Types of Organic Reactions

Introduction

Understanding reaction mechanisms is the ultimate skill in organic chemistry! Instead of memorizing thousands of reactions, you learn the patterns - how and why reactions happen. This is what separates JEE toppers from average students.

Classification of Organic Reactions

graph TD
    A[Organic Reactions] --> B[By Bond Change]
    A --> C[By Reagent Type]
    B --> B1[Substitution]
    B --> B2[Addition]
    B --> B3[Elimination]
    B --> B4[Rearrangement]
    C --> C1[Nucleophilic]
    C --> C2[Electrophilic]
    C --> C3[Free Radical]

By mechanism:

1. Substitution: One group replaces another
2. Addition: Two groups add across multiple bond
3. Elimination: Two groups leave, forming multiple bond
4. Rearrangement: Atoms rearrange within molecule

By reagent:

1. Nucleophilic: Nucleophile attacks
2. Electrophilic: Electrophile attacks
3. Free Radical: Radical chain reaction

Substitution Reactions

General: One atom/group replaced by another

Nucleophilic Substitution

Nucleophile (Nu⁻): Electron-rich species, attacks electron-poor carbon.

Two mechanisms: SN1 and SN2

SN1: Unimolecular Nucleophilic Substitution

“SN1” = Substitution, Nucleophilic, 1st order

Mechanism:

Step 1: Ionization (slow, rate-determining)

Step 2: Nucleophilic attack (fast)

Example: tert-Butyl bromide + OH⁻

Step 1 (slow):
(CH₃)₃C-Br → (CH₃)₃C⁺ + Br⁻
(Carbocation formation)

Step 2 (fast):
(CH₃)₃C⁺ + OH⁻ → (CH₃)₃C-OH

Characteristics:

Why racemization?

• Carbocation is planar (sp²)
• Nucleophile attacks from both sides equally
• 50% inversion + 50% retention = racemic mixture

Substrate reactivity:

Reason: Stability of carbocation intermediate

Solvent: Polar protic (H₂O, ROH)

• Stabilizes carbocation and leaving group
• Doesn’t need to stabilize nucleophile

SN2: Bimolecular Nucleophilic Substitution

“SN2” = Substitution, Nucleophilic, 2nd order

Mechanism:

One-step: Nucleophile attacks as leaving group departs (concerted)

Example: CH₃Br + OH⁻

HO⁻ + CH₃-Br → [HO···CH₃···Br]⁻ → HO-CH₃ + Br⁻

Backside attack
Transition state (not intermediate!)
Inversion of configuration

Characteristics:

Why inversion?

• Backside attack (SN2 mechanism)
• Nucleophile approaches from side opposite to leaving group
• Like umbrella flipping in wind

Substrate reactivity:

Reason: Steric hindrance - nucleophile needs access to carbon

Solvent: Polar aprotic (DMSO, DMF, acetone)

• Doesn’t solvate nucleophile
• Keeps Nu⁻ “naked” and reactive

Electrophilic Substitution

Electrophile (E⁺): Electron-poor species, attacks electron-rich site.

Most important: Electrophilic Aromatic Substitution (EAS)

Mechanism of EAS (General)

Step 1: Generation of electrophile Step 2: Electrophile attacks benzene Step 3: Deprotonation restores aromaticity

Example: Nitration

Step 1: Electrophile generation
HNO₃ + H₂SO₄ → NO₂⁺ + HSO₄⁻ + H₂O
(Nitronium ion)

Step 2: Attack on benzene
C₆H₆ + NO₂⁺ → C₆H₆NO₂⁺ (carbocation, arenium ion)

Step 3: Deprotonation
C₆H₆NO₂⁺ → C₆H₅NO₂ + H⁺
(Nitrobenzene)

Common EAS Reactions:

Addition Reactions

General: Two groups add across multiple bond (C=C or C≡C)

Electrophilic Addition

Most common for alkenes and alkynes

Mechanism

Step 1: Electrophile attacks π-bond, forms carbocation Step 2: Nucleophile attacks carbocation

Example: CH₂=CH₂ + HBr

Step 1:
CH₂=CH₂ + H⁺ → CH₃-CH₂⁺
(More stable carbocation)

Step 2:
CH₃-CH₂⁺ + Br⁻ → CH₃-CH₂-Br

Markovnikov’s Rule

“Rich get richer” - H adds to carbon with MORE hydrogens.

Example: Propene + HCl

CH₃-CH=CH₂ + HCl

Could form:
CH₃-CHCl-CH₃  (2° carbocation, Markovnikov)  ← Major
CH₃-CH₂-CH₂Cl (1° carbocation)               ← Minor

Markovnikov product: CH₃-CHCl-CH₃

Reason: Electrophile adds to form more stable carbocation (2° > 1°)

Anti-Markovnikov Addition

With peroxides (free radical mechanism) - ONLY for HBr!

Example: Propene + HBr (peroxide)

CH₃-CH=CH₂ + HBr --ROOR--> CH₃-CH₂-CH₂-Br

Anti-Markovnikov product!

Mechanism:

Initiation:
ROOR → 2RO·
RO· + HBr → ROH + Br·

Propagation:
Br· + CH₃-CH=CH₂ → CH₃-CH·-CH₂-Br (2° radical, stable!)
CH₃-CH·-CH₂-Br + HBr → CH₃-CH₂-CH₂-Br + Br·

Why only HBr?

• HCl: H-Cl too strong, won’t abstract
• HI: H-I too weak, reaction reverses

Addition Reactions Summary

Elimination Reactions

General: Two groups leave, forming multiple bond (opposite of addition)

Most common: Dehydrohalogenation (removing HX)

E1: Unimolecular Elimination

Similar to SN1 - goes through carbocation

Mechanism:

Step 1: Ionization (slow)

Step 2: Deprotonation (fast)

Example: tert-Butyl bromide + OH⁻ (heat)

Step 1:
(CH₃)₃C-Br → (CH₃)₃C⁺ + Br⁻

Step 2:
(CH₃)₃C⁺ → (CH₃)₂C=CH₂ + H⁺

Characteristics:

E2: Bimolecular Elimination

Similar to SN2 - concerted, one-step

Mechanism:

One-step: Base removes H as leaving group departs (anti-periplanar)

Example: Ethyl bromide + KOH (alcohol, heat)

          H
          |
KOH + CH₃-CH-Br → CH₂=CH₂ + H₂O + KBr
       (Anti-periplanar geometry required)

Characteristics:

Zaitsev’s Rule

“More substituted alkene is more stable” → Major product

Example: 2-Bromobutane + KOH

    CH₃
     |
CH₃-CH-CH₂-CH₃ + KOH
     |
     Br

Could form:
CH₃-CH=CH-CH₃   (Trisubstituted) ← Zaitsev (Major, 80%)
CH₂=CH-CH₂-CH₃  (Disubstituted)  ← Hofmann (Minor, 20%)

Zaitsev product: More substituted, more stable (hyperconjugation!)

Hofmann Rule

With bulky bases (like (CH₃)₃CO⁻) → Less substituted alkene

Reason: Steric hindrance - base attacks less hindered H

Rearrangement Reactions

Atoms/groups move within molecule to form more stable structure.

Types

1. Wagner-Meerwein Rearrangement

Carbocation rearrangement - hydride or alkyl shift

Example: Pinacol-Pinacolone Rearrangement

  CH₃   CH₃              CH₃   CH₃
   |     |                |     |
  C  -  C      H⁺        C  =  O
  |     |      →         |
 OH    OH               CH₃

Pinacol             Pinacolone

Mechanism:

1. Protonation of OH
2. Loss of H₂O → carbocation
3. Methyl shift → more stable carbocation
4. Deprotonation → ketone

2. Beckmann Rearrangement

Oxime → Amide (used in nylon production!)

R-C=N-OH  → R-NH-CO-R'
   |
   R'

3. Benzilic Acid Rearrangement

α-Diketone → α-Hydroxy acid

Free Radical Reactions

Chain mechanism: Initiation → Propagation → Termination

Halogenation of Alkanes

Example: CH₄ + Cl₂ –hν–> CH₃Cl + HCl

Mechanism:

Initiation:

Propagation:

Termination:

Reactivity: F₂ > Cl₂ > Br₂ > I₂

Selectivity: I₂ > Br₂ > Cl₂ > F₂ (opposite!)

Why?

• F₂ too reactive → no selectivity
• I₂ too slow → very selective

Selectivity in alkanes:

Summary Table

Practice Problems

Level 1: Identification

1. Identify the mechanism (SN1, SN2, E1, E2):

   • a) (CH₃)₃C-Br + OH⁻ (aqueous, room temp)
   • b) CH₃-Br + OH⁻ (acetone)
   • c) (CH₃)₃C-Br + (CH₃)₃CO⁻ (heat)

2. Predict major product:

   • a) CH₃CH=CH₂ + HBr
   • b) CH₃CH=CH₂ + HBr (peroxide)

3. Which gives faster SN2? CH₃Br, CH₃CH₂Br, (CH₃)₂CHBr, (CH₃)₃CBr

Level 2: Application

4. Explain why:

   • SN2 doesn’t work with 3° substrates
   • E2 requires anti-periplanar geometry
   • Carbocation rearrangements occur in SN1 but not SN2

5. Draw mechanism:

   • a) 2-Bromo-2-methylpropane + H₂O → tert-Butanol
   • b) Bromomethane + OH⁻ → Methanol

6. Predict product with stereochemistry: (R)-2-Bromobutane + OH⁻ (SN2) → ?

Level 3: JEE Advanced

7. Which reaction is fastest?

   • (a) (CH₃)₃C-Br + H₂O (SN1)
   • (b) CH₃-Br + OH⁻ (SN2)
   • (c) (CH₃)₂CH-Br + OH⁻ (SN2)
   • (d) Cannot compare (different mechanisms)

8. What happens when (S)-2-bromobutane undergoes SN2 with CN⁻?

   • (a) (S)-2-cyanobutane
   • (b) (R)-2-cyanobutane
   • (c) Racemic mixture
   • (d) Achiral product

9. Assertion (A): E2 elimination requires anti-periplanar geometry. Reason (R): This allows maximum orbital overlap during elimination.

   • (a) Both true, R explains A
   • (b) Both true, R doesn’t explain A
   • (c) A true, R false
   • (d) Both false

10. In the reaction CH₃-CH(Br)-CH₂-CH₃ + KOH (alc., heat), major product is:

    • (a) CH₂=CH-CH₂-CH₃
    • (b) CH₃-CH=CH-CH₃
    • (c) CH₃-CH₂-CH=CH₂
    • (d) CH₃-CH₂-CH₂-CH₂-OH

Memory Tricks

“1 vs 2” - The Ultimate Rule

“1” reactions:

• One molecule in rate equation
• One step is slow (first step)
• Best with 3° (most stable carbocation)
• Racemization (SN1) or Zaitsev (E1)

“2” reactions:

• Two molecules in rate
• Two species in transition state
• Best with 1° (least hindered)
• Inversion (SN2) or anti geometry (E2)

SN vs E Decision Tree

Is substrate 1°?
├─ Yes → SN2 (good Nu⁻, no heat) or E2 (strong base, heat)
└─ No (3°) → SN1 (weak Nu⁻) or E1/E2 (heat, base)

Heat + Strong Base = Elimination
Good Nu⁻, No heat = Substitution

Interactive Demo: Visualize Reaction Mechanisms

Watch SN1, SN2, E1, and E2 mechanisms unfold with animated electron movement.

Markovnikov

“Rich Get Richer”

• H adds to carbon with MORE H atoms
• Forms more stable carbocation

Related Topics

Within Organic Principles

• Hybridization - sp² in carbocations
• Electronic Effects - Inductive and resonance effects on reactivity
• Reaction Intermediates - Carbocations in SN1, E1
• Isomerism Types - Stereochemistry in SN2

Halogen Compounds (Key Applications)

• SN1 and SN2 Reactions - Detailed mechanisms
• Elimination Reactions - E1 and E2 details
• Alkyl Halides - Substrate reactivity
• Haloarenes - Aromatic substitution

Hydrocarbon Reactions

• Alkenes - Addition reactions, Markovnikov
• Alkynes - Addition across triple bonds
• Benzene - Electrophilic aromatic substitution
• Directive Effects - Ortho-para and meta directors

Oxygen Compounds

• Alcohols - Dehydration (E1), substitution
• Ethers - Williamson synthesis (SN2)
• Aldehydes and Ketones - Nucleophilic addition

Nitrogen Compounds

• Amines - Nucleophilic reactions
• Diazonium Salts - Substitution reactions

Foundation Topics

• Chemical Equilibrium - Reaction reversibility
• Chemical Kinetics - Rate equations for SN1, SN2