Organic Chemistry Basics and GOC
Why This Chapter Matters
General Organic Chemistry (GOC) underpins all of JEE Organic — 8-12 marks. Electronic effects (inductive, resonance, hyperconjugation), reaction mechanisms (addition, substitution, elimination), and stereochemistry are all tested.
Core Concepts
1. Electronic Effects
Inductive Effect (+I, -I):
Through sigma bonds. Weakens with distance.
+I (electron donating): alkyl groups, -O⁻, -COO⁻. Makes compound more negative/electron-rich.
-I (electron withdrawing): -F, -Cl, -Br, -I, -CN, -NO₂, -COOH. Makes compound electron-poor.
Resonance (+M, -M):
Delocalisation of π electrons through conjugated system.
+M (electron donating to ring): -OH, -OR, -NH₂, -NR₂ (lone pair donation)
-M (electron withdrawing from ring): -NO₂, -CN, -COOH, -CHO
Hyperconjugation:
Delocalisation of σ electrons from C-H bond adjacent to π system or +ve charge.
More H atoms on carbon → more hyperconjugation → more stability.
Explains: stability of carbocations (3° > 2° > 1°), alkyl benzene stability.
2. Reactive Intermediates
Carbocation: Carbon with +charge, 3 bonds. sp² hybridised. Planar.
Stability: 3° > 2° > 1° > methyl (more alkyl groups → more electron donation)
Allyl/benzyl carbocations: stabilised by resonance (very stable)
Carbanion: Carbon with -charge, 3 bonds, 1 lone pair. sp³ hybridised.
Stability: methyl > 1° > 2° > 3° (opposite to carbocation!)
sp hybridised carbanion most stable (more s character → electrons closer to nucleus)
Free radical: Carbon with 1 unpaired electron, sp² hybridised.
Stability: 3° > 2° > 1° > methyl (same as carbocation)
3. Reaction Mechanisms
Nucleophilic Substitution:
SN2: bimolecular, one step, inversion of configuration (Walden inversion), favoured by 1° substrates
SN1: unimolecular, two steps (form carbocation, then Nu attack), racemisation, favoured by 3° substrates
Electrophilic Addition (to alkenes):
Add to π bond. Markovnikov's rule: H adds to carbon with MORE hydrogens.
Anti-Markovnikov (with peroxide/radical): H adds to carbon with FEWER hydrogens.
Elimination:
E1: similar to SN1 conditions, weak base, higher T favoured
E2: bimolecular, strong base, anti-periplanar geometry required
4. Stereochemistry
Chirality: Non-superimposable mirror images. Chiral carbon = 4 different groups.
Enantiomers: Mirror images of each other. Same physical properties except optical rotation.
Diastereomers: Stereoisomers that are NOT mirror images.
R/S configuration: based on priority rules (atomic number). CIP rules.
Optical activity: Rotates plane polarised light. (+) dextrorotatory, (-) levorotatory.
PYQs
2024: Arrange in order of decreasing stability: CH₃CH₂⁺, (CH₃)₂CH⁺, (CH₃)₃C⁺
(CH₃)₃C⁺ > (CH₃)₂CH⁺ > CH₃CH₂⁺ (3° > 2° > 1°)
2023: Which effect explains that toluene is more reactive than benzene towards electrophilic substitution?
Hyperconjugation of CH₃ group donates electrons to ring → ring more electron-rich → more reactive.
2022: Which is the correct order of acid strength: CH₄, C₂H₂, C₂H₄, NH₃?
More s character → more electronegative → better at retaining negative charge → stronger acid.
C₂H₂ (sp, 50% s) > C₂H₄ (sp², 33% s) > NH₃ > CH₄