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Lab IX: Oxidation & Identification of an Unknown Alcohol

Pre-Lab Work

Reading Assignment:

  1. Description of Experiment--see below.
  2. Oxidation Reactions:
    Hornback; 2nd ed.; pp 380-385.

Introduction
In this experiment, you will use common household bleach as the oxidizing agent. Most bleaches are aqueous sodium hypochlorite, NaOCl(aq). This is created by adding chlorine to sodium hydroxide:

When sodium hypochlorite is added to acetic acid, the following acid-base reaction occurs:

NaOCl, Cl2, and hypochlorous acid (HOCl) are all possible sources of positively charged chlorine (Cl+). Although the existence of discrete Cl+ ions in aqueous solutions has never been found, it is apparent that a key step in these reactions is the transfer of electrons from the organic substrate to the Cl+ species to generate Cl-. One possible transformation for the oxidation of an alcohol with these reagents, using cyclopentanol as an example, is shown below:



The first step in the mechanism shows the replacement of the hydroxyl proton with the positive chlorine. The following step is the elimination of HCl from the resulting alkyl hypochlorite to form the ketone, cyclopentanone. In the first step, Cl+ is transferred to the substrate; and in the second, Cl- is lost. The change is a reduction by two electrons. Cyclopentanol provides those two electrons and is therefore, oxidized.

Hypochlorite oxidations provide a distinct advantage over other oxidizing reagents, such as Cr(VI) reagents, since the toxicity of the chromium reagents provide difficulties in handling and waste disposal. The salt formed in the hypochlorite oxidations can be rinsed down the sink.

In this experiment, you will use the NaOCl in a water/acetic acid mixture to oxidize one of seven possible alcohols into a ketone. Due to the possible low boiling points of some of the ketone products, we will never completely isolate your ketone product. Instead, your product will remain in a solution of dichloromethane (methylene chloride) that can be injected into the GC/MS for separation and identification of individual compounds in your product. Therefore, the GC/MS will be the only instrument that will allow us to identify the compounds present in your product mixture. Identification of your product by GC/MS should allow you to determine the ketone you have created. From this, you should be able to determine which one of the following seven alcohols you were given at the beginning of the lab.

Table 1: List of Possible Alcohols
cyclopentanol
2-methylcyclohexanol
2,4-dimethyl-3-pentanol
3-hexanol
cyclohexanol
2-heptanol
3,3-dimethyl-2-butanol

Experimental Work
Weigh out 1.5 grams of your unknown alcohol into a 125-mL Erlenmeyer flask. Add 1.0 mL of glacial acetic acid and place the flask on a stir plate. Add a stir bar to the flask and gently stir your solution.

Add 20 mL of aqueous sodium hypochlorite solution dropwise to the flask over a 10-minute span making sure to keep the temperature of the solution below 50oC.

After complete addition of the aqueous sodium hypochlorite, stir the mixture for an additional 20 minutes. After approximately 20 minutes of stirring, add 5 mL of isopropanol and stir for 1-2 minutes. (What does the isopropanol do?)

Pour the product mixture from the flask into a 125-mL separatory funnel. Add 20-30 mL of NaCl solution. Shake and vent frequently! (Why is the salt solution added?)
Extract your organic products from the aqueous layer by adding approximately 10 mL of dichloromethane. Shake and vent, then remove the aqueous layer which should now be void of any organic products. Wash the organic layer with approximately 10 mL of saturated sodium hydrogen carbonate solution. Vent often! Dry the organic layer with phase paper and anhydrous magnesium sulfate and fill a labeled GC/MS vial with the dried organic layer.

Run a GC/MS on this organic product mixture.

Analyze your GC/MS to determine the ketone that you have formed.


Special Waste Disposal
Place all aqueous solutions down the sink rinsing with plenty of water.


Post-Lab Questions

  1. What was the purpose of adding isopropanol to your reaction?
  2. Why did you add salt before trying to isolate your organic product from the aqueous solution?
  3. Using your mass spectrum, prove which ketone was synthesized in your reaction?
  4. Using your mass spectrum, can you identify any other compounds, or any functional groups on other compounds, in your product mixture?
  5. Using your partner's mass spectrum, prove which ketone was synthesized in her reaction.
  6. Using your partner's mass spectrum, can you identify any other compounds or any other functional groups on other compounds in her product?
  7. Which alcohols did you and your partner start with in this oxidation reaction? (Make sure to mention their unknown numbers.)
  8. What errors occurred in your oxidation and what would you do differently if you were to do this reaction again?