Fundamental Concepts in Organic Reaction Mechanism

⚡ Fission of Covalent Bonds

When bonds break, it happens in 2 ways:

  • Heterolytic cleavage: Both electrons go to one atom.
    • Forms charged particles: carbocations (+) or carbanions (-).
    • Carbocation stability order: Tertiary > Secondary > Primary > Methyl \[ (\ce{CH3})3\overset{+}{\ce{C}} > (\ce{CH3})2\overset{+}{\ce{CH}} > \ce{CH3CH2+} > \ce{CH3+} \]
    • Example: \[ \ce{CH3-Br -> CH3+ + Br-} \]
  • Homolytic cleavage: Each atom gets 1 electron.
    • Forms free radicals (neutral, unpaired e⁻).
    • Radical stability: Tertiary > Secondary > Primary > Methyl

🔬 Substrate vs. Reagent

  • Substrate: Organic molecule being attacked (supplies carbon for new bond).
  • Reagent: Attacking molecule.
  • Example: \[ \ce{CH2=CH2 + Br2 -> BrCH2-CH2Br} \] (Ethene = substrate, Bromine = reagent)

🎯 Nucleophiles & Electrophiles

  • Nucleophile (Nu:): Electron-rich, attacks positive sites.
    • Examples: \(\ce{HO-}\), \(\ce{CN-}\), \(\ce{H2O}\), \(\ce{NH3}\)
  • Electrophile (E+): Electron-deficient, attacks negative sites.
    • Examples: \(\ce{CH3+}\), \(\ce{BF3}\), \(\ce{CH3COCH3}\) (carbonyl carbon)

Electrophilic centers in: \(\ce{CH3CHO}\) (C in CHO), \(\ce{CH3CN}\) (C in CN), \(\ce{CH3I}\) (C bonded to I)

➡️ Electron Movement

Shown with curved arrows:

  • Full arrow: Movement of electron pair.
  • Half-headed arrow: Movement of single electron.

Example: \[ \ce{HO- + CH3Cl -> HO-CH3 + Cl-} \]

⚡ Electron Displacement Effects

  • Inductive Effect: Permanent bond polarization.
    • \(\ce{-NO2}\), \(\ce{-CN}\) = electron-withdrawing.
    • \(\ce{-CH3}\) = electron-donating.
    • Weakens over distance: \(\ce{CH3-CH2-CH2->Cl}\) (least effect on C3-H).
  • Resonance Effect: Delocalization of π-electrons.
    • +R groups donate electrons (\(\ce{-OH}\), \(\ce{-NH2}\)).
    • -R groups withdraw electrons (\(\ce{-NO2}\), \(\ce{-CHO}\)).
    • Example: Benzene’s true structure is a hybrid of two resonance forms.
  • Electromeric Effect (E): Temporary polarization during attack.
    • +E: Electrons move toward attacking reagent.
    • -E: Electrons move away from attacking reagent.
  • Hyperconjugation: Stabilization via σ-bond delocalization.
    • Explains carbocation stability: More alkyl groups = more stabilization.
    • \(\ce{(CH3)3C+ > CH3CH2+ > CH3+}\) (9 C-H bonds > 3 > 0).

🧪 Types of Organic Reactions

  1. Substitution
  2. Addition
  3. Elimination
  4. Rearrangement

🔍 Methods to Purify Compounds

  1. Sublimation
  2. Crystallization
  3. Distillation
  4. Chromatography

Purity check: Sharp melting/boiling points.

🚀 NEET Super-Ready Concepts

  1. Carbocation Stability: Hyperconjugation & inductive effects.
  2. Resonance Hybrids: Real structure ≠ single Lewis structure (e.g., benzene).
  3. Nucleophile/Electrophile ID: Spot Nu: (e⁻-rich) and E+ (e⁻-deficient) in reactions.
  4. Inductive Effect: Predict bond polarity and group influence.
  5. Curved Arrows: Track e⁻ flow in mechanisms (polar reactions).