a) Write the structure of the radical formed

when Br. radical reacts with ethyl benzene.

b) Why is there no radical formation in the beta position of ethyl benzene?

MiniQuiz 2 Questions:MiniQuiz 2 Questions: CH2CH3

NBS

CCl4

CHCH3

Br

CCl4

NBSCCH3

Br

Br

Today:

•Diels-Alder (cont.)

•Esterification (Exp.5)

•MiniQuiz

Diels-Alder Reaction

I

+ O

O

OO

H

H

O

Odiene dienophile D.-A. adduct

The Diels-Alder reaction requires a 1,3-diene and a dienophile.The Diels-Alder reaction involves a 1,4-addition of an alkene to a 1,3-diene The Diels-Alder reaction involves a 1,4-addition of an alkene to a 1,3-diene

in an electrocyclic reaction (a “cycloaddition”).in an electrocyclic reaction (a “cycloaddition”).

The 1,3-diene must be in s-cis

conformation

The dienophile is an alkene; it is most reactive if EWG’s

are attached

1144

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 (Exp.4)

Which structural feature is always formed as the result of Diels-Alder reactions?

Diels – Alder ReactionsSynthesis of Natural Products

A B

C D

Cholesterol

A B

C D

CH3

CH3

CH3

CH3

HO

Diels – Alder Reactions

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

The H-NMR of the dimer of 1,3-cyclopentadiene (dicyclopentadiene) 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 …!

Exp.5:Methyl Benzoate Synthesis(first step for Exp.6 Grignard)

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

Next time:

Conclusion of “Esterification” (Exp. 5)

Grignard reactions (Exp. 6)

MiniQuiz on today’s lab lecture!