Aldol and Mixed Aldol Condensation

This experiment was about synthesizing dibenzalacetone through an aldol condensation reaction between acetone and benzaldehyde. This was done by mixing the two reactants with NaOH and ethanol, then allowing the reaction to sit for thirty minutes. The crystals were then washed with water three times and recrystallized using ethanol. The aldol condensation for this experiment was successful.

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INTRODUCTION

Aldol Condensation is an extremely useful carbon-carbon bond-forming reaction in organic chemistry. An aldol condensation is a reaction that is named based on the type of product formed when two aldehydes (or ketones), in the presence of dilute base, yields a molecule having both aldehyde (ald-) and alcohol (-ol) functional groups. The aldol products are β-hydroxyaldehydes (or β-hydroxyketones). This reaction is used extensively for the synthesis of new C--C bonds and to make larger organic molecules.

The reaction of an aldehyde with a ketone employing sodium hydroxide as the base is an example of a mixed aldol condensation reaction. In this experiment, a double mixed-aldol condensation reaction between acetone and benzaldehyde to produce Dibenzalacetone will be performed. Dibenzalacetone is a conjugated molecule. This conjugation allows the molecule to absorb sunlight and UV rays. Thus, when dibenzalacetone is used in sunscreen, the conjugation in the molecule allows the dibenzalacetone, and thus the sunscreen, to absorb the sunlight before it reaches the skin, thus preventing sun burns from occurring. 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. The mechanism looks like this

Then repeating the mechanism steps from the first to the fourth mechanism, it will arrive to dibenzalactone;

The mechanism is from left to right. The overall reaction will look like this:

An extension of a simple aldol condensation reaction is the mixed aldol condensation reaction. A mixed aldol reaction leads to a number of different products unless one of the carbonyl compounds involved cannot form an enolate ion (i.e. the compound has no α-hydrogens). A good choice for such a compound is an aromatic aldehyde. Even more specificity can be accomplished by having the other carbonyl compound form only one possible enolate ion (ex. - acetophenone as opposed to 2-butanone). Once formed, the nucleophilic enolate ion attacks carbonyl carbon to form a β-hydroxy carbonyl product (aldol addition product).The β-hydroxy carbonyl product then eliminates a molecule of water to form a conjugated system composed of a double bond and the carbonyl group. The conjugation is extended through two benzene rings as well, producing a very stable product. This compound exists in different forms, the most prominent being a trans- form about the C=C double bond. An example of the mechanism looks like this:

The mechanism is from left to right.

METHODOLOGY

Part 1

Prepare an ice-water bath in a 100-mL beaker. Place 5 mL 95% ethanol and 5 mL 5M NaOH in a test tube. Prepare a mixture of 2.1mL benzaldehyde and 0.75mL acetone. Add this mixture to the ethanol-NaOH solution in a vial with a magnetic stirrer. A yellowish precipitates will be observed that time. Then, let the solution sit for 30 -45 minutes, stirring occasionally. Once the thirty minutes is up, cool the mixture in the prepared ice-water bath. Then, isolate the product through filtration by using a Buchner funnel. Add 2 mL distilled water to the tube in order to wash the crystals. Rinse the crystals two more times with 1 mL portions of distilled water each time. Make sure to remove all the solvent from the crystals.

Part 2

Place 2.5 mL unknown aldehyde and 12.5 mL of 95% ethanol in a 50 - mL Erlenmeyer flask. Add 8.75 mL of 2 M KOH in it and mix thoroughly. Then add 0.65 mL of the ketone which is the acetone