Chemistry 125: Lecture 53February 18, 2011

IsoprenoidsTuning Polymer Properties

AcetylenesPreliminary This

For copyright notice see final page of this file

e.g. J&F Sec. 12.13 pp. 554-562

R-L and R+ Electrophiles in*

Terpene/Steroid Biogenesis

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

Isopentenyl Pyrophosphate

GeranylPyrophosphate

C5

C10

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)

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

Axe

l Bol

dt

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

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

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?

View from Yale Health Center

Goodyear Inventor

Goodyear to Gibbs

Gibbs Mathematical PhysicsJ. Willard GibbsB.A. Yale,1858

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

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

SyntheticRubber

Thermoplastic Ionomers

Malleablecross links

Julius NieuwlandJulius NieuwlandCl

Neoprene

http

://la

mb.

arch

ives

.nd.

edu/

phot

os/0

5A-0

14.h

tm

Natural Rubber vs. Synthetics

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 ().

Hydration and Hydrogenation

of Alkynes

+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

Alkyne Acidity and Isomerization

e.g. J&F Sec. 12.4 pp. 516-518

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.

C C

Conjugation & Aromaticity(Ch. 12-13)

Conjugated Pi Systems

OC

Yoke

Jungere

Jugóm

(to Join)

The Localized Orbital Picture(Pairwise MOs and Isolated AOs)

Is Our Intermediate betweenH-like AOs and Computer MOs

When must we think more deeply?

When does conjugationmake a difference?

Experimental Evidence

Conjugation worth

~5 kcal

Conjugation worth

<7 kcal

Conjugation worth

~ 4 kcal

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