Benzene: Structure & Electrophilic Substitution (AQA A Level Chemistry): Video

Hi, I'm Eleanor with 3 years of experience teaching chemistry, and this video is about the structure of benzene and its electrophilic substitution reactions.

The three parts go together because benzene's delocalised ring is what makes it react by substitution, and nitration and Friedel–Crafts acylation are that same substitution with a different electrophile.

Aromatic compounds contain a benzene ring, in which the π electrons are delocalised over the ring. That delocalisation is why arenes usually undergo substitution rather than addition. The electrophilic substitution reaction in arenes consists of three steps: generation of an electrophile, electrophilic attack, and regenerating aromaticity. Those same three steps happen every time, and the only thing that changes is the electrophile.

We start with the structure of benzene and the evidence for the delocalised ring. Then the general electrophilic substitution mechanism steps. Then nitration, where the electrophile is the nitronium ion, and Friedel–Crafts acylation, where it is the acylium ion.

Benzene consists of six carbon atoms arranged in a hexagonal ring. The Kekulé structure, proposed in 1865, shows alternating single and double carbon–carbon bonds, which suggests benzene should react like an unsaturated alkene.

That is not the case. Benzene is a regular, planar molecule with bond angles of 120 degrees. Each carbon atom forms three sigma bonds, and the remaining p orbitals overlap laterally with the p orbitals of neighbouring carbon atoms to form a π system. That sideways overlap lets the electrons spread over the entire ring, as two ring-shaped clouds of electron density, one above the plane and one below it.

The bond lengths are the evidence. A carbon–carbon double bond is 0.134 nanometres and a single bond is 0.154, but every carbon–carbon bond in benzene is 0.140. The bonds all being the same length is evidence for the delocalised ring, and it is that delocalised ring that decides how benzene reacts.

The main reactions of benzene replace one of the six hydrogen atoms on the ring with an electrophile. That is different from the reactions of unsaturated alkenes, which break the carbon-carbon double bond and add atoms across it.

The delocalisation of electrons in arenes explains why they undergo substitution rather than addition. In electrophilic substitution the aromatic ring is temporarily disrupted, but aromaticity is restored in the final product, so the ring keeps its stabilised delocalised π system. Addition permanently breaks that system: hydrogenation converts benzene into cyclohexane, and the product has no aromatic stabilisation, which makes addition less favourable under normal conditions.

The reaction runs in three steps: generation of an electrophile, electrophilic attack, and regenerating aromaticity. The delocalised π system is a region of high electron density, and the electrophile usually cannot be added directly, so it is generated in the reaction mixture. Those three steps are the same for every reaction of this type.

The nitration of benzene is one example of an electrophilic substitution reaction. A hydrogen atom is replaced by a nitro group, NO2, giving nitrobenzene and water.

The electrophile is the nitronium ion,NO2 plus, generated by reacting concentrated nitric acid with concentrated sulfuric acid. The nitrating mixture is heated under reflux with the benzene at 25 to 60 degrees Celsius.

The three steps run as before, with the nitronium ion as the electrophile.

Friedel–Crafts reactions are also electrophilic substitution reactions. Because of the aromatic stabilisation in arenes they are often unreactive, so to use arenes as starting materials for the synthesis of other organic compounds, their structure needs changing into something more reactive.

A Friedel–Crafts reaction substitutes a hydrogen atom in the benzene ring for an acyl group, which is an alkyl group containing a carbonyl. The benzene ring is reacted with an acyl chloride in the presence of an aluminium chloride catalyst, giving phenylethanone and hydrogen chloride.

The aluminium chloride reacts with the acyl chloride to produce the acylium ion, and that is the electrophile. The three steps run as before; only the electrophile has changed.

Make sure you understand the general steps of the electrophilic substitution mechanism, and that you can explain what is happening. The same steps happen every time. The only difference is the electrophile used in the reaction.

Benzene is a planar ring with its π electrons delocalised over all six carbon atoms, and all of its carbon–carbon bonds are the same length.

That delocalisation is why benzene undergoes substitution rather than addition: the ring is temporarily disrupted, but aromaticity is restored in the product.

Nitration replaces a hydrogen atom with a nitro group, using the nitronium ion. Friedel–Crafts acylation replaces a hydrogen atom with an acyl group, using the acylium ion.

Both are the same three steps: generate the electrophile, electrophilic attack, regenerate aromaticity. Only the electrophile changes.