1. 7. Alkenes: Reactions and Synthesis

2. Diverse Reactions of Alkenes
Alkenes react with many electrophiles to give useful products by addition (often through special reagents)
alcohols (add H-OH)
alkanes (add H-H)
halohydrins (add HO-X)
Dihalides or dihaloalkanes (add X-X)
halides or haloalkanes(add H-X)
diols (add HO-OH)
cyclopropane (add :CH2)

3. 7.1 Preparation of Alkenes: A Preview of Elimination Reactions
Alkenes are commonly made by
elimination of HX from alkyl halide (dehydrohalogenation)
Uses heat and KOH
elimination of H-OH from an alcohol (dehydration)
require strong acids (sulfuric acid, 50 ºC)

4. Saytzeff Rule or Zaitsev Rule: In eliminations (dehydration or dehydrohalgenation) when a mixture of product alkenes can form, the major product will be the more substituted alkene: discussed again in chapter 11.

5. Addition of Halogens to Alkenes
Bromine and chlorine add to alkenes to give 1,2-dihalides, an industrially important process. The bond strengths in the diatomic halogens are relatively small.
F2 is too reactive and reacts explosively. The addition I2 is thermodynamically unfavorable and does not occur
Cl2 reacts as Cl+ Cl-
Br2 is similar
The solvent used is an inert solvent such as CH2Cl2 which dissolves the reactants but is not a reactant.

6. Addition of Br2 (Cl2)to Cyclopentene
Nucleophile: alkene; Electrophile: One of the Br of Br2(Cl of Cl2)
The other Br (Cl) leaves as Br- (Cl-)
Nucleophile: a lone pair on the attached Br (Cl); electrophilic: carbocation
An unstable cyclic bromonium ion(chloronium) is formed.
Nucleophile: Br- (Cl-); electrophile: more substituted C of double bond
Addition is exclusively anti: stereospecific +

7. Bromonium Ion Mechanism
Electrophilic addition of bromine to give a cation is followed by cyclization to give a bromonium ion
This bromoniun ion is a reactive electrophile and bromide ion is a good nucleophile

8. Halohydrin Formation
When the addition of halogens to alkene is carried out in the presence of water as solvent rather than the inert CH2Cl2, the halohydrin forms.
This is formally the addition of HO-X to an alkene (with +OH as the electrophile) to give a 1,2-halo alcohol, called a halohydrin
Nucleophile: alkene; Electrophile: One of the Br of Br2(Cl of Cl2)
The other Br (Cl) leaves as Br- (Cl-)
Nucleophile: a lone pair on the attached Br (Cl); electrophilic: carbocation
An unstable cyclic bromonium ion(chloronium) is formed.
Nucleophile: H2O; electrophile: more substituted C of double bond
Addition is exclusively anti: stereospecific

9. Mechanism of Formation of a Bromohydrin
Br2 forms bromonium ion, then water adds
Orientation toward stable C+ species
Aromatic rings do not react

10. An Alternative to Bromine
Bromine is a difficult reagent to use for this reaction
N-Bromosuccinimide (NBS) produces bromine in organic solvents and is a safer source

11. Acid-catalysed Addition of Water to Alkenes: Markovnikov product
Hydration of an alkene is the addition of H-OH to to give an alcohol
Acid catalysts are used in high temperature industrial processes: ethylene is converted to ethanol

12. Addition of Water to Alkenes by Oxymercuration-Reduction: Markovnikov product
For laboratory-scale hydration of an alkene
Use mercuric acetate in THF followed by sodium borohydride
Nucleophile: pi electrons of double bond; electrophile: Mercury(II)cation of mercury(II)acetate
A cyclic mercurinium ion is formed; No carbocation intermediate formed
Nucleophile: water; electrophile: more substituted C of double bond; Markovnikov orientation
NaBH4 converts the C-Hg bond to a C-H bond;
Solvent: water and THF

13. Addition of Water to Alkenes: Hydroboration-Oxidation: non-Markovnikov regiochemistry; syn stereochemistry
Herbert Brown invented hydroboration (HB)
Borane (BH3) is electron deficient is a Lewis acid
Nucleophile: pi electrons of double bond; electrophile: borane
H adds to more substituted C of double bond; BH2 to less substituted C
Borane adds to an alkene to give an organoborane; solvent is THF
Oxidation of the organoborane by hydrogen peroxide in basic medium( H2O2 in presence of NaOH) yields the alcohol
In the oxidation step, The C-B is replaced by C-OH such that both the added groups (H from the hydroboration step and the OH from the oxidation step) add to the same face: syn addition

14. BH3 Is a Lewis Acid. It has only 6 electrons
BH3 Is a Lewis Acid. It has only 6 electrons. It forms a coordinate bond with a lone pair on Oxygen in THF. In hydroboration, it coordinates with the pi electrons of the double bond Although it attaches itself so that a positive charge can form at the more substituted C atom of the double bond, there is no carbocation intermediate in the reaction. In a concerted manner the more substituted C becomes more electron-deficient and the B as it gains the pi electrons becomes more ready to allow the transfer one of the H attached to it to the C. The attachment of the BH3 to the less substituted C is motivated both by the stability of the incipient carbocation and steric factor.

15. Hydroboration-Oxidation Forms an Alcohol from an Alkene
Addition of H-BH2 (from BH3-THF complex) to three alkenes gives a trialkylborane
Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene

16. Orientation in Hydration via Hydroboration
Regiochemistry is opposite to Markovnikov orientation
OH is added to carbon with most H’s
H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)

17. Mechanism of Hydroboration
Borane is a Lewis acid
Alkene is Lewis base
Transition state involves anionic development on B
The components of BH3 are across C=C

18. Hydroboration: Orientation in Addition Step
Addition in least crowded orientation, syn
Addition also is via most stable carbocation

19. Hydroboration, Electronic Effects Give Non-Markovnikov
More stable carbocation is also consistent with steric preferences

20. Hydroboration - Oxygen Insertion Step
H2O2, OH- inserts OH in place of B
Retains syn orientation

21. Summary for hydration of Alkenes:
Acid-catalyzed hydration: Advantages: cheaper chemicals; disadvantages: carbocation intermediate in mechanism: carbocations can rearrange which can result in a mixture of products. This method is regiospecific (Markovnikov addition: H adds to less substituted C of double bond; OH adds to the more substituted C of the double bond). But the addition is not stereospecific as the mechanism involves the addition of the OH (from H2O) to a trigonal planar carbocation. Reagents/solvent: alkene, H3PO4/H2O
Oxymercuration-demercuration: regiospecific (Markovnikov addition: H adds to less substituted C of double bond; OH adds to the more substituted C of the double bond). No carbocation intermediate; intermediate is cyclic Mercurium ion and hence no rearrangements. Alkene, Hg(OAc)2/THF, H2O; NaBH4/H2O
Hydroboration-Oxidation: regiospecific (nonMarkovnikov addition: H adds to more substituted C of double bond; OH adds to the less substituted C of the double bond). No carbocation intermediate ion and hence no rearrangements. Stereospecific syn addition. Alkene, BH3/THF; H2O2, NaOH

22. 7.6 Addition of Carbenes to Alkenes
The carbene functional group is “half of an alkene”
Carbenes are electrically neutral with six electrons in the outer shell
They symmetrically across double bonds to form cyclopropanes

23. Formation of Dichlorocarbene
Base such as t-butoxide removes proton from chloroform
Stabilized carbanion remains
Unimolecular Elimination of Cl- gives electron deficient species, dichlorocarbene
The addition of the carbene is stereospecific: syn addition

24. Simmons-Smith Reaction
Equivalent of addition of CH2:
Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species
Forms cyclopropanes by cycloaddition

25. Reaction of Dichlorocarbene
Addition of dichlorocarbene is stereospecific cis

26. 7.7 Reduction of Alkenes: Hydrogenation
Addition of H-H across C=C
Reduction in general is addition of H2 or its equivalent
Requires Pt or Pd as powders on carbon and H2
Hydrogen is first adsorbed on catalyst
Reaction is heterogeneous (process is not in solution)

27. Hydrogen Addition- Selectivity
Selective for C=C. No reaction with C=O, C=N
Polyunsaturated liquid oils become solids
If one side is blocked, hydrogen adds to other

28. Mechanism of Catalytic Hydrogenation
Heterogeneous – reaction between phases
Addition of H-H is syn

29. 7.8 Oxidation of Alkenes: Hydroxylation and Cleavage
Hydroxylation adds OH to each end of C=C
Catalyzed by osmium tetroxide
Stereochemistry of addition is syn
Product is a 1,2-dialcohol or diol (also called a glycol)

30. Osmium Tetroxide Catalyzed Formation of Diols
Hydroxylation - converts to syn-diol
Osmium tetroxide, then sodium bisulfate
Via cyclic osmate di-ester

31. Summary of Hydroxylation Reactions:
Syn Addition to create the syn-diol: 2 methods:
Osmium tetroxide catalysed reaction & Potassium permanganate oxidation under alkaline conditions , in the presence of OH-(look at handout for mechanism)
2. Anti addition to form the anti-diol: epoxidation; Reagents: peroxy acid (peroxy benzoic acid; acidic medium, H2O)

32. Alkene Cleavage: Ozone
Ozone, O3, adds to alkenes to form molozonide
Reduce molozonide to obtain ketones and/or aldehydes

33. Examples of Ozonolysis of Alkenes
Used in determination of structure of an unknown alkene

34. Structure Elucidation With Ozone
Cleavage products reveal an alkene’s structure

35. Permangante Oxidation of Alkenes
Oxidizing reagents other than ozone also cleave alkenes
Potassium permanganate (KMnO4) can produce carboxylic acids and carbon dioxide if H’s are present on C=C

36. Cleavage of 1,2-diols
Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compinds
Sequence of diol formation with OsO4 followed by diol cleavage is a good alternative to ozonolysis

37. 7.10 Addition of Radicals to Alkenes: Polymers
A polymer is a very large molecule consisting of repeating units of simpler molecules, formed by polymerization
Alkenes react with radical catalysts to undergo radical polymerization
Ethylene is polymerized to poyethylene, for example

38. Free Radical Polymerization of Alkenes
Alkenes combine many times to give polymer
Reactivity induced by formation of free radicals

39. Free Radical Polymerization: Initiation
Initiation - a few radicals are generated by the reaction of a molecule that readily forms radicals from a nonradical molecule
A bond is broken homolytically

40. Polymerization: Propagation
Radical from intiation adds to alkene to generate alkene derived radical
This radical adds to another alkene, and so on many times

41. Polymerization: Termination
Chain propagation ends when two radical chains combine
Not controlled specifically but affected by reactivity and concentration

42. Other Polymers
Other alkenes give other common polymers

43. Unsymmetrical Monomers
If alkene is unsymmetrical, reaction is via more highly substituted radical

44. Chain Branching During Polymerization
During radical propagation chain can develop forks leading to branching
One mechanism of branching is short chain branching in which an internal hydrogen is abstracted

45. Cationic Polymerization
Vinyl monomers react with Brønsted or Lewis acid to produce a reactive carbocation that adds to alkenes and propagates via lengthening carbocations

46. Calculate degree of unsaturation; If this number is ≥ 4 one good guess is the presence of a benzene ring in the structure; One benzene ring has a degree of unsaturation = 4 (1 ring + 3 double bonds)
See if the structure if you have guessed is consistent with the products specified for the given reactions.