isn’t true that -OCH3 (methoxy group) The methoxy group is electron withdrawing by the inductive effect of the oxygen atom, since the electronegativity of oxygen is 2.6??? However in EAS, -OCH3 is EDG (electron donating group), why
The Correct Answer and Explanation is :
You’re correct that the methoxy group (-OCH₃) contains an oxygen atom, which is highly electronegative (with an electronegativity of 2.6). However, the behavior of the methoxy group in Electrophilic Aromatic Substitution (EAS) reactions may seem confusing at first glance due to the combination of both inductive and resonance effects.
Inductive Effect:
The oxygen atom in -OCH₃ is indeed electronegative, and it pulls electron density through the sigma bond (inductive effect), making the carbon attached to the oxygen slightly electron-deficient. This would suggest that -OCH₃ is electron-withdrawing by induction.
Resonance Effect:
However, the methoxy group can also donate electron density to the aromatic ring through resonance. The lone pairs on the oxygen atom can delocalize into the ring, increasing electron density on the ring, particularly at the ortho and para positions relative to the methoxy group. This resonance donation is a strong electron-donating effect and dominates over the inductive electron-withdrawing effect.
In EAS reactions, electron-donating groups increase the electron density of the aromatic ring, which stabilizes the carbocation intermediate formed during the reaction, facilitating nucleophilic attack by the electrophile. Because of this resonance donation, the methoxy group is classified as an electron-donating group (EDG).
Summary:
In the case of the methoxy group (-OCH₃), while oxygen is electronegative and might withdraw electron density inductively, the resonance effect of the lone pairs on oxygen outweighs this inductive effect. Therefore, -OCH₃ is considered an electron-donating group (EDG) in Electrophilic Aromatic Substitution reactions. It enhances the electron density on the benzene ring, making it more reactive towards electrophiles at the ortho and para positions.
This explanation is backed by the fact that the methoxy group, due to its resonance donation, stabilizes the intermediate carbocation formed during EAS, thereby increasing the rate of these reactions at the positions where the electron density is increased.