Selective Aldol Completed

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Organic Synthesis Experimental Activity: Enantioselective Aldol Reaction

1. Provide the chemical reaction and mechanism of the enantioselective Aldol reaction

being performed in this video.

2. Explain what the term “enantioselective” means? Does it mean something different from

“enantiospecific”?

Enantioselective means that the reaction formed two enantiomers in unequal amounts. The

main difference between enantioselective and enantiospecific is that in enantioselective,

two products are formed (pair of enantiomers) while enantiospecific leads to the formation

of a single product.

3. Which enantiomer is being formed preferentially?

In the reaction, the R-isomer was favored.

4. Explain how the reaction can be enantioselective?

The reaction was enantioselective due to the hydrogen-bonded transition state. The O atom

of the isobutyraldehyde interacted with the OH group of the enamine. Because of this, the

attack of the nucleophile to one of the sides of the carbonyl carbon is blocked, favoring the

formation of the R-isomer.

P a g e | 2

5. What is being used in place of an enol or enolate? Explain the reactivity of this nucleophile

in comparison to an enol and enolate using resonance contributors of each.

Instead of the enolate ion, an enamine was used in the reaction. The enolate is a nucleophile

that is capable of attacking the carbonyl carbon on both sides while in the case of the

enamine, it forms H-bonding with the carbonyl group which leads to the preferential attack

on the one side of the carbonyl carbon over the other.

6. Write out the reaction procedure and fill in the following reaction table.

Place 1 mL of isobutyraldehyde in a 25 mL Erlenmeyer flask and then add about 14 mL of

acetone. After that, add about 230 mg of L-proline to the reaction mixture. Stir the mixture

at room temperature. Place the mixture inside the cabinet for 1 week ensuring that no light

enters the reaction mixture. To the reaction mixture, add 50 mL of saturated NaCl solution

and then extract once with 15 mL diethyl ether. Then, dry using magnesium sulfate. Decant

the organic layer to a 25 mL Erlenmeyer flask and concentrate the mixture thru evaporation.

Subject the concentrated product to a polarimeter using chloroform as solvent. Lastly,

obtain the

H-NMR and

C-NMR spectra and IR spectrum of the product.

Name

Amount

density

mmol

equivalents

BP/MP

CAS#

Safety Info

Appearance

isobutyraldehyde

72.11

g/mol

1 mL

0.79

g/mL

78-

84-2

Toxic.

Flammable

clear liquid

acetone

58.08

g/mol

14 mL

0.79

g/mL

190

67-

64-1

Toxic.

Flammable

clear liquid

L-proline

115.13

g/mol

230 mg

N/A

0.18

228

147-

85-3

N/A

white solid

Note: Catalysts can be the limiting reactant as they are not part of the chemical reaction overall.

Often, less than a stoichiometric amount is added (less than 1 equivalent)

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Page |1 Organic Synthesis Experimental Activity: Enantioselective Aldol Reaction 1. Provide the chemical reaction and mechanism of the enantioselective Aldol reaction being performed in this video. 2. Explain what the term “enantioselective” means? Does it mean something different from “enantiospecific”? Enantioselective means that the reaction formed two enantiomers in unequal amounts. The main difference between enantioselective and enantiospecific is that in enantioselective, two products are formed (pair of enantiomers) while enantiospecific leads to the formation of a single product. 3. Which enantiomer is being formed preferentially? In the reaction, the R-isomer was favored. 4. Explain how the reaction can be enantioselective? The reaction was enantioselective due to the hydrogen-bonded transition state. The O atom of the isobutyraldehyde interacted with the OH group of the enamine. Because of this, the attack of the nucleophile to one of the sides of the carbonyl carbon is blocked, favoring the formation of the R-isomer. Page |2 5. What is being used in place of an enol or enolate? Explain the reactivity of this nucleophile in comparison to an enol and enolate ...
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