JEE Main 2024 · 4 April · morning · Single correct · +4 / −1
Which among the following is incorrect statement?
- (a) The electromeric effect is, temporary effect
- (b) Hydrogen ion (H⁺) shows negative electromeric effect
- (c) The organic compound shows electromeric effect in the presence of the reagent only.
- (d) Electromeric effect dominates over inductive effect
Answer and solution
Answer (b) Hydrogen ion (H⁺) shows negative electromeric effect
Solution
Chemistry ma’am · 3 steps
Ask where the π electrons go: towards H⁺, so it is a +E effect, not −E.
- What E-effect is. When a reagent attacks a multiple bond, the π electron pair shifts completely to one atom. This shift happens only while the reagent is present, and it disappears when the reagent is removed.
- Sign of the effect. If the π electrons move towards the atom the reagent attaches to, it is +E. If they move away from that atom, it is −E. An electrophile pulls the pair towards itself, so it gives +E. A nucleophile such as CN⁻ attacking C=O pushes the pair onto oxygen, so it gives −E.
Statement Check Verdict (a) Temporary effect It lasts only while the reagent is present correct (b) H⁺ shows −E In CH₂=CH₂ + H⁺, the π pair moves to the carbon that H⁺ bonds to, so it is +E incorrect (c) Needs the reagent There is no shift without an attacking reagent correct (d) Dominates inductive A full shift of the π pair outweighs a partial σ shift when the two oppose correct - So. The incorrect statement is (b). H⁺ is an electrophile, so it produces a positive electromeric effect.
The idea to keep. In the electromeric effect, the attacking reagent decides the sign. An electrophile gives +E because the π pair moves towards it. A nucleophile gives −E because the π pair moves away from where it attacks.
Answer from the NTA final answer key. Solution by PrepWiser.