Organic Reaction Mechanism 

1) Reaction Mechanism – Basic Terms

• Substrate: Part of reactant which is under attack.
• Reagent: Part of reactant which attacks the substrate.
• Mechanism: Stepwise description of bond breaking and bond formation.
• General path: Substrate + Reagent → Transition State → Intermediate → Product

2) Types of Reagents

Electrophiles

• Electron pair acceptors (Lewis acids).
• Examples: H⁺, CH₃⁺, AlCl₃, BF₃.

Nucleophiles

• Electron pair donors (Lewis bases).
• Examples: OH⁻, NH₃, H₂O, CN⁻.

Free Radicals

• Species with at least one unpaired electron.
• Examples: H·, CH₃·, Cl·.

3) Bond Cleavage (Fission)

Homolytic Fission

• Bond electrons divided equally.
• Forms two free radicals.
• Favoured by non-polar bonds, high temperature, light or radical initiators.

Heterolytic Fission

• Bond electrons taken by one atom.
• Forms ions (cation + anion).
• Favoured by polar bonds and polar solvents.

4) Electronic Effects

Inductive Effect (I)

• Permanent effect transmitted through σ-bonds.
• Decreases with distance.
• –I groups: –NO₂, –COOH, –F, –Cl, –Br, –I.
• +I groups: Alkyl groups.

Mesomeric Effect (M)

• Permanent effect due to delocalisation of π-electrons.
• +M groups: –OH, –OR, –NH₂ (o/p directing, activating).
• –M groups: –NO₂, –CN, –CHO, –COOH (meta directing).

Electromeric Effect (E)

• Temporary effect in presence of attacking reagent.
• Complete transfer of π-electrons.

5) Resonance

• Delocalisation of electrons in conjugated systems.
• Actual structure is a resonance hybrid.
• More resonance structures → greater stability.
• Bond order = (Total bonds in all structures) / (No. of structures).

Conditions for Resonance

• Same arrangement of atomic nuclei.
• Same number of paired and unpaired electrons.
• Structures must have comparable energy.
• Atoms involved must be planar and follow octet rule.

6) Hyperconjugation (No-Bond Resonance)

• Interaction of σ-bond electrons with adjacent π-system.
• Requires sp³ carbon with at least one H attached to sp² carbon.
• Explains stability of carbocations and free radicals.
• Stability ∝ number of hyperconjugative structures.

7) Organic Reaction Intermediates

Free Radicals

• Formed by homolytic cleavage.
• sp² hybridised, planar.
• Stability: Allyl ≈ Benzyl > 3° > 2° > 1° > Methyl > Vinyl.

Carbocations

• Formed by heterolytic cleavage.
• Electron deficient.
• Stability: Allyl ≈ Benzyl > 3° > 2° > 1° > Methyl.

Carbanions

• Electron-rich species.
• sp³ hybridised, pyramidal.
• Stability increases with –I groups (–CN, –NO₂).

Carbenes

• Neutral species with incomplete octet.
• Obtained from diazomethane or ketenes.
• Exist as Singlet and Triplet.

8) Types of Organic Reactions

Addition Reactions

• Electrophilic addition
• Nucleophilic addition
• Free radical addition

Elimination Reactions

• α-elimination (forms carbene)
• β-elimination (forms alkene)
• γ-elimination (forms cyclic compound)

Substitution Reactions

• SN – Nucleophilic substitution
• SR – Radical substitution
• SE – Electrophilic substitution

Other Reactions

• Condensation reactions
• Rearrangement reactions
• Polymerisation reactions