Reduction-Oxidation

        Reduction-Oxidation (Redox) reactions play an important role in the daily life of mankind. In the general chemistry, redox reaction is defined using the acronym LEORA-GEROA (Loss Of Electrons-Oxidation-Reducing Agent)- (Gain of Electrons-Reduction-Oxidizing Agent). In organic chemistry, electron transfer is still involved but the definition of Redox reactions, specifically the oxidation, may became confusing and inconsistent because the valence electrons of carbon atom in organic reactions usually remains four (Journal of Organic Chemistry, 1980). Thus, it is more appropriate to define oxidation as (1) loss of Hydrogen atoms or (2) addition of Oxygen or other electronegative atoms to Carbon (Hill and Barbaro, 2000). Even though the definition of oxidation of an atom is attuned with organic chemistry, the process of assigning the oxidation number does not change. For example, the Cl in this experiment will undergo _____ (oxidation/reduction) given by the reaction:

        ( insert drawing in lab manual 2. Loss of electrons (Cl-))

The oxidation no. of Cl in the reaction above is ____.  

        Ketones can be prepared from coupling of acid chlorides with organocopper reagents and the hydration of alkynes while aldehydes can be prepared by reduction of acid chlorides with certain metal hydrides or by catalytic hydrogenation (Carey, n.d.). However, both of them can be prepared by the oxidation of alcohols Different oxidizing reagents are used to oxidize alcohols and some examples of it are: [pic 1]

The chromic acid is the most widely used oxidizing agent for alcohols but studies shows that chromium salts produced by the reactions are suspected to be mutagen and carcinogen. As an alternative, three chemists named Stevens, Chapman and Weller introduced the use of sodium hypochlorite with acetic acid as the oxidizing agent of alcohols in 1979 (Journal of Organic Chemistry, 1980). The advantages of sodium hypochlorite over chromic acid is that first no hazardous waste produced because the end products are only the desired aldehyde/ketone and chloride ion whereas the other produces Cr(III) compounds which are proven to be toxic especially to aquatic life. Secondly, Cl2 gases which can be contained relatively easier than Cr(VI) offers low hazard compared to Cr(VI) compound which are skin and membrane irritants. Thirdly, the oxidation of secondary alcohols is more rapid and accompanied with high yield in sodium hypochlorite. And lastly, the cost of of usage of household bleach is lower than the use of Cr(VI) compounds (Stevens, et al., 1979). In this exercise, calcium hypochlorite will be used instead of sodium hypochlorite but nevertheless the advantage of sodium hypochlorite is also the same as calcium hypochlorite. The general reaction for the protonation of Ca(OCl)2 is given by the equation:

                                ( nasa lab manual!)

        When a primary alcohol is oxidized, aldehydes are produced which can further oxidize into carboxylic acids by some oxidizing agents including HOCl-. On the other hand, when a secondary alcohol is oxidized, ketones are produced (which is the target of this experiment) which do not undergo further oxidation due to its stability. Unfortunately for the tertiary alcohols, they cannot undergo oxidation because their carbon-carbon bond will break if they do so (Hill & Barbaro, 2000). The alcohol that will be used in this experiment is cyclohexanol which is a secondary alcohol. Thus, this experiment will produce a ketone specifically cyclohexanone.

        The mechanism for the generation of cyclohexanone from cyclohexanol using HOCl- from calcium/sodium hypochlorite is still under further studies but the proposed mechanism and the overall reaction will be (Experiment 8 Preparation of Cyclohexanone by Hypochlorite Oxidation, n.d.):

[pic 2]

[pic 3]

There are some important notes to remember in performing the procedure of this experiment. Firstly, this experiment is exothermic and is affected by how fast of slow the hypochlorite solution is added. Adding too fast will make the temperature high and will make the ketone product react further to give these side products (Experiment 9Oxidation of Cyclohexanol toCyclohexanone, n. d.):[pic 4]

(insert general reaction of these side reactions here!)

Correspondingly, adding too slow will produce low yield of the desired ketone product. Secondly, the presence of excess hypochlorite ions OCl- ions is detected using the addition of small amount of Jones reagent. Presence of opaque green mixture indicates the further addition of hypochlorite solution. The addition should also be gradual to avoid too much evolution of Cl2 gases. Thirdly, the NaHSO3 is added to react with excess Ca(OCl)2 in the solution. The bisulfite ion will act as ____ (oxidizing or reducing ????) agent and the reaction is indicated below (Univ. of Colorado, 2004):