questions of the day: a) diels-alder reactions are what type of reactions? b) which structural...
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Questions of the Day: I
+ O
O
OO
H
H
O
Odiene dienophile
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 Diels-Alder)?
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
Diels Alder: Reaction of 1,3-cyclopentadiene with maleic anhydride
Hydrolysis of Diels-Alder product
I
+ O
O
OO
H
H
O
Odiene dienophile
H
H+
I
room temp.
high temp.
dicyclopenatdiene O
H
H
O
O
H2O
CO2H
CO2H
H
H
II
Dimer
D.-A. adduct
Cis endo
Compare:
• 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 ReactionsSynthesis of Natural Products
A B
C DSteroids!
Cholesterol
A B
C D
CH3
CH3
CH3
CH3
HO
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?
H
H+
I
room temp.
high temp.
dicyclopenatdiene
I
+ O
O
OO
H
H
O
Odiene dienophile
O
H
H
O
O
H2O
CO2H
CO2H
H
H
II
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 …!
Methyl Benzoate Synthesis (Exp.5) the first step of a two- step synthesis
CO2CH3 + H 2O+ CH 3OHCO2HH
+
You will heat your reaction under reflux for 1 hour! Do Practice Problems, calculations …
Why sulfuric acid and not HCl?
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?
Esters and Esterifications
• Fats and Oils
• Waxes
• Ripening process in fruits
• phosphate esters in DNA
Salicin and Aspirin OH
O -D-glucose
Salicinin willow bark (Salix)
C
O
CH 3O
CO OH
Next time:
•Conclusion of “Esterification” (Exp. 5)
•Grignard reactions (Exp. 6)
•MiniQuiz on today’s class
MiniQuiz 4 Problems: CH2CH3
NBS
CCl4
CHCH3
Br
CCl4
NBSCCH3
Br
Br
1) Draw a simple, but clear H-NMR for alpha - monobromoethylbenzene
2) Draw a simple, but clear H-NMR for alpha - dibromoethylbenzene