Adipic Acid Synthesis 1v2h5a

Synthesis of Adipic Acid via Oxidative Cleavage of Cyclohexanone The oxidation of a ketone by basic permanganate to the dicarboxylic acid takes place through the enolate form of the ketone. The reaction can be followed as the bright purple permanganate solution reacts to give a brown precipitate of manganese dioxide. A possible mechanism for this reaction is shown in Scheme 1. O O H

O

O

O

H

O Mn

O

O

O

O Mn O

O O

OH

H2O

MnO3

MnO3

Enolate

H2O

O

O O

O HO

H3O+ OH

O

Adipic Acid

(work-up)

O

MnO4/ HO

OH

O O

H

Oxidation of aldehyde O

Scheme 1. Partial mechanism of oxidative cleavage of cyclohexanone by permanganate Adipic Acid from Cyclohexanone Procedure The reaction to prepare adipic acid is conducted with 10.0 g of cyclohexanone, 30.5 g of potassium permanganate, and amounts of water and hydroxide that can be adjusted to provide an attended exothermic reaction period of 1/2 h. Performing the Oxidation Mix the cyclohexanone and potassium permanganate with 125 mL of DI water in a 250 mL Erlenmeyer flask containing a magnetic stir bar. Adjust the temperature to 30˚C by running a little warm water on the outside of the flask. Note that there is no spontaneous temperature rise. Place the flask on a stir plate. Get the mixture stirring and then add 1 mL of 3 M sodium hydroxide solution. Make sure the stir bar does not bang into your thermometer. A temperature rise should soon be ed. You may want to determine the temperature at which you can just detect warmth, by feeling the flask with your hand. When the temperature reaches 45˚C (after about 15 minutes) slow the oxidation process by brief ice/water bath cooling, and keep the temperature at 45˚C for 20 min. (Don’t over-cool it.) Wait for a slight further rise (to about 47˚C) and an eventual drop in temperature (after another 25 min). Then heat the mixture gently by turning on the hot plate at a low to medium setting to complete the oxidation and to coagulate the precipitated manganese dioxide. Do not bring the solution to a boil.

Testing for unreacted permangenate. Make a spot test by withdrawing reaction mixture on the tip of a stirring rod and touching it to a filter paper. If permanganate is present, it will appear in a purple ring around the brown spot of manganese dioxide. If permanganate is still present, add small amounts of sodium bisulfite until the spot test is negative. CAUTION! Any time you add a reducing agent to an oxidizer there is the potential for a vigorous reaction. Start with a small amount, swirl, and see what happens before adding more in small amounts. When you are convinced that all the excess permangenate has reacted, filter the mixture by vacuum filtration using a Büchner funnel. (Be sure to clamp your filter flask and use only one sheet of properly fitting filter paper.) to wet the filter paper with a little water before pouring in the MnO2 slurry. Wash the brown precipitate well with water. Add a boiling chip, and evaporate the filtrate (the solution that came through) on your hotplate in a large beaker to a volume of about 35 mL. If the solution is not clear and colorless, add a little water, clarify it with decolorizing charcoal, filter out the charcoal, and evaporate again to 35 mL. One partner can clean your Büchner funnel and get another piece of filter paper during this process because you will be using it again. Leave your filter flask set up. Getting the goods out of solution At this point, your dicarboxylic acid is deprotonated because the solution is still very basic. Thus, the charged polar dicarboxylate is water-soluble. By protonating the two carboxylic acids, the molecule becomes uncharged and much less water-soluble. Acidify the hot solution with concentrated hydrochloric acid to pH 1-2. Add another 5 mL HCL in excess, and let the solution stand to crystallize. After it has cooled to room temperature, you can put it in an ice/water bath to maximize crystal formation. Collect the crystals on your Büchner funnel using a new circle of filter paper and wash them with a very small quantity of ice-cold water (adipic acid is still pretty water-soluble). Press the crystals between sheets of filter paper to remove excess water, and set them aside to dry. Analyze the goods Determine the melting point of your purified product. Also, obtain an IR spectrum. Cleaning Up Since everything was water-based, your waste can go into the aqueous waste container. Your lab instructor will collect all the adipic acid in one container.