Question 1

Why is concentrated sulfuric acid needed for nitration if it's a catalyst?

Accepted Answer

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.

Question 2

What's the difference between the halogenation of alkanes and arenes?

Accepted Answer

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).

Question 3

Can I use Friedel-Crafts alkylation to add a propyl group to benzene?

Accepted Answer

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.

Question 4

Why doesn't Friedel-Crafts acylation suffer from polysubstitution like alkylation does?

Accepted Answer

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.

Arenes

What you need to know

Arenes