Chemistry · General Principles Of Organic Chemistry · NEET
| Bonds involved | Inductive: sigma bonds only | Resonance: pi bonds or lone pairs in conjugation |
| Strength | Inductive: weak, dies after 3-4 bonds | Resonance: strong, reaches far positions |
| Nature | Inductive: partial (small) shift | Electromeric: complete transfer of a pi pair |
| Permanence | Inductive/Resonance/Hyperconjugation: permanent | Electromeric: temporary (needs reagent) |
| When both oppose | Inductive: loses | Electromeric: predominates |
Inductive effect is the shift of sigma (single) bond electrons because of an electronegative atom. It is weak, works through the chain of sigma bonds, and dies out after 3-4 bonds. Resonance (mesomeric) effect is the shift of pi electrons or a lone pair through a conjugated (alternate double-single bond) system. It is stronger and can pass its charge to far positions in the chain. Simple rule: inductive works on sigma bonds, resonance needs pi bonds or lone pairs in conjugation.
Only the electromeric effect (E) is temporary. It happens just when an attacking reagent comes near a double or triple bond, and it disappears the moment the reagent leaves. The other three - inductive, resonance and hyperconjugation - are permanent effects that stay in the molecule all the time, even with no reagent around. NEET loves this one line.
Hyperconjugation is a permanent effect. It is the delocalisation of the sigma electrons of a C-H bond (of an alkyl group) into an empty p orbital or a neighbouring pi system. It is also called 'no bond resonance'. Because it is always present, it explains the fixed stability order of carbocations and alkenes. Do not confuse it with the electromeric effect, which is the only temporary one.
+I effect (electron pushing) groups release electrons toward the chain: alkyl groups like -CH3, -C2H5, and -(CH3)3C. -I effect (electron pulling) groups pull electrons away: halogens (-F, -Cl, -Br, -I), -NO2, -CN, -COOH, -OH, -OR. Key NEET use: +I groups make acids weaker and bases stronger; -I groups make acids stronger and bases weaker.
Inductive effect is permanent, only a partial shift of sigma electrons (electrons are just pulled a little, not fully moved), and needs no reagent. Electromeric effect is temporary, a complete transfer of a whole pi electron pair to one atom, and happens only when an attacking reagent is present at a multiple bond. When the two effects act in opposite directions, the electromeric effect wins (predominates) - this exact point was asked 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. Choose the correct answer.
The correct order of decreasing acidity of the following aliphatic acids is:
A tertiary butyl carbocation is more stable than the secondary butyl carbocation because of which one of the following?
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Four: inductive effect (I), resonance or mesomeric effect (R/M), electromeric effect (E) and hyperconjugation. Three are permanent (I, R, H) and only the electromeric effect is temporary.
The electromeric effect. It appears only when an attacking reagent is near a double or triple bond and vanishes when the reagent is removed. All the others are permanent.
It decreases fast with distance. The inductive effect passes through sigma bonds but becomes very weak after about 3-4 bonds, so it is almost negligible far from the electronegative atom.
They decide acid strength, base strength, and the stability of carbocations, carbanions and free radicals. NEET asks acid/base order and carbocation stability almost every year, and all of these are solved using +I/-I, +R/-R and hyperconjugation.
They mean the same thing. NCERT uses both names - resonance effect and mesomeric effect (M effect) - for the polarity produced by shifting pi electrons or a lone pair through a conjugated system.
