Wednesday, November 16, 2011

Synthesis of Dibenzalacetone by Aldol Condensation

Objective:

1. To carry out a mixed aldol condensation reaction

2. To study the mechanism of aldol condensation reaction

Introduction:

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The reaction of an aldehyde with a ketone employing sodium hydroxide as the base is an example of a mixed aldol condensation reaction, the Claisen-Schmidt reaction. The double mixed-aldol condensation reaction between acetone and benzaldehyde was carried out. Acetone has α-hydrogens (on both sides) and thus can be deprotonated to give a nucleophilic enolate anion. The alkoxide produced is protonated by solvent, giving a β-hydroxyketone, which undergoes base-catalyzed dehydration. The elimination process is particularly fast in this case because the alkene is stabilized by conjugation to not only the carbonyl but also the benzene. In this experiment, excess benzaldehyde such that the aldol condensation can occur on both sides of the ketone.

Dibenzalacetone is readily prepared by condensation of acetone with two equivalent of benzaldehyde. The aldehyde carbonyl is more reactive than that of the ketone and therefore reacts rapidly with the anion of the the ketone to give a β-hydroxyketone, which easily undergoes base catalyzed dehydration. Depending on the relative quantities of the reactants, the reaction can give either mono- or dibenzalacetone.

Dibenzalacetone is a fairly innocuous substance in which its spectral properties indicate why it is used in sun-protection preparations. In the present experiment, sufficient ethanol is present as solvent to readily dissolve the starting material, benzaldehyde and also the intermediate, benzalacetone. The benzalacetone once formed, can then easily to react with another mole of benzaldehyde to give the desired product in this experiment, dibenzalacetone.

Apparatus: Erlenmeyer flask, Buchner funnel, glass funnel, melting point apparatus, UV/Vis spectrometer, FTIR spectrometer

Materials: Benzaladehyde, acetone, sodium hydroxide, 95% ethanol, ethyl acetate, ice

Procedure:

1. 5g of NaOH was added to 25ml of H­2O in an Erlenmeyer flask and the solution was swirled.

2. 25ml of 95% ethanol was added and the solution was allowed to come nearly to room temperature.

3. 2.9g of acetone and 10.5ml of benzaladehyde were added.

4. After 15 minutes of occasional swirling, the products was filtered on a Buchner funnel.

5. The product was washed with cold ethanol and was allowed to suck dry.

6. The yellowish product was recrystallized from ethyl acetate.

7. After recrystallization, a yellow crystalline was obtained.

8. The weight, yield, and melting point of the product were determined.

9. The UV and IR spectra of dibenzalacetone were ontained.

Results:

Weight of watch glass = 36.1291 g

Weight of watch glass + products = 45.6878 g

Weight of products = 9.5587

Melting point of products = 109 °C

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Moles of benzaldehyde used = 0.1 mol

Moles of acetone used = 0.05 mol

Moles of dibenzalacetone produced = Moles of acetone used

= 0.05 mol

Theoretical weight of dibenzalacetone produced = 0.05 mol * 234.29 g mol-1

= 11.7145 g

Percentage yield of dibenzalacetone = 9.5587 g / 11.7145 g × 100 %

= 81.60 %

Discussion:

Condensation is a process which joins two or more molecules usually with the loss of a small molecule such as water or an alcohol. Aldol condensation (Claisen-Schmidt reaction) definitely is a process which join two carbonyl groups with a loss of water molecule in order to form β-hydroxyketone. The product is also known as adol because it containing two functional groups which includes aldehyde (or ketone) group and alcohol group. The product dibenzalacetone was formed from the reaction between an acetone molecule and two benzaldehyde molecules. Generally, the aldol condensation is carried out under a base condition.

Sodium hydroxide was mixed with distilled water then was used to react with sufficient ethanol as the first step. The particular reaction is an exothermic reaction which released the heat energy to the surrounding from the reaction. The sodium hydroxide was functioned as a catalyst in the reaction. The ethanol acts as a solvent which allows the acetone and benzaldehyde to dissolve and react with each other. After that, acetone and benzaldehyde were mixed in the solvent which turns to yellow colour quickly. Eventually, the product was formed with a yellow precipitate appear in the reaction after a few seconds. However, there are some impurities and side products were formed in the yellow precipitate. So, recrystallization was carried out by using ethyl acetate as solvent in order to purify the product and hence a pure product could be obtained for the ultraviolet (UV) and IR spectra analysis. In the recrystallization process, the yellow precipitate in ethyl acetate was immersed into an ice-bath in order to obtain a higher yield of product. This is because the heat energy in the precipitate easily to be released since the precipitation formation is an exothermic reaction and hence it maximizes the formation rate of the product.

Acetone is considered as a stable and unreactive compound, so it should be converted into anionic form to increase its nucleophile properties to initiate the reaction. The sodium hydroxide dissolves in water to produce hydroxide ion and it tends to attack the α-hydrogen in acetone and to form water molecule. The deprotonation of acetone caused the enolate ion was produced as nucleophile which will be used in the synthesis of dibenzalacetone. An enolate ion was formed which it exists as resonance-stabilized structure which shown in the following diagram:

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Diagram 1

The acetaldehyde enolate ion attack to the benzylic carbon of benzaldehyde via nucleophilic addition to form the intermediate as shown in below:

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Diagram 2

The oxygen attached to the benzylic position of carbon tends to attract one proton from water molecule to form hydroxide group in the intermediate. This is the formation of an aldol since the molecule consists of a carbonyl group and an alcohol group. In the basic condition, the hydroxide ion tends to remove one proton from the α-carbon resulting the formation of C=C double bond at the α and β carbon. At the same time, the hydroxide group attached to the β carbon forms a leaving group. After the condensation, benzalacetone was formed after two water molecules leaved as shown:

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Diagram 3

The benzalacetone tends to form benzalacetone enolate ion after the hydroxide group from the surrounding attack the proton which attached to the carbon at benzylic position.

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Diagram 4

The same process has been take place as in the Diagram 2 but with the more bulky benzalacetone enolate ion as the material. The benzalacetone enolate ion acts as a nucleophile which attacks another benzaldehyde. The protonation of the aldol took place followed by the hydroxide groups have been eliminated as leaving groups. As a result, the nucleophilic addition and base-catalyzed dehydration lead to the formation of the desired product which is dibenzalacetone. The mechanism of dibenzalacetone formation was shown in the Diagram 5:

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Diagram 5

The overall mechanism of the dibenzalacetone was summarized in the Diagram 7 as shown in below:

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Diagram 7

The percentage yield of dibenzalacetone in this experiment is 83.27%. Some of the product has been lost during the process of recrystallization. In recrystallization, some of the product dissolved in the ethyl acetate. The melting point of the product is lower than the actual melting point (110 °C ~ 111 °C). This is because there is some impurities exist in the particular compound which will tend to lower the melting point of the dibenzalacetone.

Precaution steps:

1. Avoid to carry out the experiment near the fire since the organic solvent are mostly flammable.

2. Avoid to smell benzaldehyde directly.

3. Handle carefully with sodium hydroxide since it is corrosive.

2 comments:

  1. Glorious. Simply glorious. Well explained.

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