Arenes: Directing Effects of Substituents
- Arenes readily undergo electrophilic substitution of one of their hydrogen atoms with another species
- Substituents that are already present on the arenes can affect where the substitution of the hydrogen atom on the arene takes place
- These groups are said to direct substitution reactions to different ring positions
Electron-withdrawing & -donating groups
- The substituents on the arenes can either be electron-withdrawing or electron-donating groups
- Electron-withdrawing substituents remove electron density from the π system in the benzene ring making it less reactive
- These groups deactivate attack by electrophiles and direct the incoming electrophile to attack the 3 and/or 5 positions
- For example, the nitro group in nitrobenzene is an electron-withdrawing group
- Upon bromination of nitrobenzene, the bromine electrophile will be directed to the 3 and/or 5 position
- The products are 3-bromonitrobenzene and 5-bromonitrobenzene
- Electron-donating substituents donate electron density into the π system of the benzene ring making it more reactive
- These groups activate attack by electrophiles and direct the incoming electrophile to attack the 2, 4 and/or 6 positions
- For example, the methyl group in methylbenzene is an electron-donating group
- Upon bromination of methylbenzene, the bromine electrophile will be directed to the 2 and/or 4 position
- The products are 2-bromomethylbenzene and 4-bromomethylbenzene
Electron-withdrawing & -donating substituents table
