chemoleo - 19-12-2004 at 12:39
Well, as the title says, I am interested in decarboxylating fatty acids such as stearic & palmitic acids (which are, for instance, extracted from
oil similar to procedures from the Biodiesel thread).
The use of these long chain alkanes could be various, i.e. as unreactive high BP oils, as fuels, or whatever. I am after the former 
. These alkanes should have a boiling point between 280 and 350 deg C (taken from here, and will be on the borderline of the solid/liquid transition at room temperature - which is perfect!)
I am predominantly interested in long-chain saturated fatty acids, to make things simple.
There are various decarboxylation reactions of cyclic systems, but they do not normally apply to aliphatic ones.
Apart from the Kolbe reaction ( electrolysis of a F.A. salt solution in H2O/DMF), I can't think of other methods that would work well.
Would, for instance, a reaction similar to benzoate/NaOH work, where benzene would distill off?
I don't have access to my usual reportoire of information, so forgive me if I overlooked the obvious.
Thoughts anyone?
[Edited on 19-12-2004 by chemoleo]
Esplosivo - 19-12-2004 at 12:53
'Aliphatic' decarboxylation is usually carried out by fusing the sodium salt of the carboxylic acid with sodium hydroxide. The alkane would
distil off and Na2CO3 will be left behind - just as in the case of the benzoate. So I think that you can actually use soap and heat that to fusion
with NaOH. Simple procedure, seems quite straight forward. I'm not sure if the carboxylic acid salt need be anhydrous though, although most
probably the heat required to melt it will probably be more than enough.
chemoleo - 19-12-2004 at 13:09
Hmm, ok, that sounds simple enough.
I am just confused by it because I did search, and it always came up with decarboxylations of cyclic systems - and I started to get the impression
that this wouldn't work with aliphatic ones.
The problem with distillation here of course is that quite some high temperatures are required.
Esplosivo - 19-12-2004 at 13:17
Well at the melting point of the hydroxide which falls in the range of 320 deg celcius the alkane will surely distill over - hepta and octadecane have
boiling points of approx 310 deg celcius. The most efficient way would probably be to set-up the apparatus made by Organikum, simple, efficient and
cheap. The problem is with the heat distribution throughout the mixture - similarly to the decarboxylation of benzoates.
S.C. Wack - 19-12-2004 at 14:24
AFAIK, a,b-methylene acids don't react so very well. Good luck finding a direct or indirect method that you have materials on hand for.
neutrino - 19-12-2004 at 14:30
Another possibility is using UV radiation. The reaction looks like this:
2RCOO<sup>-</sup> -> R-R + 2CO<sub>2</sub> + 2e<sup>-</sup>
The carbon dioxide simply effervesces out.
chemoleo - 19-12-2004 at 16:00
Neutrino, I think this reaction is, in principle, similar to the Kolbe reaction.
S.C.Wack, what do you mean by alpha/beta methylene acids, and don't react so well with which method? NaOH decarboxylation?
Btw, does anyone know decarboxylation reactions that do not involve excessive heat, or UV/electrons, but instead would involve a simple solvent based
decarboxylation?
S.C. Wack - 19-12-2004 at 17:42
Methylene is CH2, I was trying to say RCH2CH2COO-. AFAIK, such compounds do not react in the hot method being discussed in a meaningful way - one
needs an H replaced with something else alpha or beta to the carboxyl.
EDIT: Although not a decarboxylation, you might want to take a look at a long Ber. article:
http://gallica.bnf.fr/document?O=N090695
typing 1687 in the Aller Page field. Red P/I2 is used here. It might be more chemical- and apparatus-wise accessible than hydrides, H2, Ag and CCl4 (a
decarboxylation, at least), other etc. indirect methods to alkanes using expensive or somewhat exotic stuff. Or it might not, I don't speak
German. I hate being reminded of how cheap and legal Euro individuals get their RP.
[Edited on 20-12-2004 by S.C. Wack]
unionised - 21-12-2004 at 09:48
The data I have on photolysis of acids gives RCOOH --> RH +CO2 as the dominant process and even that has a poor yield. Even to get this reaction
you need hard UV ie below 240nm and the absobtion max is at about 205. Not an easy solution
ru9 - 20-10-2005 at 00:02
Gasoline fron Coconut Oil
This may interest you.
www.cocogas.com
Regards.
Luis Matias