diggafromdover
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methyl 4-aminobenzoate synthesis report
This past weekend I made what I believe to be methyl 4-aminobenzoate by the Fisher esterification of methanol and 4-aminobenzoic acid using
concentrated sulfuric acid as the catalyst.
Yield was 64% in tenth molar quantities. A higher value should have been achieved, but was not owing to a small geyser which shot out the top of the
Kjedahl flask I was using to reflux the reaction mixture when a boiling stone was added.
I used a large excess of methanol ~100 ml (Heet), 1/10 mole PABA and 10 ml H2SO4 (Rooto) and refluxed for 2 hours. Then I added some 5% sodium
bicarbonate solution. The product precipitated voluminously. I brought the solution to pH 8 by adding solid bicarb in small quantities until gas no
longer evolved, then filtered off the precipitate by gravity and dried it atop the steam radiator.
9.7 g of white powdery product was saved. 50 ml of waste methanol was recovered from the reaction waste by distillation. I determined that an
anesthetic product had been produced by using several mg super-lingually. A numbing sensation was noted.
I do not have a melting point apparatus at present, but will test a sample when I am capable.
[Edited on 4-4-2017 by diggafromdover]
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CharlieA
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First, I admire your attempt at doing chemistry. That said, there is almost no time when one should taste any product of a chemical reaction,
especially one that has not been identified.
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BlackDragon2712
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You tasted a compound that could or could not have been your product ... just wow ... though very brave you do not want to end up like this guy https://en.wikipedia.org/wiki/Carl_Wilhelm_Scheele 
It would be interesting to compare the anesthetics effects with the ethyl ester benzocaine
[Edited on 05/12/2013 by BlackDragon2712]
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Texium
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One thing I was wondering, regarding esterifications of amino acids, I assume bicarbonate is sufficient to freebase the amine in the workup? Otherwise
you'll have the sulfate salt as your product instead, and you couldn't freebase it with a stronger base since that would hydrolyze the ester.
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DJF90
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Quote: Originally posted by zts16  | | One thing I was wondering, regarding esterifications of amino acids, I assume bicarbonate is sufficient to freebase the amine in the workup? Otherwise
you'll have the sulfate salt as your product instead, and you couldn't freebase it with a stronger base since that would hydrolyze the ester.
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You're joking right? Proton transfer is a diffusion controlled process, i.e. about as fast as possible. Ester hydrolysis will be negligable if you're
careful (avoid prolonged contact with hot or concentrated base).
The only time I've had difficulty with freebasing a salt was when there was another functionality present that could be deprotonated (notably a
phenolic tetrahydroisoquinoline bydrochloride - the amine hydrochloride and phenol are both of very similar acidity and getting a neutrally charged
species for extraction into organic solvent was not possible in my hands)
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diggafromdover
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Tasting product is very, very old school. I read somebody's thesis from back in the 20's where he did a survey of 4-aminobenzoates and that is how he
evaluated analgesic efficacy - one crystal on the tip of the tounge.
I checked the LD50 for the reactants and products. With the exception of the Sulfuric acid and Methanol, everything was in the g/KG range. Nothing
happened out of turn. But don't any of you all try this, unless, like me, you are bat shit crazy.
As for the bicarb freeing the base, I read up on Benzocaine, it's ethyl brother and bicarb does the trick there. I'll do a quick check with calcium
chloride solution. If there's any base hung up as a sulfate, that ought to smoke it out.
Next up is propyl 4-aminobenzoate aka "Risocaine".
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AvBaeyer
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zts16,
For esters of normal amino acids, bicarbonate does not do a very effective job of liberating the free aminoester from its salt. This is because the
pKa's of the protonated aminoester amino group is around 8 and is comparable to the basicity of bicarb. A much more effective way for many but not all
aminoester salts is to treat a concentrated aqueous solution of the salt with saturated potassium carbonate solution. The amino esters typically oil
out and can be extracted with ether or other suitable solvent. As DJF90 mentions, however, the pKa's of other functional groups present can present
problems when trying to isolate amino compounds by base treatment of salts.
Bicarbonate will work fine with aminobenzoates because the pKa of the protonated amino group is much lower than bicarbonate.
AvB
[Edited on 6-4-2017 by AvBaeyer]
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Texium
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Thanks for the elucidation, AvB. I hadn't considered the relatively low pKa of anilines.
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