๐น What is a Reaction Mechanism?
A reaction mechanism is a step-by-step description of how a chemical reaction takes place. It shows how bonds are broken and formed, which intermediates are generated, and what the transition states are.
> ๐ In simple terms: It tells us how reactants convert into products on a molecular level.
---
๐ Why Study Reaction Mechanism?
To understand reaction rates and pathways
To predict products of organic reactions
To identify reactive intermediates like carbocations, carbanions, free radicals
Helpful in naming and designing reactions in synthetic organic chemistry
Essential for competitive exams like NEET, JEE, CSIR-NET
---
๐งช Basic Concepts in Reaction Mechanism
1️⃣ Types of Bond Fission
a) Homolytic Cleavage (Radical Fission)
Each atom gets one electron
Forms free radicals
Represented by: →•
Example:
Cl₂ → 2Cl•
b) Heterolytic Cleavage (Ionic Fission)
One atom gets both electrons
Forms carbocation and carbanion
Example:
CH₃Cl → CH₃⁺ + Cl⁻
---
2️⃣ Types of Reactive Intermediates
Intermediate Stability Order Example
Carbocation 3° > 2° > 1° > methyl CH₃⁺
Carbanion methyl > 1° > 2° > 3° CH₃⁻
Free Radical 3° > 2° > 1° > methyl CH₃•
Carbene Neutral (Singlet/Triplet) :CH₂
Nitrene Neutral (similar to carbenes) :NH
---
⚙️ Reaction Types Based on Mechanism
➤ 1. Substitution Reactions
a) SN1 (Unimolecular Nucleophilic Substitution)
Two-step reaction
Involves carbocation intermediate
Rate = k[alkyl halide]
Example:
(CH₃)₃CBr + OH⁻ → (CH₃)₃COH + Br⁻
b) SN2 (Bimolecular Nucleophilic Substitution)
One-step reaction (backside attack)
No intermediate, inversion of configuration
Rate = k[substrate][nucleophile]
Example:
CH₃Br + OH⁻ → CH₃OH + Br⁻
---
➤ 2. Elimination Reactions
a) E1 (Unimolecular Elimination)
Two-step: carbocation formation then elimination
Follows Zaitsev's Rule (more substituted alkene preferred)
b) E2 (Bimolecular Elimination)
One-step: base abstracts a proton while leaving group departs
---
➤ 3. Addition Reactions
Typical of alkenes and alkynes
Electrophilic addition
Example:
CH₂=CH₂ + HBr → CH₃CH₂Br
(follows Markovnikov’s rule)
---
๐ Common Named Reaction Mechanisms (Advanced Level)
Reaction Name Type Key Feature
Aldol Condensation Addition + Elimination ฮฑ-H of aldehyde/ketone
Cannizzaro Reaction Redox No ฮฑ-H in aldehyde
Friedel–Crafts Alkylation Electrophilic Substitution Aromatic systems
Wurtz Reaction Coupling Forms alkanes (R–R)
Reimer–Tiemann Substitution Phenol + CHCl₃ → Salicylaldehyde
SNAr Mechanism Aromatic Substitution Involves Meisenheimer complex
---
๐ Energy Profile of a Reaction
Reactants → Transition State → Intermediates → Products
Activation energy (Ea) must be overcome
Catalyst reduces Ea but doesn’t alter mechanism
---
๐ฌ Stereochemistry in Reaction Mechanism
SN2 shows Walden Inversion
E2 can be anti-periplanar
Optical activity affected by reaction mechanism
---
๐ก Tips to Master Reaction Mechanisms
1. Always identify the reactive site (electrophilic or nucleophilic)
2. Use curved arrows (arrow pushing) to show electron movement
3. Understand reaction conditions (acidic/basic, solvent)
4. Practice mechanism-based MCQs
5. Visualize intermediates and transition states
6. Focus on stability trends (resonance, hyperconjugation)
---
๐ Example Question
Q: What is the correct order of reactivity for SN1 mechanism?
A:
3° > 2° > 1° > CH₃⁺
Explanation: SN1 forms a carbocation intermediate; more substituted carbocations are more stable.
---
๐ Conclusion
Understanding reaction mechanisms is fundament
al in organic chemistry. It helps predict product formation, reaction rates, and stereochemistry. For NEET, JEE, or even advanced competitive exams, mastering this topic will boost your confidence and scores.
.jpg){kind=avatar}
No comments:
Post a Comment