Quick Summary
This chapter explains why organic reactions happen — electronic effects determine reactivity, orientation, and stability of intermediates. NEET carries 2–3 questions directly on these effects and carbocation stability. Board exams test 3-mark explanations of each electronic effect.
Key Concepts and Definitions
- Homolytic fission: Bond breaks equally; each atom gets one electron → free radicals
- Heterolytic fission: Bond breaks unequally; one atom gets both electrons → ions (carbocation / carbanion)
- Carbocation: Carbon with +ve charge; sp² hybridised; planar; stabilised by +I, +M, hyperconjugation
- Carbanion: Carbon with −ve charge; sp³ hybridised; stabilised by −I, −M effects
- Free radical: Carbon with unpaired electron; sp² hybridised; stabilised by hyperconjugation
- Nucleophile: Electron-rich species; attacks positive centre (e.g., OH⁻, NH₃, CN⁻)
- Electrophile: Electron-deficient species; attacks negative centre (e.g., H⁺, NO₂⁺, BF₃)
Electronic Effects Summary
| Effect | Mechanism | Groups | Consequence |
|---|---|---|---|
| Inductive (−I) | σ bond polarisation | –F, –Cl, –NO₂, –COOH | Electron withdrawal along chain |
| Inductive (+I) | σ bond polarisation | –CH₃, –C₂H₅, alkyl | Electron donation along chain |
| Mesomeric (+M) | π/lone pair delocalisation | –OH, –NH₂, –OR | Activates ring; ortho/para direction |
| Mesomeric (−M) | π electron withdrawal | –NO₂, –CHO, –CN | Deactivates ring; meta direction |
| Hyperconjugation | σ–π conjugation | Alkyl groups (C–H) | Stabilises alkenes, carbocations |
Stability Order of Intermediates
Carbocations: 3° > 2° > 1° > CH₃⁺ Carbanions: CH₃⁻ > 1° > 2° > 3° (opposite order) Free radicals: 3° > 2° > 1° > CH₃· (same as carbocations)
Alkene stability (Saytzeff's rule): More substituted alkene is more stable due to hyperconjugation.
Types of Organic Reactions
| Type | Reagent | Example |
|---|---|---|
| Substitution | Replace one group with another | CH₄ + Cl₂ → CH₃Cl (free radical) |
| Addition | Two reactants → one product | CH₂=CH₂ + HBr → CH₃CH₂Br |
| Elimination | One reactant → two products | CH₃CH₂Br → CH₂=CH₂ + HBr |
| Rearrangement | Structural change | Carbocation rearrangement |
Important Questions
One Mark (5)
- What is the order of stability of carbocations?
- Define nucleophile.
- Which type of bond fission produces free radicals?
- Give one example of a group with +I effect.
- What is the hybridisation of carbon in a carbocation?
Short Answer 3-mark (3)
- Explain the inductive effect with −I and +I examples. How does it affect acidity?
- Define hyperconjugation. Explain why isobutene is more stable than propene.
- Distinguish between nucleophile and electrophile with two examples each.
Long Answer 5-mark (2)
- Explain mesomeric (+M and −M) effects with examples. How do they direct electrophilic substitution in benzene?
- Define and explain hyperconjugation. Use it to explain the stability order of carbocations.
Common Mistakes
- Confusing +I and +M effects — they operate through different bonds (σ vs π).
- Thinking carbocation sp² means it is linear — sp² is planar (trigonal), not linear.
- Forgetting that the inductive effect decreases with distance (beyond 3 carbons, negligible).
- Confusing carbanion stability with carbocation stability — they are opposite.
Board Exam Tips
- All three electronic effects with examples appear as 3-mark questions in board exams.
- Carbocation stability order and reasoning is a frequent NEET objective.
- Free radical stability (Cl₂ > Br₂ selectivity) links to halogenation reactions in Class 12.
- NEET: +M effect of –OH in phenol and –M effect of –NO₂ in nitrobenzene are tested.
Revision Checklist
- [ ] Homolytic vs heterolytic fission explained with arrow notation
- [ ] Inductive effect: −I and +I groups listed separately
- [ ] Mesomeric +M and −M groups listed with one example each
- [ ] Carbocation stability order written with reason (hyperconjugation)
- [ ] Nucleophile and electrophile distinguished with 3 examples each
Frequently Asked Questions
The inductive effect is the permanent displacement of electrons along a chain of carbon atoms due to an electronegative or electropositive atom or group. –I effect: electron-withdrawing (e.g., –F, –Cl); +I effect: electron-donating (e.g., –CH₃, –C₂H₅).