Sciencemadness Discussion Board

Fluorine and organic alkanes

Edward Elric - 11-10-2009 at 13:58

So, I'm reading in a book that if I were to mix fluorine with an organic alkane, then the reaction would be incredibly exothermic. Furthermore, the book seems to jump off the topic after about three sentences and say, "...difficult to control...will not be discussed further."

So, how exothermic is this reaction?

About the only thing I found on google videos was someone mixing chlorine gas with an organic gas. It sparkled and popped: Not too explosive.

[Edited on 11-10-2009 by Edward Elric]

Joauml - 11-10-2009 at 14:07

i think that the book refers to a reaction between elemental fluorine and an alkane, not fluorine(fluoride) ions and an alkane

entropy51 - 11-10-2009 at 14:08

Quote: Originally posted by Edward Elric  
So, I'm reading in a book that if I were to mix fluorine with an organic alkane, then the reaction would be incredibly exothermic. Furthermore, the book seems to jump off the topic after about three sentences and say, "...difficult to control...will not be discussed further."

So, how exothermic is this reaction?

Is it possible for me to take gasoline, mix in some mouth wash, and watch some fireworks? I know it sounds kewlish, but still, I want to know.


No dude, not kewlish at all. We thought you were a theoretical physical chemist! They all ask this very same question all the time. They all think that there are both organic and inorganic alkanes.

How did you get from fluorine to mouthwash? What, you think the 0.001% fluorine SALT is the same as elemental fluorine?

PS: Too late. I had to capture that one for posterity.:P

[Edited on 11-10-2009 by entropy51]

JohnWW - 11-10-2009 at 15:11

Except for CF4, fluorinated alkanes (or alkenes etc.) cannot be made simply by their reaction with F2 gas, because the F2 also breaks the C-C bonds, resulting in CF4 being the principal product, as well as the reaction being dangerously exothermic. The various patented methods for making fluoroalkanes and fluoroalkenes (the most important of which latter are vinyl fluoride, CH2=CHF, and tetrafluoroethylene, CF2=CF2, which polymerize to the important high-temperature and resistant plastics PVF and PTFE or Teflon) mainly involve carefully controled electrolysis of fluorides in the presence of the organic compounds. In addition, HF can be made to add across double and triple alkene and alkyne bonds as per Markownikoff addition, although the reaction is slower than with the other halo-acids.

Edward Elric - 11-10-2009 at 18:07

So, I found these things:

(scientific)
http://www.metacafe.com/watch/463291/underwater_fireworks_ex...

(not too scientific)
http://www.youtube.com/watch?v=ggmMu5nyO5g

I'm wondering about how much more powerful (magnitude of exothermic property) the use of fluorine is. Let's say chlorine with the acetylene gas gives out 123 kcal/mol and that fluorine would have given 1337 kcal/mol or something...

[Edited on 12-10-2009 by Edward Elric]

1281371269 - 12-10-2009 at 10:10

Rockets used to be powered by Hydrazine + F2 (with an enthalpy change of -1174/kJ mol-1 using average bond enthalpies)

entropy51 - 12-10-2009 at 12:06

Mossydie, I don't believe F2 was used except in test engines that never flew. I think LOX and N2O4 may have been commonly used with N2H4 as oxidizers.

1281371269 - 12-10-2009 at 12:13

It was a claim made in my chemistry textbook so I took it to be true but it certainly doesn't seem practical for storage problems as well as for the fact that the products are N2 and HF -> which Nasa presumably doesn't want hanging around the atmosphere in their space stations.

Edward Elric - 12-10-2009 at 19:08

I found out that the rxn's magnitude is about 4x greater than Cl when using F.

That would have been one fucking nasty brake fluid explosion.

Thanks, anyway.
Anyway, I should have originally created this in organic chemistry.

[Edited on 13-10-2009 by Edward Elric]

12AX7 - 12-10-2009 at 20:25

As I recall, fluorination is done with milder agents. Like... ohh I don't even remember, transition metals are a start I suppose. Regular F2 burns everything to hell, it's no good if you want to put fluorine on a carbon backbone while keeping the carbon backbone.

Tim

entropy51 - 13-10-2009 at 05:38

Quote:
As I recall, fluorination is done with milder agents


like SbF5 or Hg2F2 acting on alkyl chlorides or bromides in the Swartz reaction. There are probably better ways now, but that is a classic.

My favorite is the Balz-Schiemann, where HBF4 is used to form the fluoborate salt of an aromatic diazonium (from the aniline) and decomposition of the salt yields a fluorobenzene. This is in Vogel. A nice touch is that HBF4 can be used in glassware, which surprised me.

DJF90 - 13-10-2009 at 05:58

SbF5 is a pretty common reagent for organic fluorination. Immediate use that comes to mind is converting benzotrichlorides to the corresponding trifluoro compounds, and as such is very useful in medicinal synthesis if I've remembered correctly.

Elemental fluorine also has uses in organic chemistry - I seem to recall trifluoroacetic acid is produced electrolytically from HF and acetic acid - the fluorine being generated in situ. Fluorine is also used as an oxidising reagent, generally as a complexed HOF - basically dissolve your organic in MeCN-H2O and gas with fluorine. Can't remember what transformations this effects off the top of my head, although I believe oxidation of secondary alcohols to ketones and ketones to esters (albeit more slowly) are amongst them, as well as oxidation of primary amines to nitro groups and also the epoxidation of alkenes.

Jor - 13-10-2009 at 06:12

Talking about fluorine... Look at this very interesting:
http://www.youtube.com/watch?v=u2ogMUDBaf4

You can see carbon, steel and hydrogen instantly in fluorine. But 1 thing: I'm not sure if the colorless gas is actually dilute F2. Look at the bottom, there is a yellow liquid. Maybe NCl3 (suggested by woelen) ? You can see the Al-powder ignite when it hits the bottom.

DJF90 - 13-10-2009 at 08:41

I dont think there is a liquid in there jor. And I dont see how they'd get NCl3 at the bottom of a jar of F2... I think its much more likely that its just refraction, the glass at the bottom of a gas jar is much thicker than the rest, and this could exaggerate the colour of the fluorine... after all, fluorine is not a colourless gas.

12AX7 - 13-10-2009 at 08:48

The "hydrogen fluorine" fireball is certainly the hydrogen balloon popping and burning in air. The jet of fluorine burns the rubber (which will be particularly reactive with all those interesting disulfides and carbon bonds and such), which ignites the hydrogen. I'm disappointed that we don't get to see a real HF flame. The HCl flame is really neat. I wonder what HBr and HI look like, if any different. I'm also surprised at the heat generated by AlCl3 formation: I thought it would've smoldered (gah, after hearing the British accent in that video, I almost typed 'smouldering'! :P ) rather than shining with bright burning.

Tim

woelen - 13-10-2009 at 10:08

I also like this video very much and it is well filmed. The only thing is that I have doubts on the fluor. I suspect it is another chemical compound. All other chemicals can be seen clearly and have very good colors, but the fluorine looks a little bit fake to me.

@DJF90: I think that there is no bottle of F2 at all. The refraction you mention is not present in the bottle of chlorine, in that bottle there should be an even stronger effect, but it is not there. You just clearly see the color of the Cl2 and that is how it should be. For F2 I would expect a little bit of color for the gas, but I only see color at the bottom. Another point is that the powder ignites when it reaches the bottom of the flask, I would expect it to ignite earlier.

DJF90 - 13-10-2009 at 10:51

You make very valid points woelen. And I can see why the wouldnt want to actually use fluorine gas... but to use NCl3 instead? That is a little crazy... Perhaps another substitute?

entropy51 - 13-10-2009 at 11:28

Chlorine dioxide??? On a laser at work we use a mixture of a low percentage of F2 in an 'inert' gas, Helium IIRC. Could it be that?

[Edited on 13-10-2009 by entropy51]

DJF90 - 13-10-2009 at 11:32

I dont think it would react violently like that with aluminium? If at all? and ClO2 is much more yellow than that, unless its diluted, but then even less likely to react with aluminium?

Picric-A - 13-10-2009 at 13:50

It is F2. I went to a lecture where a man filled a gas jar with this gas and added Al, S and iron wool to it (at different times :P) the result looked similar to on the video.
I guess the reason why it looks like it inites on the bottom is becuase it only has enough energy to produce the flash when its been in the gas for so long, however it does start to react on contact.

Jor - 13-10-2009 at 15:20

But how do you explain the burning of the charcoal? They seem like serious people, no fakers. Isn't fluorine in cylinders always diluted with some inert gas like He, Ar or N2 ?
Even in pure form it only has a faint color:

http://www.webelements.com/_media/elements/element-pics-theo...

Maybe I will retry the experiment sometime. I can buy CoF3, although it's quite expensive (few euros per grams). This can be decomposed to elemental fluorine.

entropy51 - 13-10-2009 at 15:28

Quote:
I can buy CoF3, although it's quite expensive (few euros per grams). This can be decomposed to elemental fluorine.


Really! To macroscopic amounts of F2? That's extremely interesting. I knew it could be used to fluorinate alkanes, but not to yield the element.

Edward Elric - 13-10-2009 at 15:37

Anyone willing to make a video of mixing some fluorine and some alkanes?
I don't have elemental fluorine hanging around me.
And for where I'm living, I wouldn't want to store it.

Here is another video describing the exothermic properties of halogens (non-scientific), since some people like watching halogens do things:

http://www.youtube.com/watch?v=mY7o28-l_WU

[Edited on 13-10-2009 by Edward Elric]

Jor - 13-10-2009 at 15:54

I'm not sure if CoF3 decomposes quantitive to F2. However, it does decompose to a high degree to F2.
The price has rosen since I last checked. It's now 25g for 100 EUR and 5g for 30 EUR. I'm not gonna spend that much.

However, some salts also decompose to fluorine, like K2NiF6 and some lead(IV) -fluoride complex salt, I think it was something like X3HPbF8 or XHPbF6 (X=K or Na) wich can be prepared from the higher oxides and HF AFAIK.
Someone who has HF (Ozonelanbs) should try to make them from either PbO2 or NiO2 (made from Ni(OH)2 and persulfate).