IV. Oxidation• Three types

A. Epoxidation

B. Hydroxylation

C. Oxidative cleavage

A. Epoxidation• Formation of epoxide

• Cyclic ether

• Example:

• Reagent is peroxy acid (RCO3H)

• Stereochemistry = syn

A. Epoxidation• Another method: treat halohydrin with base:

B. Hydroxylation• Formation of a 1,2-diol/glycol/vicinal diol• Methods:

1. Opening of epoxide using aqueous acid• Product is trans diol

Hydroxylation

2. Addition of osmium tetroxide (OsO4) or potassium permanganate (KMnO4)

• How do you know these are both oxidizing agents?• Reaction includes some appropriate work-up

• H2O2 or NaHSO3, H2O for OsO4

• HO- (aq) for KMnO4

• Stereochemistry = syn

• Draw the major product of the following reaction.

1. KMnO4

2. HO-, H2O

C

C

H CH2CH3

CH2CH3H

C. Oxidative Cleavage• Oxidize an alkene and split the C=C• Results in formation of 2 carbonyls

• Type of carbonyls depends on alkene structure and the oxidizing agent used

• Two types of oxidizing agents1. Ozone

2. Potassium permanganate (not in Klein text!)

Y

ZX

W

O

X

W

O

Y

Z

+

Oxidative Cleavage

1. Ozone

• Ozonolysis• Reagents: 1. O3

2. (CH3)2S or Zn, H3O+

• Products = 2 carbonyls (ketones or aldehydes)• Terminal alkenes give CO2

Example• Draw the products of the ozonolysis of 1-butene.

Oxidative Cleavage

2. KMnO4 (not in Klein text!)

• Reagents: KMnO4 (excess or concentrated) and heat or acid• Use heat and excess KMnO4 to split intermediate glycol

• Products = 2 carbonyls (ketones or carboxylic acids)• Aldehydes oxidize to carboxylic acids in KMnO4

• Terminal alkenes still give CO2

• Draw the major product for each of the following reactions.

KMnO4 (conc)

1. O3

2. (CH3)2S

CH3

V. Polymerization• Polymer = large molecule synthesized by covalently

linking single parts (monomers)• Biological polymers: proteins, cellulose, nucleic acids• Organic polymers: plastics• Addition polymers: made from alkene monomers

• Chain-growth polymerization reactions• Cationic, anionic, or radical, depending on conditions/catalyst

• Example: Radical polymerization of ethylene

Radical Polymerization Mechanism

Radical Polymerization Mechanism

Alkene Polymers

• Draw the structure of poly(vinyl chloride).

Review of Alkene Reactions

Draw the major organic product formed (showing stereochemistry where applicable) for the reaction of the following alkene under each of the reaction conditions listed below.

CH

CH3

CH CH2

HBr

H2O

H+

Cl2

CCl4

Br2

H2O

1. Hg(OAc)2, H2O

2. NaBH4

1. BH3 THF

2. H2O2/NaOH

1. OsO4

2. H2O2

1. O3

2. (CH3)2S

H2

metal catalyst

HBr

peroxides

MCPBA

conc. KMnO4

Preparation of Alkynes• Alkenes from elimination of alkyl halides with strong base

• Alkynes from elimination reactions of alkyl dihalides with strong base• Vicinal or geminal

Preparation of Alkynes• A two-step process from alkenes

1. Alkenes undergo addition of X2 to make a vicinal dihalide

2. The vicinal dihalide undergoes 2 elimination reactions to yield the alkyne

• How could you prepare 2-butyne from 2-butene?

• How could you prepare 2-pentyne from 3-pentanol?

Reactions of Alkynes• Similar to alkenes, but can also react a second time• Mechanism:

• Alkene:

• Alkyne:

• Which is faster, reaction with alkenes or alkynes?• Alkenes have more stable carbocation intermediate

Reactions of Alkynes

I. Addition of HX

II. Hydration

III. Halogenation

IV. Reduction

V. Oxidation

I. Addition of HX

• Terminal alkynes: regiochemistry = Markovnikov• Anti-Markovnikov if peroxides are present

• Internal alkynes (unsymmetrical) = mixture of products

C CRHX

H C CR HHX

C CR H

X H X

X H

H

reaction can stop here

(vinyl halide) excess HX will form geminal

dihalide

Addition of HX

CH3 C C CH2 CH3

HBrCH3 C C CH2 CH3 CH3 C C CH2 CH3+

Br Br

H H

CH3 C C CH2 CH3 CH3 C C CH2 CH3+

Br Br

H HBr Br

HH

HBr

II. Hydration• Markovnikov addition of water

• With alkenes: H2O in H2SO4 or Hg(OAc)2, H2O with reduction

• With alkynes: H2O in H2SO4 with HgSO4

• Enol (a vinylic alcohol) rearranges to form a carbonyl• Keto-enol tautomerism

Keto-enol Tautomerism• Tautomers

• Constitutional isomers which rapidly interconvert• Rearrangement reaction, not resonance structures

• Keto tautomer typically more stable than enol

• Catalyzed by acid or base

Keto-enol Tautomerism• Acid-catalyzed mechanism

• Base-catalyzed mechanism

• Draw the major product for each of the following reactions.

H2O, H2SO4

HgSO4CH3 C C H

H2O, H2SO4

HgSO4CH3 C C CH2 CH3

Hydration• Anti-Markovnikov addition of water

• With alkenes: 1. BH3•THF, 2. H2O2, NaOH

• With alkynes: 1. BH3•THF or Sia2BH, 2. H2O2, NaOH

• Sia2BH = disiamylborane• Sia = siamyl = sec-isoamyl• More hindered than BH3, so prevents

addition of 2 borane molecules

• Product still undergoes keto-enol tautomerism• Internal alkynes yield ketones• Terminal alkynes yield aldehydes

• Draw the major product for each of the following reactions.

1. Sia2BH

2. NaOH, H2O2

CH3 C C H

CH3 C C CH3

1. Sia2BH

2. NaOH, H2O2

Hydration Summary• Internal alkynes, both reagents give the same products• Terminal alkynes, different products

III. Halogenation

• Addition of X2

• X2 = Br2 or Cl2

• Still anti addition

IV. Reduction

• Alkyne reduce to alkene or alkane• Depends on the reagent/conditions used

• Types of reduction:• Catalytic reduction• Chemical reduction

Catalytic Reduction• Alkyne → alkane

• Cannot stop reaction at alkene with these catalysts• Can form alkene with Lindlar catalyst

• Pd + BaSO4/CaCO3 + Pb salt + quinoline

• Syn addition gives cis alkene

R C C RH2

Pd, Pt, NiR C C R

H

H

H

H

N

quinoline

Chemical Reduction• Aka Dissolving metal reduction• Alkyne → trans alkene• Reagents = Li or Na in NH3(l)

Summary of Reduction Reactions

V. Oxidation

• Oxidize with O3 or KMnO4

• Both cleave C≡C • Both oxidize to carboxylic acids• Terminal alkynes give CO2

• Draw the major product for each of the following reactions.

1. KMnO4, H2O, KOH,

2. H+CH3 C C H

CH3 C C CH3

1. O3

2. H2O

Acidity of Alkynes• Terminal alkynes are weak acids

• More acidic than alkenes or alkanes

• Conjugate base = acetylide ion or alkynide ion

• Conjugate base is somewhat stable • Electron pair closer to nucleus with more s character

Acetylide Ion• Strong base

• Stronger than HO- or RO-

• Not as strong as -NH2

• Acetylide ion can act as a base or a nucleophile

Acetylide Ion as a Nucleophile• React with methyl or primary alkyl halides• Undergo substitution reaction

• Form a new, larger alkyne• Alkylation reaction• C-C bond making reaction

• Example:

Alkylation Reaction Examples

Acetylide Ion as a Base• React with secondary or tertiary alkyl halides• Undergo elimination reaction

• Dehydrohalogenation (eliminate H-X)

Review of Alkyne Reactions

Synthesis• You will be given a product. Your goal is to determine how

to make that product from simpler starting materials using reactions we have studied.

• Consider:• How many carbons are in the starting material and product? Do you

need to make any C-C bonds? If so, how will you do that?• What functional groups are in the starting material? What can you do

with those functional groups?• What functional groups are in the product? How do you know how to

make those functional groups? Try working backwards (retrosynthesis).

• Look in Klein section 9.13, 10.11, 12.1-12.6 for strategies and worked examples

• Remember there may be more than one correct answer!

• Propose a synthesis of 3-methyl-1-butane from 2-methyl-2-butene.

• Provide structures or reagents (including solvent and/or special conditions, such as heat) in the empty boxes below to complete the following reaction scheme.

HC C

1. NaNH2

2. CH3Br

C C

CH3

CH3

H

H3C

H

OH

NBS

CCl4h

• Propose a synthesis of 1-bromo-2-methylpropane from 2-methylpropane.

• Propose a synthesis of cis-3-hexene from acetylene.

C C HH

CH2CH3

HH

CH3CH2

• Propose a synthesis of 1,2-dichloropropane from acetylene.

• Propose a synthesis of 2-butanone from ethylene.