Google Adsense

Wednesday, December 19, 2012

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.


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.

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

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.
 Use a vacuum filtration apparatus to filter the solution first

 Soak the filter cake under water-alcohol solution for 
a while to dissolve more diol from the residue

 The filtrate should be looks clean like this

6.      Distil the filtrate until the volume of the residue is about 16ml. Transfer the hot residue into a flask. Weight the residue.

 The azeotropic point of cis-1,2-cyclohexanol and water is about 80

 The distillate, cis-1,2-cyclohexanol and water

7.      Add the same grams of potassium carbonate into the residue and cool down the solution.

 Add potassium carbonate to react with some acid product and make
 them become ionic compound(it would not dissolve into organic solvent)

 cis-1,2-cyclohexanol is crystallized from the solution

8.      Filter the solution and take the solid. Use 7ml hot ethyl acetate to dissolve the solid, filter it and collect filtrate.

 The residue looks a little dirty

 Put the filter paper into a beaker and than dissolve it with ethyl acetate

  Heat the ethyl acetate gentely to dissolve more solid

 Filter the hot solution quickly
9.      Evaporate ethyl acetate from the solution. Now, we get crude cis-1,2-cyclo-hexanol.  

 This is the filtrate

After used a rotary to evaporate solvent thoroughly, 
the final product looks white and clean.

Experimental Record
Theory Weight of cis-1,2-cyclohexanol
Weight of cis-1,2-cyclohexanol
Melting Point

No comments:

Post a Comment