Is there any chemical test that can indicate the pressence of an aromatic fluorine ?Sergei_Eisenstein - 25-1-2007 at 11:54
In analytical chemistry, there are a few techniques that can help you. There is NMR and MS, and to some extent GC-ECD can assist you as well. I'm not
aware of any "traditional" color test.Ozone - 25-1-2007 at 16:25
Well,
I hate this method, but there is an EPA method (which should be available on-line, free) whereby the sample is refluxed in conc. H2SO4 and then
assayed using a F- electrode (in a pinch, hooked up to a pH meter). It is not specific for aromatic-F, per-se, but if there is any F- in there, you'll
know.
Ah, 63Ni at home...(3H is a much crappier ECD source, but could, theoretically, be handled by an amateur with little fear (puny-est beta there is)).
Cheers,
O3vulture - 26-1-2007 at 15:21
NMR is your best bet if you know what's attached to the aromatic core. The 1H-spectrum will show more complicate splitting patterns than usual because
of coupling with F.JohnWW - 1-2-2007 at 05:17
More to the point, you should use F-19 NMR, which uses a different band of frequencies from H-1, D-2, C-13, or N-14 NMR spectroscopy.unionised - 1-2-2007 at 11:57
My best guess would be that anyone with access to a 19F NMR machine would know that.anti_vizsla - 1-2-2007 at 13:06
Quote:
Originally posted by samsung
Is there any chemical test that can indicate the pressence of an aromatic fluorine ?
NMR is the best for aromatic fluorine but a need a fairly concentrated solution(about 10^(-2) M).
If however you have low concentration there is no proper way. Fluorine ions are at best determined using a fluorine-selective membrane electrode at
10^(-4) - 10^(-5) M.Nicodem - 2-2-2007 at 13:05
The original question however was about a "chemical test" and not a spectroscopic one. In the archaic times of chemistry the sodium quench for organic
substances used to be one of the simple tests for detection of some elements like N, S and the halogens. The sample would be heated on a flame in a
long test tube with a small piece of sodium until it all reacted (can be quite dangerous!). The tube is broken in cold water so that the sodium reacts
(often very violently!). The so formed solution is tested/titrated for halide, cyanide, thiocyanide, sulphide and other anions by the usual chemical
tests. We actually did this at school for pedagogical reasons (and never again). That is the only chemical test for fluorine organic compounds I can
think of. Unfortunately such a test says nothing on whether the F is attached to an aromatic ring or not. For that I think you can only resort to
spectroscopical tests, especially 13C NMR which nicely shows the couplings with the 19F attached to the aromatic ring.vulture - 2-2-2007 at 14:11
You people are making this way too complicated. If the fluorine is attached to the aromatic core you can easily spot this in 1H-NMR. No 13C or 19F
needed.
F-H coupling:
ortho: 6-11Hz
Meta:3-9Hz
para:0-4Hz
F-CH3:
ortho: 2.5Hz
meta: 1.5Hz
para: 0Hz
[Edited on 2-2-2007 by vulture]unionised - 4-2-2007 at 06:30
Would those people advocating the use of NMR (on any nucleus) as a (remember what the original question said) "chemical test" please let me know what
chemistry is involved ie what reacts with what to give what product.
I don't know the person who posed the original question but I doubt he has access to NMR; if he had then he would have been unlikely to ask about a
chemical test.Ozone - 4-2-2007 at 08:48
The big problem is getting that F- off of anything--this is quite difficult (Teflon, for example). F- is the hardest nucleophile there is, and so, it
is a *terrible* leaving group. The only way I know of to do this is to boil the sample in concentrated H2SO4 (metals grade); this is a terrible mess
and resulted in the ruination of every piece of clothing I wore to work during this time. HClO4 will also work. As for other methods, neutron
activation works (directly), as does fusion of the sample (brought to dryness) with Na2CO3 or NaOH to prevent the formation of insoluble refractory
materials (like the acid, completely destroys the matrix, viz. benzene rings leaving a soluble NaF).
Following such harsh preparation, the solutions are usually treated with pH buffered EDTA and either:
1. subject to analysis via elective electrode
2. colorimetry via alizarin fluoride blue method outlined by:
BELCHER, R., LEONARD, M.A., & WEST, T.S. (1959)
Submicro-methods for the analysis of organic compounds. Part
X. Determination of fluorine. J. Am. Chem. Soc., 81: 3577.
Since Ce salts might be hard to get for the amateur, I still stand with the electrode method following (to avoid boilng acid, it is also *relatively*
cheap, requiring only a pH meter and a selective electrode) fusion of your bone-dry material with Na2CO3 or NaOH in a muffle furnace. The resulting
material is solvated and analyzed for F- as NaF and calculated on the dry weight of the original sample.
Inchem has a good overview of the topic (with many references) here:
Related to Ozone's post - cerium can be found as a polishing agent for gemstones, check out 'rock shops', and glass such as lens, and at ceramic
supply houses. It's usually in the form of CeO2, which means some processing to get it to a Ce3+ salt. You actually do better to get the lowest grade
of CeO2 you can find, the buff or tan coloured stuff is good, as these grades often dissolve fairly quickly in aqueous HCl to give CeCl3 or in HNO3 to
give Ce4+ wich can be crystallized out as CAN by adding ammonium nitrate.
