Basics of Reaction Mechanism

Mechanism

Basics of Reaction Mechanism — CHEMASH

Reaction mechanism — the step-by-step sequence of elementary reactions converting reactants to products, showing intermediates and transition states.

Why learn reaction mechanisms?
They help chemists understand how and why a reaction occurs, predict products, and control conditions to improve yield or selectivity.

Key Terminology

• Reactants: Starting substances
• Products: Substances formed
• Intermediates: Temporary species formed during the reaction
• Transition state: High-energy configuration between reactant and product
• Reaction pathway: Series of steps from reactants to products

Types of Organic Reaction Mechanisms

1. Nucleophilic Substitution (SN1 & SN2): A nucleophile replaces a leaving group
2. Electrophilic Addition: Electrophile adds to a double/triple bond
3. Elimination (E1 & E2): Removal of groups forming double/triple bonds
4. Free Radical Reactions: Involves species with unpaired electrons
5. Rearrangement Reactions: Atoms/groups shift positions in a molecule

Example: SN2 Reaction

In the reaction of CH3Br with OH-, the hydroxide ion attacks the carbon from the opposite side of the leaving group (Br-), forming CH3OH.

Key features: one-step reaction, no intermediates, backside attack leads to inversion of configuration.

Energy Profile Diagram

Energy diagrams show energetic changes during a reaction. Transition states are the highest points; intermediates appear as valleys between peaks. A multi-step reaction has multiple peaks (transition states) and valleys (intermediates). Reactants Transition State Products

Tip: Multi-step reactions may involve reactive intermediates like carbocations or carbanions.

Factors Affecting Reaction Mechanisms

• Nature of reactants: Size, charge, functional groups
• Solvent: Polar vs non-polar solvents influence ion stability
• Temperature: Affects rate and favored pathway
• Steric factors: Bulky groups can hinder attacks
• Electronic effects: Inductive and resonance effects influence reactivity

Common Intermediates

• Carbocations: Positively charged carbon species
• Carbanions: Negatively charged carbon species
• Free radicals: Neutral species with unpaired electrons
• Carbenes and nitrenes: Electron-deficient species

Applications of Reaction Mechanisms

• Designing drugs and pharmaceuticals
• Optimizing industrial synthesis
• Predicting and reducing side products
• Understanding biochemical pathways

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Quiz: Basics of Reaction Mechanism

1. What is a reaction mechanism?
2. What are intermediates?
3. Name one type of substitution reaction mechanism.
4. What does an energy profile diagram show?
5. What type of species is formed in free radical reactions?
6. What is the main difference between SN1 and SN2?
7. What factors influence a reaction mechanism?
8. What is a transition state?
9. Why is understanding mechanisms important?
10. Give one real-world application of reaction mechanisms.

Answers:

1. The detailed step-by-step process of how a reaction occurs
2. Temporary species formed during the reaction pathway
3. SN1 or SN2
4. It shows the energy changes during a reaction step
5. Free radicals (unpaired electrons)
6. SN1 is two-step with a carbocation intermediate; SN2 is one-step with backside attack
7. Nature of reactants, solvent, temperature, sterics, electronics
8. A high-energy state between reactant and product
9. To control reactions and predict products
10. Drug synthesis, polymer production, biochemical studies

Published by CHEMASH • Last updated: September 6, 2025

Factors Affecting Reaction Mechanisms

• Nature of reactants: Size, charge, functional groups
• Solvent: Polar vs non-polar solvents influence ion stability
• Temperature: Affects rate and favored pathway
• Steric factors: Bulky groups can hinder attacks
• Electronic effects: Inductive and resonance effects influence reactivity

For deeper study, check resources like Khan Academy, ChemLibreTexts.