Question

1.In today experiment you will perform the aldol condensation between benzaldehyde (PhCHO) and acetone to synthesize dibenzalacetone. Answer the following questions regarding this reaction:

a.This reaction is an example of a mixed aldol condensation. Why is only one product formed and no mixture of compounds are obtained like in other aldol condensations? (Hint: Benzaldehyde cannot undergo “ALDOL” reaction by itself in presence of strong base (NaOH). Why?)

b.Look at the mmols of the reagents used in this reaction and specify the limiting reagent.

c.Explain why the NaOH is a catalyst in this reaction, rather than a reagent. (Hint - look at the reaction mechanism.)

d.Usually an aldol condensation must be heated in order to dehydrate the beta-hydroxy ketone to form the C=C. What is different about this product that causes it to dehydrate at room temperature?

e.Why does this product have a yellow color?

f.The IR spectrum of dibenzalacetone shows the C=O peak at lower wavenumbers (1639 cm^-1) than standard C=O frequency (1715 cm^-1)? Why do you think?

g. How would you change the procedures in this experiment if you wished to synthesize benzalacetone, C₆H₅CH=CHCOCH₃? Benzalacetophenone, C₆H₅CH=CHCOC₆H₅?

h.The name dibenzalacetone does not completely characterize the molecules made in this experiment. There are 3 isomeric dibenzalacetones, one melting at 100-110 °_max 330 nm,34,300; another melting at 60 °C, _max= 295 nm20,000; and a third, a liquid with_max= 287 nm Draw the structures of the three geometric isomers of dibenzalacetone (Z,Z; Z,E; E,E), and assign each one to the three molecules described in the previous paragraph.

i.Write the detailed mechanism for the formation of dibenzalacetone from benzalacetone and benzaldehyde.

Answer 1

1. Aldol condensation reaction between benzaldehyde and acetone to synthesize dibenzalacetone.

a. Only one product is formed in this case as only acetone has an alpha hydrogen to the C=O which can be picked by the base. Acetone is symmetrical molecule. Benzaldehyde on the other hand does not have any alpha hydrogen and thus cannot form the enolate for the aldol reaction thus acting as the sole electrophile in the reaction.

b. The limiting reagent in this reaction would be the acetone.

c. NaOH is used to pick the alpha hydrogen in the reaction. It is catalytic. After the formation of the first enolate and attack of enolate onto benzaldehyde. The O- formed picks up an proton from the water moelcule thereby regerating the catalyst OH- in the reaction. Thus the OH- only acts as the catalyst and not the reactant.

d. Here the product alcohol formed is dehydrated easily because the alpha,beta-unsaturated product formed by dehydration is stabilized by resonance with the benzene ring. This is the driving force for the dehydration reaction.

e. The longer the conjugated structure formed, higher would be the wavelength of the light absorbed. Since the dibenzalacetone formed is highly conjugated, the color is intense yellow.

f. The IR frequency for the C=O is lower than the usual free C=O due to the fact that here C=O is in conjugation with the double bond and the benzene ring, which reduces the strength of the C=O bond by decreasing the double bond character of the C=O bond. As the strength reduces the frequency goes down in the IR spectrum.

g. To synhteize benzalacetone we would add more of benzaldehyde in the reaction, so the ratio of acetone to benzaldehyde would be somwhere around 1 : 1 from the previous 2 : 1. So more molecules of aldehyde is now available which would yield a monoaldol product.

To synhtesize benzalacetophenone we would use, acetophenone instead of acetone in the reaction as an electrophile.

h. Given below are the structure for the thress isomeric dibenzalacetone and the lmax for each. E,E would have higher resonance than E,Z whichch would have higher resonance than Z,Z-molecule.

i. Given below is the detailed mechanism for the aldol condensation reaction.