1. Unit 5 - 2 Synthesis of Alkenes
4/16/2017
Synthesis of Alkenes
E2 dehydrohalogenation
Debromination of vicinal dibromides
E1 dehydrohalogenation
Acid-catalyzed dehydration of an alcohol
Dehydrogenation of alkanes
Reduction of alkynes

2. E2 Dehydrohalogenation
Unit Synthesis of Alkenes
4/16/2017
E2 Dehydrohalogenation
Most synthetically useful
One-step and requires a strong base
Best transition state is anti-coplanar.
Stereospecific
Example: t-butyl bromide + methoxide

3. E2 Dehydrohalogenation
Unit Synthesis of Alkenes
4/16/2017
E2 Dehydrohalogenation
Example: t-butyl bromide + methoxide

4. E2 Dehydrohalogenation
Works best with bulky 2° alkyl halides and 3° halides.
For 2° alkyl halides, a bulky base can
minimize the SN2 product.
give rise to the Hoffman product.
bulky bases

5. E2 Dehydrohalogenation
base not bulky
29%
71%

6. E2 Dehydrohalogenation
bulky base
72%
28%

7. E2 Dehydrohalogenation
is stereospecific.

8. E2 Dehydrohalogenation
requires trans-diaxial configuration in a cyclic alkyl halide.
When drawing this mechanism, you must show the trans-diaxial (anti) configuration.

9. E2 Dehydrohalogenation

10. Debromination of Vicinal Dibromides (a reduction)
E2 mechanism: one-step and best transition state is anti-coplanar
Stereospecific
Rarely used to make alkenes

11. Debromination of Vicinal Dibromides
NaI/acetone or Zn/acetic acid
Acetone can dissolve both the iodide and the alkyl halide (if small).
If Zn is used, reaction is heterogeneous and takes place on the surface of the Zn.
Reduction because “Br2” is removed.

12. Debromination of Vicinal Dibromides

13. E1 Dehydrohalogenation
2° or 3° alkyl halides
requires a good ionizing solvent: alcohol or water.
no strong nucleophile or base
Rearrangements can occur.
will be accompanied by SN1 products.

14. E1 Dehydrohalogenation?

15. Acid-Catalyzed Dehydration of Alcohols
Common method for making alkenes.
Reversible, water must be removed as it forms by using a dehydrating agent. Or you can distill the alkene as it is formed…it will be lower boiling than the alcohol…why?
Conc H2SO4 or conc H3PO4 act as both acid catalyst and dehydrating agent.
After protonation of the alcohol group, the reaction is E1.

16. Acid-Catalyzed Dehydration of Alcohols
Step 1: protonation of the alcohol
Fast equilibrium
Converts -OH to a good leaving group

17. Acid-Catalyzed Dehydration of Alcohols
Step 2: ionization to a carbocation
slow, rate-limiting
leaving group is H2O
+ H2O

18. Acid-Catalyzed Dehydration of Alcohols
Step 3: deprotonation to give alkene
fast
The carbocation is a strong acid: a weak base like water or bisulfate can abstract the proton.
What else forms?

19. Acid-Catalyzed Dehydration of Alcohols
Write the mechanism for the product shown.

20. Catalytic Cracking of Alkanes
Common industrial method for making small alkenes from petroleum.
Catalyst = aluminosilicates
Mixture of products makes it unsuitable for the lab.
smaller alkane
from petroleum
alkene

21. Dehydrogenation of Alkanes
Similar to catalytic cracking
Catalyst = metal such as Pt
Mixture of products makes it unsuitable for the lab.

22. Reduction of Alkynes
This will be addressed in reactions of alkynes.