Lu Le Laboratory: Hydroxylation-Oxidation of Alkenes-Reaction of Organic Chemistry

The Hydroxylation of Cyclohexene with KMnO₄ to yield the cis-1,2-diol was first carried out by the Russian chemist Vladimir Markovnikov in 1878. Although this reaction has been known for over 100 years, its mechanism is still not completely understood. The reaction is believed to involve the initial addition of the permanganate ion to the pi bond to yield an unstable, cyclic manganite ester, which is rapidly hydrolyzed to cis-diol.

Mechanism

The fact that a cis-diol (and not trans) results from the permanganate hydroxylation supports the proposed mechanism. More support arises from experiments usind ¹⁸O(oxygen 18). When KMnO₄ labeled with ¹⁸O is used to hydroxylate an alkene, both oxygen atoms in the product diol are ¹⁸O. This observation means that both oxygens of the diol must have come from MnO₄^- ion and not from the water or hydroxide ions in the reaction mixture.

Although we can trace the steps of the alkene oxidation quite well, the reactions that the permanganate ion undergoes are not as well understood. From the balanced reaction , we can see that two permanganate ions are neddedto oxidize three cyclohexene molecules. Therefore, the reaction must be more complex than a simple cyclic ester would suggest. In addition, Mn is at a +7 oxidation state in permanganate ions; +5 in H₂MnO₄^- (from the hydrolysis of the manganite ester); but +4 in the observed product, MnO₂.

It has been suggested that the H₂MnO₄^- ion reacts with permanganate ion to yield manganite ion(+6 state), which also can oxidize an alkene to a diol and thus be converted to MnO₂.

Another hypothesis is the H₂MnO₄^- undergoes disproportionation (an oxidation-reduction reaction) to yield MnO₄^- and MnO₂.

Chemical

1. t-butyl alcohol: 40ml

2. Celite or other filter aid: 0.33~0.66g

3. Cyclohexene: 0.86g

The cyclohexene was made by the dehydration of cyclohexanl

4. Ethyl acetate: 8~10ml

5. Potassium carbonate: 8~33g

6. Potassium permanganate: 1.08g

7. Sodium hydroxide: 0.18g

Procedure

1. Dissolve 1.08g potassium permanganate into 33ml warm water, then cool the solution in an ice bath to about 15℃.

2. Measure 33ml of t-butyl alcohol into a 100ml flask and add 0.83g of cyclohexene. In a separate flask, dissolve 0.16g NaOH into 1ml water, then add this solution into the cyclohexene-alcohol solution and cool the combined solution to -5~0℃ and also add about 16g of ice.

3. Place the flask in a ice bath and add potassium permanganate solution into the flask at a rate of 2 drops/sec, and stir it continuously. Don’t let the temperature of the solution become higher than 15℃.

This color is caused by Mn(VI)

Now it becomes dark green

It starts turning brown, because the forming of MnO₂

_{Most Mn ions changed into Mn(IV)}

4. After the addition, add ~0.7g of celite(filter aid), heat the solution almost to boil and stir it frequently.

_{Celite is a white and fine powder, it makes}_{MnO₂ is easier to filter}

5. Filter the hot solution quickly, and use the mixture of 7ml water and 7ml t-btuyl alcohol to wash the filter cake and combine the wash with filtrate.