SODIUM HYPOCHLORITE OXIDATION: PREPARATION OF CAMPHOR Oxidation has a somewhat different meaning in organic chemistry than you have been used to in inorganic, where oxidation is defined as a process involving loss of electrons, as in the conversion of Fe+2 to Fe+3. Oxidation of organic compounds also normally involves electron transfer, but because the valence of carbon usually remains at four, it is more useful to define oxidation as either (a) loss of hydrogen atoms or (b) addition of oxygen or other electronegative atoms. Thus, all of the following reactions are classified as oxidations: H CrO3 OH O KMnO4 OH CH 3 C O OH H2O2 OsO4 OH Oxidations of alcohols provides one of the most general methods for the preparation of carbonyl compounds; oxidation of primary alcohols affords aldehydes (or carboxylic acids, if oxidation is continued), while oxidation of secondary alcohols affords ketones. Tertiary alcohols cannot be oxidized without breaking carbon-carbon bonds. H OH R O R R OH O Primary alcohol Aldehyde Carboxylic Acid R' R' R R O OH The most common oxidizing agents for the conversion of alcohols to aldehydes and ketones are chromium trioxide (CrO3) and its relatives, chromate and dichromate ions (CrO4-2 and Cr2O7-2), as well as a series of modified forms of CrO3, such as Collins reagent, in which CrO3 is complexed with pyridine (Collins reagent is used in nonaqueous media, and is especially useful for oxidizing primary alcohols to aldehydes without overoxidation to the carboxylic acid). KMnO4 can also be used to oxidize alcohols. An alternative oxidizing agent for the preparation of ketones, sodium hypochlorite (NaOCl) in acetic acid, was introduced by Stevens, Chapman, and Weller in 1980 (J. Org. Chem. 45, 2030). This reagent offers several advantages: it is cheap (sodium hypochlorite is the reagent in Clorox and "swimming pool chlorine"); it oxidizes secondary alcohols rapidly and in high yield; and it avoids the problem of disposing of toxic wastes associated with chromium and manganese reagents. The example chosen to illustrate the oxidation of alcohols is the sodium hypochlorite oxidation of isoborneol to camphor: H3C H3C CH 3 CH 3 NaOCl CH3CO2H OH H3C H3C O Camphor is a bridged bicyclic ketone widely distributed in nature, especially in trees of the Far East. It is used as a plasticizer for the production of celluloid film, for smokeless powders and explosives, as an insect repellent, and for medicinal purposes (you will recognize the characteristics odor of Vicks). Borneol, one of the stereoisomeric related alcohols, is found in certain trees of Borneo, and isoborneol, used in today's experiment, is made commercially from pinenes (the C10 hydrocarbons of turpentine). Safety and Waste Disposal 1. Sodium hypochlorite (Clorox) is a strong oxidizing agent and bleach; be careful to keep it off your skin and clothing. Wash spills with water. 2. Excess Clorox, as well as the filtrate from the collection of camphor, can be poured down the drain if diluted well with water. 3. If you distill the methylene chloride at the end of the experiment, put the distillate in the bottle labeled "RECOVERED METHYLENE CHLORIDE" EXPERIMENTAL PROCEDURE Dissolve 5 g of isoborneol in 15 mL of glacial acetic acid in a 125-mL Erlenmeyer flask. Add 50 mL of Clorox by the mL over 5 minutes, cooling the flask as necessary to keep the internal temperature in the range 15-25ºC. Allow the mixture to stand at room temperature for one hour with occasional swirling. A positive KI-starch test should be obtained at this point. Add saturated NaHSO3 solution carefully until the yellow color of the reaction mixture disappears and the KI-starch test is negative. Pour the mixture over 100 mL of brine and ice, collect the solid by filtration on a Buchner funnel, and wash it with saturated NaHCO3 solution until foaming is no longer evident. Press the solid as dry as possible on the funnel. Dissolve the solid 20 mL of methylene chloride, separate from a water layer if necessary, and dry the solution over anhydrous sodium sulfate. After filtering the drying agent, boil off the methylene chloride on a hot plate or in a hot-water bath in the hood. Alternatively, you may set up a simple distillation, heating with a hot-water bath, to remove the methylene chloride. Camphor is quite volatile, so be careful not to overheat or some of your product will be lost. Weigh the dry product and calculate the yield. After determining the percent yield, prepare the 2.4 Dinitrophenylhdrazone derivative: Dissolve about 0.2 g of your crude camphor in 5 mL of 95% ethanol, add 10 mL of the 2,4-dinitrophenylhydrazine reagent solution, heat to boiling on a hot plate for 3 minutes, and allow the mixture to cool. Add water dropwise until the turbidity just clears. Crystallization usually occurs within a few minutes. Filter the yellow precipitate by suction, wash with 2-3 mL of water, and allow drying to occur. If the time permits, determine the melting point (reported mp 164 ºC). Questions: (Answer these in your notebook write-up). 1. Write a balanced equation for the oxidation for the oxidation is isoborneol by NaOCl. 2. What do you actually observe in a "positive Kl-starch test"? What does it mean is the Kl-starch test mentioned at the end of the first paragraph in the Experimental Procedure is positive? 3. What is the purpose of adding NaHSO3 at the end of the oxidation? Write a balanced equation for the reaction it undergoes. Notes. 1. Commercial Clorox contains about 6.5% NaOCl. 2. Preparation of 2,4-dinitrophenylhydrazine reagent: Add 50 mL of concentrated sulfuric acid to 10 g of 2,4- dinitrophenylhydrazine in a 500-mL Erlenmeyer flask and mix well. Add 75 mL of water dropwise (CAREFUL! HEAT EVOLUTION), with stirring or swirling, until solution is complete. To this warm solution add 250 mL of 95% ethanol. This scale provides enough reagents for 35 students. The solution should be prepared fresh each day. 3. For alternative oxidations of secondary alcohols with Clorox, see (a) Oxidation of cyclohexanol: N.M. Zuczek and P.S. Furth, in J. Chem Educ. 58, 824 (1981). (b) Oxidation of cyclic and acyclic secondary alcohols: R.A. Perkins and F. Chau, in J. Chem. Edu. 59, 981 (1982). (c) Oxidation of 9-fluorenol: C.S. Jones and K. Albizati, in J. Chem. Educ. 71, A271 (1994).
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