Electrophilic substitution in benzene

Why substitution, not addition

An alkene adds a reagent, and its pi bond is replaced by two sigma bonds. If benzene did the same, it would lose the delocalised ring and its $150\ \text{kJ/mol}$ of stability. So benzene prefers to substitute: one hydrogen atom leaves, and a new group takes its place. The ring is untouched.

The cloud of pi electrons above and below the ring is a source of electrons. So the usual attacker is an electrophile, a reagent that is short of electrons. This is electrophilic substitution.

The mechanism, in two steps

1. The pi cloud gives a pair of electrons to the electrophile $\text{E}^{+}$. A bond forms between $\text{E}$ and one carbon of the ring. That carbon becomes $\text{sp}^3$, with both $\text{H}$ and $\text{E}$ on it. The other five carbons carry a positive charge, spread by resonance. This arenium ion (also called the sigma complex) has lost the aromatic ring. So this step is slow.
2. A base takes the proton from the $\text{sp}^3$ carbon. The two electrons of the C-H bond go back into the ring. The aromatic ring is made again. This step is fast.

Every reaction in this lesson follows this plan. They differ in how the electrophile is made.

Nitration

Benzene is warmed with a mixture of concentrated nitric acid and concentrated sulphuric acid, called the nitrating mixture. Sulphuric acid, the stronger acid, protonates nitric acid, which then loses water and leaves the nitronium ion:

The ion $\text{NO}_{2}^{+}$ is the electrophile. The product is nitrobenzene, a pale yellow oily liquid with a smell like bitter almonds.

The temperature is kept at about $323\ \text{K}$. A higher temperature, with fuming nitric acid, puts in a second nitro group and gives mainly m-dinitrobenzene.

Halogenation

Benzene does not react with bromine or chlorine alone. A Lewis acid, such as iron(III) bromide or aluminium chloride, is needed. The Lewis acid pulls on one halogen atom and makes the other halogen end strongly positive. Iron filings are often added, because they make iron(III) bromide with bromine in the flask.

The product is bromobenzene. The reaction is done in the dark. In sunlight, chlorine does something quite different, as you will see in the next lesson.

Sulphonation

Benzene is warmed with fuming sulphuric acid, also called oleum. This is sulphuric acid with sulphur trioxide dissolved in it. $\text{SO}_{3}$ is the electrophile. The product is benzenesulphonic acid.

Unlike the other reactions, sulphonation is reversible. Heating the product with dilute acid or steam takes the $\text{SO}_{3}\text{H}$ group away. A chemist can use this to block a position on the ring for a while.

Friedel-Crafts alkylation

An alkyl halide and anhydrous aluminium chloride put an alkyl group on the ring. The aluminium chloride pulls the halide away from the alkyl group and leaves a carbocation, which is the electrophile.

The product is methylbenzene, or toluene. The reaction has three weaknesses.

• The carbocation can rearrange. 1-Chloropropane gives mainly isopropylbenzene, not propylbenzene, because the primary cation shifts to a secondary one.
• More than one alkyl group goes on. The alkyl group makes the ring more reactive, so the product reacts faster than benzene.
• It fails on a ring with a strong electron-withdrawing group. Nitrobenzene is so unreactive that it is used as a solvent for Friedel-Crafts reactions.

Friedel-Crafts acylation

An acyl chloride or an anhydride, with anhydrous aluminium chloride, puts an acyl group on the ring.

The product is acetophenone, a ketone. The electrophile is an acylium ion, $\text{CH}_{3}\text{CO}^{+}$. It does not rearrange, and the product does not react further, because the $\text{C}\text{=}\text{O}$ group makes the ring less reactive. To make a straight-chain alkylbenzene, a chemist first adds an acyl group, then reduces the C=O to $\text{CH}_{2}$, for example by the Clemmensen reduction with zinc amalgam and hydrochloric acid.

Practise with new numbers

Try it in Python

The program follows the nitration. It finds the mass of nitrobenzene, the nitric acid used, and the atom economy: the share of the reactant mass that ends up in the product.

Common mistakes

• Writing addition for benzene. The product keeps the ring. A hydrogen atom is replaced.
• Forgetting the Lewis acid in halogenation. Without it, bromine does not react with benzene.
• Using Friedel-Crafts alkylation to make propylbenzene. The cation rearranges. Use acylation, then reduction.
• Running a Friedel-Crafts reaction on nitrobenzene. The ring is too deactivated.
• Saying that sulphonation is irreversible. The group can be removed with dilute acid or steam.

Remember these

• Aromatic stability is kept: electrophilic substitution, in two steps (slow attack, fast loss of $\text{H}^{+}$).
• Nitration: conc. $\text{HNO}_{3}$ and conc. $\text{H}_{2}\text{SO}_{4}$, electrophile $\text{NO}_{2}^{+}$.
• Halogenation: $\text{X}_{2}$ and a Lewis acid ($\text{FeX}_{3}$ or $\text{AlCl}_{3}$). Sulphonation: oleum, $\text{SO}_{3}$, reversible.
• Friedel-Crafts: $\text{RCl}$ or $\text{RCOCl}$ with anhydrous $\text{AlCl}_{3}$. Alkylation can rearrange. Acylation cannot.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Which species attacks the ring in nitration, and how is it made?

2. Why is the first step of electrophilic substitution slow?

3. What is the product of benzene with ethanoyl chloride and anhydrous aluminium chloride?

4. Why does the Friedel-Crafts reaction of 1-chloropropane give isopropylbenzene?

5. How can you take a sulphonic acid group off a ring?

References

• OpenStax Organic Chemistry (McMurry), chapter 16, the section "Alkylation and Acylation of Aromatic Rings (Friedel-Crafts)".
• NCERT Chemistry, Class 11, Part II, the chapter "Hydrocarbons" (electrophilic substitution reactions of benzene).