Synthesis and Recrystallisation of Adipic Acid - Lab Report Example

Name: Lab Instructor: Date: Experiment: Synthesis and Recrystallization of Adipic Acid Objective The objective of this lab was to carry out the oxidative cleavage of cyclohexene to yield adipic acid using sodium tungstate as a catalyst and hydrogen peroxide, and then characterize the crude and also re-crystallized product. Introduction The carbon-carbon double bond of an alkene is susceptible to oxidation due to the fact that it is a site with a relatively high electron density (Sato, Aoki and Noyori). The oxidation product obtained depends on the reagents and the conditions under which the oxidation process takes place. Under mild oxidation conditions, only the pi bond is cleaved to form epoxides or 1, 2-diols. When the oxidation condition is rigorous, a complete cleavage of both pi and sigma bonds may occur (Anastas and Williamson). The complete cleavage of these bonds leads to formation of different compounds of carbonyl depending on the pattern of alkene substitution and type of oxidizing agent used. The oxidation process is generally complete when the cleavage product is a ketone, but if the aldehyde is produced as the cleavage product, it is usually further oxidized to produce the corresponding functional group of carboxylic acid, as shown below. Oxidation of alkenes and the resulting products It is generally believed that the oxidative cleavage of alkenes proceeds by an initial oxidation of the pi bond to produce 1, 2-diol, and continued oxidation of this product causes cleavage of the sigma bond between carbon-carbon. The laboratory scale production of Adipic acid is typically accomplished using hot, basic KMnO4 solution, which is a harsh oxidizer and produces large quantities of MnO2 waste. In industrial production of Adipic acid, nitric acid is used. Nitric acid is a strong oxidizing acid with many chemical safety hazards as well as environmental risks, and can violently react with some organic compounds resulting to accidents (Hutchison). In addition, the use nitric acid to prepare Adipic acid causes emission of nitrous oxide, a greenhouse gas that contributes to global warming. In this experiment, hydrogen peroxide (H2O2) was used as an alternative oxidant to carry out oxidative cleavage of cyclohexene (a simple alkene), with sodium tungstate (Na2WO4) as a catalyst, to produce 1, 6-hexanedioic acid (adipic acid) as shown below: Formation of adipic acid This oxidation process avoids the requirement of strongly basic mediums and only generates water as a byproduct. Tungstate acts as an oxidizing agent, having a similar structure as that of permanganate, though, it functions as a catalyst, with its intermediate products being re-oxidized back to the original state by H2O2. Adipic acid is utilized in the production of “Nylon 6, 6”, a polymer used in the production of upholstery, carpet fibers, auto parts, apparel, tire reinforcements and other products. Nylon 6, 6 It was found that the melting point of the crude adipic acid was 141.30oC and upon re-crystallization, the melting point was 158.6oC. The percentage yield of the crude product was 1.9% while that of pure, re-crystallized product was 0.4%. The percent recovery of the pure Adipic acid was 21.57%. Experimental Procedure Synthesis of Adipic acid: A 0.50 g weight of sodium tungstate dehydrate was accurately weighed and transferred to a 50 mL round bottom flask with a magnetic stir bar. Using a 1 mL syringe, 0.6 mL of Aliquat 336 was transferred into the round-bottom flask. The flask and its contents was then placed in an oil bath. 11 mL of 30% hydrogen peroxide was then measured and added to the flask, followed by 0.37 g of potassium bisulfate and the mixture stirred using the magnetic stirrer. Finally, 2.5 mL of cyclohexene was added to the mixture using an automatic dispenser and the mixture stirred before turning on the hotplate to heat the reaction. The mixture was then heated to reflux on the oil bath, for about 1 hour while stirring the mixture vigorously and rinsing down any cyclohexene that was trapped in the condenser using drops of water. The round-bottom flask was then raised from the condenser and then the condenser carefully removed. The figure below shows a picture taken during the experimental showing the experimental set-up. Figure 1: A picture of the experimental set-up taken during the experiment Workup The mixture in the round bottom flask was transferred into a small beaker by use of a glass pipette, leaving behind any PTC that may have separated to avoid the risk of contamination. The beaker with the reaction mixture was then rapidly cooled in an ice bath to form a precipitate. The crude product was then collected using Büchner funnel by vacuum filtration. The product material was then air-dried and then weighed to determine its yield and melting point. Finally, the crude product was recrystallized using about 15 mL of water. The mass of the recrystallized product was then determined and used to determine the final yield and the final melting point. Results The tables below show the measurements of mass and melting point for the crude product obtained after the reaction was complete. Table 1: Mass of the crude product Mass of glass 34.79 g Mass of glass + crude product 34.86 g Mass of crude product 0.07 g Melting point 141.30oC Observations: The colour of the solid crude product of adipic acid after vacuum filtration was white, with slight odor. The melting point is 141.30oC, lower than that of pure, the recrystallized product. Yield: Density of cyclohexene = 0.811g/cm3 2.5 mL of cyclohexene = (2.5 0.811) = 2.0275 g Molecular weight of cyclohexene = 82.143 g/ mol Moles of cyclohexane in the reaction = Molecular weight of adipic acid = 146.14 g/ mol Mass of crude product and theoretical yield = (0.025 mol 146.14 g/ mol) = 3.65 g % theoretical yield of crude product = ( 100 %) = 1.9% Table 2: Mass of the re-crystallized product Mass of glass 34.3813 g Mass of glass + crude product 34.3964 g Mass of re-crystallized product (pure product) 0.0151 g Melting point 158.6oC Observations: The pure, re-crystallized product formed is a white crystalline solid. The solid is odourless. The melting point of the pure, recrystallized product is higher than that of the crude product. % yield of pure, recrystallized product = ( 100 %) = 0.4% % Recovery = Discussion The literature range of melting point of adipic acid is 152 – 154oC. The melting point of the crude product is lower than this range (141.30oC), a fact that may be attributed to the presence of impurities. Possible impurities that may be present include traces of the catalyst and other reactants used. It is known that impurities lower the melting point of any substance. Recrystallization may not be very effective method of purification for highly impure compound. The melting point of the pure product (158.6oC) is higher than the literature range. The percentage yields for both crude and pure product are very low ( Read More