The cyclic ketone cyclohexanone was oxidized to adipic acid using the oxidizing agent nitric acid. The experiment yielded 0.2667 grams of adipic acid, giving a percent yield of 113.97%. Although the product was allowed to dry for one week, residual moisture was still present in the sample and a melting point could not be obtained. This error in the experiment either resulted from adding too much water or not allowing the product to remain in the Hirsch vacuum filtration for long enough to sufficiently dry.
Introduction
Redox reactions are an important class of reactions in organic chemistry that involve the transfer of electrons from one molecule to another. This experiment focused on an oxidation reaction. In organic chemistry, oxidation reactions occur when a carbon atom becomes bonded to a more electronegative atom, causing the electron density on the carbon atom to decrease (McMurray, 2012). Examples of oxidation reactions in organic chemistry are the breaking of a carbon-hydrogen bond, or the formation of a carbon-oxygen bond (McMurray, 2012).
This reaction oxidized a secondary alcohol to a ketone (Williamson, 2003). The oxidation of a ketone, in this case, cyclohexanone, can be accomplished by using various oxidizing agents, however, this experiment made use of the powerful oxidizing agent nitric acid. An intermediate is formed during this oxidation, which is further oxidized to adipic acid (Figure 1) (Williamson, 2003). This reaction is of particular importance, as it breaks the cyclic carbon ring when adipic acid is formed (Williamson, 2003).
The ketone reagent used in this experiment was cyclohexanone, a cyclic six-carbon ring with an attached oxygen group (Figure 1). Cyclohexanone is an oily clear liquid at room temperature. Cyclohexanone is known to have the molecular formula, C6H10O, a molecular weight of 98.14 grams/mole, a melting point of -47 °C and a density of 0.9 grams/mL. (PubChem.com) The oxidizing agent used in this experiment was 15 M nitric acid (Figure 1), a colorless or yellowish, fuming, corrosive liquid at room temperature. Nitric acid has the chemical formula HNO3, a molecular weight of 63.01 grams/mol, a melting point of -42 °C, and a density of 1.5027 grams/mL. (PubChem.com)
The product yielded in this reaction was adipic acid (Figure 1), a white crystalline solid substance at room temperature, which can be deprotonated twice. Adipic acid is an important industrial chemical used in the synthesis of nylon. Adipic acid has the molecular formula, C6H10O4, the molecular weight 146.14 grams/mol, a melting point of 153 °C, and a density of 1.36 g/cm3. (PubChem.com) Figure 1. Reaction mechanism for the reduction of cyclohexanone to adipic acid, using the oxidizing agent nitric acid. Figure 1. Reaction mechanism for the reduction of cyclohexanone to adipic acid, using the oxidizing agent nitric acid.
Experimental
This experiment was conducted under conditions described by Williamson, 2003. To begin, approximately 150 mg of cyclohexanone was placed into a vial. In a separate 10 x 100 mL reaction tube, 1.0 mL of HNO3 was added by pipette, along with a pre-weighed boiling chip. The reaction tube containing the nitric acid was clamped into a sand bath under the fume hood and heated at a low setting. One drop of cyclohexanone was careful added to the nitric acid. The presence of a brown oxide indicated that the reaction had begun, at which point the reaction tube was removed from the sand bath. Cyclohexanone was added to the reaction tube drop wise.
Additional cyclohexanone was not added to the reaction tube until the previous reaction had completed. After the last drop of cyclohexanone was added, the reaction tube was placed back into the sand bath and boiled for one minute. The tube was then removed from the sand bath and allowed to cool to room temperature before being placed in an ice bath for three minute. Three drops of water were added to the reaction tube while it remained in the ice bath. The product was collected by Hirsch vacuum filtration and allowed to dry for the period of one week before weight and melting point were obtained.
Results
The experiment yielded 0.2667 grams of adipic acid, giving a percent yield of 113.97% yield. The product contained residual moisture and was not in a solid state, so that a melting point could not be obtained.
Discussion
The adipic acid produced by this experiment still contained moisture and was not in a solid state after one week, giving a large degree of inaccuracy in the percent yield and not allowing for a melting point to be taking. This is somewhat limiting, in that a comment on the purity of the product cannot be given without a melting point. The product was likely too wet because it was not allowed to dry long enough in the Hirsch vacuum filtration process. Adipic acid is also very soluble in water, so that adding too much water while the product was one the ice bath may have dissolved some of the crystals (Gaivoronskii & Granzhan, 2005). The degree to which the adipic acid is soluble in water has also been found to depend on the temperature (Gaivoronskii & Granzhan, 2005). In future experiments, it is recommended to allow the product to dry longer in the vacuum filtration process and to exercise more caution when adding water to the crystals to ensure product is not lost.
References
Gaivoronskii AN, Granzhan, VA (2005). Solubility of Adipic Acid in Organic Solvents and Water. Russian Journal of Applied Chemistry. 78 pp 404-408. McMurray, JM (2012). Organic Chemistry, 8th Edition. CENGAGE Learning. Williamson, KL (2003). Macroscale and Microscale Organic Experiments, 4th Edition. Houghton Mifflin Custom Publishing. PubChem.com (2012). National Center for BioTechnology Information. http://pubchem.ncbi.nlm.nih.gov/summary/summary.cgi?cid=944&loc=ec_rcs Accessed February 20, 2013.