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I have an office hour on Tues 10:45 am - 11:45 am in Chem Room 759. This is to handle primarily administrative details. I will hold a general office hour on Friday 1:15-2:15 pm in the Chemistry Learning Center, Chemistry Room 312. The Chemistry Learning Center is the place to go for help with problem sets, lectures and workshops. The TAs will have office hours there through out the week. Hours will be posted on the web site soon.
Br H-Br + Br
-80C
80%
20%
+ Br
What is the second product? h h d d A. C.
Br Br
B. D.
Br
Br
Br HBr + Br
1,2 addition
1,4 addition
-80C 80% 1,2 20% 1 4 1,4 +40C 20% 1,2 80% 1 4 1,4
Br HBr + Br
1,2 addition
1,4 addition
-80C 80% 1,2 20% 1 4 1,4 +40C 20% 1,2 80% 1 4 1,4
Kinetic Product
Thermodynamic y Product
or H Br
Br
Br
Br
+ DCl
Ion Pair
H-Br HB D Cl
Br
Never far apart Proximity P effect
+ kinetic product thermodynamic product
Br
H
D
H
D
equal contributions
Cl-
Predict the kinetic product y Predict the thermodynamic product
Br Br Br Br
D + Cl Cl
Cl- simply reacts with the closest carbon of the delocalized cation.
65%
35%
A
B
C
D
plus enantiomers
The Diels Alder reaction is a [2 + 4] cycloaddition reaction reaction.
O O
O O
+
+
4 the diene
2 the dieneophile 4 the diene 2 the dieneophile
2 new bonds formed f d at once!
The reaction was discovered by German chemists, Otto Diels and Kurt Alder. They received the Nobel Prize in 1950.
Arrows can go either way
or
A A
LUMO
The diene must be in the s-cis configuration.
HOMO
A S
LUMO
LUMO
A S
HOMO
HOMO
S
LUMO
s-trans
0 kJ
+1.5 kJ 15
+1 kJ 1
s-cis
S S
HOMO
A A
HOMO
LUMO
Arrows can go either way
or
A S
LUMO
LUMO
The diene must be in the s-cis configuration.
A S
HOMO
HOMO
S
LUMO
S
S
HOMO
s-trans
0 kJ
+1.5 kJ 15
+1 kJ 1
s-cis
CH3 O H3C H3C cyclohexa-1,3-diene cyclopenta-1,3-diene furan CH3
Atoms would collide!
Some cyclic dienes work very well because they are "locked" into the s-cis conformation.
But with big subsituents sometimes you can not make s-cis. No Diels Alder possible. cis.
s-trans
s-cis
s-trans
s-cis
+
200o s C
closed vessel
A
Base B
A
LUMO
LUM O
The i l Th simplest Diels Alder reaction between Di l Ald i b butadiene and ethane is not very favorable. It has a very large energy of activation. How can we make the Diels Alder reaction more favorable? We can add substituents that enhance the flow of electrons electrons.
S
LUMO
A S
HOM O
A
HOMO
HOMO
S
Raise this level Acid How do you make something a better base? Add more electrons. Add an El t Electron D Donating G ti Group. (EDG)
A
Lower this level Base B
A
LUMO
LUM O
The simplest Diels Alder reaction between butadiene and ethane is not very favorable.
LUMO
S
It has a very large energy of activation. activation How can we make the Diels Alder reaction more favorable?
A S
HOM O
A
HOMO
HOMO
S
Acid How do you make something a better acid? Remove electrons. Add an Electron Withdrawing G El t Withd i Group. (EWG)
We W can add Electron Donating Groups to the dd El D G h Diene and make it a better base. And we can add Electron Withdrawing Groups to the Dienophile and make it a better acid.
We can add Electron Donating Groups to the Diene and make it a better base base.
O O O CH3 H3C O NH2
We can add Electron Donating Groups to the Diene and make it a better base base.
O O
A B C D E Here are ...