Chemistry · General Principles Of Organic Chemistry · NEET
It is the complete transfer of a shared pair of pi electrons in a multiple bond (double or triple bond) to ONE of the two atoms, and it happens only when an attacking reagent is near. As soon as the reagent goes away, the electrons come back to normal. That is why it is called a temporary effect. It is shown by a curved arrow starting from the pi bond.
Inductive effect is PERMANENT, works through sigma bonds, and involves only a small partial shift of electrons (electrons never fully leave). Electromeric effect is TEMPORARY, works through pi bonds (needs a double/triple bond), needs an attacking reagent to appear, and involves a COMPLETE transfer of the electron pair. Also important for NEET: when the two act in opposite directions, the electromeric effect wins.
Both involve pi electrons, but resonance/mesomeric effect is PERMANENT and exists in the molecule on its own (from the ground state structure). The electromeric effect is TEMPORARY and only shows up when a reagent attacks; it vanishes when the reagent leaves. Think: resonance is always there, electromeric appears on demand.
In the +E (positive) effect, the pi electrons move TOWARD the atom to which the attacking reagent gets attached (for example, when H+ adds to C=C). In the -E (negative) effect, the pi electrons move to the atom AWAY from where the reagent attaches (for example, when a nucleophile attacks a C=O carbonyl group, electrons go onto oxygen).
Because it only happens in the presence of an attacking reagent. The reagent 'demands' electrons, so the pi pair shifts. The moment you remove the reagent, there is no demand, so the electrons return to their normal position. No reagent means no electromeric effect.
Yes. When the inductive effect and the electromeric effect pull electrons in opposite directions, the electromeric effect predominates (wins). This is a direct NEET fact from NCERT and was tested in NEET 2023.
Statement I: In an organic compound, when inductive and electromeric effects operate in opposite directions, the inductive effect predominates. Statement II: Hyperconjugation is observed in o-xylene. In the light of the above statements, choose the correct answer.
Which of the following is correct with respect to the -I effect of the substituents? (R = alkyl)
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. The electromeric effect only shows in molecules that have a multiple bond (a double or triple bond), because it involves shifting the pi electron pair. A single (sigma) bond alone cannot show it.
It is also called the polarisability effect, because the attacking reagent temporarily polarises (shifts) the pi electrons of the multiple bond.
By the letter E, and the movement of the pi electron pair is shown by a curved arrow that starts from the pi bond and points to the atom that receives the electrons.
When H+ (a proton) attacks a C=C double bond during addition, the pi electrons shift toward the carbon to which H+ gets attached. This is the positive electromeric (+E) effect.
