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Home , Ozonolysis, Syn and anti addition

The Reactions of

The of Addition Reactions

1 Diverse Reactions of Alkenes Alkenes react with many electrophiles to give useful products by addition (often through special reagents)

2 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 (dehydration) • requires strong acids (sulfuric acid, 50 ºC)

3 A Regioselective Reaction

A reaction in which one structural isomer is favored over another, leading to its predominance in the mixture of products.

4 A Stereoselective Reaction

A reaction in which one stereoisomer in a mixture is produced more rapidly than another, resulting in predominance of the favored stereoisomer in the mixture of products.

5 A Stereospecific Reaction

A reaction in which a particular stereoisomeric form of reactant gives one specific stereoisomer of product, while a different stereoisomeric form of reactant leads to a different single pure streoisomer of product.

Stereospecific reaction is also stereoselective; however, stereoselective reaction is not stereospecific. 6 An Electrophilic Addition Reaction

where HX = HF, HCl, HBr, and HI

Reactivity of HF << HCl < HBr < HI since HF is less acidic and HI is most acidic. The rate of addition of HI is too fast to measure.

7 The Mechanism of the Reaction

8 Relative Stabilities of Carbocations

9 Hyperconjugation Stabilizes a Carbocation

10 The Difference in Carbocation Stability Determines the Products

11 The Product Does Not Have Stereoisomers

Not Regioselective or Regiospecific

12 Formation of a

13 The Major Product is a Surprise

14 The Major Product is a Surprise

15 The Carbocation Does Not Rearrange (No Improvement in Carbocation Stability)

16 Anti-Markovnikov Regiochemistry; Radical Intermediate

17 Mechanism of the Reaction

18 Radical Stabilities

19 Addition of Halogens to Alkenes • Bromine and chlorine add to alkenes to give 1,2-dihaldes, an industrially important process

• F2 is too reactive and I2 does not add + - • Cl2 reacts as Cl Cl

– Br2 is similar

20 Addition of Br2 to Cyclopentene • Addition is exclusively trans

21 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 The Mechanism for the Addition of a Halogen

The intermediate is a cyclic bromonium ion.

23 The Reality of Bromonium Ions • Bromonium ions were postulated more than 60 years ago to explain the stereochemical course of the addition (to give the trans- dibromide from a cyclic )

• Olah showed that bromonium ions are stable in liquid SO2 with SbF5 and can be studied directly

24 No Carbocation Rearrangements

25 Alkenes Do Not Add I2

26 Halohydrins from Alkenes: Addition of HOX • This is formally the addition of HO-X to an alkene to give a 1,2-halo alcohol, called a halohydrin

• The actual reagent is the dihalogen (Br2 or Cl2) in water in an organic solvent)

27 Formation of Halohydrins

Markovnikov regiochemistry (OH group) and Anti-addition

28 Mechanism for Halohydrin Formation

29 Why Does it Follow the Same Rule?

The electrophile adds to the sp2 carbon bonded to the most hydrogens. 30 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

31 More Reactions

32 from Alkenes

1) Acid Catalyzed Hydration of Alkene Markovnikov regiochemistry and Carbocation formation

2) Oxymercuration –demurcuration (reduction) Markovnikov regiochemistry

3) Hydroboration-oxidation anti-Markovnikov regiochemistry and syn-addition

33 Hydration of Alkenes: Addition of H2O • Hydration of an alkene is the addition of H-OH to give an alcohol • Acid catalysts are used in high temperature industrial processes: is converted to ethanol

34 Acid-Catalyzed Addition of Water

Mechanism for the Acid-Catalyzed Addition of Water

35 Acid-Catalyzed Addition of an Alcohol

36 Oxymercuration Intermediates • For laboratory-scale hydration of an alkene • Use mercuric acetate in THF followed by • Markovnikov orientation – via mercurinium ion

37 Mechanism for Oxymerucation

The final step of this mechanism is not well understood. In another words, just know how it works.

38 Hydration of Alkenes: Addition of H2O by Hydroboration . Borane (BH3) is electron deficient . Borane adds to an alkene to give an organoborane

39 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 in water produces the alcohol derived from the alkene

40 Orientation in Hydration via Hydroboration • Regiochemistry is opposite to Markovnikov orientation (anti- Markovnikov regiochemistry) – 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)

41 Mechanism of Hydroboration • Borane is a Lewis acid • Alkene is Lewis base • Transition state involves anionic development on B

• The components of BH3 are added across C=C • More stable carbocation is also consistent with steric preferences R2BH Allows Only Monoalkylation

Because of its bulky R groups, it has a stronger preference for the less substituted sp2 carbon. 43 The Mechanism is the Same

44 OH Replaces BR2

45 Mechanism for the Oxidation Reaction

46 No Carbocation Rearrangements

47 The Electrophile Adds to the sp2 Carbon Bonded to the Most Hydrogens

The reagents are numbered because the second set of reagents is not added until the first reaction is over.

48 Reduction of Alkenes: • 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)

49 Addition of Hydrogen

catalytic hydrogenation a reduction reaction 50 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

51 Mechanism for Hydrogen Addition

catalytic hydrogenation

52 Relative Stabilities of Alkenes

53 The Most Stable Alkene Has the Smallest Heat of Hydrogenation

54 Relative Stabilities of Alkenes

relative stabilities of dialkyl-substituted alkenes

55 Formation of an Epoxide

the mechanism is similar to that for the addition of Br2

56 Oxidation of Alkenes: Epoxidation and Hydroxylation • Epoxidation results in a cyclic with an oxygen atom • Stereochemistry of addition is syn

57 Nomenclature of Epoxides

58 Catalyzed Formation of Diols • Hydroxylation - converts to syn-diol • Osmium tetroxide, then sodium bisulfite • Via cyclic osmate di-

59 Hydroxylation Reaction-Syn Diol

60 Ozonolysis

61 Mechanism for Formation

62 The Ozonide is Converted to and/or

63 Oxidation of Alkenes: Cleavage to Carbonyl Compounds • , O3, adds to alkenes to form molozonide • Molozonideis converted to ozonide that may be reduced to obtain ketones and/or aldehydes

64 Examples of Ozonolysis of Alkenes • Used in determination of structure of an unknown alkene

65 Ozonolysis

66 Permangate 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

67 Cleavage of 1,2-diols

• Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO4 , cleaves the diol into two carbonyl compounds

• Sequence of diol formation with OsO4 followed by diol cleavage is a good alternative to ozonolysis

68 Addition of to Alkenes: Cyclopropane Synthesis • The is “half of an alkene” • Carbenes are electronically neutral with six electrons in the outer shell • They add symmetrically across double bonds to form cyclopropanes

69 Reaction of Dichlorocarbene • Addition of dichlorocarbene is stereospecific cis

70 Formation of Dichlorocarbene • Base removes proton from chloroform • Stabilized carbanion remains • Unimolecular elimination of Cl- gives electron deficient species, dichlorocarbene Simmons-Smith Reaction

• Equivalent of addition of CH2: • Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species • Forms cyclopropanes by

72 Electrophilic Addition

73 Four Stereoisomers are Obtained if the Reaction Forms a Carbocation Intermediate

syn and anti addition 74 Syn Addition to a Cis Isomer Forms Only the Erythro Stereoisomers

75 Syn Addition to a Trans Isomer Forms Only the Threo Stereoisomers

76

Radical Additions to Alkenes: Chain-Growth

• A 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 polyethylene, for example

78 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

79 Polymerization: Propagation • Radical from initiation adds to alkene to generate alkene derived radical • This radical adds to another alkene, and so on many times

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

81 Other Polymers • Other alkenes give other common polymers

82 Biological Additions of Radicals to Alkenes

• Severe limitations to the usefulness of radical addition reactions in the lab • In contrast to electrophilic additions, reactive intermediate is not quenched so it reacts again and again uncontrollably

83