Questions of the Day:
H
H
O
O
+
diene
O
O
dienophile
O
I
O
a) Diels-Alder reactions are what type of reactions?
b) Which structural feature is always formed as the result of
Diels-Alder reactions?
c) What is a “good diene” (i.e. one that reacts fast in DielsAlder)?
d) What would be a good dienophile?
Today:
• Diels-Alder (cont.), NMR
• Esterification (Exp.5), a first introduction
• MiniQuiz
Diels-Alder reactions require a
1,3-diene (in s-cis) and a
dienophile (an alkene, best when EWG is attached).
Reaction equations for our experiment:
Retro Diels Alder: Cracking of dimer
H
H
room temp.
+
high temp.
I
Dimer
dic yclopenatdiene
Diels Alder: Reaction of 1,3-cyclopentadiene with maleic anhydride
H
H
O
O
+
O
diene
O
Compare:
Hydrolysis of Diels-Alder product
H
H
H
H2O
H
O
O
O
Cis endo
O
I
O
dienophile
D.-A. adduct
II
CO2H
CO2H
• Polarity of I
versus II
• Mp. of I versus II
Diels – Alder Reactions
Which of the following dienes would be most reactive? why?
1,3-cyclopentadiene, 1,3-butadiene
(see problems on Report Form)
Which of the following dienophiles would be more reactive?
Ethene or maleic anhydride
Diels – Alder Reactions
Synthesis of Natural Products
C
Steroids!
A
B
D
CH3
C
CH3
A
CH3
D
CH3
B
HO
Cholesterol
Diels Alder
The H-NMR of the dimer of 1,3-cyclopentadiene is quite complex, with
many signals.
In contrast, the proton NMR of our Diels-Alder product is simple, with
few signals; the same is true for its hydrolysis product. How can you
explain this?
emp.
H
H
H
H
O
O
O
mp.
O
I
O
dic yclopenatdiene
dienophile
O
H
H
H2O
H
O
O
I
O
H
II
CO2H
CO2H
Methyl Benzoate Synthesis (Exp.5)
the first step of a two- step synthesis
CO2 H
+ CH 3 OH
H
+
CO2 CH3 + H 2 O
You will heat your reaction under reflux for 1 hour!
Do Practice Problems, calculations …
In order to increase the yield of the ester, which of the
following would help?
•
use an excess of methanol
•
use an excess of conc. sulfuric acid
Calculations …!
Why sulfuric
acid and not
HCl?
Esters and Esterifications
1. General properties of esters?
2. Compare the boiling points of carboxylic acids and
esters of similar molecular weight and explain.
3. Examples of esters in biological systems?
• Fats and Oils
• Waxes
• Ripening process in fruits
• phosphate esters in DNA
Salicin and Aspirin
OH
O
-D-glucose
O OH
C
O
O
Salicin
in willow bark
(Salix)
C
CH 3
Next time:
•Conclusion of “Esterification” (Exp. 5)
•Grignard reactions (Exp. 6)
•MiniQuiz on today’s class
MiniQuiz 4 Problems:
Br
Br
CH 2 CH 3
NBS
CCl 4
CHCH 3
NBS
CCl 4
1) Draw a simple, but clear H-NMR for
alpha - monobromoethylbenzene
2) Draw a simple, but clear H-NMR for
alpha - dibromoethylbenzene
CCH 3
Br