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kaviaari
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O-toluic acid synthesis is looking quite odd
Hi!
I have been refluxing a mixture containing 10ml of xylene and 20ml of 60% nitric acid in order to make o-toluic acid. The procedure that I have scaled
down is found from orgsyn.org. But I did some modifications to synthesis. First of all I used water bath and second I don't have a clue what isomers of xylene does
my solvent grade xylene have.
Now I have been refluxing the mixture and reached 12h. I took the RB-flask from water bath as usual when I am finished. After a while I went to the
lab and saw that the once liquid organic layer had turned to yellow goo. I wonder if my changes have done this or is this just normal? I wouldn't like
to reflux it for another 12h if it's not working right 
Bigger picture of the attachment can be found from http://koti.mbnet.fi/kaviaari/Tyo/very_odd.jpg
[Edited on 13-1-2007 by kaviaari]
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Nicodem
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| Quote: | Originally posted by kaviaari
I have been refluxing a mixture containing 10ml of xylene and 20ml of 60% nitric acid in order to make o-toluic acid. The procedure I have scaled down
is found from orgsyn.org. I did some modifications to synthesis. |
Indeed. You seem to have forgotten to dilute your nitric acid with water as instructed in that Org.Syn. procedure so you got also a bunch of nitrated
products instead of oxidation products.
Besides that, the commercial xylene is a mixture of all three regioisomers (the para and ortho predominating) so you will not obtain o-toluic acid but
a mixture of two or three toluic acids. Since the meta isomer is a very minor one, a simple recrystallization will completely remove it (if present at
all), but the ortho and para isomers might not be easy to separate. There ought to be a solvent for recrystallization capable of doing so since the
two compounds should have notably different solubilities, but you will need to perform a couple of recraystalizations to obtain a pure sample of
either of the two toluic acids. Note also that in order to remove colored side products you need to use activated charcoal ('Darco' is a special
commercial type of activated charcoal).
Edit: Actually you probably obtained only the nitration products since the Org. Syn. procedure calls for a reflux of 55h. The reflux of such an
reaction mixture is at about 130°C, while you say you used a steam bath (100°C) for 12h only.
[Edited on 13-1-2007 by Nicodem]
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garage chemist
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Is this dilute HNO3 oxidation of methylaromatics applicable for toluene ---> benzoic acid? Could a PTC speed up this biphasic reaction?
[Edited on 13-1-2007 by garage chemist]
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Nicodem
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Yes, it can be (and was) applied for toluene oxidation to benzoic acid. However, I don't think PTC catalysts would do anything at all. They are only
useful when the reaction mechanism requires the transfer of anionic species among the phases which is obviously not the case in this oxidation. If I
remember correctly, the oxidation of benzylic positions with diluted HNO3 starts trough NO2 (from decomposing HNO3) abstracting the benzylic hydrogen.
This step being the rate limiting one since the benzylic radicals are then very readily oxidized further (creating more nitrogen oxides in the
process). Both HNO3 and NO2 are soluble in toluene to some degree so transfer is not a terrible problem. Perhaps irradiation with light speeds up the
rate limiting step but I really would not know. Heat certainly does.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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garage chemist
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OK, this might be worth a try, especially with substituted toluenes like o- and p- nitrotoluene this has the potential to make KMnO4 unnecessary.
Ordinary toluene might partially get nitrated as a side reaction.
To make clear what kaviaari did wrong, this is the relevant text bit from orgsyn:
"In a 5-l. round-bottomed flask are placed 1.6 l. of water, 800 ml. of concentrated nitric acid (sp. gr. 1.42), and 400 ml. (364 g., 3 moles) of
commercial 90% o-xylene"
The "1,6 l. of water" is what was lacking.
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kaviaari
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| Quote: | Originally posted by garage chemist
The "1,6 l. of water" is what was lacking. |
True, very true. I threw out the stuff from RB-flask and made new coctail. This time with water Tomorrow I am going to take some oil and check if I can do this right.
Well it's my first organic synthesis but I still wonder why on earth I did't notice the water even though I red the instructions through about twenty
times.
Let's see what I forget on second try 
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kaviaari
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Mmkay, the new mixture has been refluxing for about ten hours. I coulnd't use an oil bath so I planted the flask on a heat plate. Not a very good
idea! Yet again there are some white particles floating in the solution. Could the unaccurate temperature cause nitration in dilluted nitric acid
solution?
Oh dear, oh dear. It just might be the time to buy a heat plate with magnetic stirrer and good temperature control.
[Edited on 14-1-2007 by kaviaari]
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Nicodem
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Reflux on a hot plate will do as well as long as you maintain reflux for those 55 hours. You will not see much crystals dropping out at cooling before
more that half of the xylene is consumed since the products are soluble in it. No noteworthy nitration should occur with diluted nitric acid. Besides
any such minor side product get removed after one recrystallization. The problem will be separating the 4-methylbenzoic and 2-methylbenzoic acids.
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jjspek
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Is it possible that one of you gives me the reaction equation of the reaction: o-xylene to o-toluic acid? I can't find it anywhere!
Thanks in advance!
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garage chemist
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Do you mean with nitric acid as the oxidiser?
It would be difficult to say what the reduction product of the nitric acid will be (NO would be possible), and not very sensible as a considerable
excess of nitric acid will have to be used in order for its concentration to still be high enough towards the end of the oxidation. Stochiometric
calcultions make not much sense with this type of reaction.
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jjspek
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Yes, I mean this synthesis. It is for a report (:$), and they want to see a reaction mechanism and reaction equations. It doesn't have to be stochiometric
correct i.m.o., but the basic route should be correct.
I am a university chemist student, but I don't know how to solve this one...
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Nicodem
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Have you considered doing the most obvious first thing to do, checking the literature?
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jjspek
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I am a student, we have a university library. First thing I do when I don't get it is dive into our library. I could find some basic theory about
oxidations, but not about this case, in the different books. They mention that this synthesis is a possible route, but the mechanism stays unrevealed.
It maybe looks like I want to get an easy way by asking here on the discussion board instead of just doing some research. That is not the case. I
tried very hard, but as I said: I can't solve this one.
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IPN
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Take a look at Fittig and Bieber, Ann. 156, 242 (1870). It should have the original procedure (oxidation with nitric acid) with further details.
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jjspek
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I study at the University of Twente in The Netherlands, and unfortunately, we don't have the book you suppose. But thanks for the effort!
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not_important
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A number of books have chapters on oxidation processes used in organic chemistry, and a few books are dedicated to the subject. Seems likely that the
information you're looking for would show up in some of those.
Alternatively checking the patent lit. for processes using nitric acid as the oxidiser, then checking those for references to the research literature.
might look at http://www.springerlink.com/content/t0r65534k8p52384/
http://www.osti.gov/energycitations/product.biblio.jsp?osti_...
and European Patent EP1324968
Also look into the production of adipic acid via nitric acid oxidation, which has the general handwaving equation of
(CH2)5CO + (CH2)5CHOH + wHNO3 => HOOOC(CH2)4COOH + xN2O + yH2O
[Edited on 29-6-2007 by not_important]
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Nicodem
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| Quote: | Originally posted by jjspek
I study at the University of Twente in The Netherlands, and unfortunately, we don't have the book you suppose. But thanks for the effort!
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That is not a book! It's a paper in the famous old journal I find hard to believe of being unavailable in the Netherlads. But you could have requested
it in the References section if you would have really been interested. Anyway, I'm pretty sure it contains nothing about the mechanism given that it
is from the pre-mechanistics era of chemistry. Nevertheless, I attached it for your convenience.
You don't seem as having put much effort into doing a literature search. Try at least finding some references in Oxidations in Organic Chemistry by
Hudlicky. If there is nothing, then take a day or two of searching with SciFinder&Beilstein.
Tip: The majority of mechanistics papers are published in a couple of ACS journals.
Attachment: Ueber die Constitution der Xylylsäure und Paraxylylsäure und eine neue Modification des Dimethylbenzols.pdf (622kB)
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jjspek
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Dear previous writer,
I have no acces to the reference section on this discussion board. It requested a password.
And: we don't have either scifinder or beilstein available here, as far as I know. So those routes are not possible. I did much research, but for me
it is hard to understand that a simple reaction mechanism like an oxidation of a dimethylbenze to toluic acid is not know in the world of organic
chemistry. I looked at the ACS journals as well, but no good results so far.
I'll try the book of Hudlický next week.
I am grateful for your help, but on the other hand: I don't like it at all that you judge my research skills. I'm new in this forum, so I will stay
humble, but a little help is more welcome than some easy prosecution.
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not_important
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The nitric acid oxidation methods are mostly likely well documented, but in older literature. As it has been used to make benzoic acid, toluic acids,
and the phthalic acids on a lare scale, I assume it was extensively investigated by industrial researchers.
However, it was also used a lot in research where all that was important was the oxidised product, the mechanisms and equations were not important or
even interesting. When I used Google to try to find some references, I found dozens and dozens of papers where dilute HNO3 had been used to oxidise
some organic compound, but no mention of the reaction details outside of how to perform it.
Seeing some of the side products in those cases, I suspect that it is a very messy reaction so far as to mechanism, in many cases taking several paths
to the same main product.
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jjspek
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| Quote: | Originally posted by not_important
[...] I suspect that it is a very messy reaction so far as to mechanism, in many cases taking several paths to the same main product.
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That makes things clear. I will put the information you just gave me in the report, together with my findings in literature. The assistent will maybe
not be happy with the result, but I will ask him for the right mechanism afterwards . We'll see what he will say, then.
I'll keep you updated. Thanks anyhow, all of you, for your kind advise!
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leu
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The applicable section from Hudlicky:
Nitric acid, HNO3, is available as concentrated (68%) nitric acid (d 1.41, azeotropic mixture with water, by 120.5 °C),
fuming (100%) nitric acid (d 1.52), and, rarely, red fuming nitric acid, containing nitrogen oxides. For many oxidations, dilute nitric acid
(approximately 35%) obtained by dilution with water in a 1:1 ratio is employed. Most of the oxidations are carried out at atmospheric pressure in
glass equipment or in porcelain dishes. If higher temperatures are needed, either sealed glass tubes or stainless steel or glass-lined autoclaves must
be used.
Nitric acid is a very strong but not very selective oxidant. The advantages of its use are simple and usually clean-cut isolation of the products. The
obnoxious fumes generated during the oxidations are a disadvantage. The main applications of nitric acid are the dehydrogenation (aromatization) of
dihydropyridines [458]; the degradation of aromatic rings to carboxylic acids [459]; the oxidation of aromatic side chains to carboxyls [460, 461,
462, 463, 464]; the oxidation of alcohols [465, 466, 467, 468, 469], aldehydes [467,470], ketones [471], and esters [472] to carboxylic acids; and the
oxidation of aromatic amines to quinones [473], thiols to sulfonic acids [474], and iodo compounds to iodoso compounds [475]. In combination with
silver oxide, nitric acid is used for the oxidative cleavage of ozonides to carboxylic acids [77].
[460.] Valkanas, G.; Hopff, H. J. Chem. Soc. 1963, 3475 [19, 105, 106].
[461.] Hopff, H.; Valkanas, G. J. Org. Chem. 1962, 27, 2923 [19, 105, 106].
[462.] Bengtsson, E. B. Acta Chem. Scand. 1953, 7, 774; 1954, 8, 842; 1955, 9, 832 [19, 105, 106, 108, 109].
[463.] Zaugg, H. E.; Rapala, R. T. Org. Synth., Collective Volume 1955, 3, 820 [19, 105, 106].
[464.] Tuley, W. F.; Marvel, C. S. Org. Synth., Collective Volume 1955, 3, 822 [19, 105, 106].
[465.] Degering, E. F.; Boatright, L. G. J. Am. Chem. Soc. 1950, 72, 5137 [19, 127].
[466.] Coates, R. R.; Cook, J. W. J. Chem. Soc. 1942, 559 [19, 50].
[467.] Kent, W. H.; Tollens, B. Justus Liebigs Ann. Chem. 1885, 227, 221 [19, 128, 184].
[468.] Ellis, B. A. Org. Synth., Collective Volume 1932, 1, 18 [19, 150].
[469.] Powell, S. G.; Huntress, E. H.; Hershberg, E. B. Org. Synth., Collective Volume 1932, 1, 168 [19, 127].
One of these references might shed further light on your question.
[Edited on 1-7-2007 by leu]
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Nicodem
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The mechanism of oxidation of alcohols with HNO3 (and HNO2) is the only one I know for sure of having been documented, but I have no references and no
time to search for them (you can try the ones above: 465-469; perhaps one of those papers talk about the mechanism). With alcohols, the first step is
the esterification. The second step which is rate limiting is the beta-elimination of HNO2 from the ester. HNO2 in acidic ambient disproportionates to
all kind of nitrogen oxides including HNO3 as well, so that 2 equivalents of HNO3 can oxidize 1 equivalent of the alcohol (but in practice some NO2
escapes as well). This is only valid for secondary alcohols since the primary ones get oxidized up to carboxylic acid.
PS: The mechanism of oxidation of toluene with HNO3 that I assumed in reply to Garage_chemist's question above was nothing but an assumption so please
don't include it in the report without finding a reference first. It was only based on the fact that the oxidation of aromatic non-acidic side chains
generally proceeds by radical mechanism (H-abstraction as the first step) and that NO2 can be formally considered a radical species (you can draw its
Lewis structure and you can see it has one uncoupled electron). If you talk about it in the report make sure you discuss it as a hypothetical
mechanism and back it up with tangible theoretical and experimental arguments.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Nicodem
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| Quote: | Originally posted by jjspek
I looked at the ACS journals as well, but no good results so far. |
Hmm... what about http://dx.doi.org/10.1021/op000085o ?
If the conditions are bellow the ones required for dibenzyl ether hydrolitic cleavage you can assume this kinetic study quite relevant for your
report. I have no time to read it all, but note the rate enchancement by NaNO2 supporting the radical mechanism as suggested in this thread. Also, the
H-abstraction in benzyl ethers is even less endothermic than in toluene.
…there is a human touch of the cultist “believer” in every theorist that he must struggle against as being
unworthy of the scientist. Some of the greatest men of science have publicly repudiated a theory which earlier they hotly defended. In this lies their
scientific temper, not in the scientific defense of the theory. - Weston La Barre (Ghost Dance, 1972)
Read the The ScienceMadness Guidelines!
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Pyridinium
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I looked this up in March's "Advanced Organic Chemistry". It says the oxidation mechanism is still not known (this was 1992). The book implies
that the mechanism of R-CH2 to R-COOH via HNO3 is the same as with other oxidizers, not taking into account the nitration side reactions of course.
According to the book, it's not certain whether it's a carbocation or a radical that is the intermediate. Again, that was in 1992.
Like Nicodem, I'd lean towards its being a radical, but we can't be sure without a literature search. This mechanism is probably going to be in the
post-1992 literature, rather than the early lit.
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Cesium Fluoride
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I found this patent stating that 30-70% HNO3 can be used at about 130C for producing toluic acids from xylene. The HNO3 is added gradually from a
dropping funnel or such throughout the course of the reaction. Is this the reason why no nitrated products are formed in this case?
[Edited on 4-7-2007 by Cesium Fluoride]
Attachment: Oxidation of Xylenes.pdf (420kB)
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