Chemistry 351 Laboratory - Spring 2013 – Professor T. Nalli, Winona State University

Expt #6. Preparation of Dibenzalacetone by a Double Aldol Condensation

Relevant textbook reading - Klein, Chapter 22

References - Adapted from http://web.mnstate.edu/jasperse/Chem365/Aldol%20Reaction.doc.pdf

Overview – In this experiment, we will carry out a double crossed aldol condensation reaction (eq 1). The reaction uses the simple starting materials, acetone and benzaldehyde, to form the 15-carbon product, dibenzalacetone, in diastereoselective fashion. The product will be recrystallized and characterized by IR, NMR, and melting point determination.

Background - The base-induced reaction of an aldehyde with a methyl ketone to form a larger conjugated ketone is an example of a crossed aldol condensation (eq 2). The mechanism (eq 3-5) involves deprotonation of an α-hydrogen on the ketone to form an enolate (eq 3), which then adds nucleophilically to the aldehyde carbonyl to form a β-hydroxy ketone (eq 4). Deprotonation of a remaining α-hydrogen is followed by expulsion of hydroxide to form the carbon-carbon double bond of the final product (eq 5). The reaction works best when there is no possibility of the aldehyde forming an enolate, e.g., when R' = Ph (as in this experiment).

 

Acetone has α-hydrogens on both sides and, thus, can react in aldol fashion with two equivalents of benzaldehyde to give the compound known as dibenzalacetone (eq 1). Dibenzalacetone is a yellow solid, which has found use in commercial sunscreen ointments.

Benzaldehyde is used in a slight excess (2.2 equiv) in the recommended procedure. This allows the reaction to reach completion in a reasonable amount of time. If excess aldehyde is not used, then the reaction slows down dramatically as it nears completion because the aldehyde gets used up and its concentration approaches zero. (The rate determining step is addition of the enolate to the aldehyde.) Using excess aldehyde ameliorates this problem because the aldehyde never gets all used up and the concentration always stays well above zero. If the reaction is worked up before completion is reached, then the product will be contaminated with benzalacetone (the monosubstitution product in which only one benzaldehyde has been incorporated).

 

Procedures

Running the reaction.

  • In a 125-mL Erlenmeyer flask equipped with a magnetic stirring bar, prepare a mixture of 15 mL EtOH, 3 mL 10% NaOH, and 3 mL water.
  • To the stirring solution add 0.0275 mol benzaldehyde.
  • Add 0.0125 mol acetone and note the time. (You will want to time your observations throughout the reaction.)
  • Continue stirring for 30 min.

Work-up procedures.

  • Add 20 mL water, and then vacuum filter the mixture.
  • Wash the solid three times with 50-mL of water each time.
  • Allow the vacuum to pull air through and press the solid dry by pressing a filter paper or rubber glove to squeeze out more water.
  • Measure the crude yield and save a small sample for melting point determination.
  • Recrystallize the crude product from ethanol.
  • Allow the crystals to dry thoroughly on the vacuum and pressing as before.
  • Measure the final yield.

Characterization of Product

Obtain the 1H and 13C NMR spectrum using CDCl3 as the solvent.

Obtain an IR spectrum using the dry film method.

Obtain the mp of both the curde and final products.

Report

Report the spectroscopic results in tables as usual (and as specified in the report guidelines). For this report I am expecting you to also report the coupling constant for each NMR multiplet.

Remember to also report the yield, theoretical yield, and percent yield as well as the refractive index and observed bp of the product. Use the equation given in Mohrig to correct the measured refractive index so it can be directly compared to the literature value given at 298K.

Assigned Questions

1. Why is it important to add the benzaldehyde first and then the acetone rather than in the opposite order?
2. What is the purpose of the water washes in step 2 of the work up procedure?
3. What does the coupling constant observed for the alkene proton doublets reveal about the stereochemistry of the product?