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A Green Method for Synthesis of Cyclohexanone Oxidation of Cyclohexanol Using Sodium Hypochlorite

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Green Method For sythesis of cyclohexene
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  1 A Green Method for Synthesis of Cyclohexanone: Oxidation of Cyclohexanol using Sodium Hypochlorite Oxidation-reduction reactions form an extremely diverse group, and they proceed by a variety of mechanisms. Oxidation is defines as a loss of electrons or an increase in oxidation number, while in a reduction electrons are gained and oxidation number is decreased. Primary alcohol can readily be oxidized to aldehydes and carboxylic acids. The oxidation of aldehydes to carboxylic acids in aqueous solutions is easier than oxidation of primary alcohols to aldehydes. It is difficult to stop the oxidation of a primary alcohol at the aldehyde stage unless specialized agents are used. Primary alcohols can be oxidized to carboxylic acids by chromic acid (H 2 CrO 4 ) and potassium permanganate (KMnO 4 ). The reaction with KMnO 4  is usually carried out in basic aqueous solution, from which MnO 2   precipitates as the oxidation is complete. The aldehyde is formed as an intermediate, but it is unstable under the reaction conditions and cannot be isolated. There is a color change that accompanies the reaction - the  purple solution (KMnO 4 ) changes to a brown precipitate (MnO 2 ). Many of the oxidizing agents in organic chemistry are based on Cr. All forms of Cr(VI) are powerful oxidizing agents. Chromic acid is a commonly used oxidant and is prepared by dissolving chromium(VI) oxide (CrO 3 ) or sodium dichromate (Na 2 Cr  2 O 7 ) in a mixture of sulfuric acid and water, a mixture known as Jones reagent. In non-aqueous solutions, oxidation  by Cr(VI) does not go to completion and   under these conditions, primary alcohols may be oxidized to aldehydes without forming carboxylic acid. The most common agents for this partial oxidation are: PCC, or  pyridinium chlorochromate (formed by dissolving CrO 3  and HCl and then treated with pyridine). Primary alcohols are also oxidized by atmospheric oxygen to aldehydes and carboxylic acids.   This reaction is very slow and it is catalyzed by enzymes (Acetobacter).   This is how wine turns to vinegar!!! Secondary alcohols are oxidized to ketones - and that's it. The reaction stops at the ketone stage because further oxidation reaction requires the braking of carbon-carbon bond. Scheme 1. Oxidation of Alcohols  2 Tertiary alcohols, because they bear no  -hydrogen, do not easily undergo oxidation. There aren't oxidized  by acidified sodium or potassium dichromate(VI) solution, there is no reaction at all.   In order to oxidize, there must be at least one C - H bond in the functional group. Chromium-based oxidations are reliable and well established, but the toxicity associated with chromium salts meant that they are generally considered the second choice. Chromium compounds have been shown to be carcinogenic, they must be disposed of carefully, and they cannot biodegrade. One experiment, found in virtually all organic chemistry laboratory programs, is the oxidation of an alcohol with chromium(VI).   Chromic acid has been used in introductory chemistry labs since the 1940's. Probably the most popular experiment is the oxidation of cyclohexanol to cyclohexanone, using sodium dichromate in an acidic medium. Waste disposal, safety, and cost considerations associated with the Cr(VI) oxidation  procedures provided the motivation to find some alternative oxidizing agents. In 1980 Stevens, Chapman, and Weller reported in the  Journal of Organic Chemistry   (Stevens, R.V., Chapman, K.T., Weller, H.N.  J. Org. Chem. , 1980, 45, 2030-2032) that swimming pool chlorine (calcium hypochlorite) as the oxidizing agent is a convenient and inexpensive method of preparing ketones in good yields from secondary alcohols. Jerry Mohrig was teaching an introductory organic chemistry laboratory course at Carleton College at that time and decided to use the experimental procedure from the  Journal of Organic Chemistry . The adapted procedure using “swimming pool chlorine”  to oxidize cyclohexanol to cyclohexanone encountered several problems. First, the large amount of solvent and the ether used in multiple extractions made the experiment too expensive, and second, storage of “swimming pool chlorine”  causes its concentration to decrease by about 20% per month. It was found that household bleach functions even better that “swimming pool chlorine”  as the oxidant for cyclohexanol (Mohrig, J.R., Mahaffy, P.G.,  Nienhuis, D.M., Linck, C.F., Van Zoeren, C. Fox, B.G.  J. Chem. Edu.  1985, 62, 519-521). Yields were comparable to slightly better and atmospheric Cl 2  concentrations were diminished considerably. Bleach is inexpensive and has low toxicity if it is handling with care, and it reduces to products that are nontoxic as well. There is no need for the large amount of acetic acid; 0.5 mL AcOH/g alcohol is sufficient. In this experiment you will use household bleach in the presence of acetic acid as the oxidizing agent. The reaction is oxidation of cyclohexanol to cycohexanone. Scheme 2. Oxidation of Cyclohexanol to Cycohexanone  3 Most bleach is an aqueous solution of sodium hypochlorite (NaOCl) prepared by adding Cl 2  to aqueous sodium hydrohide solution: Cl 2 +   2NaOH    NaOCl + NaCl + H 2  Eq. 1 When NaOCl is added to acetic acid, the following acid-base reaction produces hypochlorite (HOCl):  NaOCl + CH 3 COOH HOCl + CH 3 COO -  Na + Eq. 2 In acid solution, HOCl is in equilibrium with Cl 2 : HOCl + HCl Cl 2  + H 2 O Eq. 3 Chlorine gas is toxic, but when household bleach is used to oxidize alcohols, Cl 2  is present in very low and safe concentrations in the atmosphere if the reaction is performed in the fume hood. Both Cl 2  and hypochlorous acid are sources of positive chlorine, which has two fewer electrons than does chlorine anion. A key step in reactions of positive chlorine reagents is the transfer of Cl +  to the substrate. It is reasonable to expect that the first step in the oxidation of cyclohexanol is exchange of Cl +  with the hydroxyl  proton. Subsequent E2 elimination of HCl from the resulting alkyl hypochorite forms the ketone, cyclohexanone. If HOCl is not present, the first step cannot take place and the overall oxidation is very slow. In the first reaction, Cl +  is transferred to the substrate, and in the second reaction, Cl -  is lost. The change is a reduction by two electrons. Cyclohexanol provides the two electrons as it is oxidized to cyclohexanone. Scheme 3. Mechanism of Hypochlorite Oxidation of Cyclohexanol Oxidation of cyclohexanol to cyclohexanone using sodium hypochlorite is an example of green chemistry. In this reaction water is used as a reaction medium and household bleach as the oxidizing agent. The by- products of the reaction, water and sodium chloride, are nonhazardous wastes. Any excess acetic acid remaining in the aqueous solution is neutralized with sodium hydroxide to form acetate ion, also a nonhazardous waste that can be washed down the sink. Cyclohexanone has been separated from the two  phase reaction mixture by extraction with an organic solvent such as DCM. Steam distillation is a green alternative for separating the cyclohexanone. The compromises are a lower yield (50-60%) instead of the 70-  4 80% when extraction was used, as well as higher energy costs, versus no organic solvent waste that would require disposal. The new catch phrases “green chemistry”   and “green engineering”  are really about reaction optimization with respect to materials and energy usage, waste reduction from all sources, and overall cost minimization. Also included are minimization of toxicity and hazards and maximization of safety practices. When these terms are applied to the performances of individual chemical reactions, chemical processes, and chemical synthesis plans they really deal with the same issues. All of these ideas are implemented routinely by process chemists and chemical engineers in the chemical industry who operate under a code of good laboratory  practice (GLP). Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances. Green chemistry is also known as sustainable chemistry. The Twelve Principles of Green Chemistry, srcinally published by Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998, p.30; provide a road map for chemists to implement green chemistry. For more information on the 12 Principles of Green Chemistry, you can visit: http://www.acs.org/content/acs/en/greenchemistry/about/principles/12-principles-of-green-chemistry.html and to learn more about the basics of green chemistry visit:  http://www2.epa.gov/green-chemistry  Purpose of the Experiment    Synthesize cyclohexanone by oxidation of cyclohexanol using an environmentally friendly reaction. Learning Objectives    Perform an oxidation reaction; functional group modification Procedure. To a 500 mL Erlenmeyer flask adds cyclohexanol (16 mL) and glacial acetic acid (7 mL). In a hood, pour sodium hypochlorite solution (bleach, 100.6 mL of 6%) into a 125 mL dropping (separatory) funnel. Prepare an ice-water bath for cooling the flask if the reaction becomes too warm. Introduce a thermometer and slowly add to the reaction flask with swirling approximately one-fourth of the bleach solution. Add the rest of the sodium hypochlorite solution over a period of 15 min. Adjust the rate of addition so that the temperature remains between 40  C and 45  C during the addition. Cool the reaction flask briefly with the ice-water bath if the temperature exceeds 45  C; however, allowing the temperature of reaction mixture to fall below 40  C for any period of time slow down the reaction and may results in a lowered yield. When the addition of sodium hypochlorite solution is complete, pour another 100.6 mL of bleach into the separatory funnel and add this portion of bleach to the reaction flask over a period of 15 min, monitoring the
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