Aldol Condensation Conclusions The purpose of the aldol condensation reaction was to induce the formation of a new carboncarbon
bond between an enolate and an aldehyde, producing an alkoxide ion. The reagents used in the reaction were acetophenone and 4-nitrobenzaldehyde. Acetophenone is a ketone with three hydrogens, whereas 4-nitrobenzaldehyde is an aldehyde with no carbon to which hydrogens might had been bonded. Only an aldehyde or a ketone with hydrogens is able to lose its hydrogens in an acid-base reaction, yielding an enolate. Therefore, in this reaction, the acetophenone was first reacted with a base to form the enolate, and after that, the enolate, being a nucleophile, reacts with the electrophilic carbonyl carbon of 4-nitrobenzaldehyde. The inability of 4-nitrobenzaldehyde to form an enolate ensures the purity of the final product. This was already a low yield reaction, with only 17% yield in my case, and having only one of the reagents as enolate ensures that the other reagent will not be able to react as a nucleophile in the form of an enolate possibly and likely attacking the acetophenones electrophilic keto carbon. The reason for having the nitro group para to the carbon of 4-nitrobenzaldehyde is so that its keto carbon would be more electrophilic than the keto carbon of acetophenone, thus decreasing the likelihood of the enolate to attack molecules of acetophenone (that have not reached their enolate form) at their keto carbon, and instead have them attack the more positively charged 4-nitrobenzaldehyde keto carbon. The nitro group is deactivating, meaning it repels electrophilic groups at the ortho and para positions by creating positive charge in these positions thus making them electrophilic. This positive charge surely greatly increases the electrophilicity of the keto carbon as well. I added 400 mg of 4-nitrobenzaldehyde dissolved in ethanol to a flask, making sure the 4nitrobenzaldehyde was fully dissolved by heating the flask. I then added 300 l acetophenone, followed by a few drops of 1M NaOH. Upon the addition of NaOH, a base, the acetophenone turned into its enolate form and, acting as a nucleophile, it attacked the keto carbon of the other reagent that was incapable of forming an enolate itself. Since the enolate is very negatively charged, in case the other reagent also formed an enolate, the two negative charges would had repelled one another and the reaction would have had an even lower percent yield. I observed that, as the new carbon bond was being formed, the solution turned transparent orange, previously being transparent bright yellow. I continued stirring until a precipitate formed. A few drops of 1M HCl was added for the purposes of neutralizing the base. Upon adding the acid the solution turned transparent bright yellow again. The precipitate was isolated using vacuum filtration and was then dissolved in 100% ethyl acetate as a recrystallization solvent. Ethyl acetate is a convenient recrystallization solvent because of its ability of dissolving the product when heated contrasted to its inability to dissolve it when cooled in an ice bath. The reason why the addition of the acid did not have the effect of protonating the alkoxide ion was the fact that, at the point of the addition of the acid two reactions have already been carried out due to the presence of the NaOH base in the solution: not only has the alkoxide ion been protonated by water molecules, but the formed aldol from this protonation has also been dehydrated to yield an ,-unsaturated carbonyl. Therefore, the new bond formed in the overall reaction is a double carbon-carbon bond. The dehydrated product was favored over the aldol because the loss of water produced extended conjugation. The color of the crystals obtained was yellow to bright green. The reason why their color was not white was because the molecule of the product represents a system of eight conjugated double bonds, 6 coming from the two phenyl groups, 1 newly formed, and one from the carbonyl, all of these at a one bond distance from each other. Such systems of eight or more conjugated double bonds absorb light in the visible region. With every double bond added the system absorbs light of longer wavelength, moving farther away from the UV region of the spectrum. If it abrobs the violet-blue color, it means that the compound will be yellow-ish in appearance.
