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Sunday, June 2, 2013

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






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