Oxidation & Identification of an Unknown Alcohol
of Experiment--see below.
Hornback; 2nd ed.; pp 380-385. theoretical
1. What are the molecular weights of the seven possible alcohols and their corresponding ketones?
(Please turn in the answers to these questions when you arrive to lab.)
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?
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
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,
step in the mechanism shows the replacement of the hydroxyl proton with
the positive chlorine. The following step is the elimination of HCl from
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.
1: List of Possible Alcohols
Safety Note: The alcohols used in this experiment are volatile, so all work should be done in the hood.
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 50oC?
After complete addition of the aqueous sodium hypochlorite, stir the
mixture for an additional 20 minutes. After approximately 20 minutes
add 5 mL of isopropanol and stir for 1-2 minutes. What does the isopropanol
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,
remove the aqueous layer which should now be void of any organic products.
Wash the organic layer with approximately 10 mL of saturated
hydrogen carbonate solution. Vent often! Dry the organic layer with phase
paper and anhydrous magnesium sulfate and fill a labeled GC/MS vial
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
- Place all aqueous solutions and extracts in the sodium hydrogen carbonate waste bottle.
- Dispose of all remaining products in the flammables waste bottle.
- Rinse your glassware with acetone and dispose of the acetone wash in the flammables waste bottle.
- Clean your glassware with hot soapy water and return it to the appropriate storage area.
- Dispose of empty used Pasteur pipets in the glass disposal box
(Please turn in the answers to these questions when you arrive at your NEXT lab.)
- What was the purpose
of adding isopropanol to your reaction?
- Why did you add
salt before trying to isolate your organic product from the aqueous
- 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.
- 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.
- Which alcohols
did you and your partner start with in this oxidation reaction? (Make
sure to mention their unknown numbers.)