1Alkenes: Polymers A polymer is a very large molecule consisting of

repeating units of simpler molecules (the monomer) Ethylene is polymerized to polyethylene, for example

Polymers have been a part of life since prehistoric times.

Cellulose in wood and starches obtained from vegetables are carbohydrate polymers made from many thousands of glucose molecules.

Silk and wool are polymer made from amino-acids.

DNA, the molecule that carries genetic information, is a polymer of nucleotides.

POLYMER

Synthetic polymers provide a wide variety of items that we use every day.

This alkene makes what common products?

a. plastic bottlesb. garden hosesc. nonstick coatingsd. coffee cups

Question

Answer: b. Vinyl chloride makes plastic pipes and tubing, garden hoses and garbage bags.

The abbreviation PETE appears on many consumer products. What does it stand for?

a. phenyl ether tetraethylene

b. phosphorus ethyl tertiary ethylene

c. polyethylene terephthalate

d. polyester tetramer ethene

Question

Answer: c. PETE stands for polyethylene terephthalate.

8Free Radical Polymerization of Alkenes

9 Alkenes combine many times to give polymer Reactivity induced by formation of free radicals

10

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

11

Polymerization: Propagation Radical from intiation adds to alkene to generate alkene

derived radical This radical adds to another alkene, and so on many

times

12

Polymerization: Termination Chain propagation ends when two radical chains

combine Not controlled specifically but affected by reactivity and

concentration

13

Free-Radical Polymerization of Propene

polypropylene

H2C CHCH3

CH CH CHCHCHCH CH

H CH3 H CH3 H CH3 H

14

Mechanism..

RO..

H2C CHCH3

15

H2C CHCH3

Mechanism..

RO:

16

H2C CHCH3

Mechanism..

RO:

H2C CHCH3

17

H2C CHCH3

H2C CHCH3

Mechanism..

RO:

18

H2C CHCH3

H2C CHCH3

Mechanism

H2C CHCH3

..RO:

19

H2C CHCH3

H2C CHCH3

H2C CHCH3

Mechanism..

RO:

20

H2C CHCH3

H2C CHCH3

H2C CHCH3

Mechanism

H2C CHCH3

..RO:

21

Other Polymers Other alkenes give other common polymers

22

Unsymmetrical Monomers

If alkene is unsymmetrical, reaction is via more highly substituted radical

23

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

24

Long Chain Branching

In long chains, a hydrogen from another chain is abstracted

25

Cationic Polymerization

26

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

27

H2SO4

Cationic Polymerization

monomer(C4H8)

Dimerization of 2-methylpropene

(CH3)2C CH2

28

Mechanism of Cationic Polymerization

+

CH3

H2C C

CH3

CH3C

CH3

+

CH3

H+

29

Mechanism of Cationic Polymerization

CH3CCH2C

CH3

CH3

CH3

CH3

+

+

CH3

H2C C

CH3

CH3C

CH3

+

CH3

30

+

Mechanism of Cationic Polymerization

CH3CCH

CH3

CH3

C(CH3)2 CH3CCH2C

CH3

CH3

CH2

CH3

CH3CCH2C

CH3

CH3

CH3

CH3

+

31

32

33

34

Epoxidation of Alkenes

Epoxides

are examples of heterocyclic compounds three-membered rings that contain oxygen

ethylene oxide propylene oxide

H2C CH2

O

H2C CHCH3

O

Epoxide Nomenclature

Substitutive nomenclature: named as epoxy-substituted alkanes.epoxy precedes name of alkane1,2-epoxypropane 2-methyl-2,3-epoxybutane

H2C CHCH3

O

CHCH3

O

C

H3C

H3C

1

2 3 4

cis-2-Methyl-7,8-epoxyoctadecane

OH H

Problem 6.17 Give the IUPAC name, including stereochemistry, for disparlure.

Epoxidation of Alkenes

peroxy acid

C C +

O

RCOOH

CC

O

+

O

RCOH

Example

+ CH3COOH

O

(52%)

+ CH3COH

O

O

Epoxidation of Alkenes

C C +

O

RCOOH

CC

O

+

O

RCOH

syn addition

Relative Rates

ethylene H2C=CH2 1

propene CH3CH=CH2 22

2-methylpropene (CH3)2C=CH2 484

2-methyl-2-butene (CH3)2C=CHCH3 6526

More highly substituted double bonds react faster.Alkyl groups on the double bond make itmore electron rich.

Epoxidation

Mechanism of Epoxidation

Mechanism of Epoxidation

Mechanism of Epoxidation

Mechanism of Epoxidation

Mechanism of Epoxidation

Ozonolysis of Alkenes

Ozonolyis has both synthetic and analytical applications.

synthesis of aldehydes and ketones identification of substituents on the

double bond of an alkene

First step is the reaction of the alkene with ozone. The product is an ozonide.

+ O3 C CO

O O

Ozonolysis of Alkenes

C C

Second step is hydrolysis of the ozonide. Two aldehydes, two ketones, or an aldehyde and a ketone are formed.

+ O3 C CO

O O

Ozonolysis of Alkenes

C C

C O CO+

H2O, Zn

As an alternative to hydrolysis, the ozonidecan be treated with dimethyl sulfide.

+ O3 C CO

O O

Ozonolysis of Alkenes

C C

C O CO+

(CH3)2S

Example

1. O32. H2O, Zn

(38%)(57%)

CH3

CH2CH3H

C C

CH2CH3

CH2CH3

CO

CH2CH3

C O

CH3

H

+

Oxidation/Reduction

by using a balanced half-reaction1. write a half-reaction showing one reactant and its product(s)

2. complete a material balance; use H2O and H+ in acid solution, use H2O and OH- in basic solution

3. complete a charge balance using electrons, e-

How do you recognize the reaction whether oxidation/reduction or not

Oxidation/Reduction

three balanced half-reactions

2013 lect4b polymerization of alkenes-Alkenes: PolymersSlide Number 2Slide Number 3Slide Number 4Slide Number 5Slide Number 6Slide Number 7Free Radical Polymerization of AlkenesSlide Number 9Free Radical Polymerization: InitiationPolymerization: PropagationPolymerization: TerminationSlide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Slide Number 19Slide Number 20Other PolymersUnsymmetrical MonomersChain Branching During PolymerizationLong Chain BranchingCationic PolymerizationSlide Number 26Slide Number 27Slide Number 28Slide Number 29Slide Number 30Slide Number 31Slide Number 32Slide Number 33Slide Number 34

2013 lect4c epoxidation s of alkenes-Epoxidation of AlkenesSlide Number 2Slide Number 3Problem 6.17 Give the IUPAC name, including stereochemistry, for disparlure.Slide Number 5Slide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Slide Number 12Slide Number 13Slide Number 14Slide Number 15Slide Number 16Slide Number 17Slide Number 18Oxidation/ReductionOxidation/Reduction