The Aldol Condensation

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Lawrence Parsons
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1 The Aldol ondensation Synthesis and Analysis of 2,3,4,5-Tetraphenylcyclopentadienone Yakety Sax Bennie ill theme song TPP eactions of Aldehydes and Ketones ' 1. Nucleophilic Addition. 2. Substitution at the -arbon 3. ondensation eactions 4. xidation/eduction eactions 1

song TPP eactions of Aldehydes and Ketones ' 1.

2 ondensation eactions In condensation reactions, two molecules are condensed to form one larger molecule. In many condensations, a small molecule such as water or an alcohol are eliminated. In carbonyl condensation reactions, one molecule acts as a nucleophile while the other acts as an electrophile at the carbonyl carbon. Acetaldehyde Aldol Treatment of acetaldehyde with base generates 3-hydroxybutanal ( aldol ) This reaction will also occur under acidic conditions, but we will not discuss that mechanism here. 2

In carbonyl condensation reactions, one molecule acts as a nucleophile while the other acts as an electrophile at the carbonyl

3 Aldol Mechanism Protons on a carbon adjacent to a carbonyl group are relatively acidic because the resulting anion is stabilized by resonance that delocalizes the negative charge onto an electronegative oxygen atom. + 2 Aldol Mechanism

stabilized by resonance that delocalizes the negative charge

4 Aldol Dehydration Step The dehydration step results in the overall equilibrium favoring product Aldol eactions with Ketones Ketones also undergo the aldol reaction, and dehydration is even more important here

eactions with Ketones Ketones also undergo the aldol

5 rossed Aldols If we have two carbonyl reactants, A & B, and both can form enolates, there are four possible products. A-A B-A B-B A-B Therefore, the yield of either B-A or A-B is not likely to be high. If one of the carbonyl components (e.g. B) cannot form an enolate there are only two possible products - A-A and B-A and the yield of B-A may be reasonable. Tetraphenylcyclopentadienone (TPP) In today s experiment, we are going to do a crossed aldol in which only one component (dibenzyl ketone) can form an enolate and the other component (benzil) is more reactive towards enolates. In addition, a second intramolecular aldol step gives a cyclic product with two -unsaturated bonds adjacent to the ketone. Ph 2 2 Ph dibenzyl ketone Ph Ph benzil Ph Ph Ph Ph TPP 5

. If one of the carbonyl components (e.g. B) cannot form an enolate there are only two possible products - A-A and B-A and the yield of B-A may be reasonable.

6 Today s Procedure A. Synthesis and haracterization of TPP B. Spectrometric Analysis of TPP. omputer Visualization of the TPP Structure Synthesis of TPP You will prepare an ethanolic solution of potassium hydroxide and add it to an heated ethanolic solution of benzil and dibenzyl ketone. The solution will be heated for 15 minutes, cooled and filtered to isolate deep purple crystals. The crystals will be washed with cold ethanol, and dried by drawing air through the filter. 6

hydroxide and add it to an heated ethanolic solution of benzil and dibenzyl ketone.

7 haracterization of TPP You will determine the weight of your product and its melting point (above 200 o ) You will also obtain a UV-Vis spectrum of the product. Spectrometric Analysis of TPP The TPP product has a very high epsilon value, so a very low concentration of TPP must be used. For quantitative analysis, we would need to do multiple dilutions of a weighed amount of TPP. Since we are only wanting to see the wavelengths absorbed by TPP, we will just dissolve a small amount in a solvent into which TPP dissolves slowly. 7

Spectrometric Analysis of TPP The TPP product has a very high epsilon value, so a very low concentration of TPP must be used.

8 omputer Visualization of TPP The final component of this experiment is to go to one of the two computers in the lab room to record some observations on the computergenerated structure for TPP. Not everyone can wait until the end of lab to do the computer part. Some students will need to do this first (no waiting) or while the reaction mixture is heating. Follow the steps on the Instruction Sheet to complete the Worksheet. Safety Potassium hydroxide and the ethanolic solution of K are toxic corrosives. Eye hazard! BE AEFUL in crushing the K pellets in making the K solution. Don t break the beaker! exanes are flammable irritants. 8

Some students will need to do this first (no waiting) or while the reaction mixture is heating.

9 lean-up Dispose of filter papers and product in the appropriate waste bottles in the hood. Solvents from filtration go into the aqueous waste bottles in hood. The aqueous ethanol filtrate goes into a waste bottle in the hood. The hexane solutions go into an organic waste bottle in the hood. lean all glassware with acetone. 9

The aqueous ethanol filtrate goes into a waste bottle in the hood.

Reactions of Aldehydes and Ketones

Reactions of Aldehydes and Ketones Structure Deduction using lassification Tests 1 Determination of Structure Determining the structure of an unknown organic compound is an exercise in deductive reasoning.