panziandi
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Fluoromethanes and fluoro-halomethanes
I thought I would dedicate a thread to this class of compounds for anyone who is interested in fluoro-organic chemistry.
Tri-iodomethane (iodoform) and tribromomethane (bromoform) can be purchased or prepared by the infamous haloform reaction from methyl ketones and the
appropriate halogen in basic conditions leading to polyhalogenation of the methyl radical followed by cleavage to the trihalomethane and the sodium
salt of a carboxylic acid.
Now tri-iodomethane (and the bromine analogue) can undergo a process known as halogen exchange whereby the each halogen can be replaced for a fluorine
stepwise yielding the monofluorodihalomethane, the difluoromonohalomethane and finally trifluoromethane.
Trifluoromethane was first obtained by reacting dry tri-iodomethane with silver fluoride. The reaction being repeatedly explosive! Trifluorometahne
can be generated from trifluoroacetic acid by decarboxylation (expensive! 100mL TFA cost me £20!)
Later (Henne, A. L. J. Am. Chem. Soc.; 1937; 59(7) pp 1200 - 1202. Fluoroform) described the fluorination of tribromomethane using antimony
trifluoride to gently replace the first two bromine atoms then using mercury fluoride to replace the last bromine to yield trifluoromethane without
the violence of the traditional method.
The convenience of this reaction is that antimony trifluoride replaces two halogen with fluorine to yield the difluorohalomethane.
This reaction is on my list of things to do. Unfortuently it requires antimony trifluoride which is toxic and very corrosive but I think it is quite
doable. Silver and mercury fluorides are easily obtainable or can be made. Other fluorinating agents could be tried too.
The driving force behind silver fluoride is the high latice energy between AgI vs AgF and also the formation of a gaseous product form solid
reactants.
Perhaps it is possible to perform the reaction with the tetrahalomethanes too? Tetraiodomethane is easily prepared from tetrachloromethane and
iodoethane. Tetrabromomethane is readily obtained by bromination of tribromomethane with hypobromite or bromine. Trifluorobromomethane maybe
interesting? It can form a Grignard with Reke Magnesium (ref needed I'll dig it out)
Anyway I hope anyone interested in this who has any of these chemicals at hand, or related chemicals, who wishes to have a go please do! And any ideas
anyone has to this topic please post them!
Attachment: fluoroform.pdf (312kB)
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Klute
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Has tetrabromomethane similar properties than CCl4? Or is it more reactive/less usefull solvent-wise?
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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panziandi
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Tetrabromomethane is a colourless crystaline solid which i think decomposes slightly when heated to boiling at atmospheric pressure m.p. 88-90*C
b.p.190*C. Its easily obtained when you make tribromomethane from the haloform reaction. If you add excess bromine you will see that the dense layer
of tribromomethane collects crystals/becomes crystaline. When I first did this reaction when I was younger and more niave I thought it was salt
crystallising out. I would expect it to be somewhat more reactive than tetrachloromethane since the C-Br bonds are much weaker, I expect it would have
similar solvent properties as CCl4 but would be suitable at temperatures above CCl4's boiling point. Tetraiodomethane is a beautiful red crystaline
material, and as we all know tri-iodomethane is bright yellow.
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Klute
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Thank you, that's something I didn't know. I guess CBr4 could be used as a mild brominating agent? Although its preparation would be a bit wastefull
on bromides..
\"You can battle with a demon, you can embrace a demon; what the hell can you do with a fucking spiritual computer?\"
-Alice Parr
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Sauron
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Give me a little time to dig out my papers on this topic and I will do my best to help.
Sic gorgeamus a los subjectatus nunc.
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panziandi
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Something which may be of interest is introducing trifluoromethyl groups into compounds. Org Syn lists a procedure for doing just that by reacting
trifluoroiodomethane with unsaturated ketones. Which ok requires quite some skill, inert atmosphere etc and diethylzinc but still... doable.
http://www.orgsyn.org/OrgSyn/default.asp?formgroup=basenpe_f...
Trifluoroiodomethane is a gas b.p. -22.5*C and can be prepared by slowly distilling a an equimolar mixture of dry silver trifluoroacetate and iodine,
the yield of pure trifluoroiodomethane being 80% according to the following reference. Albert L. Henne, William G. Finnegan, J. Am. Chem. Soc.; 1950;
72(8); 3806-3807.
Attachment: trifluoroiodomethane.pdf (294kB)
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vulture
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CBr4 was originally used in the Appel synthesis to make alkylbromides from alcohols. It's now usually replaced with NBS and PPh3 in DCM. Word of
caution, CBr4 reacts violently with metals at RT.
[Edited on 20-10-2008 by vulture]
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panziandi
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Yes I stumbled upon that synthesis too.
I didn't know it was violently reactive with metals at rt  ... What metals does
it react with at this temperature? I would have expected a violent reaction with alkali and alkaline earths, but is it violent towards other metals
too? Obviously anyone working with unfamiliar chemicals should always read a MSDS.
I don't know if I will be able to do anything practical in this field until christmas time due to uni 
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