Question

Prelab questions:
1. Make sure the reaction table described above is completed in your notebook.
2. Two benzaldehyde molecules combine with one acetone molecule in this aldol reaction. If someone uses 5.0 mL of benzaldehyde and 2.5 mL of acetone to try to improve the yield of product, is this the correct ratio of molecules so that there is no excess? Why or why not?

The Aldol Reaction: Synthesis of Dibenzalacetone
Purpose: Introduce the student to the hands-on chemistry of carbonyl condensations.
Warning: Dispose of organic material only in waste containers, not in the sink or trash can.
Background: Aldol reactions are a powerful and biochemically useful way to construct large carbon molecules. They involve the combination of an aldehyde with another carbonyl compound, typically a ketone or aldehyde, to generate a -hydroxycarbonyl compound, which in many cases, including this experiment, dehydrates to the ,-unsaturated carbonyl product:
   RCHO + R’C(O)CH2R’’ [RCHOHCHR’’C(O)R’] RCH=CR’’C(O)R’
You and your classmates will explore the reaction of benzaldehyde with acetone. 1) Write the specific reaction equation for this experiment (no “R”’s), and predict the product using the above generic equation, the title of the experiment, and the fact that both sides of the ketone react. Do NOT use a computer search engine or get other help until you have proposed a structure. 2) Use the next 5 lines below each species to fill in the bold values: Under each species list the mass used of each compound (see procedure below), FM (formula mass), mmol, and reacting equivalents, setting the ketone compound as 1 equivalent. 3) Calculate a) the mmol of product that can form if all the limiting reagent is converted to dibenzalacetone, b) the formula mass of the product, and then c) the theoretical yield of product (fill in mass value under product structure).
An aldol reaction between two different carbonyl compounds is called a mixed- or crossed aldol condensation. Such reactions generally result in an undesirable mixture of products, although in some cases such as this one a single product may be obtained in good yield. Mixed condensation in this case is successful because benzaldehyde has no alpha hydrogens to undergo self-aldol condensation, and any formed enolate reacts much faster with benzaldehyde than with acetone. The subsequent protonation and dehydration readily occurs to conjugate the aromatic ring, C double bond C, and carbonyl. The extensive conjugation of the final product allows it to absorb visible violet light, so the product appears yellow to the observer.
Procedure:
Reaction
Add 24 mL of absolute ethanol and 16 mL of deionized water to a 125 (or 250) mL Erlenmeyer flask. Add a magnetic stir bar and 0.5 g of KOH (base catalyst), and dissolve the potassium hydroxide in the aqueous alcohol by magnetic stirring. Add 5.0 mL of benzaldehyde and 2.0 mL of acetone. Use a magnetic stirrer for at least 30 minutes, noting any changes in the appearance of the reaction mixture over the course of the reaction.

Isolation and Purification

Add 25 mL of water, stopper, and shake the flask vigorously. Transfer the solid to a Buchner funnel with filter paper under aspiration. Wash the reaction flask and crystals twice more with 25-mL portions of water. Rinse the entire solid out of the flask into the funnel using your squeeze bottle of water. Remove the paper and solid from the Buchner funnel, and place them on a large piece of filter paper or Kimwipe. One need not try to scrape the solid off of the filter paper. Compress the collected solid and filter paper between another sheet of larger filter paper to remove most of the water and then recrystallize the solid in a clean flask with at least 25 mL 70% ethanol/water. Heat the mixture to gentle boiling on a hot plate. Remove the filter paper once the solid has dissolved. If solid remains, add a 3 mL portion of the 70% ethanol and heat for several minutes. Once a clear yellow solution is obtained, allow the mixture to slowly cool to room temperature on the benchtop, and when the amount of solid stops increasing then cool in an ice water bath. If the product separates as an oil (“oils out”), reheat to a homogenous solution, start cooling and add a seed crystal of the product from the instructor. Add several drops more ethanol if seed crystals are not available. Vacuum filter the solid onto a tared filter paper. Wash the crystals with about 10 mL of ice-cold 70% ethanol. Dry the solid in the vacuum oven for at least 10 minutes. Weigh the recrystallized product, and take a melting point. Grind a small spatula-tip-worth of solid with 2 g of potassium bromide to a fine homogeneous powder, and place some of this powder in the bolt/die press to prepare a single crystal for infrared analysis. Obtain the IR spectrum of the aldol product.

Answer 1

Prelab questions,

1. Table of reactants/products

Reagent                  FW (g/mol)                density            Eq.                

Acetone                      58.08                      0.791               1               

benzaldehyde            106.121                   1.04                 2

dibenzalacetone          234.29                      -                    1

2. With 5.0 ml benzaldehyde and 2.5 ml acetone

moles of benzaldehyde = 5.0 ml x 1.04 g/ml/106.121 g/mol = 0.049 mmol

moles of acetone = 2.5 ml x 0.791 g/ml/58.08 g/mol = 0.034 mmol

From the reaction we need 1 mole of acetone per 2 moles of benzaldehyde

So for 0.034 mmol of acetone one would need = 2 x 0.034 = 0.068 mmoles of benzaldehyde

But we only have 0.049 mmol available, so the ratio is not correct.

We have excess of acetone in this case.