Chemistry 125: Lecture 53February 19, 2010

Tuning Polymer Properties.Alkynes, Dienes& Conjugation

This

For copyright notice see final page of this file

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

Permanent Wave

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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

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

Radical Copolymerization

CO2CH3 CO2CH3CO2CH3

CO2CH3

Block

CO2CH3

MethylMethacrylate Styrene

CO2CH3

105

[1]2

krelative

CO2CH3CO2CH3CO2CH3CO2CH3

Alternating

?

fastest

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!

AcetylenesReview Sec. 10.6-10.7 pp. 444-448

Sections 10.8-10.11 pp. 448-455

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

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

But as shown in text, 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?

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

The schizophrenic nature of a Br substituent.

Br is both electron withdrawing () and electron-donating ().

Hydration of Alkyne

Markovnikov or anti-Markovnikov

Initial enol undergoes acid-catalyzed isomerization.

Because C=O is so stable(compare average bond energies)

10.8 to 10.11

Reg

iose

lect

ion

Ste

reos

elec

tion

First e-

First H+

Second e- Second H+

Alkyne Acidity and Isomerization

Sec. 12.4 pp. 516-518

Approximate “pKa” Values

CH3-CH2CH2CH2H ~ 52

CH3-CH2CH=CHH ~ 44

CH3-CH2C CH ~ 25

~ 34 H2NH

= 16 HOH

CH3-CH=C=CHH

CH3-C C-CH2H ~ 38

sp3 C_

sp2 C_ (no overlap)

sp C_ (no overlap)

C_ HOMO - overlap(better E-match N-H)

(bad E-match O-H)

(best E-match C-H)

* Values are approximate because HA1 + A2- = A1

- + HA2 equilibria for bases stronger that HO- cannot be measured in water. One must

“bootstrap” by comparing acid-base pairs in other solvents.

50

40

30

20

10

pKa

*

:

:

(allylic)

(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]

H+(aq) +

+ HO-

favors dissn. by 21 kcal

(4/3 16)

Equilibrium & Rate

kcal

/mol

40

30

20

10

-10

50

0

+ H2N-

favors dissn. by 45 kcal (4/3 34)

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

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

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