Question

Dibenzalacetone Synthesis PURPOSE: purpose of this lab exercise is to synthesize dibenzalacetone by means of an aldol condition. INTRODUCTION: Wow, is it wet in here or what? That's what happens when you condense, right? No, that is not right, you say! You can condense things without getting wet in organic chemistry. Think about aldol condensations, you tell me. Yes. yes..of course you are correct: organic condensations dont get you wet. They get you a new product. Silly me Aldol condensation is the forth type of carbonyl reaction that you've learned about in lecture: nucleophilic addition, nucleophilic acyl substitution, alpha-substitution, and condensation. As the name implies, aldol condensations are a reaction of aldehydes and ketones, and as can be seen in Figure 1, they involve the formation of a beta-hydroxy product. Mechanistically, an aldol con- densation is really a combination of an alpha-substitution reaction and a nucleophilic addition reaction: one of the carbonyls undergoes alpha-substitution while the other undergoes nucle- ophilic addition. (Can you tell which reactant in Figure 1 is alpha-substituted, and which under- went nucleophilic addition?) The reaction shown in Figure 1 is really an example of a mixed aldol condensation, in that the reactants are different. Mixed aldol condensations only make synthetic sense when the reactants are chosen so that only one product is possible. ?? ?? H-C-CH,--CH Figure 1: A Simple Aldol Condensation Reaction In this laboratory, you'll synthesize dibenzalacetone from benzaldehyde and acetone via an aldol condensation (the overall reaction is shown in Figure 2). As you can see in Figure 2, the beta- hydroxy product is not obtained. Instead, the beta-hydroxy product is dehydrated to form the dibenzalacetone product. This reac- tion is really a two step synthesis, which is shown in Figure 3. The first step of the synthesis is the aldol condensation, while the second step is the dehydration. As you might expect, the highly con-

Answer 1

Purpose of four acetic acid: ethanol washes is to completely neutralize the NaOH present in the crude product it is important to break up crystal as NaOH may get happed between crystals and thus by breaking the crystals, we are making sure that NaoH is completely removed from the crystals Moles of acetone = Mass of acetone/Molar mass of acetone = 0.791/58.08g/mol = 0.0136 moles Moles of benzaldehyde = 1.044g/106.12 g/mol = 0.00984 moles Since mole of benzaldehyde < moles of acetone: benzaldehyde is limiting reagent Theoretical yield = moles of limiting times Molar mass of product reagent = 0.00984 moles times 234.28 g/mol = 2.305g % yield = Actual yield/Theoretical yield times 100 = 1/2.305 times 100 % yield = 43.38%=