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Unformatted text preview: Dibenzalacetone by Aldol Condensation 37 ALDOL SYNTHESIS of DIBENZALACETONE, AN ORGANIC SUN SCREEN H 3 C C CH 3 O 1 Acetone molar mass: 58 g/mol density: 0.79g/mL limiting reagent + 2 C H O Benzaldehyde molar mass: 106 g/mol density: 1.04 g/mL an excess will be used NaOH, H 2 O, CH 3 CH 2 OH C C C O C H C H H H original acetone carbons Dibenzalacetone molar mass: 234 g/mol melting point: somewhere between 80 and 120ºC 1 Overview : The reaction of an aldehyde with a ketone employing sodium hydroxide as the base is an example of a mixed aldol condensation reaction. You will do a double mixed-aldol condensation reaction between acetone and benzaldehyde. Acetone has α-hydrogens (on both sides) and thus can be deprotonated to give a nucleophilic enolate anion. The aldehyde carbonyl is much more electrophilic than that of a ketone, and therefore reacts rapidly with the enolate. The alkoxide produced is protonated by solvent, giving a β-hydroxyketone, which undergoes base-catalyzed dehydration. The elimination process is particularly fast in this case because the alkene is stabilized by conjugation to not only the carbonyl but also the benzene. In today’s experiment you will use excess benzaldehyde, such that the aldol condensation can occur on both sides of the ketone. Mechanism for Aldol Condensation H 3 C C C H 2 O H OH H 3 C C CH 2 O C H O CH C O H 3 C C H 2 O enolate CH C O H 3 C C HO H 2 O alkoxide hydroxy- ketone OH H H CH C O H 3 C C H HO hydroxyenolate H C C O H 3 C C H Benzalacetone "enone" H C C O C H H C C H Dibenzalacetone Step 1 Step 2 Step 3 Repeat Steps 1-5 Again Summary: Step 1: Deprotonation (makes nucleophilic enolate) Step 2: Attack by nucleophile on electrophile Step 3: Protonate to give neutral hydroxy-ketone Step 4: Deprotonate again (makes enolate) Step 5: Eliminate hydroxide to generate alkene pi bond Step 5 Step 4 Dibenzalacetone by Aldol Condensation...
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This note was uploaded on 06/05/2011 for the course CHEM 2 taught by Professor Chemistry during the Spring '11 term at Abilene Christian University.
- Spring '11