123.202 lecture 11 alkynes and dienes

1. ..LECTURE ELEVEN
dienes & alkynes
©canyon289@flickr

2. PREVIOUSLY...
Br radicals...

3. 12 dienes
H
H
C
H
H
isolated
conjugated cumulated
diene diene
diene
allene

4. cumulated dienes
allenes
mirror
H H
Ph Ph
C C
H H
Ph Ph

5. π(2py+2py)
Ph H
C C C
H
Ph
orbitals
H
Ph
C
H
Ph

6. π(2py+2py)
sp
Ph H
sp2
C C C
H
Ph
π(2pz+2pz)
shape
90°
H
Ph
C
H
Ph

7. conjugated
dienes
stabilisation

8. resonance stabilisation

9. resonance stabilisation
Observed Expected
kJmol–1 kJmol–1
127
253 254
254 254
239 254

10. 13 electrophilic addition to conjugated dienes
HX
X X
H
H
HX
X H
H
X
regioisomers

11. how many
?
regioisomers ?
X X
H H
H
H
HX X
X
X
H

12. how many
regioisomers ?
X X
H H
H
H
HX X
X
X
H

13. how many
regioisomers ?
X X
H H
H
H
HX X
X
X
least H
favoured

14. 13 electrophilic addition to conjugated dienes
Br2
Br
2 x Br2 Br
Br
Br

15. 13 electrophilic addition to conjugated dienes
Br2
Br
Br
Br2
Br
Br
regioisomers

16. mechanism Br
Br
Br
Br
Br
Br
Br
Br
Br

17. alkynes
©SlamEye@flickr

18. 14 acidity of alkynes
R H Na NH2
very strong
base
R Na NHH2

19. X
not
acidic
H H
H
R H R H

20. ©Horia Varian@flickr

21. sp sp2 sp3
(50% s) (33% s) (25% s)
molecular
orbitals

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. strong acid + strong base
weaker acid + weaker base

24. OH
strong strong base
acid R
O H
weak
weak acid
base
R

25. H O
weak
weak acid
base
R
X
OH
strong
strong base
R
acid

26. http://www2.lsdiv.harvard.edu/labs/evans/
best pKa
table I
know

27. good nucleophile
R H Na NH2 R
Br
R
SN2

28. DANGER
©SlamEye@flickr

29. strong base
R

30. Br
R
SN2 X E2
R
side reaction

31. 15 hydration of alkynes
HgOAc
H3O...
Hg(OAc)2
H2O
OH
HgOAc
Markvonikov
addition

32. tautomerisation
OH O
enol ketone

33. hydroboration/oxidation
BH3 BH2
H
anti
Markvonikov H2O2
NaOH
addition
OH
O
H

34. good
luck
©fd@flickr