Elimination and Addition Reactions
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Elimination reactions proceed by either an E1 or E2 mechanism. E1 reactions are favored with weak nucleophiles and follow Zaitsev's rule, while E2 reactions occur with strong nucleophiles and favor the formation of trans alkenes. E2 reactions follow both Zaitsev's and Hofmann's rules in determining the most stable alkene product. The choice between substitution and elimination depends on factors like the strength of the base/nucleophile and steric effects. Addition reactions allow for functionalizing alkenes and alkynes through hydration, halogenation, oxidation, hydroboration, conjugate addition and reduction reactions. Industrial processes exploit specific addition reactions, like the synthesis
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Elimination and Addition Reactions
- 1. Eliminations
- 2. Elimination ReactionsElimination Reactions E1 Reaction E2 Reaction Weak nucleophile Strong nucleophile General Mechanism General Mechanism Most commonly used bases:
- 3. E1 Reaction: Stereochemistry Substitution predominates largely And Elimination is a minor process! Zaitsev’s Rule is followed here, strictly!
- 4. E2 Reaction: Stereochemistry Syn Elimination: Uncommon Anti Elimination: Very common Example:Example: The requirement of trans relationship
- 5. E2 Reaction: Direction of Elimination Zaitsev’s rule: More substituted olefin is favored! Hofmann’s rule: Least substituted olefin is favored! Reasons: Acidity and Sterics! Steric and Electronic Factors in Eliminations: More conjugated and not More substituted!
- 8. Elimination Reactions: Synthetic Applications Dehydrohalogenations: Dehydrations: Oxidations: Loss of H2
- 9. Competition Between Elimination and Substitutions SN2 versus E2N • A good base/nucleophile (avoids SN1) favors SN2/E2 • Sterically bulky base can favor E2 (pentacoordinated TS not possible)! All SN2 or E2! Tertiary, yet E2 with a strong base/nucleophile! All SN2 or E2! • Strong base/nucleophile always promotes elimination and a weaker base/nucleophile always yields substitution products prediminantly!
- 10. Competition Between Elimination and Substitutions SN1 versus E1N CH3L: No -carbon, exellent substrate for SN2, no SN1 possible, elimination cannot occur! RCH2L: Less sterics, exellent substrate for SN2, but predominantly E2 with a strong base!2 , , p y g R2CHL: SN2 with CH3COO-, CN-, RS-, etc.) E2 with OH-, OR-, DBU, DBN, etc. In a polar solvent: SN1 and E1 in the absence of a good nucleophile or base R3CL: No SN2, no SN1 if substittuion is needed (no base!) E cellent Elimination ith a strong base b E2!Excellent Elimination with a strong base by E2!
- 12. Addition of Oxygen to Olefins: Epoxidation Dih d l ti f Ol fiDihydroxylation of Olefins: Trans‐dihydroxylation Cis‐dihydroxylation
- 13. Hydrationby Oxymercuration Hydroboration by Hydroboration Terminal Alkyne: Internal Alkyne:
- 14. Conjugate Addition: 1,2 versus 1,4 Addition
- 15. Conjugate Additions (Michael Reaction Mi h l Additi )Michael Addition)
- 17. Addition of Br2 to Triple Bonds
- 18. Reduction of Alkynes Lindlar Catalyst: Pd/CaCO3 or BaSO4 deactivated with Lead Tetraacetate and QuinolineLindlar Catalyst: Pd/CaCO3 or BaSO4 deactivated with Lead Tetraacetate and Quinoline Trans addition of H2Trans addition of H2
- 19. Industrial Synthesis of Vitamin A by Hoffmann-LaRoche Pharmaceutical Company Involves reduction (Lindlar) followed by thermal isomerization!
- 21. Simmons-Smith Reaction Carbenoid, a carbene like! Additions are stereospecific!