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site map read first about course lab manual appendix

Lab I

Lab II

Lab III

Lab IV

Lab V

Lab VI

Lab VII

Lab VIII

Lab IX

Lab X

Lab XI

Lab XII

Lab X: 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.
    theoretical

Pre-lab Questions:
(Please turn in the answers to these questions when you arrive to lab.)

1. What are the molecular weights of the seven possible alcohols and their corresponding ketones?
2. What is the purpose of the isopropanol in this experiment? 
3. You and your partner were assigned the following two alcohols to oxidize in your Oxidation lab:

What are the 3 most important masses that you would see in EACH of the mass spectrum for the oxidized products of these two alcohols?   Could you tell the two oxidized products apart using MS?  If so, how?

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

 

Safety Note: The alcohols used in this experiment are volatile, so all work should be done in the hood.

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. Do you need to measure out the reactant amounts precisely? Why or why not?

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. Why do you need to try to keep the temperature below 50
oC?

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.


Waste Disposal and Clean-up

  1. Place all aqueous solutions and extracts in the sodium hydrogen carbonate waste bottle.
  2. Dispose of all remaining products in the flammables waste bottle.
  3. Rinse your glassware with acetone and dispose of the acetone wash in the flammables waste bottle.
  4. Clean your glassware with hot soapy water and return it to the appropriate storage area.
  5. Dispose of empty used Pasteur pipets in the glass disposal box

Post-Lab Questions
(Please turn in the answers to these questions when you arrive at your NEXT lab.)

 
  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 the mass spectra, prove which ketone was synthesized in each reaction for each unknown alcohol? To help justify your answer, please include the mass spectral data for each unknown and use structures/fragments to argue for the structure of each unknown ketone. Please make sure to clearly label on your MS data the M+ peak and any fragments that helped you confirm the structure of each ketone.
  4. Pick ONE random GC peak from either of your unknown’s GC/MS product data, other than your ketone, and tell me as much as possible about the structure of that compound.  Some “peaks” can be extremely small and not even get a retention time label on them in your Percent Report. 
  5. Which alcohols did you and your partner start with in this oxidation reaction? (Make sure to mention their unknown numbers.)