1. Chapter 8 Alkenes and Alkynes II: Addition Reactions

2. ALKENES Addition Reactions
Addition of H2
Addition of HX
Addition of ICl
Addition of H2SO4
Addition of H2O/H+
Addition of X2
Halohydrin Formation
Polymerization
Oxymercuration-Demercuration
Hydroboration Oxydation
Oxydation with OsO4 and KMnO4
Ozonolysis

3. Introduction: Additions to Alkenes
Generally the reaction is exothermic because one p and one s bond are converted to two s bonds.
E+ Nu- E Nu

4. Some Specific Addition Reactions of Alkenes

5. The p electrons of the double bond are loosely held and are a source of electron density, i.e. they are nucleophilic
Alkenes react with electrophiles such as H+ from a hydrogen halide to form a carbocation
The carbocation produced is an electrophile
It can react with a nucleophile such as a halide

6. 1- Addition of Hydrogen
Hydrogen adds to alkenes in the presence of metal catalysts
Heterogeneous catalysts: finely divided insoluble platinum, palladium or nickel catalysts
This process is called a reduction or hydrogenation
An unsaturated compound becomes a saturated (with hydrogen) compound

7. Hydrogenation: The Function of the Catalyst
The catalyst provides a new reaction pathway with lower DG‡ values

8. 2- Addition of Hydrogen Halides HX
Addition of HBr to propene occurs to give 2-bromopropane as the major product
Markovnikov’s Rule (Original): addition of HX to an alkene proceeds so that the hydrogen atom adds to the carbon that already has the most hydrogen atoms

9. Example:

10. Theoretical Explanation of Markovnikov’s Rule
The product with the more stable carbocation intermediate predominates.
The most stable carbocation is formed fastest because it has a lower ∆G‡.
The transition state for the rate determining step (first step) resembles
a carbocation and is stabilized by factors which stabilize carbocations.
Donating group

11. In addition reactions the alkene changes from a nucleophile in the first step to an electrophile in the second.

12. Mechanism for Hydrogen Halide Addition to an Alkene
Rate determining step

13. Energy diagram for the addition of HX to alkene
The reaction has two steps:
In step1: the alkene donates a pair of electrons to the proton of the hydrogen halid and forms carbocation.
This step is highly endergonic(endothermic) and has a high free energy of activation.
In step2: the highly active carbocation stabilizes itself by combining with a halide ion.
This step is highly exergonic(exothermic) and has a very low free energy of activation and takes place very rapidly.
Energy diagram for the addition of HX to alkene

15. Stability of Carbocation
Relative carbocation stability: 3° > 2° >> 1° > methyl
>
>>
> 3° 2° 1°
methyl
Most stable
Less stable
Result
The final product comes from the most stable carbocation intermediate

16. Addition of HBr to 2-methylpropene gives only tert-butyl bromide

17. Anti-Markovnikov’s Rule
An Exception of Markovnikov’s Rule
when alkenes are treated with HX (X= F, Cl, Br, I)
in the presence of peroxides,
an anti-Markovnikov addition takes place.
Markovnikov’s Rule
ROOR
Anti-Markovnikov’s Rule

18. 3- Addition of ICl to 2-methylpropene
2-chloro-1-iodo-
2-methylpropene

19. Give the structure and name of the product that would be obtained from the ionic addition of IBr to propene.
Give the structure and name of the product that would be obtained from the following addition:

20. 4- Addition of Sulfuric Acid
Addition of concentrated sulfuric acid to alkenes leads to alkyl hydrogen sulfates which are soluble in the acid.
The addition follows Markovnikov’s Rule.
The sulfate can be hydrolyzed by heating with water
The net result is Markovnikov addition of water to an alkene

21. 5- Addition of Water to Alkenes: Acid-Catalyzed Hydration
The reaction of alkenes with dilute aqueous acid leads to Markovnikov addition of water .

22. The Mechanism of Acid-Catalysed Hydration
The first step in which a carbocation is formed is rate determining.
Step 1
Formation of the most stable C+
Step 2
Formation of oxonium ion O+
Step 3
Formation of Markovnikov product

23. The hydration of alkenes and the dehydration of alcohols are simply reverse reactions of one other.
The reaction is governed by the position of all the equilibria.
Hydration is favored by addition of a small amount of acid and a large amount of water.
Dehydration is favored by concentrated acid with very little water present (removal of water produced also helps favor dehydration).
H2O/dil. H+
H-OH H OH
Alkenes
Alcohols
conc. H+

24. 6- Addition of Bromine and Chlorine to Alkenes
Addition produces vicinal dihalides (each halide next to other one).
This reaction is used as a test for alkenes because the red color of the bromine reagent disappears when an alkene (or alkyne) is present
Alkanes do not react with bromine in the dark

25. Mechanism of Halogen Addition
A bromonium ion intermediate results instead of the carbocation seen in other addition reactions

26. 7- Halohydrin Formation
If halogenation is carried out in aqueous solvent.
The main product is a halohydrin
In unsymmetrical alkenes, the bromonium ion will have some of its d+ charge density on the most substituted of the two carbons
The most substituted carbon can best accommodate d+ charge
The water nucleophile will tend to react at the carbon with the most d+ charge

27. 8- Polymerization of an Alkene

28. Hydroxy alkyl mercuric compound
9- Oxymercuration-Demercuration
Markovnikov Addition
The procedure gives high yields of alcohols and avoids rearrangements.
The reaction shows Markovnikov selectivity
Step 1: Oxymercuration
Mercuric acetate
Hydroxy alkyl mercuric compound
Step 2: Demercuration

29. Write the structure of the appropriate alkenes and specify the reagents needed for synthesis of the following alcohols by oxymercuration-demercuration

30. 10- Hydroboration-Oxidation
Anti-Markovnikov Syn Hydration
The reaction leads to anti-Markovnikov addition of water to alkenes.

31. 11- Oxidation Reaction with OsO4 or KMnO4
Syn 1,2-Dihydroxylation (cold oxidation)
Syn 1,2-Dihydroxylation is an important oxidative addition reaction of alkenes.
Strong oxidizing agents can be used such OsO4 (ozmium tetroxide) or KMnO4 (potassium permenganate).
This type of oxidation used to synthesize 1,2-diols (1,2-dihydroxylation), sometimes called (glycols).
cold

32. Mechanism for Syn Hydroxylation of Alkenes
Cyclic intermediates result from reaction of the oxidized metals.
The initial syn addition of the oxygens is preserved when the oxygen-metal bonds are cleaved and the products are syn diols
Cyclic intermediate

33. Oxidative Cleavage of Alkenes (hot oxidation)
Reaction of an alkene with hot KMnO4 results in cleavage of the double bond and formation of highly oxidized carbons.
Monosubstituted carbons become carboxylates .
Disubstituted carbons become ketones .
Unsubstituted carbons become CO2 .
This be used as a chemical test for alkenes in which the purple color of the KMnO4 disappears and forms brown MnO2 residue if alkene (or alkyne) is present.
heat
heat

34. Solved Problem 1
An unknown alkene with formula C8H16 was found on oxidation with hot basic KMnO4, to yield only the following products.
What was the structure of this alkene ?

35. Solved Problem 2
An unknown alkene with formula C7H12 yields only the following product on oxidation with hot KMnO4
Answer: Since no carbons are missing in the product, the alkene must be part of a ring in the original. molecule

36. 12- Ozonolysis of Alkenes
Cleavage of alkenes with ozone and workup with zinc in acetic acid leads to less highly oxidized carbons than products from cleavage with hot KMnO4
monosubstituted carbons are oxidized to aldehydes
disubstituted carbons are oxidized to ketones
unsubstituted carbons are oxidized to formaldehyde

37. Aldehyde and/or ketone
Mechanism of Ozonolysis of an Alkene
Initial ozonide
ozonide
(1) O3
Aldehyde and/or ketone
(2) Me2S

38. Predict the products of the following
ozonolysis reactions:

39. Give the structure of an unknown alkene with the formula C7H12 that undergoes ozonolysis to yield the following compounds:

40. Write the structures of the alkenes that would yield the following carbonyl compounds when treated with ozone and then with dimethyl sulfide:

41. Summary of the Addition Reactions of Alkenes
polymer
H2/ catalyst
H2O/H+
KMnO4 O3
polymerization HX X2
H2O/X2

42. 1- Addition of Bromine and Chlorine to Alkynes
Addition of halogen to alkynes can occur once or twice depending on how many equivalents (mol) of the halogen are added.
Addition of one equivalent usually proceeds to give the trans-dihalide.

43. ALKYNES Addition Reactions
Addition of X2
Addition of HX
Ozonolysis
Catalytic Hydrogenation

44. 1- Addition of Bromine and Chlorine to Alkynes
Addition of halogen to alkynes can occur once or twice depending on how many equivalents (mol) of the halogen are added.
Addition of one equivalent usually proceeds to give the trans-dihalide.

45. 2- Addition of Hydrogen Halides to Alkynes
Addition of hydrogen halides occurs once or twice depending on how many molar equivalent of hydrogen halide are added.
Both additions are Markovnikov and give geminal-dihalides.
HBr can be generated by reaction of acetyl bromide and alumina instead of aqueous HBr.
gem-dihalides

46. Anti-Markovnikov addition
Anti-Markovnikov addition of HBr occurs in the presence of peroxide
Anti-Markovnikov addition

47. 3-Ozonolysis of Alkynes
Reaction of alkynes with ozone or basic potassium permanganate (KMnO4/OH-) both lead to formation of
carboxylic acids.
carboxylic acids
carboxylic acids

48. Mechamism of Ozonolysis of Alkyne

49. 4- Catalytic Hydrogenation of Alkyne
Catalytic Hydrogenation of an alkyne using a typical transition-metal catalyst, such at Pt, Pd, of Ni, yields an alkane.
alkyne
alkane
Syn Addition of Hydrogen: Synthesis of cis-Alkenes
Hydrogenation of the alkene can not be prevented by using one molar equivalent of H2. To prevent hydrogenation of the alkene, a less active catalyst must be employed.  Lindlar catalyst, which is a mixture of Pd/CaCO3, and lead salts, is the catalyst of choice in catalytic hydrogenation of alkynes to give only cis- alkenes.
alkyne
cis-alkene

50. Anti Addition of Hydrogen: Synthesis of trans-Alkenes
A dissolving metal reaction which uses lithium or sodium metal in low temperature ammonia or amine solvent produces trans-alkenes
Net anti addition occurs by formal addition of hydrogen to the opposite faces of the double bond
alkyne
trans-alkene

51. Summary of the Addition Reactions of Alkynes

52. Summary of the Addition Reactions of Alkynes