Chapter 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 (add X-X) –halides (add H-X) –diols (add HO-OH) –cyclopropane (add :CH 2 )

3 Reactions of Alkenes

4 Preparation of Alkenes

5 Part 1 - Synthesis of Alkenes These reactions are used to produce alkenes.

6 Synthesis of Alkenes: Synthesis 1# Alkenes are commonly made by elimination of HX from alkyl halide (dehydrohalogenation) Uses heat and KOH

7 Synthesis of Alkenes: Synthesis 2# –elimination of H-OH from an alcohol (dehydration) require strong acids (sulfuric acid, 50 ºC)

8 Part 2 - Reaction of Alkenes These reactions react alkenes to form a series of alkane products.

9 Addition of Halogens to Alkenes Bromine and chlorine add to alkenes to give 1,2- dihaldes F 2 is too reactive and I 2 does not add.

10 Example: Mechanism of Bromine Addition 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. –Gives trans addition.

11 Example: Addition of Br 2 to Cyclopentene Addition is exclusively trans

12 Halohydrin Formation 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. The actual reagent is the dihalogen (Br 2 or Cl 2 in water in an organic solvent) AlkeneHalohydrin

13 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.

14 Addition of Water to Alkenes: Oxymercuration 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

15 Addition of Water to Alkenes: Oxymercuration Hg(OAc) 2 is used as an electrophillic sink. The double bond is then attacked by the water creating an alchohol. This is then REDUCED by NaBH 4 that adds an H to the molecule.

16 Addition of Water to Alkenes: Hydroboration Herbert Brown (HB) invented hydroboration (HB) Borane (BH 3 ) is electron deficient is a Lewis acid. Borane adds to an alkene to give an organoborane.

17 BH 3 Is a Lewis Acid Six electrons in outer shell Coordinates to oxygen electron pairs in ethers

18 Addition of H-BH 2 (from BH 3 -THF complex) to three alkenes gives a trialkylborane Oxidation with alkaline hydrogen peroxide in water produces the alcohol derived from the alkene Hydroboration-Oxidation Alcohol Formation from Alkenes

19 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)

20 Mechanism of Hydroboration Borane is a Lewis acid Alkene is Lewis base Transition state involves anionic development on B The components of BH 3 are across C=C

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

22 Hydroboration - Oxygen Insertion Step H 2 O 2, OH - inserts OH in place of B Retains syn orientation

23 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

24 Formation of Dichlorocarbene Base removes proton from chloroform Stabilized carbanion remains Unimolecular Elimination of Cl - gives electron deficient species, dichlorocarbene

25 Simmons-Smith Reaction Equivalent of addition of CH 2 : Reaction of diiodomethane with zinc-copper alloy produces a carbenoid species Forms cyclopropanes by cycloaddition

26 Reaction of Dichlorocarbene Addition of dichlorocarbene is stereospecific cis

27 Reduction of Alkenes: Hydrogenation Addition of H-H across C=C Reduction in general is addition of H 2 or its equivalent Requires Pt or Pd as powders on carbon and H 2 Hydrogen is first adsorbed on catalyst Reaction is heterogeneous (process is not in solution)

28 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

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

30 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)

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

32 Section 3: Breakdown of Alkenes These Reactions are used to breakdown alkenes into two products.

33 Alkene Cleavage: Ozone Ozone, O 3, adds to alkenes to form molozonide Reduce molozonide to obtain ketones and/or aldehydes

34 Examples of Ozonolysis of Alkenes Used in determination of structure of an unknown alkene

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

36 Permanganate Oxidation of Alkenes Oxidizing reagents other than ozone also cleave alkenes Potassium permanganate (KMnO 4 ) can produce carboxylic acids and carbon dioxide if H’s are present on C=C

37 Cleavage of 1,2-diols Reaction of a 1,2-diol with periodic (per-iodic) acid, HIO 4, cleaves the diol into two carbonyl compinds Sequence of diol formation with OsO 4 followed by diol cleavage is a good alternative to ozonolysis

38 Mechanism of Periodic Acid Oxidation Via cyclic periodate intermediate

39 Biological Alkene Addition Reactions Living organisms convert organic molecules using enzymes as catalysts Many reactions are similar to organic chemistry conversions, except they occur in neutral water Usually much specific for reactant and stereochemistry

40 Biological Hydration Example Fumarate to malate catalyzed by fumarase Specific for trans isomer Addition of H, OH is anti

41 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

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

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

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

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

46 Other Polymers Other alkenes give other common polymers

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

48 Take Home Message Learn the REACTIONS (ALL OF THEM)

49 Synthesis of Alkenes 1) dehydrohalogenation 2) dehydration

50 Part 2 - Reaction of Alkenes 1) Addition of Halogens to Alkenes 2) Halohydrin Formation

51 Part 2 - Reaction of Alkenes 3) Addition of Water to Alkenes 4) Hydroboration-Oxidation Alcohol Formation 5) Carbene Formation – Cyclopropane synthesis

52 Part 2 - Reaction of Alkenes 6) Catalytic Hydrogenation 7) Hydroxylation and Cleavage

53 Part 3 - Breakdown of Alkenes 1) Ozonolysis 2) Permangante Oxidation 3) Periodic Acid Oxidation, Cleavage of 1,2-diols