Aldol Condensation Introduction As you've learned in lecture, a condensation reaction is one where two molecules are joined together while expelling another small molecule such as water or an alcohol in the process. Many condensations involve the alpha carbon of one carbonyl compound adding to another carbonyl group. This is classification of reactions is known as the aldol condensation. The reaction we'll investigate is generally known as a mixed aldol condensation and more specifically a Claisen-Schmidt reaction. This occurs when an aldehyde is reacted with a ketone upon the addition of sodium hydroxide functioning as a base. The product, dibenzalacetone, is prepared by condensation of acetone with two equivalents of benzaldehyde. The aldehyde carbonyl is more reactive than the ketone carbonyl in the acetone. This means that the aldehyde will react more rapidly with the anion of the ketone to produce the ẞ-hydroxy ketone, which will then undergo base-catalyzed dehydration. Depending on the ratio of reactants, the reaction will either give mono or dibenzalacetone. In this reaction, enough solvent, ethanol, is present to dissolve both the starting material (benzaldehyde) and the intermediate (benzalacetone). Once the intermediate has formed due to the first addition, another equivalent of benzaldehyde can be added to yield the product dibenzalacetone. This product is conveniently insoluble in aqueous ethanol and precipitates out of the reaction mixture once it has formed. Both the starting material and the intermediate are soluble in aqueous ethanol. 2 H NaOH H₂O ethanol acetone benzaldehyde Figure 1: General reaction scheme for the aldol condensation. dibenzalacetone The aldol condensation begins when the acidic proton is removed from the alpha carbon of the carbonyl. This generates a powerful nucleophile, an enolate. The enolate will then attack the carbonyl carbon. The reaction involves several general steps: 1. base-catalyzed generation of the enolate 2. nucleophilic attack of the enolate on a carbonyl carbon 3. protonation of the resulting anion to give the initial aldol product, an α, ß -hydroxy carbonyl compound. This reaction exists in equilibrium and is reversible. The a, ẞ -hydroxy carbonyl can be isolated because the next dehydration step is generally slower than the initial addition reaction. The final step of the reaction is a hydroxide catalyzed dehydration of the product by way of its enolate. We know that typically a hydroxide ion is not a great leaving group, however in this case the alpha hydrogen to the ketone is acidic and can be removed under strongly basic conditions. The elimination step also produces a stable conjugated α, ß -unsaturated ketone. Because of the conditions and these two factors, the hydroxide is able to function as a leaving group. :O: my of oth H H H надн TOH O H-OH :OH Figure 2: Mechanism for the formation of dibenzalacetone. H There are three isomeric dibenzalacetones, each with a different melting point. One melts at 110°C, another at 60 °C and the final one is a liquid. In the final step of the aldol condensation, loss of water from the ẞ -hydroxy ketone can form molecules in which the alkene hydrogen atoms are either cis or trans to another. Thus, along with the melting point, the 1H NMR spectrum provides the necessary evidence for the stereochemistry of the product. The J value or coupling constant of cis protons for alkenes is 10-12 Hz, whereas the J value of trans protons is 15 20 Hz. Experimental Using a syringe, measure 1.2 mL of benzaldehyde. Add this to a 50 mL beaker along with the theoretical quantity of acetone needed (see prelab question #2) and set aside. Next, add 1.2 g of sodium hydroxide to 12 mL of water and 10 mL of ethanol in a 100 mL beaker. Add a stir bar and allow it to stir on a stir plate until the sodium hydroxide is completely dissolved. Once completely dissolved, add half of the acetone/benzaldehyde mixture to the sodium hydroxide