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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