Chemistry 125: Lecture 52February 17, 2010
Additions by Radicals & Electrophilic Carbon;
Isoprenoids; andPolymer Properties This
For copyright notice see final page of this file
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.
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)
Gooey in heatBrittle in cold
Thomas Hancock(England -1820) “Masticator”
Goodyear
(1839)Vulcanization
Charles Macintosh(Scotland - 1823)
Sandwiched rubber between clothlayers for waterproof garments
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
SomehowVulcanizationjoins adjacent chainswith sulfur “cross-links”
S
Latexpolymer
Radical Addition and Allylic Substitution?
H ?
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)
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”).
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)
End of Lecture 52Feb. 17, 2010
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