[limpet chicken]

I am currently following a synthesis that calls for the use of glacial acetic acid, there is a byproduct of water during the synth, which is taken up by polyphosphoric acid, or phosphorus trichloride.

I will be doing the synthesis in a reasonably confined lab, so PCl3 is NOT an option
and have no polyphosphoric acid, nor the budget to buy it, so, I had an idea, why not use an excess of acetic anhydride, rather than glacial acetic acid, as the reagent for the actual synthesis.

Hopefully, the acetic anhydride would take up the excess water would it not? Probably recycle the acetic anhydride by drying over molecular sives.

Can anyone point out an gaping buggerups in my alteration? as I really don't want to waste acetic anhydride, asI only have a half liter, and that cost me almost £50, which is a damned lot, considering I only get £45 a week

[hmx9123]

Well, you may actually have trouble getting the reaction started, because some of them need a little water to get going, strangely enough.  Also, you need to watch out because acetic anhydride will give a much greater exotherm when uptaking water from the reaction than glacial acetic acid will.

I have no idea what you're doing synthesis wise or if this would work, but why not add some concentrated sulfuric acid?  It sucks up water quite nicely.  Depending on how carefully you pour it, you can get two layers to form--the bottom being the dense sulfuric acid, the top being your glacial acetic acid.

Also, I am not sure if you can get acetic anhydride from glacial acetic acid by drying over molecular sieves.

[limpet chicken]

 I don't think using H2SO4, that it is particularly safe, as there is a possibility of the use of KMnO4 during the synthesis, and we all know what happens with KMnO4/H2SO4 when it just happens to be sat in a beaker full hard to come by organic chemicals, and the combination of A: getting a faceful of acetic anhydride, Mn2O7 and o-toluidine, and B: blowing two weeks money in a corrosive mess all over the ceiling doesn't really appeal to me much


The reaction in particular, actually needs strong heating, and is done in glacial acetic acid as the solvent usually, so lack of water in there isn't such a bad thing.

 

[cbee]

Instead of using just the anhydride, why not calculate mols of water produced in the reaction and use, um, say 2 equiv. of anhydride in acetic acid to mop up the water.  Saves money and the reaction conditions are much closer to the original.  One potential problem would be if your substrate/ product has any nuleophilic groups present which could react with the anydride.

You could also dump in some benene or toluene and use a dean-stark trap and azeotrope the water away.  I can't rememeber if acetic acid is soluble in benzene (maybe not) but long as you have vigorous stirring you should still be able to dry it ok.



Cisco

[limpet chicken]

I do not have a dean-stark trap, and unfortunately, the same goes for acetic acid, save for spirit vinegar (5%), my only options, are to use excess acetic anhydride (I am attempting to acetylate 2-aminobenzoic acid), OR, to stick the reaction flask on a chair on my lawn, and let the phosphorus trichloride fume its merry way on hydrolysation, I suppose I could collect the HCl gas for use whilst I am at it, but I would have to make the phosphorus trichloride by first distilling red phosphorus, condensing the resulting white P, and then passing a stream of dry Cl2 through a glass condensor into a flask before even using it.

So, will using excess acetic anhydride work for acetylating 2-aminobenzoic acid? I am 99% sure it will, but I really cannot afford to mess up the synthesis.

[cbee]

I'm not sure about this, so somebody speak up if it appears that I am full of poop.

There is a good chance that the 2-amino benzoic acid exists as a zwitterion (COO- and NH4+) in which case the amine will not be nucleophilic and therefore no reaction will occur unless you add base to deprotonate the ammonium salt.  I would suggest triethylamine or a similar amine base.  If you use an aqueous base (NaOH) be sure to use only one equivalent because any more would hydroylse your acetic anydride. In fact, the best way would be to take aminobenzoic acid, make the sodium salt via treatment with one equivalent of NaOH, isolate the salt (so there is no water or NaOH in the reaction) and then treat that with the anhydride.

One question:  You mentioned an oxidation with KMnO4.  Is this one pot with the acetylation?  Before?


Cisco

[limpet chicken]

Uhhhhh, if I were to use KMnO4 and acetic anhydride in a onepot, the AA would of course, get raped, right to the aldehyde

There is only a chance I may use KMnO4.

I GUARANTEE the anthranilic acid is acetylatable, it has been done before,  I will not post what I am doing, but acetylation of 2-aminobenzoic acid has been done with glacial acetic acid, which I am now using (ebay listing of the acetic anhydride has ceased )

I will now have to buy some polyphosphoric acid, or better, I found one of those u-tube shaped glass lightbulbs, one half of which has been made into a new condensor, and the other half, will be used for making PCl3, and i shall bee done outdoors now (and I will collect the HCl fumes in water, heh, I like free reagents as byproducts )