Reactions of Benzene

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. TNT has a low melting point of only 80 °C, meaning it can be used in liquid form, and is insoluble in water. These properties enable it to be used in a variety of situations, from mining endeavours to military tasks in wet environments.

The decomposition of TNT produces a variety of products: the gases nitrogen, hydrogen, and carbin monoxide, as well as solid carbon. The hydrogen and carbon atoms come from TNT's benzene ring. The nitrogen and oxygen atoms come from three nitrate groups attached to the benzene ring. TNT also contains a methyl group, as shown below.

The structure of TNT. It contains three nitrate groups and one methyl group attached to a benzene ring. StudySmarter Originals

But how do we get from a planar benzene molecule to such a highly branched structure? To understand this, we need to look at the reactions of benzene.

• This article is about the reactions of benzene in organic chemistry.
• First of all, we’ll recap the structure of benzene.
• After that, we'll look at why benzene reacts in electrophilic substitution reactions.
• We'll explore specific examples of electrophilic substitution reactions, including nitration, chlorination, Friedel-Crafts acylation, and Friedel-Crafts alkylation.
• We’ll then look at other reactions of benzene, such as combustion and hydrogenation.
• Finally, we'll consider reactions of benzene derivatives, including the reduction of nitrobenzene and the oxidation of methylbenzene.

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.

Overlapping pi orbitals in benzene form a ring of delocalisation. commons.wikimedia.org

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:

$$ H_2SO_4+HNO_3\rightarrow NO_2^++HSO_4^-+H_2O $$

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:

$$ C_6H_6+HNO_3\rightarrow C_6H_5NO_2+H_2O $$

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:

$$ Cl_2+AlCl_3\rightarrow Cl^++AlCl_4^- $$

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:

$$ C_6H_6+Cl_2\rightarrow C_6H_5Cl+HCl $$

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:

$$ AlCl_3+RCOCl\rightarrow RCO^++AlCl_4^- $$

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:

$$ C_6H_6+RCOCl\rightarrow C_6H_5COR+HCl $$

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:

$$ RCl+AlCl_3\rightarrow R^++AlCl_4^- $$

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:

$$ C_6H_6+RCl\rightarrow C_6H_5R+HCl $$

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:

$$ C_6H_6+7.5 O_2\rightarrow 6CO_2+3H_2O $$

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:

$$ C_6H_5CH_3+2.5H_2\rightarrow C_6H_{11}CH_3 $$

The hydrogenation of methylbenzene. StudySmarter Originals

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):

$$ C_6H_5CH_3+3[O]\rightarrow C_6H_5COOH+H_2O $$

The oxidation of methylbenzene. StudySmarter Originals

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:

$$ C_6H_5NO_2+6[H]\rightarrow C_6H_5NH_2+2H_2O $$

The reduction of nitrobenzene. StudySmarter Originals