1. Unit 4 - 5
4/11/2017
E2 Reactions
E2 = elimination, bimolecular
Reaction is concerted (one step)
Rate = k[substrate][base] (a second order process)
E2 can occur with SN2.
Occurs by abstraction of H+ from a C adjacent to the C with the LG.
Products follow Zaitsev’s Rule.
If the base is strong, E2 will occur instead of E1 for 3° alkyl halides.

2. E2 Reaction Mechanism One-step and requires a strong base.
Best transition state is anti-coplanar.
Example: t-butyl bromide + methoxide
Why don’t SN2, SN1, or E1 occur?

3. E2 Reaction Mechanism
Example: t-butyl bromide + methoxide

4. E2 Reaction Mechanism
Anti-periplanar alignment allows pi bond to form from sp3 orbitals.

5. E2 Reaction Profile
rate = ?
k = ?

6. Factors Affecting E2 Reactions
Structure of the substrate
Strength of the base
Nature of the leaving group
The solvent in which the reaction is run.

7. Factors Affecting E2 Reactions - Structure of the Substrate
Structure of the alkyl halide: 3° > 2° >1°
This is due to the stability of the more highly substituted alkene (Zaitsev’s Rule).

8. Factors Affecting E2 Reactions - Structure of the Substrate
Decide whether the following substrates could react by E1 or E2 (and by SN1 or SN2).

9. Factors Affecting E2 Reactions - Strength of the Base
The base must be strong. Which of the following bases favor E2 and which favor E1 reactions?

10. Factors Affecting E2 Reactions - the Leaving Group
The LG should be good. Which of the following substrates have good LGs?

11. Factors Affecting E2 Reactions - Solvent Effects
Polarity is not so important because negative charge is spread over the transition state.

12. Zaitsev’s Rule
When two or more elimination products are possible, the more stable alkene will predominate. This is usually the product with the more substituted double bond.

13. Zaitsev’s Rule Show the E2 elimination products that could come from:
Which E2 reaction would occur more quickly?

14. E2 Reactions - Summary
The structure of the substrate affects the rate.
Relative rates for E2: 3°>2°>1°.
The base must be strong.
The LG should be good.
The solvent should be polar.
Coplanar (usually anti) transition state is required.
Products will follow Zaitsev’s Rule.
Can occur with SN2 for 2° alkyl halides.

15. E2 Reactions - Summary
Predict the products

16. E2 Competes with SN2 for 2° Alkyl Halides

17. polar, aprotic is better
Unit 4 - 5
4/11/2017
E1 and E2 Reactions E1 E2
Base
weak ones work
strong
Substrate
3°>2°
3°>2°>1°
Leaving group
good
Solvent
polar, ionizing
polar, aprotic is better

18. E1 and E2 Reactions E1 E2 Rate =k[RX] =k[RX][Base]
Unit 4 - 5
4/11/2017
E1 and E2 Reactions E1 E2
Rate
=k[RX]
=k[RX][Base]
Carbocation intermediate? Y N
Stereochemistry
Zaitsev’s rule
Rearrangement
~H, ~ CH3 possible
No rearrangements

19. How Do You Decide Which Reaction(s) Happen?
The strength and structure of the nucleophile/base is paramount.
Strong ones always give a second order reaction (SN2 or E2).
If the base is bulky, E2 will occur instead of SN2.

20. How Do You Decide Which Reaction(s) Happen?
Now, look at the structure of the substrate.
1° alkyl halides will undergo SN2 and perhaps E2 unless rearrangement is possible.
3° alkyl halides will NOT undergo SN2.
2° alkyl halides are the toughest to predict.

21. How Do You Decide Which Reaction(s) Happen?
Evaluate, in order,
Nucleophile, strength and structure
-or- Base, strength and structure
Structure of the alkyl halide/carbocation
Leaving group
Solvent
YOU MUST BE ABLE TO DRAW THE MECHANISM!

22. Elimination Reactions
Predict the products

23. Did this product come from the reaction of 1-chlorobutane with a) sodium acetate or b) sodium t-butoxide?

24. This product came from the reaction of 1-chlorobutane with sodium acetate. The C=O and C-O peaks show ester.

25. Did this product come from the reaction of 1-chloropentane with a) sodium acetate or b) sodium t-butoxide?

26. This product came from the reaction of 1-chloropentane with sodium t-butoxide. The sp2C-H and C=C peaks show alkene.