Substituents on a benzene ring significantly influence the reactivity and orientation of monosubstituted benzene compounds in electrophilic aromatic substitution (EAS) reactions.
The substituents can be broadly classified as either electron-donating groups (EDGs) or electron-withdrawing groups (EWGs), and their effects on reactivity and orientation are crucial for predicting the outcome of these reactions.
Types of Substituents on Benzene
1) Electron-Donating Groups (EDGs):
Characteristics: These groups donate electron density to the benzene ring, typically through resonance or inductive effects.
Examples: -OH (hydroxyl), -OCH3 (methoxy), -NH2 (amino), -CH3 (methyl), -C2H5 (ethyl).
2) Electron-Withdrawing Groups (EWGs):
Characteristics: These groups withdraw electron density from the benzene ring, again through resonance or inductive effects.
Examples: -NO2 (nitro), -CF3 (trifluoromethyl), -COOH (carboxyl), -SO3H (sulfonic acid), -CN (cyano).
Effect of Substituents on Reactivity
1) Electron-Donating Groups (EDGs):
Increased Reactivity: EDGs increase the electron density of the benzene ring, making it more nucleophilic and thus more reactive towards electrophiles.
Mechanism: Through resonance, EDGs can donate electron density to the ring, particularly enhancing the negative charge on the ortho and para positions.
2) Electron-Withdrawing Groups (EWGs):
Decreased Reactivity: EWGs decrease the electron density of the benzene ring, making it less nucleophilic and thus less reactive towards electrophiles.
Mechanism: EWGs withdraw electron density from the ring, stabilizing it but making it less attractive to electrophiles.
Effect of Substituents on Orientation
1) Electron-Donating Groups (EDGs):
Ortho/Para Directors: EDGs typically direct incoming electrophiles to the ortho and para positions relative to themselves.
Reason: The increased electron density at these positions due to resonance stabilization makes them more favorable for electrophilic attack.
Examples:
Phenol (-OH): The electrophile preferentially attaches at the ortho and para positions.
Anisole (-OCH3): Similarly, ortho and para positions are more reactive.
2) Electron-Withdrawing Groups (EWGs):
Meta Directors: EWGs typically direct incoming electrophiles to the meta position relative to themselves.
Reason: The decreased electron density at the ortho and para positions due to resonance or inductive withdrawal makes these positions less favorable, leaving the meta position relatively more reactive.
Examples:
Nitrobenzene (-NO2): The electrophile preferentially attaches at the meta position.
Benzonitrile (-CN): Meta position is more reactive for substitution.
Examples and Applications
Toluene (methylbenzene): The -CH3 group is an EDG, making the ortho and para positions more reactive. For instance, nitration of toluene primarily yields ortho and para nitrotoluene.
Nitrobenzene: The -NO2 group is an EWG, directing substitution to the meta position. For example, bromination of nitrobenzene predominantly forms meta-Bromo nitrobenzene.