Chem 125 Lecture 482/9/09

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It is not readily understood without reference to notes or the wiki from the lecture.

3. Consider the chlorination reaction :

i-Pr2NCl + RH i-Pr2NH + RCl

and these approximate bond dissociation energies (kcal/mole): N-Cl (46), R-Cl (85), N-H (92), R-H (100).

G. (3 min) Why should the BDEs of N-Cl and R-Cl be so different, when those for N-H and R-H

are so similar?

Could e-Pair Repulsion Explain BDEs?

1.79 Å

1.73 Å1.40 Å

1.53 ÅH3C CH3

HO OH H2N Cl

H3C Cl

84BDE 90 kcal/mole

51 46

Total e-Density Contour (a0-3)

0.0020.010.050.25

Drawing proton away from nucleus removes OMO-OMO e-density from overlap region.

isoe

lect

roni

c

isoe

lect

roni

c

from Wikipedia

Lone pair repulsion seems a plausible explanation for weakening O-O vs. C-C or N-Cl vs. C-Cl.

But might electronegativity help explain stronger C-Cl than N-Cl ?

C+ Cl-

Which Bond is Stronger N-Cl or C-Cl?

Cl N

Ele

ctro

n E

nerg

y

separate separate

C

Compared to What?

••••

N-Cl stronger if forming Ions (N+ Cl-)

••

C-Cl stronger if forming Atoms (Cl C)• •

together

mismatch aids Heterolysis

mismatch hinders Homolysis

•

•

“Electronegativity” and Bond Strength

First use in English (O.E.D.)

1837 J. D. Dana Syst. Mineral. 82 When chemistry has so far advanced, that the relative electro-negativity, (if I may so call it,) or electro-positivity, of the several elements, is fully known,..we shall probably be able to construct a natural arrangement of minerals on chemical principles.

J. D. Dana1813-1895

Silliman’s son-in-lawDana House

1849

“Electronegativity” and Bond Strength

H-X“normal”(average of

H-H and X-X)

actualH+ X-

1932Pauling was

pushing resonance.

Why not use to measure resonance

stabilization?

actual

“Normal”

Observed BDE

(units of electron volts)

1932Pauling was

pushing resonance.

~ additivenothing special

O-O O-F

F-F

Relative to H & F

Relative to O

Relative to C

Relative to H

1.00

1.48

0.58

1932Pauling was

pushing resonance.

Is it surprising that bond strength should correlate with Pauling electronegativity differences?

No, his Pscale was defined by differences in bond strength.

from Wikipedia

Mulliken Electronegativity (1934) average of Atomic Ionization Potential and Electron Affinity

Pau

ling

Ele

ctro

nega

tivity

But we expect energy-mismatch to strengthen bonds, so crude

correlation of P with IP and EA is hardly surprising.

Radical PolymerizationSec 5.7 (214-216)

LDPE (Problem 5.44 p. 233)

R

H

Occasional butyl side-chains inhibit close packing.

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

Vulcanizationjoins adjacent chainswith sulfur “cross-links”

S

Latexpolymer

Text pp. 708-709

Addition and allylic substitution

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

Caoutchouc expands on heating.

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”).

Physical Properties

Why?

Goodyear Plot

Goodyear Inventor

Goodyear to Gibbs

Gibbs to Onsager

Gibbs Jr. & Sr.

Statistical Mechanics

End of Lecture 48Feb. 9, 2009