Chemistry Forums for Students > Organic Chemistry Forum for Graduate Students and Professionals
Triphosgene Removal
shrent:
I performed a reaction making an isocyanate using ORGSYN process of aq. bicarbonate & dichloromethane in biphasic media. The reaction goes nicely using 1/3 eq. of triphosgene giving 85-90% final yield.
However, I'm facing severe problem of the remaining excess triphosgene in the reaction media after the completion. It is not being removed at all with any sort... :'(
I tried it giving multiple washes of aq. bicarbonate, caustic, but still it remains ! Even if I take some lower % of triphosgene in the reaction, then also it consequently gives lower yield with some excess remaining ! :P
I even tried distilling my isocyanate with high vacuum & low vacuum but at this stage also triphosgene comes alongwith the isocyanate in the receiving flask..!!!!! :-\
I just fed-up with this situation....Pls try to suggest me some solution for this.
:'(
njlgdxhy:
I have done the reasearch on this .So ,you can use the weak aqua ammonia to remove triphosgene.After reaction,drop the solution slowly
lavoisier:
Going back to your original question, you say you're making the isocyanate of an amino acid.
I saw a procedure in orgsyn, but they actually start from an amino ester.
So is it the amino acid or ester you're using?
The difference might be important. Have a look at http://www.sigmaaldrich.com/aldrich/bulletin/al_techbull_al176.pdf, they say you don't get the isocyanate, but a cyclic 'anhydride'.
On the other hand, if you're using an amino ester, you could try two different solutions:
1 - changing the alcoholic moiety of the ester to a heavier of lighter one, so that you can have a sufficiently different bp wrt TP.
2 - starting from the amino acid and see if the cyclic anhydride is easier to separate from TP, while still reacting as you want afterwards.
By all means, it's a tricky problem, and even commercial websites selling TP admit that its separation is sometimes awkward.
lavoisier:
Well, in this case I'm afraid I have no suggestions for you, IF you definitely have to use TP and not any other phosgene substitute.
However, if you can change your reagents, have a look at http://www.sigmaaldrich.com/Area_of_Interest/Chemistry/ChemFiles/Vol_2_No_7/Phosgene_and_Substitutes.html.
I particularly like CDI, but I'm not sure whether it works or not for making isocyanates.
HP:
Hm interesting question: both your isocyanate product and the triphosgene regent are very reactive compounds so you cant chemically neutralize triphosgene without desactivating the isocyanate i think...I suggest you using some selective solvent for your product which is bad solvent for TP - is your product soluble in cyclohexane? Also TP decompose at 130C in heating so if your isocyanate is stable at this temperature then the formed phosgene will flow out the flask - be careful with such a procedure throw the gases true a flask with NaOH solution for safety....One more hint: phosgene is chemically absorbed by hexamethylene triamine (urotropin) forming insolublesalt and i am not sure if TP can also react with it or only at heating ;)
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