Directing groups & activating groups in benzene

Although this is no longer part of many A-level specifications, it is something that used to be on them. Many university interviewers may not realise this or may know this and still use it because they can ask probing questions.

Try to explain why nitrobenzene is less reactive than benzene itself.

When nitrobenzene DOES react, it forms 3-dinitrobenzene in preference to other possibilities.

Clue: Think about how electron pairs can be moved around the benzene ring (mesomerism).

Help

The nitro group is electron-withdrawing. It reduces the electron density of the benzene ring making it less able to give an electron pair to the incoming electrophile. This is felt particularly badly at the 2,4 and 6 positions. Consequently, although 3 and 5 positions are less reactive than they would be in benzene itself, they are more reactive than the 1,3,5 positions and so are attacked.

Methyl (and other alkyl groups) has the reverse effect and so it adds electron density to the ring, increasing the electron pair donating ability of the benzene ring and so increasing the reactivity. This is felt particularly strongly at the 2,4 positions and so thes epositions are particularly favoured. 

Check out Vorlander's rule in books or on the internet.