1. Chemistry 125: Lecture 53 February 19, 2010 Tuning Polymer Properties. Alkynes, Dienes & Conjugation This For copyright notice see final page of this file

2. Vulcanization in the Home

3. Hair before Permanent Wave “Reduce” disulfide cross links with excess basic RSH www.softspikecurlers.com S S S S S S S S RS - - H SR - H RS-SR - H H H H H H (pK a ~11) + NH 4 HS CO 2 HS CO 2 OH or H SR - H Curl

4. Permanent Wave www.softspikecurlers.com H H H H H H BDE kcal/mole HO-OH 52 RS-SR ~ 64 RS-H 87 RO-H 105 S S S S S S S S H H Curl “Oxidize” thiols back to disulfide with HOOH 139169

5. Synthetic Rubber

6. Thermoplastic Ionomers Malleable cross links

7. Julius Nieuwland Cl Neoprene

8. Natural Rubber vs. Synthetics

9. Radical Polymerization Poly(styrene) Regiochemistry R           R   head-to-tail random ~ 13 kcal/mole more stable than

10. Radical Polymerization Poly(propylene) Tacticity CH 3 H H H H H H H H H H H H H H H H H H H H H H H H H H H Isotactic (Radical) (Ziegler-Natta) Syndiotactic Atactic

11. Radical Copolymerization     CO 2 CH 3 Block CO 2 CH 3 Methyl Methacrylate  Styrene CO 2 CH 3 105  [1]2 k relative      CO 2 CH 3 Alternating ? fastest

12. Anti-Hammond Copolymerization     ~ 20 kcal/mole CO 2 CH 3    not as stable but twice as fast!

13. Radical Copolymerization CO 2 CH 3      C=O gives unusually low LUMO. Good when SOMO is not low. “Ionic resonance structure stabilizes transition state.” COCH 3 - O  + - O N.B. This special stability applies in TS only,not in the radical product!

14. Acetylenes Review Sec. 10.6-10.7 pp. 444-448 Sections 10.8-10.11 pp. 448-455

15. Stepwise Addition of HBr to Alkyne 1-Hexyne + HBr 2-Bromo-1-hexene FeBr 3 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. CH 3 -CH 2 -Cl CH 3 -CH 2 + + Cl - gas phase 193 kcal/mole CH 2 =CH-Cl CH 2 =CH + + Cl - 225 kcal/mole

16. Stepwise Addition of HBr to Alkyne 1-Hexyne + HBr 2-Bromo-1-hexene FeBr 3 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?

17. Stepwise Addition of HBr to Alkyne 1-Hexyne + HBr 2-Bromo-1-hexene FeBr 3 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 (  ).

18. 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

21. Regioselection Stereoselection

22. First e - First H +

23. Second e - Second H +

24. Alkyne Acidity and Isomerization Sec. 12.4 pp. 516-518

25. Approximate “pK a ” Values CH 3 -CH 2 CH 2 CH 2 H ~ 52 CH 3 -CH 2 CH=CHH ~ 44 CH 3 -CH 2 C CH ~ 25 ~ 34 H 2 NH = 16 HOH CH 3 -CH=C=CHH CH 3 -C C-CH 2 H ~ 38 sp 3 C _ sp 2 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 HA 1 + A 2 - = A 1 - + HA 2 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 pK a * : : (allylic) (Acidity of 1-Alkynes Secs. 3.14 p. 129; 12.4 p. 516-518)

26. H + (aq) + Equilibrium & Rate kcal/mol 40 30 20 10 -10 50 0 CH 3 -CH=C=CH 2 CH 3 -C C-CH 3 CH 3 -CH 2 C CH CH 3 -CH 2 C C CH 3 -CH=C=CH CH 3 -C C-CH 2 pK a  38 K a  10 -38  G  4/3  38 = 51 pK a  25 K a  10 -25  G  4/3  25 = 33 4.1 4.8 0.1%0.03% k  10 13  10 -38 /sec t 1/2 = 0.69/k  10 25 sec = 10 17 yrs  10 4  time since Big Bang [0]

27. H + (aq) + + HO - favors dissn. by 21 kcal (4/3  16) Equilibrium & Rate kcal/mol 40 30 20 10 -10 50 0 + H 2 N - favors dissn. by 45 kcal (4/3  34) CH 3 -CH=C=CH 2 CH 3 -C C-CH 3 CH 3 -CH 2 C CH CH 3 -CH 2 C C CH 3 -CH=C=CH CH 3 -C C-CH 2 t 1/2  30 yrs @ 300K -7.2 0.0001%  2 min @ 150°C

28. Trick to obtain terminal acetylene: Equilibrate with RNH _ base (in RNH 2 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.

29. C C Conjugation & Aromaticity (Ch. 12-13) Conjugated Pi Systems O C Yoke  Jungere  Jug ó m (to Join)

30. The Localized Orbital Picture (Pairwise MOs and Isolated AOs) Is Our Intermediate between H-like AOs and Computer MOs When must we think more deeply?

31. When does conjugation make a difference? Experimental Evidence

33. Conjugation worth ~5 kcal Conjugation worth <7 kcal

34. Conjugation worth ~ 4 kcal

35. Allylic Stabilization: Cation R-Cl  R + + Cl - (gas phase kcal/mol) Cl 193 172 171 Anion pK a 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

36. Why is conjugation worth more in allylic systems? Because we can draw reasonable resonance structures? good bad

37. Conjugation & Aromaticity (Ch. 12-13) http://www.chem.ucalgary.ca/SHMO/index.html Simple H ü ckel MOs

38. : : Sum is same as localized : : Secondary mixing is minor (because of poor E-match) Two Ways to Think about Butadiene  System 4 p-orbitals          How different in overall stability?Very Little! (~3 kcal/mole max) : : Localized  bond picture 4 Delocalized   ::        

39. Two Ways to Think about Butadiene  System 4 p-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

40. 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 picture 4 Delocalized   :: far UV (167 nm) nearer UV (210 nm)

41. Is There a Limit to  1 Energy for Long Chains? 8 1/  8 1/8 77/84 1/  4 1/4 33/4 Chain length 2 Normalized AO size 1/  2 Overlap per  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 H 2 C=CH 2  bond. Similarly, the LUMO destabilization limit is twice that of the H 2 C=CH 2   MO.. N.B. Here we are using our own “overlap stabilization” units, which are twice as large as conventional “  ” units.

42. End of Lecture 53 Feb. 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).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