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|Subject: Phenylacetic Acid - Stability and Solubility
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|Phenylacetic Acid - Stability and Solubility
Swim has recently come across a quantity of phenylacetic acid that may be as old as 20 years. It has that unmistakeable smell and is bone dry, but about 20% of the crystal masses have taken on a varying shade of light brown. Could this paa be substantially degraded? Or could the color change represent a only a small degree of degradation? Swim has no problem recrystallizing it, but would prefer not to if it's not neccessary since it is already dry.
Swim plans to use this paa in an NaOAc/Ac2O P2P synth and would like to try drip addition of the paa as discussed (but never tried, it seems) a LONG time ago here.
Post 165317 (Labrat: "Phenylacetone Synth", Chemistry Discourse)
Glacial acetic acid would appear to be the solvent of choice for this endevour, given the conditions of the reaction. Does anyone have any info or best guesses about the quantity of gaa needed to dissolve a quantity of paa? Yes, swim knows he can try it and see, but it would be nice to have a general idea before beginning. As far as swim can tell, the addition of small amounts of gaa to this reaction mix should have no adverse effects aside from slightly dilluting the anhydride concentration (AcOH is a reaction product which is included in the anhydride exchanges anyway).
Swim's reaction will go something like this:
80g fused NaOAc will be added to 300g Ac2O and brought to about 120-130c (at first aming for just below reflux temp of solution with AcOH so that CO2 evolution may be observed and drip rate can be set, followed by increasing temp to reflux.) Then, 80g PAA will be added dropwise in a solution of AcOH at such a rate so that the CO2 formation caused by the addition of one drip will have subsided by the time the next one falls. After addition, reflux will be allowed to continue for another hour. AcOH will then be distilled out followed by Ac2O. The remains of the mixture will be dilluted with water followed by extraction with dcm. This will be washed until neutral with 5% NaHCO3 followed by removal of the dcm by distillation. The p2p will be fractionally distilled hopefully in good yield.
Any comments/suggestions about swim's procedure are appreciated.
|P2P from PAA
Glacial acetic acid would appear to be the solvent of choice for this endevour, given the conditions of the reaction. Does anyone have any info or best guesses about the quantity of gaa needed to dissolve a quantity of paa? Yes, swim knows he can try it and see, but it would be nice to have a general idea before beginning.
Simply make a small-scale test using 1g of PAA to see how much GAA is needed to dissolve it at RT. That way you will know the solubility beforehand.
80g fused NaOAc will be added to 300g Ac2O and brought to about 120-130c (at first aming for just below reflux temp of solution with AcOH so that CO2 evolution may be observed and drip rate can be set, followed by increasing temp to reflux.) Then, 80g PAA will be added dropwise in a solution of AcOH
This corresponds to a 1:3.75 ratio between PAA:Ac2O - which is suboptimal for good yields. According to the procedures in https://www.rhodium.ws/chemistry/phenylacetone.html#phenylacetic you should aim for a ratio of 1:6 instead or your yields will be lower (unreacted Ac2O will be recycled anyway). You won't need a 1:1 ratio between NaOAc:PAA either, a 1:2 ratio is enough.
You should also shake the crude reaction mixture extract with 5-10% sodium hydroxide rather than sodium hydrogen carbonate, as you need to fully hydrolyze any phenyl-2-propanone enol acetates to phenyl-2-propanone before distilling (Enol acetates = Compounds 23/24 in Fig. 9 in this document (https://www.rhodium.ws/chemistry/p2p.phenylacetic.html))
The Hive - Clandestine Chemists Without Borders
|In the following post, terbium quotes his...
In the following post, terbium quotes his reaction parameters for this synth as follows:
280g PAA added in 4 portions
275g anhydrous KAcO
reflux 12 hours
Post 478232 (terbium: "Phenylacetic acid / Acetic anhydride", Stimulants)
This corresponds to an even smaller ratio of Ac2O to PAA than swim's outlined procedure.
Swim assumed that terbium added the paa in portions so as to assure that an excess of Ac2O was always present. This was swim's intent as well, only at a much more finite scale (Ac2O always present in extreme excess due to dropwise addition).