Chem 125 Lecture 48 2/9/09 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-Pr 2 NCl + RH  i-Pr 2 NH + 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 Å 1.40 Å 1.53 Å H 3 C CH 3 HO OHH 2 N Cl H 3 C Cl 84 BDE 90 kcal/mole Total e-Density Contour (a 0 -3 ) Drawing proton away from nucleus removes OMO-OMO e-density from overlap region. isoelectronic

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 Electron Energy 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. Dana Silliman’s son-in-law Dana House 1849

“Electronegativity” and Bond Strength

H-X “normal” (average of H-H and X-X) actual H + X -  1932 Pauling was pushing resonance. Why not use  to measure resonance stabilization? actual

“Normal”     Observed BDE (units of electron volts) 1932 Pauling was pushing resonance. ~ additive nothing special O-OO-F F-F

Relative to H & F Relative to O Relative to C Relative to H Pauling was pushing resonance. Is it surprising that bond strength should correlate with Pauling electronegativity differences? No, his  P  scale was defined by differences in bond strength.

from Wikipedia Mulliken Electronegativity (1934) average of Atomic Ionization Potential and Electron Affinity Pauling Electronegativity But we expect energy-mismatch to strengthen bonds, so crude correlation of  P with IP and EA is hardly surprising.

Radical Polymerization Sec 5.7 ( ) LDPE (Problem 5.44 p. 233) R H Occasional butyl side-chains inhibit close packing.

2 Isoprenes Isoprene H OH Geraniol “dimer” H OH

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 heat Brittle in cold Thomas Hancock (England -1820) “Masticator” Goodyear (1839) Vulcanization Charles Macintosh (Scotland ) Sandwiched rubber between cloth layers 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 Vulcanization from 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

Vulcanization joins adjacent chains with sulfur “cross-links” S Latex polymer Text pp Addition and allylic substitution

Gough

wordsworth “ No floweret blooms Throughout the lofty range of these rough hills, Nor in the woods, that could from him conceal Its birth-place; none whose figure did not live Upon his touch.” Wordsworth “Excursion” (1813) ( )

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 48 Feb. 9, 2009