..LECTURE ELEVEN
 dienes & alkynes



                   ©canyon289@flickr
PREVIOUSLY...




Br      radicals...
12 dienes

                       H
                               H
                           C
                        ...
cumulated dienes
allenes

             mirror
 H                          H
            Ph     Ph
      C                 ...
π(2py+2py)
    Ph                           H
            C       C       C
     H
                                 Ph

or...
π(2py+2py)
                         sp
  Ph                           H
                                    sp2
         C...
conjugated
dienes




             stabilisation
resonance stabilisation
resonance stabilisation
                Observed   Expected
                 kJmol–1    kJmol–1

                   127

 ...
13 electrophilic addition to conjugated dienes
HX
                           X            X
              H
              ...
how many




                         ?
regioisomers ?
                     X               X
                 H          ...
how many
regioisomers ?
                     X               X
                 H           H

                         H
...
how many
regioisomers ?
                     X               X
                 H           H

                         H
...
13 electrophilic addition to conjugated dienes
Br2

                                Br
            2 x Br2     Br
        ...
13 electrophilic addition to conjugated dienes
Br2

                        Br
                                           ...
mechanism                   Br
                  Br

                       Br


                        Br
       Br
    ...
alkynes


©SlamEye@flickr
14 acidity of alkynes

 R               H      Na    NH2
                              very strong

                      ...
X
not
acidic

         H     H
                   H
R        H R   H
©Horia Varian@flickr
sp       sp2          sp3
(50% s)   (33% s)      (25% s)


                    molecular
                     orbitals
decreasing acid strength
                           OH                               H
              O                    ...
strong acid + strong base




weaker acid + weaker base
OH

strong            strong base
  acid        R




         O    H

 weak
                  weak acid
base
            ...
H       O
                            weak
weak acid
                            base
              R


                  ...
http://www2.lsdiv.harvard.edu/labs/evans/




                                    best pKa
                               ...
good nucleophile


R         H Na   NH2            R
                       Br




                            R

    SN2
DANGER




©SlamEye@flickr
strong base




R
Br
      R

    SN2   X         E2




R

              side reaction
15 hydration of alkynes


                        HgOAc
        H3O...
       Hg(OAc)2
                  H2O
             ...
tautomerisation




     OH              O

  enol            ketone
hydroboration/oxidation

              BH3                BH2
                             H
anti
   Markvonikov          ...
good
            luck
©fd@flickr
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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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