chem 125 lecture 48 2/9/09 this material is for the exclusive use of chem 125 students at yale and...
TRANSCRIPT
Chem 125 Lecture 482/9/09
This material is for the exclusive use of Chem 125 students at Yale and
may not be copied or distributed further.
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