Chapter 10 – Principles of Organic Reaction Mechanism (ORM)
1. What is an Organic Reaction Mechanism?
An organic reaction mechanism is a **step-by-step description** of how a chemical reaction occurs,
showing:
- Movement of electrons
- Breaking and formation of bonds
- Formation of intermediates
- Energy changes during the reaction
In JEE, **mechanism > reaction**.
If you understand the mechanism, you can predict unknown reactions.
2. Electron Displacement Effects
Organic reactions are driven by **electron movement**.
There are four major electronic effects that control reactions.
3. Inductive Effect (I Effect)
Inductive effect is the **permanent displacement of σ-electrons**
along a carbon chain due to electronegativity difference.
$-I$ effect → electron withdrawing
$+I$ effect → electron releasing
$+I$ effect → electron releasing
$I$ effect decreases rapidly with distance and operates only through σ-bonds.
4. Resonance (Mesomeric Effect)
Resonance occurs when a molecule can be represented by two or more
Lewis structures differing only in electron positions.
Actual structure = Resonance hybrid
Resonance increases stability.
More resonance structures → more stability.
5. Hyperconjugation
Hyperconjugation is the interaction between σ-bond electrons
(C–H or C–C) and an adjacent empty or partially filled p-orbital.
Also called **no-bond resonance**.
Important for carbocation and alkene stability.
6. Electromeric Effect (E Effect)
Electromeric effect is a **temporary complete transfer of π-electrons**
under the influence of an attacking reagent.
$+E$ effect → π-electrons move towards attacking electrophile
$-E$ effect → π-electrons move away
$-E$ effect → π-electrons move away
7. Types of Organic Reactions
Major classes:
- Substitution reactions
- Addition reactions
- Elimination reactions
- Rearrangement reactions
8. Reaction Intermediates
Intermediates are **short-lived species** formed during reaction.
They do not appear in the final equation.
9. Carbocation
Carbocation is a positively charged carbon species:
$R_3C^+$
Stability order:
$$3^\circ > 2^\circ > 1^\circ > CH_3^+$$
10. Carbanion
$R_3C^-$
Stability order:
$$CH_3^- > 1^\circ > 2^\circ > 3^\circ$$
11. Free Radicals
$R\cdot$
Stability order similar to carbocation due to hyperconjugation.
12. Nucleophiles and Electrophiles
- Nucleophile: Electron-rich species (e.g. $OH^-$, $CN^-$)
- Electrophile: Electron-deficient species (e.g. $H^+$, $NO_2^+$)
13. Bond Fission
Two types:
- Homolytic fission → radicals
- Heterolytic fission → ions
$A-B \rightarrow A\cdot + B\cdot$
$A-B \rightarrow A^+ + B^-$
$A-B \rightarrow A^+ + B^-$
14. Energy Profile Diagram
Reaction progress is shown using energy diagrams.
- Peak → Transition state
- Valley → Intermediate
- Higher activation energy → slower reaction
15. Transition State
Transition state is the highest energy point during reaction.
It cannot be isolated.
16. Reaction Rate and Stability
More stable intermediate → faster reaction.
JEE often asks:
“Which reaction is faster and why?”
17. Hammond Postulate
Structure of transition state resembles the species
(intermediate or reactant) closer in energy.
18. Reaction Selectivity
Selectivity depends on:
- Stability of intermediate
- Electronic effects
- Steric hindrance
19. Role of Solvent
Polar solvents stabilize ionic intermediates,
non-polar solvents favor radical reactions.
20. Why ORM is the Backbone of Organic Chemistry
If you master ORM:
- You can predict products
- You can explain reaction speed
- You can handle unknown reactions
- Organic chemistry becomes logical, not memorization