Donation and Withdrawal of Electrons

Substituents or groups on a benzene ring can donate or withdraw electron density from the aromatic ring. This donation/withdrawal can be described by resonance and inductive effects.

Inductive Effects

This is the result of differences in electronegativity or polarizability.

Inductive Withdrawers - A Fluorine atom is more electronegative than a C atom. A fluorine atom withdraws electron density by way of induction from the benzene ring. This induction occurs through a sigma bond. Fluorine is an EWG as are most halogens and as such they would deactivate the ring (slow down the rate of an EAS reaction).

Inductive Donator - On the other hand a -CH₃ donates electron density because of hyperconjugation and the polarity of the (3) C-H bonds. A methyl group or any alkyl group is a slight EDG (Electron Donating Group) and slightly activates the ring making it a better nucleophile and speed any EAS reaction with it.

Resonance Effects

This effect results from conjugation of either a lone pair of electrons or a π bond. The push or pull of electrons can be seen with resonance structures.

Resonance Donators - Resonance donators have a lone pair of electrons in conjugation with the ring. For example, phenol has the following resonance structures. In general, the halogens (I, Br, Cl F) and -OH, -NH2, NHR, NR2 will behave as a resonance donator. Except for the halogens they are all considered EDG's and activate the aromatic ring making them more nucleophilic. Notice how the curved arrows show the electrons being pushed into the ring. This activates it and speeds up EAS reactions.

Take NoteNotice the negative charge on the ortho and para carbons in the resonance structures. This is one method of predicting the orientation of electrophilic attachment. Recall that the aromatic ring is a nucleophile so these carbons have increased electron density or populations. As we will see the -OH group is an Ortho/Para Directing Activator.

Resonance Withdrawers - Resonance withdrawers have a pi bond in conjugation with the aromatic ring. For example, acetophenone has the following resonance structures. In general -NO2, ketone, aldehyde, nitriles behave as resonance withdrawers and deactivate the aromatic ring in EAS reactions. Notice how the curved arrows show the electrons being pulled out of the ring and a net positive charge left on the ring. This deactivates it and slows down EAS reaction.

Take NoteThere's a positive charge on the ortho and para carbons in the resonance structures. These carbons have decreased electron density and would be expected to be less reactive than the meta position. As we will see the acetyl group in acetophenone is a Meta Directing Deactivator.

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