1. Synthesis of Alkenes Major approaches to the synthesis of alkenes:
Dehydrohalogenation of Alkyl Halides
E2 mechanism – most useful
E1 mechanism
Dehalogenation of Vicinal Dibromides
Dehydration of Alcohols

2. Synthesis of Alkenes
Dehydrohalogenation can occur via either an E2 or E1 mechanism.
Loss of H+ and X- ions from adjacent carbons, forming a new pi bond
NaOH D

3. Synthesis of Alkenes
The most synthetically useful dehydrohalogenation reactions occur under E2 reaction conditions.
3o or bulky 2o alkyl halide
strong bases
strong bulky bases are best when using 2o alkyl halides
less likely to undergo substitution reactions

4. Synthesis of Alkenes Common strong bulky bases triethylamine
diisopropylamine
t-butoxide ion
2,6-dimethylpyridine

5. Synthesis of Alkenes Mechanism of E2 Dehydrohalogenation
concerted reaction
anti-coplanar transition state

6. Synthesis of Alkenes
E2 elimination reactions can take place in cyclohexanes only when proton and leaving group can get into a trans-diaxial arrangement
corresponds to anti-coplanar

7. Synthesis of Alkenes
Strong, less hindered bases (MeO-, EtO-, etc) generally give the most substituted alkene (Saytzeff’s rule) as the major product.

8. Synthesis of Alkenes
Strong, bulky bases usually give the Hoffmann product (least highly substituted alkene) as the major product
bulky bases often abstract a proton from a less hindered carbon

9. Synthesis of Alkenes
Example: Predict all elimination product(s) of the following reactions. Which one is the major product?

10. Synthesis of Alkenes
Example: Predict all possible elimination products for the following reaction. Which one will be the major product?

11. Synthesis of Alkenes Dehalogenation of Vicinal Dibromides
two possible reagents
NaI (E2 mechanism)
Zn/HOAc (redox reaction)

12. Anti-coplanar conformation required
Synthesis of Alkenes
Dehalogenation using I- takes place via a concerted, stereospecific E2 mechanism
Anti-coplanar conformation required
Trans-diaxial conformation required for cycloalkanes

13. Synthesis of Alkenes
Example: Predict the major elimination product formed in the following reactions.

14. Synthesis of Alkenes Dehydration of Alcohols removal of water
equilibrium process
drive reaction to completion by removing alkene as formed (LeChatelier’s Principle)
H2SO4 D

15. Synthesis of Alkenes Typical reaction conditions alcohol substrate
Order of reactivity:
3o > 2o > 1o alcohol
acid catalyst
conc. H2SO4
conc. H3PO4
heat

16. Synthesis of Alkenes Mechanism of Dehydration (E1)
Step 1: Protonation of the hydroxyl group (fast)
Step 2: Ionization (RDS) +

17. Synthesis of Alkenes Step 3: Proton abstraction (fast)
Rearrangements to form more stable carbonium ions are common in dehydration reactions.
Saytzeff’s product preferred.

18. Synthesis of Alkenes
Example: Propose a mechanism for the following reaction. C

19. Synthesis of Alkenes Step 1: Protonation of OH group
Step 2: Ionization with Methyl Shift

20. Synthesis of Alkenes
Step 3: Abstraction of proton

21. Synthesis of Alkenes
Example: Predict the major product formed in the following reaction.

22. Reactions of Alkenes
The most common reactions of alkenes are addition reactions:
the addition of a reagent to the pi bond with subsequent formation of new sigma bonds
number of elements of unsaturation decreases

23. Reactions of Alkenes
The electrons in the p bond of C=C are delocalized above and below the sigma bond
more loosely held
In the presence of a strong electrophile, the double bond acts as a nucleophile, donating the p electrons to the electrophile and forming a new s bond.

24. Reactions of Alkenes
Most reactions of alkenes are electrophilic addition reactions.
Step 1: Attack of electrophile on pi bond forming a carbonium ion:
Step 2: Nucleophile attacks carbonium ion giving product.

25. Reactions of Alkenes
Addition of H-X to Alkenes

26. Reactions of Alkenes
In the previous example, the proton added to the secondary carbon, forming the most stable carbonium ion.
Markovnikov’s Rule:
Asymmetric reagents such as H-X add to a C=C so that the proton adds to the carbon (in the double bond) that already has the greater number of hydrogen atoms.
“The rich get richer”

27. Reactions of Alkenes Markovnikov’s Rule (extended):
In an electrophilic addition to an alkene, the electrophile adds in such a way as to give the most stable intermediate.

28. Reactions of Alkenes
Example: Predict the product formed in each of the following reactions.

29. Reactions of Alkenes
Anti-Markovnikov Addition of HBr
In the presence of peroxides, HBr adds to C=C via a free radical mechanism giving the “Anti-Markovnikov” product.
Works only with HBr (not HCl or HI) due to relative bond strengths.

30. Reactions of Alkenes Benzoyl peroxide Acetyl peroxide
Some common peroxides:
Benzoyl peroxide
Acetyl peroxide
Di-t-butyl peroxide
Diethyl peroxide

31. Reactions of Alkenes HBr
Example: Predict the product of the following reaction.
HBr