributes the negative charge of the electrons across a wider area. This means that it takes a lot of energy to disrupt the delocalisation. As a result, benzene doesn’t readily take part in any reactions that involve breaking the ring, such as addition reactions. But on the other hand, it does take part in substitution reactions. To be more precise, these tend to be electrophilic substitutions.

Electrophiles are electron-deficient species, meaning that they have a vacant electron orbital and a positive or partially positive charge on one of their atoms. They are particularly attracted to benzene's ring of delocalisation because of its high electron density. When electrophiles attack benzene, they trigger a substitution reaction. These 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 such as nitrate groups or chlorine atoms. But note that we don't disrupt the benzene ring by adding or taking away any of the delocalised electrons - this would simply require too much energy.

Electrophilic substitution reactions of benzene include:

• Nitration.

• Chlorination. We'll also look at bromination.

• Friedel-Crafts reactions.

Each reaction has three steps:

1. The electrophile is generated.

2. The electrophile reacts with benzene.

3. The catalyst is regenerated.

We'll look at each of the reactions in turn.

Nitration of benzene

Do you remember TNT from the start of the article? It has three nitrate groups and one methyl group attached to a benzene ring. We nitrate benzene in an example of an electrophilic substitution reaction. Nitrated arenes are important industrially as they are the first step in synthesising aromatic amines, used in products like dyes.

Benzene is nitrated using the nitronium ion, NO₂⁺, which acts as our electrophile. It is generated by mixing concentrated sulfuric and nitric acids (H₂SO₄ and HNO₃). 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. The reaction forms H₂NO₃⁺ and the bisulfate ion (HSO₄^-); H₂NO₃⁺ then breaks down into water and a nitronium ion. The overall equation is shown below:

$$H_{2}S{O}_4+HN{O}_3\to N{O}_2^{+}+HS{O}_4^{-}+H_{2}O$$

The nitronium ion is an electrophile - an electron pair acceptor with a vacant electron orbital and a positive or partially positive charge. The nitronium ion reacts with benzene because is attracted to benzene’s ring of delocalisation, an area of high electron density. It replaces one of the benzene ring's hydrogen atoms. The reaction involves heating benzene at 50 °C with concentrated sulfuric and nitric acids, using reflux to prevent any volatile components from escaping. This produces nitrobenzene (C₆H₅NO₂) and a hydrogen ion (H⁺). The hydrogen ion then reacts with the bisulfate ion generated earlier to reform sulfuric acid. This means that sulfuric acid is just a catalyst.

Here's the overall equation:

$$C_6{H}_6+HN{O}_3\to C_6{H}_{5}N{O}_2+H_{2}O$$

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!

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1,3-dinitrobenzene. Vaia Originals

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!

This is just one example of the directing effects of different substituents in electrophilic substitution reactions of benzene. As we mentioned earlier, you'll out more about this topic, as well as the mechanism for not only the nitration of benzene but also all the other electrophilic substitution reactions that you need to know about, over at Electrophilic Substitution of Benzene.

Chlorination of benzene

We can also swap hydrogen atoms on a benzene ring with chlorine atoms, using aluminium chloride (AlCl₃) as a catalyst. This is another type of electrophilic substitution reaction and takes place at room temperature.

Aluminium chloride reacts with chlorine to form a positive chlorine cation (Cl⁺) and a negative aluminium tetrachloride ion (AlCl₄^-). Here's the equation:

$$C{l}_2+AlC{l}_3\to C{l}^{+}+AlC{l}_4^{-}$$

The chlorine cation acts as our electrophile. It reacts with benzene, forming chlorobenzene (C₆H₅Cl) and a hydrogen ion. Like in the nitration reaction, the hydrogen ion reacts with the aluminium tetrachloride ion produced earlier to reform our catalyst, aluminium chloride. This also produces hydrochloric acid (HCl).

Here's the overall equation:

$$C_6{H}_6+C{l}_2\to C_6{H}_{5}Cl+HCl$$

Friedel-Crafts reactions of benzene

Friedel-Crafts reactions were invented in 1877 by the chemists Charles Friedel and James Crafts, of French and American origin respectively. They are a means of attaching different substituents to an aromatic benzene ring.

Friedel-Crafts reactions include:

• Friedel-Crafts acylation. We'll look at this reaction with both acyl chlorides and acid anhydrides.
• Friedel-Crafts alkylation.

Friedel-Crafts acylation of benzene

You might know from Acylation that acylation reactions involve adding the acyl group, -RCO-, to another molecule. Benzene is acylated by heating an acid derivative, such as an acyl chloride (RCOCl) or acid anhydride (RCOOCOR), with aluminium chloride at 60 °C. The reaction takes place in anhydrous conditions under reflux. We'll focus on acylation using an acyl chloride.

Our acyl chloride first reacts with aluminium chloride, our catalyst, to generate the electrophile (RCO⁺) and a negative aluminium tetrachloride ion (AlCl₄^-). Note that in the equation below, R represents the acyl chloride's alkyl group:

$$AlC{l}_3+RCOCl\to RC{O}^{+}+AlC{l}_4^{-}$$

The electrophile reacts with benzene to form a ketone with a benzene ring attached (C₆H₅COR), and a hydrogen ion (H⁺). We name the ketone using the prefix phenyl-. Like before, the hydrogen ion is used to regenerate the catalyst, which also produces hydrogen chloride (HCl).

Overall, we end up with the following reaction:

$$C_6{H}_6+RCOCl\to C_6{H}_{5}COR+HCl$$

Friedel-Crafts alkylation of benzene

Finally, let's consider Friedel-Crafts alkylation of benzene. If acylation reactions involve adding an acyl group to a molecule, then alkylation reactions involve adding an alkyl group to a molecule. We do this by reacting benzene with a halogenoalkane, typically a chloroalkane (RCl), in the presence of an aluminium chloride catalyst.

The chloroalkane first reacts with aluminium chloride to produce a positive carbocation (R⁺) and a negative ammonium chloride ion:

$$RCl+AlC{l}_3\to R^{+}+AlC{l}_4^{-}$$

The positive carbocation is an electrophile, and attacks benzene to produce an alkylarene (C₆H₅R) and a hydrogen ion. The hydrogen ion regenerates the catalyst and also releases hydrochloric acid.

Overall, we get the following reaction:

$$C_6{H}_6+RCl\to C_6{H}_{5}R+HCl$$

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Sulfonation of benzene

We can introduce the sulfonic acid group, SO₃, 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.

Summary of benzene electrophilic substitution reactions

Phew - you made it through all the electrophilic substitution reactions! Here's a handy table to help summarise the new material.

Other reactions of benzene

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:

• Combustion.
• Hydrogenation.

Combustion

Benzene burns just like any other hydrocarbon to produce carbon dioxide and water in a combustion reaction.

Try writing an equation for the complete combustion of benzene. You should get the following:

$$C_6{H}_6+7.5O_2\to 6C{O}_2+3H_{2}O$$

However, because of its high proportion of carbon, benzene often combusts incompletely. This produces a lot of carbon in the form of soot.

Hydrogenation

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 hydrogen gas (H₂), a nickel catalyst and high temperatures and pressures.

For example, hydrogenating methylbenzene (C₆H₅CH₃) produces methylcyclohexane (C₆H₁₁CH₃), as shown below:

$$C_6{H}_{5}C{H}_3+2.5H_2\to C_6{H}_{11}C{H}_3$$

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Reactions of benzene derivatives

Last of all, let's consider some reactions of benzene derivatives. These include:

• Oxidation of an alkylarene to produce benzoic acid.
• Reduction of nitrobenzene to produce a phenylamine.

Oxidation of alkylarenes

Producing carboxylic acids usually involves the Oxidation of Alcohols. But to produce benzoic acid (C₆H₅COOH), which is a molecule containing the carboxyl group (-COOH) attached to a benzene ring, we can simply oxidise the side chain of an alkylarene. This involves refluxing an alkylarene with alkaline potassium manganate(VII) (KMnO₄) followed by sulfuric acid (H₂SO₄). The same reaction can be done with almost any alkylarene and always produces benzoic acid, plus water. There is just one catch - the carbon atom directly bonded to the benzene ring must also be joined to a hydrogen atom.

Here's the equation for the oxidation of the simplest alkylarene, methylbenzene (C₆H₅CH₃):

$$C_6{H}_{5}C{H}_3+3[O]\to C_6{H}_{5}COOH+H_{2}O$$

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Reduction of nitrobenzene

Earlier in the article, we explored how we create nitrobenzene by nitrating benzene. We can replace the nitro group (-NO₂) in nitrobenzene with an amine group (-NH₂) in a reduction reaction, forming phenylamine (C₆H₅NH₂). We first heat nitrobenzene under reflux with tin (Sn) and concentrated hydrochloric acid (HCl), then add sodium hydroxide (NaOH).

Here's the equation:

$$C_6{H}_{5}N{O}_2+6[H]\to C_6{H}_{5}N{H}_2+2H_{2}O$$

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