1. Elimination reactions
Chapter 6

2. Elimination reactions
A problem that often occurs in substitution reactions is elimination

3. E1-elimination n = k [substrate]
The mechanism is similar to that of the SN1 reaction: the first step is formation of the carbocation via heterolytic cleavage

4. Substitution vs elimination
Elimination and substitution reactions are competing processes: in this case, the tertiary carbocation gives
primarily elimination in the presence of a strong base (EtO–)
mainly substitution if there is only a good nucleophile present
This can be considered as a ‘rule of thumb’

5. pKa’s revisited *The pK values are those of the conjugated acidsHI
H3O+ HF
CH3CO2H
H2S
NH4+
MeOH
HCCH
H2C=CH2
pKa
–10
–1.7
3.2
4.2
7.0
9.4
15.2 25 45
Base
CH3CO2¯
CN ¯
NH3
(CH3CH2)3N
CH3O¯
HO¯
CH3CH2O¯
(CH3)3CO¯
NH2¯
pK*
4.2
9.1
9.4
10.7
15.2
15.7
15.9 19 35
*The pK values are those of the conjugated acids
If the pK of the conjugated base is higher than the pKa of the acid deprotonation is possible (pK > 12-15: strong base)

6. The Saytzeff elimination
Saytzeff elimination: in the elimination process, formation of the most substituted double bond is favored

7. Explanation
In the transition state for elimination, the increased stability of the most substituted double bond is already felt so that there is a lower energy barrier for elimination of Ha

8. Problems
6.20: Predict the products of the E1 and SN1 reactions of the following molecules in water:

9. n = k [substrate] [base]
The E2 elimination
n = k [substrate] [base]
As in the SN2 reaction, the rate is dependent on the concentration of both reaction partners

10. E2 vs SN2
There is a strong similarity between the E2 and SN2 reactions: strong nucleophiles favor substitution, while strong bases favor elimination

11. Steric bulk favors elimination
We already saw that SN2 substitution on a tertiary carbon is not possible, therefore E2 elimination will prevail (beside E1 elimination)

12. Other examples
Note that at a primary carbon atom, only SN2 and no SN1 substitution is possible

13. Stereochemistry of the E2 rxn
Possible orientations of the proton that eliminates and the leaving group

14. E2 and the anti-conformation
The E2 elimination takes place in a single process in the anti-conformation: proton abstraction, double bond formation and cleavage of the C–L bond occur simultaneously

15. Resemblance to SN2
The mechanism is ‘more or less’ analogous to the SN2 substitution
The process is also called anti-elimination or antiperiplanar elimination

16. How to visualize this aspect ?
What are the products of E2 elimination of both diastereomers ?

17. The answer…..

18. Regiochemistry
Generally, formation of the most substituted double bond is favored
Note the occurrence of E- and Z-isomers

19. Saytzeff vs Hofmann elimination

20. Influence of the base

21. Effect of the leaving group
Especially quarternary ammonium leaving groups favor the Hofmann product

22. The E1cB elimination
The E1cB reaction resembles the SN2 reaction, with the difference that there is an anion formed prior to the loss of the leaving group

23. The three eliminations
E1cB: the proton is removed first, an anion is formed
E1: the leaving group departs first, a cation is formed
E2: all processes occur at the same time

24. Example of an E1cB reaction
This reaction is possible if there is a group present that can stabilize the negative charge

25. Stability of anions
The more substituted the carbanion, the less stable it is; this is a result of the inductively electron donating alkyl groups

26. The synthetic outlook

27. Formation of CC-bonds
Formation of CC-bonds is a synthetically important reaction

28. Summary

29. Summary (2)

30. Problems
Make problems: 6.30, 6.31, 6.32, 6.38, 6.53, 6.54, 6.56, 6.59