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helpforlab - Experiment 15 Preparation of Dibenzalacetone...

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Experiment 15 Preparation of Dibenzalacetone In this experiment we will prepare dibenzalacetone from acetone and two equivalents of benzaldehyde under basic conditions. This is an example of a crossed- aldol (or mixed-aldol) reaction. The overall reaction is shown in Figure 15.1 and a detailed mechanism is given in Figure 15.2 C O H CH 3 C O CH 3 NaOH C C O C C C H H H H 2 H 2 O Figure 15.1 Dibenalacetone Formation + + dibenzalacetone benzaldehyde acetone Acetone enolizes in the strongly basic conditions. Note that benzaldehye cannot enolize and so it must act as the electrophile. The nucleophilic alpha carbon then attacks the carbonyl of benzaldehyde. After proton transfer there is loss of water to give the α , β - unsaturated carbonyl that is stabilized by conjugation with the phenyl substituent. Notice how the π -electrons of the phenyl ring are delocalized all the way onto the carbonyl and onto the other carbonyl in the final dibenzylacetone product.
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