Substitution of Substituted Benzenes
We can perform an SEAr on an aromatic compound that already bears substituents, but we have to ask ourselves at least two questions:
- Will the substituent make the SEAr easier or more difficult?
- Will the substituent direct the incoming group to a given position?
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Let's consider the nitration of a monosubstituted benzene:
The following table contains the experimental reaction rates (krel) relative to benzene (R = H) and the ortho, meta and para regioisomer ratios obtained in the nitration of various monosubstituted benzenes. What conclusions can you draw?
The reddish color of the rings is a qualitative indication of their electron density: the redder, the higher. Click on the different figures to get an explanation.
Two kinds of groups can be distinguished regarding the reaction rate relative to benzene:
- Those that decrease it: they are electronegative groups that remove electron density from the ring thus making the SEAr more difficult (NO2, CO2H, CF3, NMe3+, Cl, Br, CH2Cl).
- Those that increase it: they are electron donors that increase the ring electron density and facilitate the SEAr (OH, CH3).
Two kinds of groups can be distinguished depending on the orientation they induce:
- Meta directors: they are generally strongly electronegative, deactivating groups (NO2, CO2H, CF3, NMe3+).
- Ortho-para directors: they can be either electron donors and activators, like OH and CH3, or slightly electronegative, weak deactivators like CH2Cl, Cl and Br.
