8. Alkenes: Reactions and 8. Alkenes: Reactions and SynthesisSynthesis

Why this chapter?Why this chapter?

To begin a systematic description of major functional groupsBegin to focus on general principles and patterns of reactivity that tie organic chemistry

2

Diverse Reactions of AlkenesDiverse Reactions of Alkenes Alkenes react with many electrophiles to give useful

products by addition (often through special reagents)

3

8.1 Preparation of Alkenes: A Preview of 8.1 Preparation of Alkenes: A Preview of Elimination ReactionsElimination 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

Preview of Elimination ReactionsPreview of Elimination Reactions

Dehydrohalogenation: Loss of HX

With and strong base (KOH)

5

Preview of Elimination ReactionsPreview of Elimination Reactions

Dehydration: Loss of HOH With strong acid cat (H2SO4)

6

8.2 Addition of Halogens to Alkenes8.2 Addition of Halogens to AlkenesBromine 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

7

Addition of BrAddition of Br22 to Cyclopentene to Cyclopentene Addition is exclusively trans

8

Mechanism of Bromine AdditionMechanism of Bromine AdditionBr+ adds to an alkene producing a cyclic ionBromonium ion, bromine shares charge with carbon

◦ Gives trans addition

9

Bromonium Ion MechanismBromonium 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

10

The Reality of Bromonium IonsThe 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 alkene

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

p. 218

Biological HalogenationBiological Halogenation•Occurs in marine organisms.•“Br1+” or “Cl1+” like compounds made by oxidizing Br1- or Cl1- with H2O2

Which product will result from the following Which product will result from the following reaction? reaction?

1. 2.

3. 4. 5.

Cl2

CH2Cl2

Learning Check:Learning Check:

Which product will result from the following Which product will result from the following reaction? reaction?

1. 2.

3. 4. 5.

Cl2

CH2Cl2

Solution:Solution:

14

8.3 Addition of Hypohalous Acids to Alkenes: 8.3 Addition of Hypohalous Acids to Alkenes: Halohydrin FormationHalohydrin Formation

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)

15

Mechanism of Formation of a BromohydrinMechanism of Formation of a Bromohydrin

Br2 forms bromonium ion, then water adds

◦ Orientation toward stable C+ species

◦ Aromatic rings do not react

16

An Alternative to BromineAn Alternative to BromineBromine is a difficult reagent to use for this reactionN-Bromosuccinimide (NBS) produces bromine in

organic solvents and is a safer source

Which of the compounds will result from the Which of the compounds will result from the reaction of NBS/Hreaction of NBS/H22O/DMSO with 1-O/DMSO with 1-methylcyclopentene?methylcyclopentene?

1. 2.

3. 4. 5.

Learning Check:Learning Check:

Which of the compounds will result from the Which of the compounds will result from the reaction of NBS/Hreaction of NBS/H22O/DMSO with 1-O/DMSO with 1-methylcyclopentene?methylcyclopentene?

1. 2.

3. 4. 5.

Solution:Solution:

19

8.4 Addition of Water to Alkenes:8.4 Addition of Water to Alkenes:Hydration of an alkene is the addition of H-OH to to

give an alcoholAcid catalysts are used in high temperature industrial

processes: ethylene is converted to ethanol

Fig. 7-2, p. 221

p. 222

Biological Addition of WaterBiological Addition of Water

p. 222

OxymercurationOxymercuration

23

Oxymercuration IntermediatesOxymercuration IntermediatesFor laboratory-scale hydration of an alkeneUse mercuric acetate in THF followed by sodium

borohydrideMarkovnikov orientation

◦ via mercurinium ion

24

8.5 Addition of Water to Alkenes: 8.5 Addition of Water to Alkenes: HydroborationHydroboration

Herbert Brown (HB) invented hydroboration (HB)Borane (BH3) is electron deficient is a Lewis acidBorane adds to an alkene to give an organoborane

p. 223

BHBH33-THF Complex-THF Complex

THF = Tetrahydrofuran; a common ether solvent

26

Addition of H-BH2 (from BH3-THF complex) to three

alkenes gives a trialkylboraneOxidation with alkaline hydrogen peroxide in water

produces the alcohol derived from the alkene

Hydroboration-Oxidation: Alkene Alcohol

27

Orientation in Hydration via HydroborationOrientation in Hydration via HydroborationRegiochemistry 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)

28

Mechanism of HydroborationMechanism 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 consistent with steric preferences

p. 225

Example:Example:

p. 225

Learning CheckLearning Check

p. 226

Solution:Solution:Options are:

So:

32

8.6 Reduction of Alkenes: Hydrogenation8.6 Reduction of Alkenes: HydrogenationAddition of H-H across C=CReduction in general is addition of H2 or its equivalent Requires Pt or Pd as powders on carbon and H2

Hydrogen is first adsorbed on catalystReaction is heterogeneous (process is not in solution)

33

Hydrogen Addition- SelectivityHydrogen Addition- SelectivitySelective for C=C. No reaction with C=O, C=NPolyunsaturated liquid oils become solidsIf one side is blocked, hydrogen adds to other

34

Mechanism of Catalytic HydrogenationMechanism of Catalytic HydrogenationHeterogeneous

– reaction between phases

Addition of H-H is syn

p. 232

Examples:Examples:

p. 232

Examples:Examples:

p. 232

Learning Check:Learning Check:What product would you obtain from catalytic hydrogenation of the following alkenes?

Fig. 7-8, p. 233

Biological Example:Biological Example:

Which compound is Which compound is the bestthe best starting starting material for hydrogenation to decalin material for hydrogenation to decalin shown below? shown below?

1. 2.

3. 4. 5.

H

HH2 (< 1atm)

Pd/C

?

Learning Check:Learning Check:

Which compound is Which compound is the bestthe best starting starting material for hydrogenation to decalin material for hydrogenation to decalin shown below? shown below?

1. 2.

3. 4. 5.

H

HH2 (< 1atm)

Pd/C

?

Solution:Solution:

41

8.7 Oxidation of Alkenes: 8.7 Oxidation of Alkenes: Epoxidation and HydroxylationEpoxidation and Hydroxylation

Epoxidation results in a cyclic ether with an oxygen atom

Stereochemistry of addition is syn

p. 234

Ways to Make EpoxidesWays to Make Epoxides

p. 234

Use Epoxides to make DiolsUse Epoxides to make Diols

44

Osmium Tetroxide Catalyzed Osmium Tetroxide Catalyzed Formation of DiolsFormation of Diols

Hydroxylation - converts to syn-diolOsmium tetroxide, then sodium bisulfateVia cyclic osmate di-ester

45

8.8 Oxidaton of Alkenes:8.8 Oxidaton of Alkenes:Cleavage to Carbonyl CompoundsCleavage to Carbonyl Compounds

Ozone, O3, adds to alkenes to form molozonideReduce molozonide to obtain ketones and/or

aldehydes

46

Examples of Ozonolysis of AlkenesExamples of Ozonolysis of AlkenesUsed in determination of structure of an unknown alkene

Compound X reacts with OCompound X reacts with O33 (followed by (followed by Zn/AcOH) to give I and II. Zn/AcOH) to give I and II.

Which of the following could be X?

1. 2.

3. 4. 5.

O O

O O

H

I II

Learning Check:Learning Check:

Compound X reacts with OCompound X reacts with O33 (followed by (followed by Zn/AcOH) to give I and II. Zn/AcOH) to give I and II.

Which of the following could be X?

1. 2.

3. 4. 5.

O O

O O

H

I II

Solution:Solution:

49

Permangante Oxidation of AlkenesPermangante Oxidation of AlkenesOxidizing reagents other than ozone also cleave alkenesPotassium permanganate (KMnO4) can produce carboxylic

acids and carbon dioxide if H’s are present on C=C

50

Cleavage of 1,2-diolsCleavage of 1,2-diolsReaction 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

What is the major product of the following What is the major product of the following reaction?reaction?

1. 2.

3. 4.5.

1. OsO4

2. NaHSO3/H2O

Learning Check:Learning Check:

What is the major product of the following What is the major product of the following reaction?reaction?

1. 2.

3. 4.5.

1. OsO4

2. NaHSO3/H2O

Solution:Solution:

Which of the following will react with Which of the following will react with periodic acid (HIOperiodic acid (HIO44) to give an aldehyde?) to give an aldehyde?

1. 2.

3. 4. 5.

Learning Check:Learning Check:

Which of the following will react with Which of the following will react with periodic acid (HIOperiodic acid (HIO44) to give an aldehyde?) to give an aldehyde?

1. 2.

3. 4. 5.

Solution:Solution:

55

8.9 Addition of Carbenes to Alkenes8.9 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 add symmetrically across double bonds to form

cyclopropanes

56

Formation of DichlorocarbeneFormation of DichlorocarbeneBase removes

proton from chloroform

Stabilized carbanion remains

Unimolecular elimination of Cl- gives electron deficient species, dichlorocarbene

57

Reaction of DichlorocarbeneReaction of DichlorocarbeneAddition of dichlorocarbene is stereospecific cis

58

Simmons-Smith ReactionSimmons-Smith ReactionEquivalent of addition of CH2: Reaction of diiodomethane with zinc-copper

alloy produces a carbenoid speciesForms cyclopropanes by cycloaddition

59

8.10 Radical Addition to Alkenes: Polymers8.10 Radical Addition to Alkenes: Polymers A polymer is a very large molecule consisting of repeating

units of simpler molecules, formed by polymerization

60

PolymersPolymers

61

Free Radical Polymerization: InitiationFree 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

Alkenes react with radical catalysts to undergo radical polymerization

Ethylene is polymerized to poyethylene, for example

62

Polymerization: PropagationPolymerization: PropagationRadical from initiation adds to alkene to

generate alkene derived radicalThis radical adds to another alkene, and so on

many times

63

Polymerization: TerminationPolymerization: TerminationChain propagation ends when two radical chains combineNot controlled specifically but affected by reactivity and

concentration

64

Other PolymersOther PolymersOther alkenes give other common polymers

Table 7-1, p. 242

Other PolymersOther Polymers

Which monomer would give the polymer Which monomer would give the polymer shown below?shown below?

1. 2.

3. 4. 5.

Cl Cl Cl

Learning Check:Learning Check:

Which monomer would give the polymer Which monomer would give the polymer shown below?shown below?

1. 2.

3. 4. 5.

Cl Cl Cl

Solution:Solution:

68

8.11 Biological Additions of Radicals to Alkenes8.11 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

69

Biological ReactionsBiological ReactionsBiological reactions different than in the

laboratoryOne substrate molecule at a time is

present in the active site of an enzyme Biological reactions are more controlled,

more specific than other reactions

70

p. 245

Natural RubberNatural Rubber

72

8.12 Stereochemistry of Reactions: 8.12 Stereochemistry of Reactions: Addition of HAddition of H22O to AlkenesO to Alkenes

Many reactions can produce new chirality centers from compounds without them

What is the stereochemistry of the chiral product?

What relative amounts of stereoisomers form?Example addition of H2O to 1-butene

73

Achiral Intermediate Gives Achiral Intermediate Gives Racemic ProductRacemic Product

Addition via carbocationTop and bottom are equally accessible

74

8.13 Stereochemistry of Reactions: 8.13 Stereochemistry of Reactions: Addition of HAddition of H22O to a Chiral AlkeneO to a Chiral Alkene

What is the sterochemical result of the addition of H2O to a chiral alkene R-4-methyl-1-hexene

Product has 2 chiral centers

Fig. 9-16, p. 313

76

SummarySummary

Which sequence of reagents is necessary Which sequence of reagents is necessary to carry out the following synthesis?to carry out the following synthesis?

multiple steps OH

OH

1. Br2/hv; NaOH; RCO3H; H3O+

2. Cl2/heat; KOH; OsO4; NaHSO3/H2O

3. H2/Pd; NBS/H2O/DMSO; H3O+

4. KOH; OsO4; NaHSO3/H2O

5. NaOH; Br2/H2O; KOH; H3O+

Learning Check:Learning Check:

Which sequence of reagents is necessary Which sequence of reagents is necessary to carry out the following synthesis?to carry out the following synthesis?

multiple steps OH

OH

1. Br2/hv; NaOH; RCO3H; H3O+

2. Cl2/heat; KOH; OsO4; NaHSO3/H2O

3. H2/Pd; NBS/H2O/DMSO; H3O+

4. KOH; OsO4; NaHSO3/H2O

5. NaOH; Br2/H2O; KOH; H3O+

Solution:Solution:

Which alkene would produce two identical Which alkene would produce two identical ketones when reacted with KMnOketones when reacted with KMnO44/H/H++? ?

1.2.

3.

4. 5.

Learning Check:Learning Check:

Which alkene would produce two identical Which alkene would produce two identical ketones when reacted with KMnOketones when reacted with KMnO44/H/H++? ?

1.2.

3.

4. 5.

Solution:Solution:

What kind of intermediate is involved in What kind of intermediate is involved in the biosynthesis of prostaglandins from the biosynthesis of prostaglandins from arachidonic acid?arachidonic acid?

1. carbocations2. carbanions3. carbon-base

radicals4. bromonium ions5. carbenes

Learning Check:Learning Check:

What kind of intermediate is involved in What kind of intermediate is involved in the biosynthesis of prostaglandins from the biosynthesis of prostaglandins from arachidonic acid?arachidonic acid?

1. carbocations2. carbanions3. carbon-base

radicals4. bromonium ions5. carbenes

Solution:Solution:

What is the best reagent to carry What is the best reagent to carry out the following reaction? out the following reaction?

1. CH2Cl2/Zn(Cu)

2. CH2l2/Zn(Cu); Cl23. CH2Cl2/KOH

4. CHCl3/KOH

5. Cl2/H2O, CH3I/KOH

ClCl

Learning Check:Learning Check:

What is the best reagent to carry What is the best reagent to carry out the following reaction? out the following reaction?

1. CH2Cl2/Zn(Cu)

2. CH2l2/Zn(Cu); Cl23. CH2Cl2/KOH

4. CHCl3/KOH

5. Cl2/H2O, CH3I/KOH

ClCl

Solution:Solution:

Which of the following reactions Which of the following reactions would notwould not yield at least one alcohol functional group? yield at least one alcohol functional group?

1. hydroboration of an internal alkene, followed by H2O2/NaOH

2. reaction of a mono-substituted alkene with NBS/H2O/DMSO

3. osmium tetroxide oxidation of a tetrasubstituted cyclic alkene, followed by NaHSO3

4. oxymercuration/demercuration of terminal alkene

5. oxidative cleavage of a tetrasubstituted alkene using KMnO4/H+

Learning Check:Learning Check:

Which of the following reactions Which of the following reactions would notwould not yield at least one alcohol functional group? yield at least one alcohol functional group?

1. hydroboration of an internal alkene, followed by H2O2/NaOH

2. reaction of a mono-substituted alkene with NBS/H2O/DMSO

3. osmium tetroxide oxidation of a tetrasubstituted cyclic alkene, followed by NaHSO3

4. oxymercuration/demercuration of terminal alkene

5. oxidative cleavage of a tetrasubstituted alkene using KMnO4/H+

Solution:Solution:

Which of the following compounds would Which of the following compounds would yield trialkylborane shown below when yield trialkylborane shown below when treated with BHtreated with BH33/THF?/THF?

1. 2-methylbut-1-ene

2. 2-methylbut-2-ene

3. 3-methylbut-1-ene

4. 2-methylpropene5. (E)-2-pentene

B

Learning Check:Learning Check:

Which of the following compounds would Which of the following compounds would yield trialkylborane shown below when yield trialkylborane shown below when treated with BHtreated with BH33/THF?/THF?

1. 2-methylbut-1-ene

2. 2-methylbut-2-ene

3. 3-methylbut-1-ene

4. 2-methylpropene5. (E)-2-pentene

B

Solution:Solution:

What is the product of reaction of 1-What is the product of reaction of 1-methylcyclohexene with Hg(OAc)methylcyclohexene with Hg(OAc)22/H/H22O/THF O/THF followed by NaBDfollowed by NaBD44? (D = ? (D = 22H)H)

1. 2.

3.

4. 5.

Learning Check:Learning Check:

What is the product of reaction of 1-What is the product of reaction of 1-methylcyclohexene with Hg(OAc)methylcyclohexene with Hg(OAc)22/H/H22O/THF O/THF followed by NaBDfollowed by NaBD44? (D = ? (D = 22H)H)

1. 2.

3.

4. 5.

Solution:Solution:

Which reaction sequence on an alkene Which reaction sequence on an alkene will accomplish the same end result as will accomplish the same end result as treating the alkene with Otreating the alkene with O33 followed by followed by Zn/AcOH?Zn/AcOH?

1. RCO3H followed by KMnO4/H3O+

2. BH3/THF followed by OsO4 followed by NaHSO3/H2O

3. OsO4/NMO followed by HIO4

4. Hg(OAc)2/H2O/THF followed by NaBH4 followed by KOH

5. Br2/H2O followed by KOH, followed by H3O+

Learning Check:Learning Check:

Which reaction sequence on an alkene Which reaction sequence on an alkene will accomplish the same end result as will accomplish the same end result as treating the alkene with Otreating the alkene with O33 followed by followed by Zn/AcOH?Zn/AcOH?

1. RCO3H followed by KMnO4/H3O+

2. BH3/THF followed by OsO4 followed by NaHSO3/H2O

3. OsO4/NMO followed by HIO4

4. Hg(OAc)2/H2O/THF followed by NaBH4 followed by KOH

5. Br2/H2O followed by KOH, followed by H3O+

Solution:Solution: