Resonance: delocalisation of π electrons; actual structure is hybrid of canonical forms. Electromeric effect (E): temporary effect during reaction with attacking reagent. +E: e⁻ donation toward reagent; -E: away.
-- NCERT Class 11 Chemistry, Ch. 8, p. 275Electromeric Effect
8 MCQs9-step worked example
Source: NCERT Organic Chemistry — Some Basic PrinciplesOfficial key: NTA-verifiedLast updated: 27 Sep 2026
Electromeric Effect, explained for NEET
The trap: Students confuse the electromeric effect with the inductive effect. The electromeric effect is a temporary, complete shift of a shared electron pair in a π-bond — it happens only in the presence of an attacking reagent and vanishes when the reagent is removed. The inductive effect, by contrast, is permanent and operates through σ-bonds.
What is the electromeric effect?
When an electrophile (or nucleophile) approaches a molecule containing a multiple bond, the π-electron pair shifts entirely to one of the two atoms of the bond. This temporary polarisation is the electromeric effect (NCERT Class 11 Chemistry, Chapter 8 — Organic Chemistry: Some Basic Principles and Techniques, page 277 of Part 2).
Two types exist:
- +E effect: The electron pair shifts toward the attacking reagent. Example: protonation of a C=C bond — the π-electrons move toward the carbon that bonds to H⁺.
- −E effect: The electron pair shifts away from the attacking reagent. Rarer; seen in carbonyl groups attacked by nucleophiles where electrons shift toward oxygen.
Key distinctions from inductive effect:
| Feature | Electromeric | Inductive |
|---|---|---|
| Bond type | π-bond only | σ-bond only |
| Duration | Temporary (reagent-dependent) | Permanent |
| Magnitude | Complete transfer of π-pair | Partial, weakens with distance |
| Trigger | Requires attacking species | Intrinsic to substituent |
Watch-out for NEET: A common confusion — treating inductive as more powerful or longer-range than resonance/electromeric effects. Remember: inductive operates through σ-bonds and weakens sharply; electromeric/resonance operates through the π-system and is often the dominant effect in conjugated systems.
Can you answer these Electromeric Effect MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
The electromeric effect is defined as:
Show answer and why every option is right or wrong
Answer: A. The electromeric effect is specifically the temporary, complete shift of a shared π-electron pair that occurs only when a reagent approaches (NCERT Class 11 Chemistry, Chapter 8, page 277).
Why B is wrong: B describes the inductive effect, which is permanent and operates through σ-bonds — not the electromeric effect.
Why C is wrong: C describes the resonance (mesomeric) effect — delocalisation across conjugated π-systems. The electromeric effect is localised to one π-bond.
Why D is wrong: D describes the inductive effect — partial, through σ-bonds, weakens with distance. The electromeric effect is complete and temporary.
MCQ 2Easy RecallPractice
Which statement correctly distinguishes the electromeric effect from the inductive effect?
Show answer and why every option is right or wrong
Answer: C. The electromeric effect is temporary (requires reagent) and involves π-electron shift; the inductive effect is permanent and operates through σ-bonds (electromeric: NCERT Class 11 Chemistry, Chapter 8, page 277; inductive: page 274).
Why A is wrong: A inverts the bond types — electromeric operates through π-bonds, inductive through σ-bonds.
Why B is wrong: B inverts the permanence — inductive is permanent, electromeric is temporary.
Why D is wrong: D is wrong because the electromeric effect is temporary, not permanent. They differ in bond type, duration, and completeness of electron shift.
MCQ 3Easy RecallPractice
The electromeric effect ceases when:
Show answer and why every option is right or wrong
Answer: D. By definition, the electromeric effect is induced by an approaching reagent and disappears completely when that reagent is removed (NCERT Class 11 Chemistry, Chapter 8, page 277).
Why A is wrong: A is irrelevant — the electromeric effect depends on the presence of an attacking reagent, not temperature.
Why B is wrong: B is irrelevant — substituent addition doesn't control the electromeric effect; the attacking reagent's presence does.
Why C is wrong: C is irrelevant — while solvent can influence reaction rates, the electromeric effect is specifically defined by the presence or absence of the attacking species.
MCQ 4Direct ApplicationPractice
In the +E effect during protonation of ethene (CH₂=CH₂), the π-electron pair shifts:
Show answer and why every option is right or wrong
Answer: D. In +E effect, the π-electrons shift toward the attacking electrophile (H⁺). Since one carbon forms a bond with H⁺, the π-pair moves to that carbon to facilitate bond formation (NCERT Class 11 Chemistry, Chapter 8, page 277).
Why A is wrong: A is wrong — the shift is asymmetric; electrons move to one specific carbon atom (the one bonding to H⁺), not away from both.
Why B is wrong: B is wrong — both carbons in ethene have equal electronegativity. The direction of shift is determined by the position of the attacking reagent, not electronegativity.
Why C is wrong: C is wrong — the electromeric effect is temporary; it doesn't permanently alter the σ-bond framework. When the reagent leaves, the molecule reverts.
MCQ 5Direct ApplicationPractice
A student claims that the −I effect of a chloro group and the electromeric effect of a C=O bond both operate through σ-bonds. The error in this claim is:
Show answer and why every option is right or wrong
Answer: C. The inductive (−I) effect does operate through σ-bonds — that part is correct. But the electromeric effect specifically involves the temporary shift of a π-electron pair. The student incorrectly attributed σ-bond operation to the electromeric effect (NCERT Class 11 Chemistry, Chapter 8, page 277).
Why A is wrong: A is wrong — the −I (inductive) effect genuinely operates through σ-bonds. That part of the student's claim was correct.
Why B is wrong: B is wrong — the inductive effect operates through σ-bonds. Only the electromeric effect operates through π-bonds.
Why D is wrong: D is wrong — both effects involve electron displacement; they differ in bond type (σ vs π) and permanence.
MCQ 6Direct ApplicationPractice
Which of the following is NOT a characteristic of the electromeric effect?
Show answer and why every option is right or wrong
Answer: B. The electromeric effect is temporary — it exists ONLY in the presence of an attacking reagent. Stating it is permanent contradicts its defining property (NCERT Class 11 Chemistry, Chapter 8, page 277).
Why A is wrong: A is a correct characteristic — the electromeric effect involves complete (not partial) transfer of the π-electron pair to one atom.
Why C is wrong: C is a correct characteristic — the electromeric effect requires an attacking species to manifest.
Why D is wrong: D is a correct characteristic — a π-bond (multiple bond) is necessary for the electromeric effect to occur.
MCQ 7Concept TrapPractice
When a nucleophile attacks the carbonyl group (C=O) of an aldehyde, the π-electrons shift toward oxygen. This is an example of:
Show answer and why every option is right or wrong
Answer: B. In the −E effect, the π-electron pair shifts away from the attacking reagent (the nucleophile approaches carbon, but electrons move to oxygen — away from the nucleophile). This contrasts with +E where electrons move toward the reagent (NCERT Class 11 Chemistry, Chapter 8, page 277).
Why A is wrong: A (+E effect) is wrong — in +E, the electrons move toward the attacking reagent. Here electrons move away from the nucleophile (toward oxygen), which defines −E.
Why C is wrong: C (+I effect) describes a permanent inductive electron donation through σ-bonds — irrelevant to the temporary π-electron shift in a carbonyl under nucleophilic attack.
Why D is wrong: D (hyperconjugation) involves σ-bond C-H electrons with an adjacent empty p-orbital — not a temporary π-electron shift triggered by a reagent.
MCQ 8Concept TrapPractice
Consider the following statements:
(i) Inductive effect weakens with increasing chain length.
(ii) Electromeric effect weakens with increasing chain length.
(iii) Electromeric effect requires a π-bond.
(iv) Inductive effect requires a π-bond.
Which statements are correct?
Show answer and why every option is right or wrong
Answer: A. Statement (i) is correct — inductive effect diminishes through σ-bond chain. Statement (iii) is correct — electromeric effect requires a π-bond. Statement (ii) is wrong — electromeric effect is localised to one π-bond, not a chain; "weakens with distance" applies to inductive. Statement (iv) is wrong — inductive operates through σ-bonds (NCERT Class 11 Chemistry, Chapter 8, page 274).
Why B is wrong: A includes statement (ii) which is wrong — the electromeric effect doesn't 'weaken with chain length' because it acts on a single localised π-bond, not through a chain.
Why C is wrong: C includes both (ii) and (iv), which are wrong. Electromeric doesn't weaken with chain length; inductive doesn't require π-bonds.
Why D is wrong: D includes statement (ii) which is wrong — the concept of 'weakening with distance' applies to inductive (σ-chain), not electromeric (localised π-bond shift).
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How do you solve a Electromeric Effect question? A worked example
- 1
Given
A proton (H⁺) attacks the C=C bond in propene (CH₃–CH=CH₂).
- 2
Required
Identify: (a) whether the effect is +E or −E, (b) which carbon receives the π-electron pair.
- 3
Concept
The electromeric effect is the temporary, complete transfer of a π-electron pair when an attacking reagent approaches. In +E, electrons shift toward the reagent; in −E, away from it.
- 4
Applicable principle
For an electrophilic attack on C=C: the π-electrons shift toward the carbon that forms a new bond with the electrophile. The direction follows Markovnikov's rule — H⁺ attaches to the carbon with more hydrogens, so the π-pair shifts to the other carbon.
- 5
Reasoning
H⁺ is an electrophile. It approaches the π-bond and the electron pair shifts to facilitate bond formation. Since electrons shift toward the attacking species (H⁺), this is a +E effect. The terminal CH₂ (bearing more H atoms) bonds to H⁺, so the π-electrons shift TOWARD that terminal carbon to make the new C–H bond. The internal carbon is the one left short of a pair, and that is where the secondary carbocation appears. Note the direction carefully: electrons move toward the carbon the electrophile attaches to, and the positive charge is left behind on the other one — a carbon that gains electrons cannot be the cation.
- 6
Application
• Electrophile: H⁺• π-bond shifts: electrons move toward the site of electrophilic attack• Type: +E effect (electrons move toward the reagent)• Result: CH₃–C⁺H–CH₃ (secondary carbocation) — more stable intermediate
- 7
Final answer
The protonation of propene exhibits the +E effect. The π-electron pair shifts toward the terminal carbon (which bonds to H⁺), generating a 2° carbocation at the internal carbon.
- 8
Common trap
Students confuse +E and −E by thinking about where the resulting charge appears rather than where the electrons move relative to the reagent. Remember: +E means electrons shift toward the attacking reagent.
- 9
Similar NEET-style question
"When HBr adds to but-2-ene, identify the type of electromeric effect at the C=C bond and the direction of π-electron shift."
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What to remember before solving Electromeric Effect questions
More in Organic Chemistry — Some Basic Principles: 9 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
Electromeric Effect questions from past NEET papers
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