Electrophilic substitution of benzeneAQA A-Level Chemistry: Revision notes

The delocalised π system gives a ring of high electron density above and below the plane of the carbon atoms. This attracts electrophiles, which are electron pair acceptors.

Because benzene is very stable, the reaction must keep the ring intact. An electrophile therefore substitutes a hydrogen atom rather than adding across the ring. This course is limited to monosubstitution, where one hydrogen is replaced.

Reagents and conditions: concentrated nitric acid and concentrated sulfuric acid, kept at about 50 °C (a water bath). Above this temperature more than one nitro group can be substituted, giving dinitrobenzene.

Benzene + HNO₃ → nitrobenzene + H₂O, that is C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O.

Generating the electrophile. Sulfuric acid is the stronger acid and protonates nitric acid, which loses water to form the nitronium ion:

HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O

1. The delocalised π electrons attack the NO₂⁺ electrophile, forming a bond from carbon to nitrogen.
2. This gives a positively charged intermediate. The delocalised ring is partly broken and the positive charge is spread over the remaining five carbons. The H and NO₂ are on the same carbon.
3. The C–H bond breaks and H⁺ is lost, restoring the stable delocalised ring and forming nitrobenzene.
4. The H⁺ reacts with HSO₄⁻ to re-form H₂SO₄, so sulfuric acid is a catalyst: H⁺ + HSO₄⁻ → H₂SO₄.

In a mechanism diagram the first curly arrow starts from the ring and points to the N of NO₂⁺. The second arrow starts at the C–H bond and points back into the ring.

Nitration is an important step in synthesis:

• Further nitration of methylbenzene is used in the manufacture of explosives such as TNT
• Nitrobenzene can be reduced to phenylamine, an aromatic amine used to make dyes and other products

The nitro group is therefore a way of putting nitrogen onto an aromatic ring.

Acylation puts an acyl group, RCO–, onto the ring, forming an aromatic ketone. Benzene reacts with an acyl chloride in the presence of anhydrous aluminium chloride, AlCl₃, as catalyst, with warming.

C₆H₆ + CH₃COCl → C₆H₅COCH₃ + HCl (product: phenylethanone)

Generating the electrophile. AlCl₃ accepts a lone pair from the chlorine of the acyl chloride to form the acylium ion:

CH₃COCl + AlCl₃ → CH₃CO⁺ + AlCl₄⁻

Conditions must be anhydrous because AlCl₃ reacts with water.

1. The π electrons of benzene attack the carbon of the CH₃CO⁺ ion.
2. A positively charged intermediate forms with the H and COCH₃ on the same carbon.
3. H⁺ is lost, restoring the delocalised ring and forming phenylethanone.
4. H⁺ reacts with AlCl₄⁻ to form HCl and regenerate AlCl₃: H⁺ + AlCl₄⁻ → AlCl₃ + HCl.

Acylation is useful because it forms a new C–C bond to the ring, which makes it a key step in building more complex molecules.