Electron Displacement Effects
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Direct answer
Electrons shift, and all of organic chemistry's reactivity follows from four displacement effects. Inductive effect: permanent polarisation transmitted along sigma bonds, fading with each bond, with −I groups (NO2, CN, halogens) pulling electron density and +I groups (alkyl) pushing it. Resonance: delocalisation of pi electrons and lone pairs through conjugated systems, stabilising structures by mixing them — benzene's extra stability of about 152 kJ per mol is its resonance energy, the NCERT number. Hyperconjugation: delocalisation of a C–H sigma bond of an alpha carbon into an adjacent empty or pi orbital, the reason more alpha hydrogens mean more stability in carbocations and alkenes. Electromeric effect: a temporary, complete shift of a pi electron pair towards one atom, induced only while the attacking reagent is present — the fourth effect and the one most often confused with the first.
What you must remember
- −I order: −NR3+ > −NO2 > −CN > −SO3H > −CHO > −COOH > −F > −Cl > −Br > −I; among halogens fluorine pulls hardest despite being least polarisable.
- +I order: −C(CH3)3 > −CH(CH3)2 > −CH2CH3 > −CH3; alkyl groups and anions (−COO−, −O−) push density outwards.
- Resonance requirements: conjugation — alternating pi bonds, or a lone pair beside a pi bond; canonical structures differ only in electron positions, never in nuclear positions or total charge.
- Resonance energy benchmark: benzene is 152 kJ per mol more stable than any single Kekule structure — the quantitative line NCERT quotes.
- Stability ladders: carbocations 3° > 2° > 1° > CH3+ (inductive + hyperconjugation); free radicals follow the same order; carbanions reverse it, 3° least stable.
- Hyperconjugation counting: stability tracks the number of alpha hydrogens — nine for tert-butyl cation, three for ethyl cation, none for methyl.
- Electromeric signature: shown by a curved arrow over a double bond towards the more electronegative atom, existing only during the reagent's attack and vanishing with it.
- Acidity application: phenol (pKa near 10) beats ethanol (pKa near 16) because the phenoxide ion delocalises its negative charge over the ring.
Effect mix-ups the paper exploits
Inductive against electromeric is the perennial confusion: inductive is permanent, partial, and travels through sigma bonds; electromeric is temporary, complete, and needs an attacking reagent — the paper's assertion-reason staple. Second, treating resonance structures as real molecules flipping back and forth; they are bookkeeping forms of one hybrid, with no equilibrium between them, and the hybrid is always more stable than any contributor. Third, quoting hyperconjugation as "no-bond resonance" without the alpha-hydrogen condition — it needs C–H sigma bonds adjacent to the cationic, radical or pi centre. Fourth, the halogen paradox in −I order: fluorine shows the strongest −I yet its +M resonance donation dominates in aryl halides, directing them ortho/para — both facts are simultaneously true and separately tested.
Frequently asked questions
What is the −I strength order among halogens?
F > Cl > Br > I — electronegativity, not polarisability, sets inductive withdrawal.
Why is benzene 152 kJ per mol extra stable?
Its six pi electrons delocalise over the whole ring; the resonance hybrid sits 152 kJ per mol below any single Kekule structure.
What is the carbocation stability order and why?
3° > 2° > 1° > methyl, from combined +I electron donation and hyperconjugation by alpha C–H bonds.
How does the electromeric effect differ from the inductive?
Inductive displacement is permanent and partial along sigma bonds; electromeric displacement is complete, pi-based, and exists only while the reagent attacks.
Why is phenol a stronger acid than ethanol?
Phenoxide's negative charge delocalises over the ring through resonance; ethoxide has no such stabilisation, so ethanol holds its proton more tightly.