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Author: Subject: Synthesis of p-nitrobenzoic acid

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[*] posted on 13-2-2009 at 07:21
Synthesis of p-nitrobenzoic acid

I'm very interested in making some anesthetics like benzocaine. Do you know a way to get easily some p-nitrobenzoic acid in large amounts ?

I tried to nitrate some toluene ion mononitrotoluene and then extract the para isomer by freezing.

I got a pale yellow cristalline solid but the reaction is not very cool and nitrotoluenes smell very bad and are quite harmful.

Any suggestions to make it easier ?

Thank you
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[*] posted on 13-2-2009 at 08:46

Hey, the whole organic chemistry smell very bad and is quite harmful.

There are threads on mononitration of toluene and regioisomers separation and problems encountered in this process. Try using the search engine. I'm not sure if I understood well, because you have not made it clear whether you want suggestions about the p-nitrotoluene preparation or alternative routes to p-nitrobenzoic acid. About alternative routes I'm afraid nothing is as simple as the oxidation of p-nitrotoluene. The only reasonable suggestion I can give you is to build a fume hood.
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[*] posted on 13-2-2009 at 08:52

I know of some oxidations of PABA with oxone in very good yields (in-situ DMDO formation) but it seems that your target is PABA anyway so perhaps this is of no use? I can find the references if you want.
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[*] posted on 13-2-2009 at 19:30

Para aminobenzoic acid can be obtained from health food stores in capsules. The capsules can be opened and the crude p-aminobenzoic poured out and recrystallized. Benzocaine can then be prepared from p-aminobenzoic acid by a Fisher esterification with ethanol using a sulfuric acid catalyst. Preparing p-nitrobenzoic acid from toluene is very low yeilding with respect to the para isomer because the nitration of toluene gives about 65% o-nitrotoluene and about 25% p-nitrotoluene. These isomers are very time consuming to purify, but the experience of making the desired p-isomer is well worth the time. The best way of producing p-aminobenzoic acid from p-nitrotoluene is to reduce the nitro compound to p-toluidine using iron powder and a very small amount of hydrochloric acid. See Fiesers text book in the library for a method of reducing nitrobenzene to aniline which is also appliable to p-nitrotoluene. The p-toluidine is then protected with a acetyl group. Acetic anhydride in aqeous solution with a sodium acetate buffer can be used. Since acetic anhydride is not readily available, glacial acetic acid can then be used for the acylation by heating the amine with an excess of the acetic acid and slowly distilling off the water formed in the reaction to drive the reaction forward. The N-acetyl p- toluidine produced is then oxidized with potassium permaganate and the N-acetaminobenzoic acid isolated. This compound is then hydrolyzed to form p-aminobenzoic acid. Since you end up with a large quantity of o-nitrotoluene, this can be used to produce anthranillic acid by the same method.

[Edited on 13-2-2009 by benzylchloride1]

Amateur NMR spectroscopist
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[*] posted on 17-2-2009 at 06:44

There is a thread started recently somewhere on this forum which talks about nitrating polystyrene to give a high % p-nitro and very little o-nitro ... you can then oxidise the polystyrene to give p-nitrobenzoic acid, this can then be reduced to p-aminobenzoic acid.

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