[tenchan]

Hi ^^

We prepared azelaic acid from ricinoleic acid in the lab.  We treated ricinoleic acid with alkaline permanganate, after which we acidified it with sulfuric acid to liberate the azelaic acid.  I can't seem to find the reaction and mechanism for this...from what I've read, ricinoleic acid is supposed to be cleaved at the unsaturation at carbon 9.  If anyone has a more detailed explanation of the process, could you please help?  Also, a lot of carbon dioxide was formed after the acid was added.  From what species would it come from?  If ricinoleic acid was cleaved at C9...then both portions would have 9 carbons each, and it doesn't seem to make sense for either one to be decarboxylated... Any ideas?  Thank you!

[tenchan]

Hi again!

I've done some more reading and learned that the experiment would involve the oxidative cleavage of alkenes by acidified KMnO4.    From what I read, the two carbons with the unsaturation will be oxidized producing a diol + manganese dioxide from KMnO4.  When you add acid, the C-C bond would be split and the C-OH groups will become COOH groups.  Could anyone explain to me why the addition of acid would have that effect?

Also, I learned that unsubstituted carbons at the unsaturation would be converted to CO2.  We observed CO2 production during the experiment, but looking at the structure of ricinoleic acid, there doesn't seem to be any unsubstituted C at the unsaturation...So where did the CO2 come from?  

[java]

I read something useful here...

http://www.cyberlipid.org/fa/acid0004.htm

[plu]

In neutral solution, KMnO₄ can only oxidize unsaturated carbon chains into diols (the MnO₄^- ions themselves being reduced to MnO₂).  However, in acidic solution, KMnO₄ can cleave carbon-carbon double bonds, forming carboxylic acid groups.  This process is very similar to the ozonolysis process (1. O₃  2. Zn,H₃O⁺) where carbon-carbon double bonds are cleaved to formed aldehydes and ketones as well as the diol formation-cleavage processes in which either neutral KMnO₄ or OsO₄,H₂S/NaHSO₃ is used to form a diol and then cleavage is performed with either HIO₄,H₂O or Pb(OAc)₄,benzene/HAc.  As for the CO₂ production, have you considered the oxidation of your glycerol side-product?

[tenchan]

I read something useful here...

http://www.cyberlipid.org/fa/acid0004.htm

Thank you ^^  I've also checked that site out  

[tenchan]

In neutral solution, KMnO₄ can only oxidize unsaturated carbon chains into diols (the MnO₄^- ions themselves being reduced to MnO₂).  However, in acidic solution, KMnO₄ can cleave carbon-carbon double bonds, forming carboxylic acid groups.  This process is very similar to the ozonolysis process (1. O₃  2. Zn,H₃O⁺) where carbon-carbon double bonds are cleaved to formed aldehydes and ketones as well as the diol formation-cleavage processes in which either neutral KMnO₄ or OsO₄,H₂S/NaHSO₃ is used to form a diol and then cleavage is performed with either HIO₄,H₂O or Pb(OAc)₄,benzene/HAc.  As for the CO₂ production, have you considered the oxidation of your glycerol side-product?

Thanks for the reply ^^  How exactly does the pH affect the outcome of the reaction?  As for the glycerol side product, I don't see where glycerol would come from...we did produce glycerol when we prepared ricinoleic acid from castor oil in the first part of the experiment...but we washed the ricinoleic acid prior to preparing azelaic acid from it.  Do you mean that there would also be glycerol from the oxidation process?

Thanks~

[plu]

Thanks for the reply ^^  How exactly does the pH affect the outcome of the reaction?  As for the glycerol side product, I don't see where glycerol would come from...we did produce glycerol when we prepared ricinoleic acid from castor oil in the first part of the experiment...but we washed the ricinoleic acid prior to preparing azelaic acid from it.  Do you mean that there would also be glycerol from the oxidation process?

Thanks~

Try writing the half-equations for the reduction of MnO₄^- to either MnO₂ or Mn³⁺.  You will see the affect of acid on the oxidating strength of MnO₄^-.

Take another look at the structures of ricinoleic acid and azelaic acid.  Write a balanced structural equation and check out the products you form