Questions of the Day: a) Diels-Alder reactions are what type of reactions? b) Which structural feature is always formed as the result of Diels-Alder reactions?

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