Organic chemistry lab 7 PDF

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Crossed-Aldol Condensation

Kanye West TA: Jeff Regier Completed On: Monday, March 2, 2020

Introduction

An aldol reaction is a reaction in which two molecules, of an aldehyde or a ketone having α-hydrogens, react with each other in the presence of a base, to form a β-hydroxyl carbonyl compound. As is the case with this experiment, the reaction of an aldehyde with a ketone, with sodium hydroxide serving as the base, is an example of a Crossed-Aldol Condensation.

In this reaction, dibenzalacetone, the β-hydroxyl carbonyl compound, will be synthesized by condensation of acetone with two equivalent of benzaldehyde. As acetone contains α-hydrogens on both sides of the molecule, it will be deprotonated to give a nucleophilic enolate anion. The resulting alkoxide will be protonated by the solvent, giving rise to a β-hydroxyketone, which will then undergo a base-catalyzed dehydration. The reaction mechanism for the synthesis of dibenzalacetone from benzaldehyde and acetone, with sodium hydroxide solution serving as the catalyst solution, is depicted in Figure 1.

The required apparatuses for this experiment include magnetic spin vanes, stirring plates, Mel-Temps, and suction filtration equipment. As the actual Crossed-Aldol Condensation reaction, which will be run for 30 minutes, will be contained inside a sample vials that will placed at room temperature, the reaction temperature will be around 20–22 °C. However, in the recrystallization phase of the experiment, the ice bath temperature will be painted around 2 - 3 ºC.

The sodium hydroxide solution, which will be prepared by dissolving 0.4 g of NaOH in 4.0 mL of water and then adding 3.0 mL of 95% ethanol, will be serving as the catalyst solution. The ethanol portion of the catalyst solution will essentially be serving as the solvent, which will be capable of dissolving the starting material, benzaldehyde and also the intermediate, benzalacetone.

The crude dibenzalacetone, which will be present as the precipitate, will be isolated through vacuum filtration with a Hirsch funnel. The isolated dibenzalacetone product will then be further purified through a twosolvent recrystallization method, in which the crude will be first dissolved in a minimum of ethanol through boiling, and then be subjected to drop-wise water addition, at which point crystallization will begin to take place. The recrystallized product will be characterized by TLC analysis, melting point determination, IR spectroscopy and, NMR spectroscopy.

Procedure The procedure given in the ON Tech CHEM 2120 February 24, 2020 laboratory manual, Crossed-Aldol Condensation, was followed without any significant changes.

Data and Results

The primary data observations were made during the product characterization phase of the experiment, durning which the recrystallized product was tested for a melting point range, IR spectroscopy and, NMR spectroscopy. The recrystallized product was additionally characterized using thin layer chromatography, in which a TLC plate (Figure 2) was spotted with benzaldehyde, crude product, recrystallized product and pure dibenzalacetone standard samples. The corresponding retention factor values were calculated for each of the TLC spots and then presented in a tabular manner (Table 2). Additionally, calculations were performed to determined the percent yields of the crude product (82.835% Yield) and the recrystallized product (35.291% Yield) (Figure 3). After collecting the crude product through vacuum filtration, it was observed that the crude product was powder-like in terms of consistency and a vibrant, neon yellow in appearance. The recrystallized product was greenish-yellow in appearance and had a completely different consistency, as it was quiet crystalline and had clear-cut edges. Once the product had been recrystallized, the MelTemp was used used to determine the melting point range of the recrystallized product (104 – 106 °C) (Table 3). Subsequent IR and NMR spectroscopies were then performed on the recrystallized product (Figures ).

Discussion

The objective of this experiment was to synthesize dibenzalacetone from benzaldehyde and acetone through a type of base-catalyzed crossed aldol condensation referred to as the Claisen-Schmidt reaction. In this crossed aldol-condensation reaction, the sodium hydroxide solution served as the catalyst solution and the

ethanol, which was part of the catalyst solution, behaved as the solvent, which allowed for the reaction between the acetone and benzaldehyde to dissolve and react with each other. After completion of the reaction, the crude product, which was present as the vibrant, neon yellow dibenzalacetone precipitate, was isolated through vacuum filtration and then further purified through a two-solvent recrystallization method. The recrystallized product was then characterized by TLC analysis, melting point determination, NMR spectroscopy and, IR spectroscopy.

An analysis of the TLC chromatograph indicates that the dibenzalacetone product was successfully formed without the presence of any impurities. Upon comparing the (Rf) values and the band patterns of the recrystallized product with those of the pure dibenzalacetone standard, it was observed that both were nearly identical. The recrystallized product pigment had the same (Rf) value of 0.63, as the pure dibenzalacetone standard, indicating that it was similar in composition to the pure dibenzalacetone standard. If the recrystallized product had contained any impurities or reactants, it would have exhibited (Rf) values and band patterns different from those of the pure dibenzalacetone standard.

As the recrystallized product had a fairly narrow melting point range (~3 0C) that closely resembled the literature melting point range (104 – 106 °C compared to 104-107 °C), it can be stated that the recrystallized dibenzalacetone product was of a relatively high purity. If the dibenzalacetone product had contained soluble impurities, it would have melted at a temperature that was much lower than that of the literature melting point.

In the structure of the dibenzalacetone molecule, there are five chemically different (inequivalent) hydrogen atoms (Figure 4). Due to the presence of the vertical plane of symmetry (σv ), all five of chemically different hydrogen atoms have corresponding symmetrically equivalent hydrogen atoms. As a result of the five chemically different hydrogen atoms, there will be five 1H-NMR peaks expected.

Through comparing the student dibenzalacetone product 1H-NMR spectrum (Figure 5) to the pure dibenzalacetone spectrum, it can be stated that the student’s dibenzalacetone product is pure and free of any significant impurities or side product peaks. Both the student 1H-NMR spectrum and the pure dibenzalacetone spectrum exhibit the presence of doublets at 7.09 ppm and 7.75~7.77 ppm, these doublets correspond to the signals of the two pairs of symmetrically equivalent hydrogen atoms present on the α & β carbons. In both spectra, multiplets corresponding to the aromatic functional groups are present at 7.43~7.44 ppm and 7.63~7.64 ppm, respectively. The multiplets exhibit an overlap at ~7.4 ppm. Additionally, the student 1H-NMR spectrum did not indicate the presence of any unreacted reactants, as it lacked the additional peaks found in the corresponding acetone and benzaldehyde NMR spectrums.

In the structure of the dibenzalacetone molecule there are seven chemically different (inequivalent) carbon atoms (Figure 7). Due to the presence of the vertical plane of symmetry (σv ), six of the chemically different carbon atoms have corresponding symmetrically equivalent carbon atoms. As a result of the seven chemically different carbon atoms, seven C-NMR peaks were expected. The expected number of C-NMR peaks were verified by the pure dibenzalacetone spectrum, which exhibited seven peaks.

Through comparing the student dibenzalacetone product C-NMR spectrum (Figure 8) to the pure dibenzalacetone C-NMR spectrum, it can be stated that the student’s dibenzalacetone product is pure and free of any significant impurities or side product peaks. Both the student product C-NMR spectrum and the pure dibenzalacetone C-NMR spectrum exhibit the presence of the carbonyl carbon (C=O) signal at 190 ppm and the two (C=C) signals present on both the α & β carbons, at 125 and 145 ppm, respectively. In regards to the aromatic carbons, four (C=C) signals were detected and all of them were located around 130~135 ppm. Additionally, the student C-NMR spectrum did not indicate the presence of any unreacted reactants, as it lacked the additional peaks found in the corresponding acetone and benzaldehyde C-NMR spectrums.

The overall objective of this experiment was to synthesize dibenzalacetone from benzaldehyde and acetone through a type of base-catalyzed crossed aldol condensation referred to as the Claisen-Schmidt reaction. Once the product had been attained, it was then characterized through melting point determination, NMR spectroscopy and TLC analysis. Through the various characterization tests, it was determined that the dibenzalacetone product was successfully formed without the presence of any impurities, thus the lab objective was successfully accomplished....