1. TOPIC 7. ELIMINATION REACTIONS (Sections 7.1-16; 6.16-20)

2. OBJECTIVES
1. Describe mechanisms for elimination of a leaving group and adjacent proton to form a pi-bond.
2. Discuss the effect of starting material (“substrate”), leaving group, and reaction conditions on the course and outcome of a reaction.
3. Describe syntheses of alkenes and alkynes.

3. REVIEW OF ALKENES: STRUCTURE, NOMENCLATURE, AND STABILITY
S:7.1-4 Prob: ,28,29
Structure
Preview of Reactivity: Additions (Topic 8)

4. Systematic Nomenclature
e.g.,
Assign Cahn-Ingold-Prelog priority to substituents on each sp2 carbon
Highest priority on
same side: Z (zusammen=same)
opposite sides: E (entgegen=opposite)
Problem: Name the following compound

5. Problem: Provide IUPAC names for the following alkenes
Problem: Determine the E/Z configurations of the double bonds in each of the following alkenes
geraniol a-pinene
                                      

6. Heats of Hydrogenation C4H8 + H2 C4H10
Relative Stability
Heats of Hydrogenation
C4H8 + H C4H10
Pt or Pd
DHo H

7. Heats of Combustion C4H8 + 6O2 4CO2 + 4H2O DHo H 4CO2 + 4H2O 2719
kJ/mol
2712
kJ/mol H
2707
kJ/mol
4CO H2O

8. Overall Stability
Number of substituents  

9. Cycloalkenes cyclopropene cyclobutene cyclopentene cyclohexene
cycloheptene
cyclooctene
Chem3D strain energies: kcal/mol 27 kcal/mol

10. FORMATION OF ALKENES & ALKYNES BY ELIMINATION REACTIONS
Preview

11. AN INTRODUCTION TO ELIMINATIONS: ALKYL HALIDES

12. The E2 Reaction

13. The E1 Reaction

14. FACTORS FAVORING SUBSTITUTION AND ELIMINATION
S:6.19 Prob:6.21,23,29
Substitution and elimination mechanisms might be competitive pathways in a reaction mixture. The outcome of a reaction depends on:
i. the structure of the substrate,
ii. the basicity of the nucleophile

15. Observations

16. Summary
3° substrates only undergo substitution with weakly basic nucleophiles (ROH, H2O, RCO2H). Stronger bases promote elimination. 1° substrates generally undergo substitution unless the base itself is sterically crowded (e.g., t-BuO-).
substrate nucleophile/base Mechanism 3° 2° 1° Me
ROH, H2O, RCO2H
Anionic bases, NH3
Less basic than HO –
(e.g.., HS –, RS –, NH3, CN, RCO2–)
More basic than HO –
ROH, H2O
All except t-BuO–
t-Bu-O–
all
SN1 (some E1 on heating) E2
SN2

17. DEHYDROHALOGENATION Base-promoted E2 2° and 3°: 1°:
S:7.6 Prob:7.22a-b,23a-b 31,32,36,37
Base-promoted E2
2° and 3°:
1°:

18. Mechanism ‡

19. Regiochemistry
Zaitsev’s Rule: DG
Reaction coordinate

20. Use of bulky bases can lead to formation of “anti-Zaitsev” products

21. Stereochemistry: Anti Elimination
t-BuOK
t-BuOK
cis reacts 500 times faster than trans

22. DEHYDRATION OF ALCOHOLS
S:7.7,8 Prob:7.22c-d,23c 26,34,35,38
Acid-Promoted Elimination
Ease of dehydration: 3° > 2° >> 1°
Proceeds under relatively mild conditions
85 °C
20% aq. H2SO4
Requires relatively harsh conditions
180 °C
conc. H2SO4

23. Mechanism: Dehydration of 3° and 2° alcohols by E1

24. Stability of Carbocations
i-Pr+,
H2O
t-Bu+,
H2O H
i-PrOH, H+
t-BuOH, H+
reaction coordinate
reaction coordinate

25. Mechanism: Dehydration of 1° alcohols by E2

26. Rearrangement during elimination

27. DEBROMINATION OF VICINAL DIBROMIDES
S:7.9,10 Prob:7.22e,23b,46
Problem: How would you convert a 1,2-disubstituted alkene to an alkyne?

28. Alternate Synthesis of Alkynes: Alkylation of Acetylide Anions (see also 4.18c)
S: c
Problem: Why is this process only useful when using 1° alkyl bromides?

29. A REVIEW OF REACTIONS
You should prepare a chart of all of the types of reactions that have been covered so far…
….we’ll add more later.

30. SYNTHETIC STRATEGIES
S:4.20 Prob:7.24,27a-c
At the end of Topic 6 you were challenged to recognize one-step synthetic transformations. But not all transformations can be achieved in one step. For example, there is no method to dehydrogenate (i.e., remove H2) alkanes. So how would you bring about the following transformation?
You need to recognize that there is a single intermediate which can:
1. be made from the starting material,
and 2. be transformed into the desired product

31. This can, of course, be extended to longer sequences of reactions
This can, of course, be extended to longer sequences of reactions. How would you prepare decane from bromohexane, bromoethane and ethyne?

32. Problem: Reaction of trans-1-bromo-2-methylcyclohexane with sodium ethoxide, a non-bulky base, results in formation of 3-methylcyclohexene, the anti-Zaitzev product. Explain.

33. TOPIC 7 ON EXAM 4 Types of Questions Preparing for Exam 4:
- Describe elimination reaction mechanisms
- Predict the products of elimination reactions
- Describe some simple reactions of alkenes (hydrogenation) and alkynes (acidity, hydrogenation)
The problems in the book are good examples of the types of problems on the exam.
Preparing for Exam 4:
- Work as many problems as possible.
- Work in groups.
- Do the “Learning Group Problem” at the end of the chapter.
- Work through the practice exam