Question

Experiment - Claisen Condensation Reactions:

In this experiment, we performed a Claisen condensation reaction of ethyl phenylacetate using potassium tert-butoxide to form ethyl 3-oxo-2,4-diphenylbutanoate and ethanol without using a solvent. This allowed us to reduce waste, have reagents that have a relatively low toxicity level, and made the reaction much more convenient.

Question 2: If this reaction were run in a solution (as opposed to solvent-free) it would require several hours of refluxing and provide a much smaller yield. Why is this the case?

I cannot seem to find an answer to this question. The only thing that I can find is that the reaction is reversible when a solvent is used (which I'm assuming means that the reaction is not reversible when there is no solvent). If this is correct, why is the reaction reversible when a solvent is used, and not reversible when a solvent is not used?

A source for the answer would be appreciated if possible.

Thank you for the help in advance!

Answer 1

Answers:

The intermolecular distance and the reactivity plays an important role. If a solvent used in the below reaction, then the ethyl phenylacetate molecules got separated by the solvent molecules which leads to lesser contacts/reactivity with the reagent or catalyst (potassium tert butoxide) to form an ion. Since the intermolecular distance is more in the diluted solution, we have to reflux the mixture for several hours to begin the reaction and also we will get very low yield because of the presence of more unreacted starting material.

If we use solvent free method to synthesis, maximum number of ethyl phenylacetate molecule participates in the reaction (since the intermolecular distance is minimum) and we get more yield of the product.

There are two chances about the reversibility of this reaction, (i) If we use ethanol as a solvent for this reaction, the exchange of ethoxy group forms two ethyl phenylacetate molecules from one molecule of product (reversible) (ii) On the other hand, the chance of reversibility is low for the below reaction, may be the unreacted ethyl phenylacetate molecules are remained in the reaction mixture. Gas chromatography is the best way to analyze the reaction pathway. If we analyze the GC-MS of the reaction mixture in the time intervals then we come to know whether the reaction is reversible or it is just an unreacted starting material. For e.g. If we compare product percentage in GC-MS of reaction mixture after 1 hr, 2hr, 3hr etc., then we can confirm the pathway