Oxidation Reactions

In this exercise, oxidation of various functional groups (alcohol, aldehyde, ketone, and acid) is tested using typical inorganic oxidants: dichromate, Tollens' reagent, and permanganate. (Note that organic oxidation reactions aren't dealt with in many texts, so lectures may require supplementary material.)

Dichromate oxidation of alcohols: Six drops of concentrated sulfuric acid are mixed with 1 mL of 5% sodium dichromate and the solution is cooled if necessary. Twelve drops of ethyl, isopropyl, or t-butyl alcohol are added. Color and odor changes are noted. Samples of acetone, acetaldehyde, and acetic acid are provided for odor comparison.  (If acetaldehyde isn't available, mix diethyl acetal, water, and sulfuric acid in a small bottle.)

Tollens' reagent: In two clean test tubes, a drop of 3 M NaOH is added to 20 drops of 5% silver nitrate, and 2% ammonium hydroxide is added dropwise until the silver oxide is almost completely dissolved. Then one drop of test substance is added to each tube. Students are assigned either glucose (10% in water) and isopropyl alcohol or propionaldehyde and ethyl alcohol. Mirrors seem to form best with glucose, fair with propionaldehyde, and not at all with ethyl or isopropyl alcohol. Comparison of results with the latter three clarify which functional group of glucose is being oxidized. We haven't yet figured out an effective but economical way for them to make flat mirrors so they can take one home. Students are told to clean up silver deposits using a few drops of nitric acid and to dispose of them appropriately since dried residues can be explosive.

Permanganate oxidation of carbonyls: To 1 mL of 1% potassium permanganate is added one drop of either propionaldehyde, acetone, or acetic acid. The muddy precipitate of manganese dioxide forms only with the first.

For the chromate and permanganate oxidations, students are asked to draw the structures and identify the functional groups of all organic substances they are testing. In each case where they see a reaction, they are to name and draw the structures of the products. For Tollens' test, they are asked which of glucose's three functional groups (primary or secondary alcohol or aldehyde) is responsible for the reaction. Finally, they are asked to draw the structures of the six functional groups used in the exercise (primary, secondary, tertiary alcohol; aldehyde; ketone; acid), state which can be oxidized, and identify the kind of bond that is missing from those functional groups that cannot be oxidized.

Revised 8/31/06