Post on 22-Dec-2015
Chemistry 125: Lecture 53February 18, 2011
IsoprenoidsTuning Polymer Properties
AcetylenesPreliminary This
For copyright notice see final page of this file
Isopentenyl Pyrophosphate
DimethylallylPyrophosphate
Adjacent unsaturationapparently speeds SN2
(as well as SN1)Cl
I
benzyl 250Cl
allyl 90Cl
n-propyl
krel for SN2 with
I- in acetone
[1]Cl
Geranyl Pyrophosphate cistrans
NerylPyrophosphate
Limonene
-H+
-Pinene
-H+
-H+
+H2O[Ox]
Camphor
"Terpene" essential oils C10
Markovnikov
anti-Markovnikov
Geranyl Pyrophosphate
Farnesyl Pyrophosphate
"head-to-headreductive dimerization"
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)
1839
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)
Hair before
Permanent Wave
“Reduce” disulfide cross links with
excess basic RSH
www.softspikecurlers.com
S S
SS
S S
S S
RS-
-
HSR-
H
RS-SR
-
H
H
H
H
H
H
(pKa~11)
+NH4HS CO2
HS CO2
OH
OHor
HSR-
H
Curl
with permission
Permanent Wave
www.softspikecurlers.com
H
H
HHH
H
BDE kcal/mole
HO-OH 52 RS-SR ~ 64 RS-H 87 RO-H 105
S
S
S S
S S
SS
H H
Curl
“Oxidize” thiols back to disulfide with HOOH
139 169
Julius NieuwlandJulius NieuwlandCl
Neoprene
http
://la
mb.
arch
ives
.nd.
edu/
phot
os/0
5A-0
14.h
tm
Radical PolymerizationPoly(styrene) Regiochemistry
R
R
head-to-tail
random
~ 13 kcal/molemore stable
than
Radical PolymerizationPoly(propylene) Tacticity
CH3H CH3
H CH3H CH3
H CH3H CH3
H CH3HCH3
H CH3H
CH3H CH3
H CH3H CH3
HCH3H HCH3 HCH3 HCH3 HCH3
CH3H CH3
H CH3HCH3
H CH3H HCH3 HCH3 HCH3 HCH3
Isotactic
(Radical)
(Ziegler-Natta)
Syndiotactic
Atactic
(Kaminsky)
Radical Copolymerization
CO2CH3 CO2CH3CO2CH3
CO2CH3
Block
CO2CH3
MethylMethacrylate Styrene
CO2CH3
2[1]
0.20.4
krelative
CO2CH3CO2CH3CO2CH3CO2CH3
Alternating
?
fastest(good radical)
Anti-Hammond Copolymerization
~ 2
0 k
cal/m
ole
CO2CH3
CO2CH3
CO2CH3
not as stable
but twice as fast!
Radical Copolymerization
CO2CH3
CO2CH3
C=O gives unusually low LUMO. Good when SOMO is not low.
“Ionic resonance structure stabilizes
transition state.”
COCH3-O
+ -
COCH3
O
N.B. This special stability applies in TS only, not in the
radical product!
Generalization to
Acetylenes
e.g. J&F Sec. 10.6-10.11 pp. 444-455
Stepwise / Markovnikov
“Keto-Enol Tautomerism”Regioselection
Addition of HBr
Addition of H2O
Addition of H2 Stepwise / Stereoselection
Acidity and base-catalyzed isomerization
Stepwise Addition of HBr to Alkyne
1-Hexyne + HBr 2-Bromo-1-hexene
FeBr3
15°C
with “inhibitor”to trap radicals isolated in 40% yield
100 to 1000x slower than comparable ionic addition to alkene, because vinyl cation is not so great.
CH3-CH2-Cl CH3-CH2+ + Cl-
Gas Phase Ionization
193 kcal/mole
CH2=CH-Cl CH2=CH+ + Cl-225 kcal/mole
Stepwise Addition of HBr to Alkyne
1-Hexyne + HBr 2-Bromo-1-hexene
FeBr3
15°C
with “inhibitor”to trap radicals isolated in 40% yield
HBr can add again to the bromoalkene (obviously more slowly) to give a second Markovnikov addition
If the bromo substituent slows addition to an alkene, why is there Markovnikov orientation?
2,2-Dibromohexane
Br is a “schizophrenic” substituent: both electron withdrawing (), and electron-donating ().
+Hg(OAc)2
H+ / H2OHC CR +
HC CR
HgOAc HgOAc
CO
R
CHH2O -H+H+
NaBH4
CO
R
CH H
H
H
Markovnikov EnolH
+
H
Ketone
an easy allylicrearrangement
“Keto-Enol Tautomerism”
+
(favors ketoneCf. Lecture 37)
ve Bond Energies
Can one sum bond energies to getaccurate"Heats of Atomization"?
H C
O
H
CCH H
H
H
H C
O
H
CCH H
H
HKetone "Enol"
C
O
CH
C
O
C
H
C=O 179
C-C 83
C-H 99
sum 361
C-O 86
C=C 146
O-H 111
sum 343
Kcalc = 10-(3/4) 18 = 10-13.5
Kobs = 10-7 = 10-(3/4) 9.3
Bonds that change(the others should cancelin taking the difference)
H C
O
H
CCH H
H
H
H C
O
H
CCH H
H
HKetone "Enol"
H
Why is Enol9 kcal/mole
"Too" Stable?
O
C=O 179
C-C 83
C-H 99
sum 361
C-O 86
C=C 146
O-H 111
sum 343
Kcalc = 10-(3/4) 18 = 10-13.5
Kobs = 10-7 = 10-(3/4) 9.3
••
C(sp2)-Hstronger than
C(sp3)-H
(they shouldn’t actually cancel)
IntramolecularHOMO-LUMO
Mixing
H C
O
H
CCH H
H
H+"ResonanceStabilization”
from
Markovnikov Enol
+Hg(OAc)2
H+ / H2OHC CR +
HC CR
HgOAc
CO
R
CH
H2O
-H+H +
H
Ketone
R’2B-HHC CR C
R
R’2B
CH
H
Anti-Markovnikov Enol
Aldehyde
HOOH
HO-
CR
HO
CH
H
H
vinylborane(hindered R’2BHadds only once)
BH3 + 2
e.g. “disiamylborane”
Hydration with Either Regiospecificity
(what is R’?)
n-Pr-C C-n-Pr
Hydrogenation with Either Stereospecificity
( Pd / CaCO3 / Pb )
H2
Lindlar CatalystCn-Pr
H
C
H
n-Pr
deactivate Pd to stop at alkene
n-Pr-C C-n-PrNa / NH3
C
n-Pr
H
CHn-Pr
%
“dissolving metal reduction”
syn addition
H H
anti addition
H
H
solvated electronNa
NH3
e-(NH3)n+ Na+
R-C C-R
First H+
R-C C-R
R-C C-R e-First e-
C C
R
RH
C C
R R
H
Vinyl radicals are sp2
but they invert easily
H
NH2NH2
Second H+ e-H
NH2
NH2
C C
R
RHVinyl anions are sp2
and invert very slowly(remember XH3)
Second e-
C C
R
RH
C C
R R
H
Vinyl radicals are sp2
but they invert easily
C C
R
RH
H
anti addition(because of radical isomerism)
H
H
Approximate “pKa” Values
CH3-CH2CH=CHH ~ 44
CH3-CH2C CH ~ 25
CH3-CH=C=CHH
CH3-C C-CH2H ~ 38
sp3 C_
sp2 C_ (no overlap)
sp C_ (no overlap)
C_ HOMO - overlap
CH3-CH2CH2CH2H ~ 52
~ 34 H2NH
= 16 HOH
(better E-match N-H)
(bad E-match O-H)
(best E-match C-H)50
40
30
20
10
pKa
*
:
:
(allylic)
(e.g. J&F Acidity of 1-Alkynes Secs. 3.14 p. 129; 12.4 p. 516-518)
H+(aq) +
Equilibrium & Rate
kcal
/mol
40
30
20
10
-10
50
0 CH3-CH=C=CH2
CH3-C C-CH3
CH3-CH2C CH
CH3-CH2C C
CH3-CH=C=CHCH3-C C-CH2
pKa 38
Ka 10-38
G 4/3 38 = 51
pKa 25
Ka 10-25
G 4/3 25 = 33
4.1 4.8
0.1% 0.03%
k 1013 10-38 /sec
t1/2 = 0.69/k 1025 sec = 1017 yrs 104 time since Big Bang
[0]
at equilibrium
H+(aq) +
+ HO-
favors dissn. by 21 kcal
(4/3 16)
Equilibrium & Rate
kcal
/mol
40
30
20
10
-10
50
0 CH3-CH=C=CH2
CH3-C C-CH3
CH3-CH2C CH
CH3-CH2C C
CH3-CH=C=CHCH3-C C-CH2
t1/2 30 yrs @ 300K
-7.20.0001%
2 min @ 150°C + H2N
-
favors dissn. by 45 kcal (4/3 34)
at equilibrium
Trick to obtain terminal acetylene:
Equilibrate with RNH_
base(in RNH2 solvent at room temp)
to form terminal anion.“Quench” by adding water which donates H+ to terminal anion and to RNH_, leaving OH_, which is too weak to allow equilibration.Or add H+, so even [OH
_] is very low.
The Localized Orbital Picture(Pairwise MOs and Isolated AOs)
Is Our Intermediate betweenH-like AOs and Computer MOs
When must we think more deeply?
Allylic Stabilization:Cation
R-Cl R+ + Cl-(gas phase kcal/mol)
Cl
Cl
Cl
193
172
171
Anion
pKa
OH
OH
16
10
5OHO
Radical
Bond Dissociation
Energy (kcal/mol)
H
H
101
89
Conjugation worth ~ 13 kcal !
as good as secondary
4/3 6 = 8 kcal
Why is conjugation worth more in allylic systems?
Because we can draw reasonable resonance structures?
good
bad
Conjugation & Aromaticity(Ch. 12-13)
http://www.chem.ucalgary.ca/SHMO/index.html
Simple Hückel MOs
::
Sum is same as localized
::
Secondary mixing is
minor
(because of poor E-match)
Two Ways to Think about Butadiene System
4p-orbitals
How different in overall stability? Very Little!(~3 kcal/mole max)
::
Localized bond picture4 Delocalized
: :
Two Ways to Think about Butadiene System
4p-orbitals
::
4 Delocalized
: :
Why ignore this mixing?
Despite better E-match, it does not
lower energy.
(What would be gained on one end
would be lost on the other)
Orthogonal
But there are substantial differences in HOMO &
LUMO energies (Reactivity), and in HOMO-LUMO gap
(color)
But there are substantial differences in HOMO &
LUMO energies (Reactivity), and in HOMO-LUMO gap
(Color).
Two Ways to Think about Butadiene System
::
How different in overall stability? Very Little!(~3 kcal/mole max)Localized bond picture4 Delocalized
: :
farUV
(167 nm)
nearerUV
(210 nm)
Is There a Limit to 1 Energy for Long Chains?
8 1/8 1/8 7 7/8
4 1/4 1/4 3 3/4
Chain length
2
Normalized AO size
1/2
Overlapper bond
(AO product)
1/2
Number of
bonds
1
Total overlap
stabilization
1/2
N 1/N 1/N N-1 (N-1)/N
Yes, the limit is 1, i.e. twice the stabilization of the H2C=CH2 bond.
Similarly, the LUMO destabilization limit is twice that of the H2C=CH2 MO..
N.B. Here we are using our own “overlap stabilization” units, which are twice as large as conventional “” units.
End of Lecture 53Feb. 19, 2010
Copyright © J. M. McBride 2010. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).
Use of this content constitutes your acceptance of the noted license and the terms and conditions of use.
Materials from Wikimedia Commons are denoted by the symbol .
Third party materials may be subject to additional intellectual property notices, information, or restrictions.
The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0
End of Lecture 53February 18, 2011
Copyright © J. M. McBride 2011. Some rights reserved. Except for cited third-party materials, and those used by visiting speakers, all content is licensed under a Creative Commons License (Attribution-NonCommercial-ShareAlike 3.0).
Use of this content constitutes your acceptance of the noted license and the terms and conditions of use.
Materials from Wikimedia Commons are denoted by the symbol .
Third party materials may be subject to additional intellectual property notices, information, or restrictions.
The following attribution may be used when reusing material that is not identified as third-party content: J. M. McBride, Chem 125. License: Creative Commons BY-NC-SA 3.0