Oxidation of cyclohexanol to cyclohexanone using sodium hypochlorite

The transformation of cyclohexanol to cyclohexanone, is a commercially important process. In

large chemical plants a catalytic dehydrogenation process is used instead of bleach.

Most of the commercial cyclohexanone goes on to make an oxime, then it undergoes a

rearrangement to form caprolactam which is the starting material of Nylon 6. Nylon-6 is one of

the most widely used nylons in the world. This polymer goes into such materials as nylon

windbreakers, shoe strings, automobile components and automobile tire cord.

Scheme-1: Oxidation of cyclohexanol to cyclohexanone

Usually NaOCl oxidation can be performed in acetic acid. An acid-base reaction of sodium

hypochlorite and acetic acid will yield hypochlorous acid (HOCl), which will be used as the

oxidizing agent (Scheme-1).

Mechanism:Positive chlorine, Cl+, is the active oxidizing agent. In the first step of the oxidation, positive

chlorine is exchanged with the hydroxyl proton. Then, E2 Elimination of HCl from the alkyl

hypochlorite yields the ketone and the chloride anion. In acidic solution, HOCl is in equilibrium

with Cl2. So, Cl+ is reduced by two electrons and the alcohol is oxidized by two electrons.

Scheme-2: Mechanism of oxidation of cyclohexanol to cyclohexanone

Reagents used in this experiment are: 1. Sodium hypochlorite: NaClO, Bleach solution. This is the oxidizing agent.

2. Acetic Acid, CH3CO2H. This further activates the sodium hypochlorite and forms

hypochlorous acid.

3. Sodium thiosulfate: Na2S2O3, is a reducing agent that reacts with and quenches any

remaining oxidizing agent.

4. Sodium Sulfate: Na2SO4, a drying agent. This removes the residual water from the

organic product.

5. Sodium bicarbonate or Sodium hydrogen carbonate: NaHCO3. This is baking soda. This

is a weak base which neutralizes any remaining acetic acid.

6. Sodium Chloride, NaCl, table salt. This reduces the solubility of the organic product in

the aqueous phase thus increasing the amount of product isolated.

Procedure:

Step-1: Pour approximately 2 mL of cyclohexanol and 1 mL of glacial acetic acid into a 250 mL

Erlenmeyer flask. (Do not waste time measuring out exactly 2.00 mL but record exactly how

much material is obtained) under a fume hood,

Step-2: Add 30 mL of household bleach (0.74 M sodium hypochlorite) into the flask,

portionwise during 2 minutes while stirring frequently. The flask will become significantly warm

because of the exothermic reaction. Stirr the flask containing the reaction mixture frequently for

20 minutes in the fume hood. During this time the reaction mixture should cool back to room

temperature.

Step-3: Wet a piece of a potassium iodide-starch test paper and put a drop of the reaction

mixture on it. This is the test for bleach solution, not for the product cyclohexanone. Potassium

iodide-starch test paper: indicates the presence of oxidizing agents by the formation of a

colored I2-starch complex, as shown here:

Fig-1

If the paper does not change its color the test for an oxidizing agent is negative. If this occurs add

another 3 mL of bleach to the reaction mixture, stir the flask for another 5 minutes, and repeat

the test. At this time the test should be positive (the iodide-starch paper turns blue-black),

signifying the presence of excess hypochlorite.

Step-4: Add 0.5-1.0 g of solid sodium thiosulfate (Na2S2O3) to the reaction mixture, swirl the

mixture for about 2 minutes, and repeat the iodide-starch test. The test should be negative at this

point (explain why). Once the test is negative you can start workup outside the fume hood.

Step-5: Check the pH of the reaction mixture with a strip of pH paper. If the reaction mixture is

acidic, add about 1.8 g of solid sodium bicarbonate. Check the pH again and add another 0.5-1 g

portion of sodium bicarbonate, if necessary to bring the pH to neutral.

Step-6: Now transfer the reaction mixture in separatory funnel after washing the reaction vessel

with EtOAc. Followed by addition of Brine solution (saturated sodium chloride solution) to the

reaction mixture to decrease the solubility of cyclohexanone in the aqueous phase. Extract the

solution three times and collect the organic layer in to a conical flask. Add Na2SO4 to remove

traces of water and filter the reaction mixture.

Step-7: After removal of solvent product will be obtained.

Product Analysis: Infrared Spectroscopy Place 2-3 drops of a neat sample of your product between two salt plates, take an IR spectrum of

your product.

Same way you can take a IR spectrum of cyclohexanol (starting material) to compare with

product obtained.

Product Analysis: 2,4-Dinitrophenyl Hydrazine (optional) Add ten drops of ethanol into each dry test tube. Add 2 drops of starting material to one test tube,

add 2 drops of product to other test tube. Add 15 drops of 2,4-dinitrophenyl hydrazine (2,4-DNP)

reagent to both the test tubes. Let sit 15 minutes. The presence of a thick orange or red

precipitate confirms the presence of a carbonyl in a ketone or aldehyde group (not as part of an

acid or ester.)

Safety:

1. Cyclohexanol is an irritant. Avoid contact with skin, eyes, and clothing.

2. Glacial acetic acid is a dehydrating agent, an irritant, and causes burns. Handle it with

care and dispense it in a fume hood and avoid contact with skin, eyes, and clothing.

3. Sodium hypochlorite solution emits chlorine gas, which is a respiratory and eye irritant.

Handle it with care, and dispense it in a fume hood.

4. Keep the reaction in the hood until all of the excess oxidizing agent has been quenched.