CHM230 – Mixed Aldol Condensation

Mixed Aldol Condensation – Preparation of dibenzyalacetone

INTRODUCTION

Like the Grignard reaction, the Aldol Condensation is an extremely useful carbon-carbon bond- forming reaction in organic chemistry. Under the reaction conditions in the experiment, two equivalents of aldehyde will react.

These products are a β-hydroxyaldehyde (or a β-hydroxyketone). This reaction is used extensively in organic synthesis to form C-C bonds and make bigger molecules. In every case, the product results from the addition of one molecule of an aldehyde (or ketone) to a second molecule in such a way that the a-carbon of the first becomes attached to the carbonyl carbon of the second.

Aldehydes and ketones, as well as esters and related compounds undergo reactions with bases due to the presence of hydrogens. These reactions are called condensations and are used to "build" large molecules with more than one functional group. Aldehydes and ketones produce beta hydroxy aldehydes and ketones, whereas esters produce beta keto esters. The former undergo dehydration readily to form conjugated alkenes. Crossed condensations involve one reactant with an alpha hydrogen, and one without. In this exercise benzaldehyde will be condensed with acetone to produce dibenzalacetone, using hydroxide as a catalyst.

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.

PROCEDURE 1. Dissolve 3.0 grams of NaOH in 15 ml of water and add 15 ml of ethanol. Cool the mixture back to room temperature. Make this solution in an Erlenmeyer flask.

2. Add 2.25 ml of acetone, and then 6.0 ml of benzaldehyde, add a magnetic stir bar, and gently stir the mixture for 10 - 15 minutes. The solid is the crude product.

3. Filter the suspension, rinsing out the flask with cold distilled water. Wash the filter cake first with additional chilled water, and then with chilled ethanol. Dry the crude product thoroughly.

4. Transfer the crude product to a beaker and recrystallize with ethyl acetate. This is done by heating the product and adding only as much ethyl acetate as will dissolve the crude product when the solvent is hot. Do not add large volumes of solvent. CHM230 – Mixed Aldol Condensation Page 2 of 3

5. Cool the ethyl acetate mixture. Crystals of dibenzalacetone will form as the solution is cooled.

6. Collect the recrystallized product using a Buchner funnel and dry it to remove any remaining ethyl acetate.

ANALYSIS 1. Run the IR spectra of the acetone, benzaldehyde and the product that you collected. Compare and contrast with the reference spectra below. Label all important bands or lack of bands in the spectrum. 2. Determine the theoretical and % yield of dibenzalacetone. 3. Determine the melting point of dibenzalacetone. 4. Write out the complete mechanism for the reaction. 5. Write the IUPAC name for dibenzalacetone.

Reference Spectrum

CHM230 – Mixed Aldol Condensation Page 3 of 3

Note: this is as a nujol mull so it contains sp3-H.