123.202 Lecture 11 - alkynes

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123.202 lecture 11 alkynes and dienes

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123.202 Lecture 11 - alkynes

  1. 1. ..LECTURE ELEVEN dienes & alkynes ©canyon289@flickr
  2. 2. PREVIOUSLY... Br radicals...
  3. 3. 12 dienes H H C H H isolated conjugated cumulated diene diene diene allene
  4. 4. cumulated dienes allenes mirror H H Ph Ph C C H H Ph Ph
  5. 5. π(2py+2py) Ph H C C C H Ph orbitals H Ph C H Ph
  6. 6. π(2py+2py) sp Ph H sp2 C C C H Ph π(2pz+2pz) shape 90° H Ph C H Ph
  7. 7. conjugated dienes stabilisation
  8. 8. resonance stabilisation
  9. 9. resonance stabilisation Observed Expected kJmol–1 kJmol–1 127 253 254 254 254 239 254
  10. 10. 13 electrophilic addition to conjugated dienes HX X X H H HX X H H X regioisomers
  11. 11. how many ? regioisomers ? X X H H H H HX X X X H
  12. 12. how many regioisomers ? X X H H H H HX X X X H
  13. 13. how many regioisomers ? X X H H H H HX X X X least H favoured
  14. 14. 13 electrophilic addition to conjugated dienes Br2 Br 2 x Br2 Br Br Br
  15. 15. 13 electrophilic addition to conjugated dienes Br2 Br Br Br2 Br Br regioisomers
  16. 16. mechanism Br Br Br Br Br Br Br Br Br
  17. 17. alkynes ©SlamEye@flickr
  18. 18. 14 acidity of alkynes R H Na NH2 very strong base R Na NHH2
  19. 19. X not acidic H H H R H R H
  20. 20. ©Horia Varian@flickr
  21. 21. sp sp2 sp3 (50% s) (33% s) (25% s) molecular orbitals
  22. 22. decreasing acid strength OH H O H H H N HCl > > > O > O > >H H > R OH H H R R R O O N O O Cl < < <H <R < < H < R O H R R increasing base strength
  23. 23. strong acid + strong base weaker acid + weaker base
  24. 24. OH strong strong base acid R O H weak weak acid base R
  25. 25. H O weak weak acid base R X OH strong strong base R acid
  26. 26. http://www2.lsdiv.harvard.edu/labs/evans/ best pKa table I know
  27. 27. good nucleophile R H Na NH2 R Br R SN2
  28. 28. DANGER ©SlamEye@flickr
  29. 29. strong base R
  30. 30. Br R SN2 X E2 R side reaction
  31. 31. 15 hydration of alkynes HgOAc H3O... Hg(OAc)2 H2O OH HgOAc Markvonikov addition
  32. 32. tautomerisation OH O enol ketone
  33. 33. hydroboration/oxidation BH3 BH2 H anti Markvonikov H2O2 NaOH addition OH O H
  34. 34. good luck ©fd@flickr

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