Question
Compare the inductive effect and the resonance effect. When both are operating in the same molecule, which one dominates? Give examples.
Solution — Step by Step
The inductive effect (I-effect) is the permanent displacement of electron density through sigma bonds due to electronegativity differences. It operates through the σ-bond framework and diminishes rapidly with distance (practically negligible beyond 3 bonds).
- –I effect (electron-withdrawing): –F, –Cl, –Br, –OH, –CN, –NO₂ pull electron density toward themselves.
- +I effect (electron-donating): –CH₃, –C₂H₅, alkyl groups push electron density toward the rest of the molecule.
The resonance effect (mesomeric effect, M-effect) is the delocalisation of π electrons or lone pairs through the π system of conjugated or aromatic molecules. It operates through p-orbital overlap and can operate over longer distances than the inductive effect.
- –M effect (electron-withdrawing by resonance): –NO₂, –C=O, –COOH, –CN — groups that pull electron density into themselves via resonance.
- +M effect (electron-donating by resonance): –OH, –OR, –NH₂, –NR₂ — groups with lone pairs that donate into the ring/π system.
| Feature | Inductive Effect | Resonance Effect |
|---|---|---|
| Bond type | σ bonds | π bonds/lone pairs |
| Range | Short (falls off rapidly) | Longer range through conjugation |
| Permanence | Always present | Only in conjugated/aromatic systems |
| Magnitude | Generally weaker | Generally stronger |
When the inductive and resonance effects work in opposite directions, resonance dominates in most cases — because resonance involves p-orbital overlap over the entire π system, a fundamentally more powerful delocalisation than through-σ-bond polarisation.
Classic example: –OH group on benzene ring
- Inductive effect of –OH: oxygen is electronegative, so it withdraws electrons through the σ bond (–I effect) → should deactivate the ring.
- Resonance effect of –OH: oxygen’s lone pair donates into the ring (+M effect) → activates the ring, directing further substitution to ortho/para positions.
The +M dominates — phenol undergoes electrophilic aromatic substitution more readily than benzene, proving resonance wins.
Another example: –COOH group
Both effects are electron-withdrawing: –I (electronegative oxygens) and –M (the C=O pulls electron density from ring). Both operate in the same direction here, making –COOH strongly deactivating and meta-directing.
Why This Works
The resonance effect is stronger because it involves delocalisation of electrons in the π system — a direct, through-space electronic interaction. The inductive effect merely polarises σ bonds sequentially and loses most of its influence within 2–3 bonds.
However, there are exceptions: when the π system is not available (no conjugation) or when the group is far from the reaction site, the inductive effect may be the dominant factor. For example, in aliphatic chain reactions, inductive effects control reactivity because there’s no ring or conjugated π system for resonance to act through.
Alternative Method
For JEE problems requiring directional prediction: first check if the group has a lone pair adjacent to the ring/π bond (resonance possible), then check electronegativity for inductive contribution. If resonance is possible, it usually dominates the orientation of electrophilic attack.
A reliable rule: groups like –NH₂, –OH, –OR are ortho/para directors (resonance donation dominates their behavior) despite having electronegative atoms that should withdraw by induction. Groups like –NO₂, –COOH, –CHO are meta directors (both inductive and resonance are withdrawing).
JEE Main and Advanced regularly test this in the context of EAS (electrophilic aromatic substitution) directing effects. The –OH group is the most common example of inductive vs. resonance conflict. In acidic/basic strength comparison questions, remember that while –OH is –I overall (due to electronegativity), it’s +M in ring systems — know the context.
Common Mistake
Students memorise “–OH is electron-withdrawing” because oxygen is electronegative (the –I effect) and apply this to benzene chemistry, predicting meta-direction for phenol. This is wrong. In the context of an aromatic ring with available p-orbitals, the +M effect of –OH wins over –I, making it ortho/para-directing. The electronegativity/induction argument only wins when there’s no conjugated path available.