Sunscreen-Preparation of Dibenzalacetone-Aldol Condensation-Lu Le Laboratory

The reaction of acetone with benzaldehyde in the presence of base is a classical aldol condensation. Depending on the stoichiometry and reaction conditions, these reagents could beused to prepare either benzalacetone or dibenzalacetone.

     The reaction conditions in this experiment are chosen to favor the formation of dibenzalacetone. The reaction is carried out by adding a mixture of two parts benzaldehyde and one part acetone to an aqueous ethanol solution of NaOH. The solvent, aqueous ethanol, also favors the formation of dibenzalacetone. The reason is that the reactants and intermediates, including benzalacetone, are soluable in aqueous ethanol and are not removed from the reaction solution by separation or precipitation. Dibenzalacetone, however, is insoluble in aqueous ethanol-while the reaction mixture is being stirred, the deep yellow dibenzalacetone slowly precipitates from the solution.

    The work-up consist of filtration, washing, and crystallization of the product. The critical part of the work-up is the washing. The reaction mixture, and thus the solid product, contains NaOH, which must be removed prior to crystallization. A number of washings may be required to remove it. To determine if the product has been washed sufficiently, each aqueous wash is tested with pH paper. Final purification of the dibenzalacetone is accomplished by crystallization from ethanol.

   

The color of dibenzalacetone is a result of the relatively large conjugated pi system, which can absorb a portion of the visible spectrum. Dibenzalacetone has been used in sun-protection prepparations because it also absorbs certain wavelengths of ultraviolet light.

Mechanism

Aldol Condensation

Reaction Equation:

Chemicals

1.      Acetone:0.400g

2.      Benzaldehyde: 1.400g

3.      95% ethanol: ~25mL

4.      Sodium hydroxide: 1.3g

Procedure

1.      Dissolve 1.3g NaOH in 13mL water in a beaker.

2.      Add 10mL 95% ethanol and cool the solution to 20℃

 

 

3.      Place 1.4g of benzaldehyde (new or distilled) and 0.4g acetone in an Erlenmeyer flask. Swirl the flask until a homogeneous solution is obtained.

4.      Add approximately one-half of the benzaldehyde solution to the hydroxide solution with vigorous stirring. Stir the mixture for 10 minutes, then add the remainder of the benzaldehyde-acetone solution. Continue stirring for another 30 minutes.

5.      Filter the yellow solid with vacuum, press it as dry as possible, and then transfer it to a clean beaker.

6.      Add 25mL water. Stir the mixture into a thick paste. Refilter and rewash it.

7.      Air-dry the product.

8.      Calculate the yield and melting point of dibenzalacetone

Experimental Record

Weight of dibenzalacetone(theory)	1.567g
Weight of dibenzalacetone	1.280g
Yield	81.7%
Melting Point(theory)	110~111℃
Melting Point	109.0~111.0℃

Synthesis of p-nitroacetanilide-Nitration-Lu Le Laboratory

        The aromatic nitration of acetanilide is an exothermic reaction ; the temperature must be carefully controlled by chilling, stirring, and the slow addition of reagents. Acetanilide is first dissolved in the solvent, glacial acetic acid, by warming. Glacial acetic acid is used because it is a polar solvent capable of dissolving acetanilide and the acetate ion is a poor nucleophile so no substitution is possible. After the solution is cooled, sulfuric acid is added; however, even with cooling, the temperature of the solution rises almost 40E. Both the acetanilide solution and the nitrating solution (a mixture of HNO3, and H2SO4) must be chilled to about 10EC before the reaction is begun.

To prevent dinitration of the acetanilide, the nitrating mixture is added in small portions to the acetanilide solution (and not vice versa) so that the concentration of HNO3, is kept at a minimum. After all the HNO3, H2SO4 solution has been added, the reaction mixture is allowed to warm slowly to room temperature. If the reaction mixture has been kept excessively cold during the addition, there will be a relatively large amount of unreacted HNO3, present, which may cause the temperature to rise above room temperature. If this should happen, the mixture must be rechilled.

The work-up procedure consists of removal of the acids and crystallization of the product. Every trace of acid must be removed because hydrogen ions catalyze the hydrolysis of the amide to p-nitroaniline or its protonated cation. Most of the acid is removed by pouring the reaction mixture onto ice and water, then filtering the flocculent yellow precipitate of p-nitroacetanilide. The last traces of acetic acid are removed by neutralization. Because bases also catalyze the hydrolysis of amides, the neutralizing agent used is disodium hydrogen phosphate (Na2HPO4). This reagent reacts with acids to yield NaH2PO4. The result is a buffered solution with a pH near neutral.

The crude product is air-dried before crystallization. If all of the acid was removed, the product will be light yellow. A deep yellow to yellow-orange product is indicative of the presence of p-nitroaniline from hydrolysis. Unfortunately, p-nitroaniline is difficult to remove from p-nitroacetanilide by crystallization.

Mechanism

Preparation of Nitronium Ion:

Nitration

Chemicals

1.      Acetanilide: 1.625g

2.      Disodium hydrogen phosphate: 3.75g

3.      95% ethanol: 15mL

4.      Glacial acetic acid: 2.5mL

5.      15M nitric acid: 0.875mL

6.      18.4M sulfuric acid: 3.75mL

Procedure

1.      Place 1.625 g of acetanilide in a Erlenmeyer flask, add 2.5 mL of glacial acetic acid(CAUTION: strong irritant), and warm the flask on a hot plate in a fume hood until the acetanilide dissolves. 

 

2.      Cool the flask in an ice bath to about 20℃

3.      Add 2.5 mL of cold, conc. sulfuric acid. The temperature of the mixture will rise to about 60℃. Chill the solution to about 10℃ in an ice bath. (The solution will become very viscous.)

4.      Mix 0.875 mL of conc. nitric acid and 1.25 mL of conc. sulfuric acid in a test tube, and chill the flask in an ice bath. 

5.      When both solutions are cold, slowly add the HNO3, H2SO4 solution, 1 mL at

a time, to the acetanilide solution. Keep the reaction flask in an ice bath so that the temperature of the reaction mixture is maintained between 10-20℃. Stir the reaction mixture carefully after each addition. The entire addition requires about 15 minutes.

 

6.      After the addition is completed, allow the reaction flask to stand at room temperature for 30 minutes. Monitor the temperature; if it rises above 25℃, chill the flask in an ice bath. Should the rechilling be necessary, allow the flask to stand for 30 minutes or more at room temperature after the rechilling.

7.      Pour the reaction mixture into a beaker containing 25 mL of water and 6.25 g of cracked ice. 

8.      Using a large Buchner funnel, filter the heavy lemon-yellow precipitate with vacuum. Press out as much aqueous acid from the filter cake as possible with a spatula or clean cork while suction is being applied. The precipitate is voluminous; use care in transferring it to the Buchner funnel or a substantial amount of product will be lost.

9.      Transfer the filter cake to a clean 250-mL beaker, and add 100 mL of 15% aqueous disodium hydrogen phosphate. Stir the mixture to a paste-like consistency and refilter using vacuum. 

 

 

   

10.  Wash the beaker with two 30-mL portions of cold water. Finally, wash the filter cake with an additional 50 mL of cold water. Press the filter cake with a spatula or clean cork to remove as much water as possible, then dry the solid overnight on a watch glass. Determine the yield and melting point.

 

 

Experimental Record

Weight of p-nitroacetanilide(Theory)	2.170g
Weight of p-nitroacetanilide	1.650g
Yield	76.0%
Melting Point of p-nitroacetanilide (theory)	216℃
Melting Point of p-nitroacetanilide	210~214℃