Chemistry 125: Lecture 52February 17, 2010

Additions by Radicals & Electrophilic Carbon;

Isoprenoids; andPolymer Properties This

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Radical PolymerizationSec 11.5 (487-489)

R

H

Occasional butyl side-chains inhibit close packing.

Cl CCl3R Cl

CCl3

CCl3

Controlling Polymer Chain LengthCCl4 is a “Chain-Transfer Agent”

shortens polymer molecules without terminating chain reaction

Properties like viscosity and melting point depend on

chain length.etc.

Cl

ktransfer/kpolymerization ~ 0.01 for styrene polymerization

When other termination is negligible, molecular length

~ kp[styrene] / kt[CCl4]“dispersity”

Alkene Oligomerization and Polymerization Using

Carbon Electrophiles

R+ (SN1)

R-L*

(SN2)

(“oligo”, a few)

CH3

CH3

H2C C

H

CH3

CH3 C CH3

CH3

CH3

H2C C

R+ Electrophile in Formation of 2,2,4-Trimethylpentane, “Isooctane”

CH3

CH3

CH3 C H

CH3

CH3

H2C C

CH3

CH3

CH3 C CH2

CH3

CH3

HCH2SO4+

CH3

CH3

CH3 C+

CH3

CH3

CH3 C +

CH3

CH3

CH2 C

H+

(defined as “100 octane”)

inter molecular hydride shift(Bartlett, 1944)

chainpoly(isobutylene)

“butyl rubber” air-tight

+

CH3

CH3

CH2 C

CH3

CH3

CH3 C

CH3

CH3

CH2 Cetc. etc.

Sec. 12.13pp. 554-562

R-L and R+ Electrophiles in*

Terpene/Steroid Biogenesis

Isopentenyl Pyrophosphate

OPP

OP

OP

O

O O O O_ _

_

OPP

DimethylallylPyrophosphate

Adjacent unsaturationapparently speeds SN2

(as well as SN1)

CH2

Nu

L

Cl

I

Clbenzyl 250Cl

Clallyl 90Cl

Cln-propyl

krel for rxn with

I- in acetone

[1]Cl

Isopentenyl Pyrophosphate

OPP

OP

OP

O

O O O O_ _

_

OPPOPP

OPP

GeranylPyrophosphate

OPP

OPP

+

_

OPP

C5

C10

Geranyl Pyrophosphate

OPPcistrans

OPP

NerylPyrophosphate

+

Limonene

-H++

+

-Pinene

-H+

+

-H+

+H2OOH

[Ox]O

Camphor

"Terpene" essential oils C10

Markovnikov

anti-Markovnikov

Geranyl Pyrophosphate

OPP

OPP

Farnesyl Pyrophosphate

OPP

"head-to-headreductive dimerization"

PPO

Squalene (shark liver oil)

new bond

C15

“sesquiterpenes”

C30 “triterpenes”

e.g. caryophyllene(clove, hemp, rosemary)

+

Squalene

H +

+

+

+

+

+

HOO

Markovnikov

Markovnikov

Anti-Markovnikov

Markovnikov

Enzyme makes “O” selective among many trisubstituted alkene groups.

Squalene

+

+

+

+

+

HO

HH

H

CH3

HH

CH3

H+

CH3H3C

H3C CH3

CH3

CH3

CH3H3C

Lanosterol(source of cholesterol& steroid hormones)

Not this time! (enzyme control)

C30 “triterpenes”

3°

3°

3°

3°3°

Squalene

+

+

+

+

+

HO

HH

H

CH3

HH

CH3

H+

CH3H3C

H3C CH3

CH3

CH3

CH3H3C

Lanosterol(source of cholesterol& steroid hormones)

Not this time! (enzyme control)

C30 “triterpenes”

3°

3°

3°

3°3°

Cute StoryIs it True?

(Wait for NMR)

2 Isoprenes

Isoprene

HOH

Geraniol“dimer”

HOH

Isoprene

OH

2 Isoprenes

Menthol“dimer” OH

Isoprene

O

4 Isoprenes

Retinal“tetramer”

O

Latex“polymer”

Isoprene

30,000 Isoprenes

Hevea braziliensis

Latex to Caoutchouc

Gooey in heatBrittle in cold

Thomas Hancock(England -1820) “Masticator”

Goodyear

(1839)Vulcanization

Charles Macintosh(Scotland - 1823)

Sandwiched rubber between clothlayers for waterproof garments

Discovery of Vulcanization

1839

The occurrence did not at the time seem to them to be worthy of notice; it was considered as one of the frequent appeals that he was in the habit of making, in behalf of some new experiment.”

He endeavoured to call the attention of his brother, as well as some other individuals who were present, and who were acquainted with the manufacture of gum-elastic, to this effect, as remarkable, and unlike any before known, since gum-elastic always melted when exposed to a high degree of heat.

“He was surprised to find that the specimen, being carelessly brought into contact with a hot stove, charred like leather.

Discovery of Vulcanizationfrom Goodyear’s Autobiographical

“Gum-Elastic” (1855)

Silliman consult

“Having seen experiments made, and also performed them myself, with the India rubber prepared by Mr. Charles Goodyear, I can state that it does not melt, but rather chars, by heat, and that it does not stiffen by cold, but retains its flexibility with cold, even when laid between cakes of ice.”

B. Silliman October 14, 1839

U.S. Pavilion Crystal Palace (1851)

Goodyear’s Vulcanite Court

India Rubber Desk

Mattatuck Museum, Waterbury

SomehowVulcanizationjoins adjacent chainswith sulfur “cross-links”

S

Latexpolymer

Radical Addition and Allylic Substitution?

H ?

Vulcanization and the Physical Properties

of Polymers

Gough

wordsworth

“No floweret bloomsThroughout the lofty range of these rough hills,Nor in the woods, that could from him concealIts birth-place; none whose figure did not liveUpon his touch.” Wordsworth “Excursion” (1813)

(1757-1825)

John Gough

Heating rubber makes it expand (more than H2O).

Heating tightly stretched rubber makes it contract!

If stretching rubber generates heat,what should letting it contract do?

A) If heat comes from internal friction, contraction should also cause

friction and generate heat.

B) If heat comes from some other cause, contraction may do the opposite

and absorb heat (“generate cold”).

Why?

Goodyear Plot

Goodyear Inventor

Goodyear to Gibbs

Gibbs Mathematical Physics

Gibbs to Onsager

Kirkwood & Onsager

PolymerStatistical Mechanics

Statistics Contracts a Stretched Chain

etc.

only one arrangement of maximum lengthmany arrangement of shorter length

Near maximum extensionthere is local Crystallization

Stretching

Contributes RigidityRigidity Releases Heat

Fixed, irregular cross-links between adjacent chains prevents crystallization (and brittleness) in the cold.

Warming “melts” the crystalline regions,and allows statistics to make the material contract.

Absorbing heat “melts” the crystalline regions,and allows statistics to make the material contract.

Lengthwise Motion by “Reptation”

Change shape by snaking along a tunnel through the tangled neighbors.

How to make a tangle flow?

Sulfur Cross-Links Stop Reptation

Vulcanization

(no flow when hot)

and inhibit crystallization.(not brittle when cold)

Vulcanization in theHome

End of Lecture 52Feb. 17, 2010

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