1. Alkenes
Lecture 4

2. Isomerism 1. Structural isomers 2. Geometrical isomers
(E)-3-methylpent-2-ene
(Z)-3-methylpent-2-ene
entgegen
zusammen

3. Methods of preparation
1. Dehydrogenation of alkanes – elimination of hydrogen
the process is usually not selective
2. Dehalogenation of 1,2-dihalogenoalkanes – elimination of halogen
3. Dehydration of alcohols – elimination of water

4. Zaitsev rule (1875) - elimination predominates in the direction that leads to the more highly substituted alkene.
Aleksander Zaitsev

5. 4. From tetraalkylammonium salts (Hofmann reaction)
Hofmann rule – elimination from tetraalkylammonium salts predominates in the direction that leads to the less substituted alkene.

6. 5. Stereoselective hydrogenation of alkynes
Reduction of Lindlar catalyst (stereoselective) :
Birch reduction (stereospecific):

7. Mechanism of Birch reduction:

8. 6. Wittig reaction – reaction of an aldehyde or ketone with a triphenyl phosphonium ylide. The configuration of a double bond is predermined by the structure of the starting materials.
Ylide – neutral molecule, in which an atom with a formal negative charge (carbon) is directly bound to an atom with a formal positive charge (N, P, S).

9. Strong bases:
n-butyl lithium
phenyl lithium
sodium hydride
sodium amide
sodium bis(trimethylsylil)amide

10. Examples:

11. Examples:

12. Mechanism
Georg Wittig,

13. Industrial example– synthesis of vitamin А1 (BASF)

14. Preparation of alkenes
Dehydrohalogenation of monohalogenoderivatives – elimination of hydrohalogenides
Regioselectivity is controlled by Zaitsev rule

15. Alkene derivatives – vinyl monomers
vinylchloride
acrylic acid nitrile
acrylic acid
methyl acrylate
methyl metacrylate
PVC
PAN
PAA
PMA
PMMA
polyvinyl alcohol
vinylacetate
PVA

16. Production of methyl metacrylate

17. Chemical properties of alkenes
1. Electrophilic addition reactions AdE
1.1. Addition of halogens
Mechanism:
1) Formation of p-complex
2) Formation of s-complex – cyclic bromonium cation

18. 3) Addition of bromide-ion to s-complex
1.2. Addition of mixed halogenides

19. 1.3. Addition of hydrogen halogenides
Mechanism:
1) Formation of p-complex
2) Formation of carbomonium cation

20. Vladimir Vasilyevich Markovnikov
3) Addition of bromide-ion to carbocation
Markovnikov's rule
In the addition of acids HX to an asymmetric alkene, the acid hydrogen (H) becomes attached to the carbon with more hydrogen substituents
Vladimir Vasilyevich Markovnikov

21. 2. Radical addition reactions AdR
Mechanism:
1) chain initiation
2) chain propagation
anti-Markovnikov addition
3) formation of product

22. 3. Radical substitution reactions SR
Mechanism:
allyl chloride
1) chain initiation
2) chain propagation
3) formation of product
allyl radical
4. Hydrogenation
see preparation of alkanes

23. peroxycarboxylic acid
5. Oxidation
5.1 Epoxidation – Prilezhaev reaction
peroxycarboxylic acid
alkene oxide
epoxide
oxiranes
Nickolai Prilezhaev

24. 5. Oxidation
5.2 Hydroxylation

25. 5. Oxidation
5.3 Oxidative cleavage of alkenes
Other reagents: K2Cr2O7 (H+), CrO3
the reaction is very unselective!

26. 5. Oxidation
5.4 Ozonolysis