Overview
This lecture covers key organic reaction mechanisms in first-year A-level Chemistry: electrophilic addition, nucleophilic substitution, and elimination. It explains how to identify, draw, and understand these mechanisms, including electron movement, product prediction, and relevant reaction conditions.
Mechanism Basics
- Organic mechanisms show electron movement using curly arrows during reactions.
- Curly arrows start at lone pairs or bonds being broken and point to new bonds or atoms taking electron pairs.
- Double-headed arrows indicate movement of an electron pair; single-headed arrows (single electrons) are rarely required.
Electrophilic Addition
- Occurs with alkenes reacting with electrophiles (e.g., HBr, Br₂, concentrated H₂SO₄).
- Electrophiles are electron-seeking species attracted to the alkene's electron-rich double bond.
- The double bond attacks the electrophile, forming a carbocation intermediate.
- The stability of carbocations determines major (more stable) and minor (less stable) products.
- Tertiary > Secondary > Primary carbocations in stability; more stable carbocation gives the major product.
- Asymmetrical alkenes can yield multiple products; major and minor separated by distillation.
Nucleophilic Substitution
- Occurs when halogenoalkanes react with nucleophiles (OH⁻, CN⁻, NH₃).
- Nucleophiles are species with a lone pair, seeking positively charged (δ⁺) carbons in polar bonds.
- The nucleophile attacks the δ⁺ carbon, displacing the halide ion.
- Ammonia (NH₃) as a nucleophile requires a two-step mechanism due to the formation of a positively charged intermediate and subsequent proton removal.
- Excess ammonia favors primary amine formation and limits multiple substitutions.
Elimination Reactions
- Small molecules (HCl, HBr, H₂O) are removed from larger ones.
- For halogenoalkanes: KOH in ethanol acts as a base, forms alkenes, water, and halide ions.
- The hydroxide ion removes a hydrogen from a carbon adjacent to the halogen, double bond forms, and halide leaves.
- Multiple alkenes (isomers) may form from unsymmetrical halogenoalkanes.
- For alcohols: concentrated H₂SO₄ or Al₂O₃ catalyst removes water, forming alkenes; proceeds via protonation, water loss, and double bond formation.
Substitution vs. Elimination: Conditions
- Elimination: Hot ethanol solvent, hydroxide base, high temperature; favors tertiary halogenoalkanes.
- Substitution: Aqueous solvent, lower temperature; favored by primary halogenoalkanes.
- Both reactions can occur with the same halogenoalkane reagent, depending on conditions.
Key Terms & Definitions
- Electrophile — Species attracted to electron-rich areas, accepts electrons.
- Nucleophile — Species with a lone pair, attracted to positive (δ⁺) centers.
- Carbocation — Positively charged carbon intermediate in reactions.
- Curly Arrow — Symbol showing direction of electron pair movement.
- Isomer — Compounds with same formula but different structure.
- Dehydration — Removal of water from a molecule, often in alcohol elimination.
Action Items / Next Steps
- Practice drawing full mechanisms for each type (addition, substitution, elimination).
- Memorize carbocation stability order: tertiary > secondary > primary.
- Review reaction conditions for substitution vs. elimination.
- Complete any assigned mechanism-based exam questions for reinforcement.