Arenes — common mistakes

When drawing the mechanism, ensure your curly arrows are precise. The first arrow must start from the delocalised ring and point to the electrophile. For the second step, the arrow must start from the C-H bond and point towards the positive charge inside the ring to reform the pi-system.

Friedel-Crafts reactions require anhydrous conditions because the AlCl₃ catalyst reacts vigorously with water. Remember that alkylation can lead to poly-substitution and rearrangement, but acylation does not because the acyl group deactivates the ring, preventing further reaction.

While it is regenerated, its role is more than just providing a surface. It's a chemical reactant in the formation of the electrophile. It's a stronger acid than nitric acid, so it forces nitric acid to act as a base, accept a proton, and then decompose to form the highly reactive NO₂⁺ ion, which is a much stronger electrophile than HNO₃ itself.

The mechanism and conditions are completely different. Alkanes undergo free-radical substitution, which requires UV light to initiate the homolytic fission of the halogen. Arenes undergo electrophilic substitution, which requires a halogen carrier catalyst (like FeBr₃) to initiate heterolytic fission and create an electrophile. Using UV light with benzene and bromine would lead to a different, addition reaction (which is not typically studied at A-level).

You can, but it's problematic. If you use 1-chloropropane, the primary carbocation initially formed can rearrange via a hydride shift to a more stable secondary carbocation. This means you will get a mixture of products, with (propan-2-yl)benzene being the major product instead of the desired (propan-1-yl)benzene. This rearrangement is a key limitation of Friedel-Crafts alkylation.

The product of acylation is a ketone (e.g., phenylethanone). The carbonyl group (C=O) is an electron-withdrawing group, which deactivates the benzene ring by pulling electron density out of the pi-system. This makes the acylated product less reactive than benzene itself, so it does not undergo a second substitution reaction. In contrast, alkyl groups are electron-donating, activating the ring and making it more susceptible to further substitution.