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

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!

7. Summary of Eliminations

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!

11. Addition Reactions

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)

16. Bromination of Olefins

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!

20. Cyclopropane Synthesis
Addition is
Stereospecific!

21. Simmons-Smith Reaction
Carbenoid, a carbene like!
Additions are stereospecific!

22. Radical Additions