Optimization of the Aldol Condensation Yield of Dibenzalacetone

  GHP CHEMISTRY PROGRAM

Optimization of the Aldol Condensation Yield of Dibenzalacetone A Look into the Key Ingredient of Sunscreen   Jim Schwoebel, Steven Lago  6/1/2008         

 

This document goes over our chemistry project that we performed at the State of Georgia Governor’s  Honors Program held during the Summer of 2008.   

 

Table of Contents       Abstract ...................................................................................... 3    Background Information ............................................................ 4    Reaction Mechanism ................................................................. 5    Experimental Protocol ............................................................... 6    H1NMR Results ........................................................................... 7    Experimental Results ................................................................. 8    Conclusion .................................................................................. 9        Page 2 of 8   

 

ABSTRACT

Aldol addition and condensation reactions are important topics in organic chemistry and in the pharmaceutical world [1]. In most basic terms, an aldol consists of alcohol in the αlpha or βeta configuration and an aldehyde (thus the name “aldol), and is universally produced from an aldol addition reaction in which a ketone enolate nucleophile is reacted with an aldehyde electrophile [2]. With an emergent popularity and of growing, global importance (in regards to the pharmaceutical world), the aldol addition reaction has become in need of a more efficient production technique (higher percent yield) to achieve fiscal viability and widespread distribution. Thus, this experiment will focus on inducing pH changes on a particular acid-catalyzed aldol addition reaction involving a control variable of temperature, and correlating the experimental data to determine if there is, in fact, a pH limit on the reaction rate (i.e. a point at which a certain pH does not speed up or slow down the reaction). As a result, we hope to define the ideal pH to catalyze an aldol reaction. Additionally, this study will attempt to perform an aldol reaction in a 50/50 ethanol/water solvent (at the ideal pH and a constant temperature) to see if the experiment performs at a quicker rate, or if the reaction yields an entirely different compound altogether. Then, the results of the acid catalyzed aldol reaction in a 50/50 ethanol/water solvent will be analyzed through a mass spectrometer and NMR to allow for the determination of the percent yield and purity.

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BACKGROUND INFORMATION

Reaction Scheme

O

O

O

+

2 H Benzaldehyde    

 

H3C

- 2 H2O

CH3

Acetone 

 

 

 

 

Dibenzalacetone 

         Dibenzalacetone is a common ingredient in sunscreen. This is because dibenzalacetone absorbs UV light and helps to protect the skin from the sun’s damaging rays. The properties that are most valuable in a compound that is used in sunscreen are the compound’s abilities to absorb, reflect, or even scatter the harmful UV rays. Another importance is for the compound to not cause an allergic reaction on a person’s skin.

 

  An aldol reaction involves a reaction between a nucleophile (which donates both pairs bonding  electrons) and an electrophile (which accepts both pairs of bonding electrons). As depicted above, the  nucleophile is usually a ketone and the aldehyde is typically the electrophile in the aldol addition  reaction (although multiple variations exist and  other compounds can be used). The reaction is  catalytically supplemented by the addition of a strong base, such as sodium hydroxide, which allows for  the hydrogen bonded to the carbonyl of the electrophile to dehydrate, as well as the hydrogen in the  nucleophile to dehydrate, and thus both components are bonded together through a 3 step mechanism.   An aldol is defined simply as an aldehyde and an alcohol combined together; for, as suggested by its  name, an aldol is  comprised of both alcohol and aldehyde functional groups.  The reaction utilized in  our experiment was an aldol condensation reaction in which a water was dehydrated from the aldol, as  visually represented above in the second reaction, and eventually an aldehyde or ketone is formed  (depending on the alpha or beta configuration of the aldol).  Page 4 of 8   

 

REACTION MECHANISM

 

       

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EXPERIMENTAL PROTOCOL Procedure: 1. Add 10 mL of CH3OH (or CH3CH2OH), 2.0 mL of benzaldehyde, 1.0 mL of acetone, and 2.0 mL of NaOH (varying molarities) together in a large test tube

2. Heat the test tube in a hot water bath at 50 °C for 15 minutes

3. Chill the solution in an ice bath and use a dropper pipet to add cold water to precipitate the product as pale yellow crystals (up to 7 mL may be added)

4. Collect the product by suction filtration (Buchner Funnel), wash with water, and dry

5. Determine the amount of precipitate formed and determine the percent yield of the aldol condensation reaction The initial experimental plan consisted of inducing change in pH (different molarities of NaOH), changing the number of cold water drops placed in the solution, changing the temperature of the hot water bath, and utilizing different solvents in order to determine the ideal situations for our product (dibenzylacetone) to form [see abstract].

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CONCLUSIONS, RESULTS AND ANALYSIS To see the in depth lab notebook, please visit http://www.schwoebel.ws/wiki/uploads/GHP_Lab_Notebook.pdf After Trials #2 - 6 Due to experimental error, we had to re-filtrate the solutions through a glass funnel to attempt to get the dibenzalacetone to precipitate out fully. Because of the nature of all organic reactions, we expected a percent yield of roughly 40-50%. However, our results generated a percent yield of only about .0114%. Thus, upon analyzing the results, we made three main deductions or conclusions from the statistical trends generated from the data: 1. 2. 3.

A higher base concentration will cause more product to form (because the mechanism allows for the dominant enolate to form an alpha or beta unsaturated aldehyde [ketone]) A higher or lower temperature may cause more or less product to form (the part of the reaction in which the aldol dehydrates into a ketone) Adding more drops of cold water to the test tube seems to cause more product to form

After Trials #7 -18 Upon analyzing the results, we have deduced that the temperature variable and the time in the beaker variable did not have much of an effect in producing a higher percent yield. However, through our results in trials #5 – 16, we have corroborated the trend in the amount of cold water drops (to the test tube) and the percent yield. Trial #11 seems to have been the ideal experiment to yield the highest amount of product, however, our evidence at this point in time is still inconclusive (because there could have been human error in combining the correct amounts of each reactant). **Experimental Error (According to Harmon’s lab book) – we could have added the reagents in the test tubes in an incorrect order (we added the methanol in first; however, we should have added the acetone and benzaldehyde in first** After Trails #19 – 27 After these 9 trials, we finally determined the problem with our experimental plan and deduced that by adding one more mL of 2.0M NaOH (2 mL total), the percent yield greatly increased. Yields with 1 mL NaOH are very low (as evidenced by trials 1-18), and this seemed to have been why the reaction did not proceed and correspond with our hypothesis. The ethanol solvent seems to yield/produce the most product, though some error could be accounted for weighing wet filter paper. The catalyst seemed to be more effective at a higher concentration – allowing for a greater number hydroxide radicals (OH-) to react with the dominant enolate ion and producing the aldol, which, in turn, is dehydrated into a ketone. **After we obtained these results, we tested the sample dibenzalacetone crystals in the NMR to corroborate our predicted products  [see data results for NMR and GC-MS results] **

After Trials #28 – 35 These trials were repeating the procedure engendered in trails #19 – 27, but with different solvents.

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NMR RESULTS See Poster

FINAL CONCLUSION: The specific aldol reaction performed best at a certain pH and in the presence of the ethanol solvent with a set number of drops of cold water added.

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