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Adipic Acid Lab Report...

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Synthesis and Recrystallzation of 1,6-hexanedioic acid (adipic acid) through Oxidative Cleavage of Cyclohexene

Results In this experiment, adipic acid was synthesized through the oxidative cleavage of cyclohexene and purified through recrystallization. As shown in Table 1, of the 3.56 g of calculated theoretical yield of crude adipic acid, only 0.608 g of crude adipic acid was obtained experimentally, which led to 17.079 % yield of crude adipic acid. The melting point of the crude product ranged from 144.9 to 147.7 °C. The 0.608 g of the crude product was then recrystallized to yield 0.190 g of purified adipic acid. Therefore, there was a 31.250 % recovery of the purified product from the crude product. While the literature value of adipic acid was 152 °C, the melting point range of purified adipic acid determined experimentally was 146.3-148.1 °C. Table 1: Theoretical, Actual and Percent Yield of Crude Product, Percent Recovery of Purified Product, and Melting Point Ranges of Crude and Purified Product Theoretical Yield (g)

3.56

Literature Value of Adipic Acid (°C)

152

Crude Product

Actual Yield (g)

0.608

Percent Yield (%)

17.079

Melting Point Range (°C)

144.9-147.7

Purified Product Actual Yield (g)

0.190

Percent Recovery (%)

31.250

Melting Point Range (°C)

146.3-148.1

Discussion In this experiment, the oxidative cleavage of cyclohexene was performed to produce adipic acid (1,6-hexanedioic acid). Generally, during the industrial production of adipic acid,

potassium permanganate and nitric acid are utilized. However, the byproducts that emerge from the use of the harsh oxidizer, such as large quantities of MnO2 waste and release of nitrous oxide (N2O), bring about a harmful impact on the environment. Therefore, in this experiment, a greener alternative oxidation technique was employed through the use of sodium tungstate dihydrate as the oxidizer for the oxidation of cyclohexene to adipic acid by hydrogen peroxide, reducing the amount of toxic waste and inducing the production of a harmless byproduct—water. Sodium tungstate dihydrate was able to replace potassium permanganate as an oxidizer throughout the oxidation reaction due to their structural similarities.

Equation 1. Oxidative Cleavey of Cyclohexene and the Formation of Adipic Acid Cyclohexene was a good reactant for this oxidation reaction because its electron density rich nature in the carbon-carbon double bond of the alkene made it prone to oxidation. There may be various products to the oxidation reaction depending on the reagents used to conduct the reaction. Under mild oxidation conditions, only the pi bond of the alkene is cleaved to form 1,2diols or epoxides. However, specifically in this experiment, the reaction occurred under a more rigorous oxidation condition, allowing the complete cleavage of the sigma and pi bonds. This complete cleavage of the alkene enabled the formation of various carbonyl compounds, such as adipic acid in this experiment, as shown in Equation 1. In this experiment, an aqueous phase, consisting of sodium tungstate, and an organic phase, consisting of cyclohexene, made up two different phases. Their insoluble nature led to the

formation of two layers when the two chemical specieis were mixed with hydrogen peroxide, making it difficult for a reaction to occur without the presence of a phase transfer catalyst (PTC). PTCs are chemical species that are soluble in both aqueous solutions and organic solvents that work to accelerate the reaction rate between chemical species in different phases by extracting and transferring the anion of the reagent in the aqueous phase to the other phase through ion pairing and exchange. In this experiment, potassium hydrogen sulfate and Aliquat 336 were used as PTCs. The ion exchange, induced by the PTC, enabled the sodium tungstate oxidizer to be transferred from the aqueous layer to the organic layer, allowing it to oxidize the cyclohexene, which made up the organic layer, to ultimately form adipic acid. Reduced sodium tungstate oxidizer, tungsten, was then released back into the aqueous layer where it was re-oxidized by hydrogen peroxide back to its tungstate state. Experimentally, a collection of the crude product was not made due to the lack of initial crystallization. Various possibilities attribute to the unsuccessful crystallization of the crude product: inaccuracy in the extraction of the organic phase containing the product and the lack of an efficient stirring rate within the reaction mixture. The two chemical species formed two insoluble phases initially, but the product was known to be soluble in hot water. Because the reaction took place under a heated environment by procedure, it was also expected of the the two phase system to naturally become a single, aqueous phase throughout the duration of the 1 hour reflux period. Although the two layers became visible as the reaction chamber cooled down, there was a possibility that the aqueous layer may have not been completely separated from the organic layer. Other than the aqueoues layer, it was essential to be mindful of the aliquat that separated as oil at the bottom of the flask at the end of the reaction. If the reaction mixture was transferred into the beaker along with some aliquat during separation, it is likely that the whole

reaction mixture was contaminated with oily aliquat and not only the purification, but also the formation of product would have been difficult. While organic and mineral molecules crystallize easily, the presence of the aqueous or oil components in the crystallization beaker may have prevented crystals from emerging at all. Additionally, since the experiment involved a phase transfer catalyst, in order to obtain a product yield, heating time and stirring intensity were essential. The reaction may have ran completely for the entirity of the 1 hour reflux period, but with the small magnetic stirring bar, the mixing of the reaction mixture may have not been vigorous enough, which would have decreased the frequency in which the two phases were in contact with one another to carry on the reaction. Ultimately, due to the lack of crude product to work with, the experiment was no longer conducted and the results and observations of another set of product were analyzed. Relatively low percent yield of the crude product was observed at the end of the experiment. Theoretically, the maximum amount of crude product would have been collected if the organic layer containing the adipic acid was completely separated from the aqueous layer and entirely extracted from the round-bottom flask. But as mentioned above, because the two phases have once become a sinlge phase, it could have been difficult to distinguish between the two phases and there may have been some product left behind in the aqueous phase, which accounts for the low percent yield of crude product. In addition to the small amount of crude product that was obtained through crystallization, the quality of the crystallization and the purity of the final product was also affected. This was determined by comparing the experimental melting point ranges of purified product to that of the literature value. The experimental melting point range of the purified product was lower than that of the literature value of the pure adipic acid most likely due to the

incomplete removal of impurities through recrystallization. However, the width of the experimental melting point range of the purified product was narrower than that of the crude product, which implied that some purification occurred through the recrystallization, based on the melting point theory which states that the presence of impurities usually mark an increase in the width of the melting point range. Finally, the removal of these impurities, such as the oil of alliquat 336 that may have been transferred with the top organic layer, through recrystallization and vacuum filtration of the crude product with water ultimately account for the low percent recovery of purified product....