[Burb]

Hello,

I'm a last year chemical engineering student. I'm doing my final project work on the epoxidation of crude rubber seed oil with a tungsten-based catalyst. I use a phase-transfer catalyst, tungsten catalyst and phosphoric acid and hydrogen peroxide.

I don't understand the reaction mechanism, can somebody help me out and explain me step by step what will happen? I think I have found the reaction mechanism, but it is not clear to me what happens in every step. I don't understand how the complexes are formed.

Thank you!

[pgk]

Forget the phase transfer for the moment.
1). Try to draw the mechanism of epoxidation of a double bond (alkene) by H2O2 in presence of an acid (acidic catalysis)
2). Now, replace the acidic proton by the tungsten Lewis acid and that’ all.
Getting back to phase transfer catalysis. The term “phase transfer” refers to the reaction conditions and not to the reaction catalysis. H2O2 and the tungsten phosphate catalyst are water soluble, while seed oil and seed oil rubber are not. So, the seed oil rubber is dissolved in and organic solvent  (organic phase) and the rest in water (aqueous phase) and they are vigorously stirred together in the presence of a quaternary ammonium salt (C27H50N+, Cl-) that is a cationic surfactant and permits the interfacial contact of the two phases, where the reaction occurs. Phase transfer reactions are schematically represented by the attached reaction scheme.
The confusion starts by wrongly naming the cationic surfactant as a ”catalyst” for phase contact (= transfer, because reactants are transferred from the one phase to the other).

[Burb]

Thank you!

Can you maybe tell me how the complex is formed between parenthesis in the attached scheme?
Is it possible that there is a triple bound?

[pgk]

Lewis complex formation. Tungsten is Lewis acid, meaning a lack of electrons that accepts electrons from water, H2O2 and phosphate anion. Thus, sodium tunsgstenate is transformed to a water soluble compound. Besides and that way, H202 is supported on the catalyst.
No, there is no oxygen triple bond. Non-exited oxygen triple bond has not still been invented. The complex in parenthesis, is a wrong symbolism by somebody who has confused Lewis acid catalysis with phase transfer catalysis.

[Burb]

Thank you very much!

So the complex between parenthesis is not correctly presented?

[Dan]

So the complex between parenthesis is not correctly presented?

The L_nW^(-)≡O⁽⁺⁾ is normally represented as L_nW=O

[pgk]

Not chemically correct. Dan’s remark is very punctual.

[Burb]

Thank you for the information!

I made a new reaction mechanism, I think that this one is correct.