Author Topic: Preparation of Boron Trifluoride/Etherate  (Read 2505 times)

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Preparation of Boron Trifluoride/Etherate
« on: July 11, 2004, 05:51:00 PM »
From Walton's Inorganic Laboratory Preparations
Boron Trifuoride

Boron trifluoride, BF3, is a gas remarkable for the many addition compounds it forms with compounds of oxygen, nitrogen, sulfur, and other elements-even with the inert gases. The electronic structure shows that the electron shell of boron is incomplete, with room for another pair of electrons. That is, it is an acceptor molecule, and its addition compounds are all formed by donation of a pair of electrons from the other molecule to form a coordinate link with the boron. They are among the simplest types of coordination compound. Because of its electron-accepting power, boron trifluoride is an excellent catalyst for many reactions such as alkylations and polymerizations. It is prepared in considerable quantities for use in the petroleum and synthetic organic chemical industries.
Boron trifluoride is similar in many ways to silicon tetra­fluoride, including its physical properties and reactions with water and alkalies. It could be prepared by an analogous method. However, a smoother reaction which gives a purer product, less contaminated by silicon tetrafluoride, this reaction will be used here. First, boric oxide and ammonium fluoborate must be prepared, since these are not generally available. (Any soluble fluoborate could be used in place of the ammonium salt.)

Boric oxide Heat 30 grams of boric acid in an iron dish over a Meker burner, stirring occasionally with an old file, until a clear viscous melt is obtained. Cool quickly, break out the glassy mass, crush it in a rock crusher or other device (it is very hard and brittle, so do not attempt this preliminary crushing in an ordi­nary mortar ; then grind to a powder in a large iron mortar.

Ammonium fluoborate Grind up 65 grams (1.15 moles) of ammonium acid fluoride, NH4HF2, with 30 grams (0.5 mole) of boric acid, and heat the mixture in a shallow iron dish over a gas ring or large burner flame in the hood. Frothing occurs, ammonia and steam escape, and the excess of NH4HF2 is volatilized. Continue to heat until a dry, puffed-up white mass remains. Break this up with a pestle and heat again, but stop as soon as dense white fumes indicate that the main product is starting to sublime. Cool; then powder in a mortar. This product is pure enough for use in preparing boron tri­fluoride. If a clean, pure preparation of ammonium fluoborate is required for another purpose, the best way to make it is to mix the theoretical amounts of ammonium acid fluoride and boric acid in aqueous solution in a plastic dish, and evaporate to crystallization on a steam bath.

Boron trifuoride Set up in the hood the apparatus shown in Fig. 14. Since boron trifluoride attacks cork less than it does rubber, cork stoppers should be used and rubber connections avoided as far as possible. (Where a very pure product is desired, an all-glass apparatus must be used. Ground joints may be lubricated with a paraffin-vaseline mixture.) The vessels and connections should be of Pyrex glass.
The generating flask A has a volume of 500 ml. The test tubes B, C, and D are 25 X 200 mm. B is empty and used as a trap; C contains some 30 ml of sulfuric acid saturated with boric acid, to absorb hydrogen fluoride from the gas stream. In A, place the entire product of ammonium fluoborate, well mixed with 6 grams of finely powdered boric oxide. In D, place 30 ml of diethyl ether, and cool the tube in ice. Arrange for the gas pass­ing out of D to be sucked away by the draft or to be absorbed in caustic soda.
When all is ready, pour 40 ml of concentrated sulfuric acid into flask A and replace the cork immediately. A vigorous reaction occurs, which, however, soon slackens and must be maintained by heating. Keep the reaction going steadily but not too fast. Some of the boron trifluoride is absorbed by the ether in D, forming a viscous solution of the addition compound (C2H5)20•­BF3. This will be used for the tests to follow.
Before all the boron trifluoride has been generated, replace the test tube of ether with a test tube containing a few milliliters of water. Note the reaction between boron trifluoride and water. If sufficient gas is passed, viscous liquids BF3-21120 and BF,-H20 are obtained. These liquids are strongly acidic, the latter ionizing to H+ and HO•BF3 . Similar compounds are formed from boron trifluoride and alcohols.
Pass a little boron trifluoride gas into a Bunsen flame. Note the characteristic color, which is given by all volatile boron com­pounds and is a sensitive test for boron.
Perform the following tests on the boron trifluoride-ether solution:
1. Pass ammonia gas into 10 ml of the solution. The ammonia can be drawn from a cylinder or prepared by dropping concen­trated aqueous ammonia onto sodium hydroxide pellets and dry­ing the gas by contact with potassium hydroxide sticks or pellets. The white precipitate is the complex NH3•BF3. Filter the solid on a Buchner funnel, wash it with a little ether, and allow it to dry. Heat the solid in a dry test tube; it sublimes unchanged.
2. Mix a few drops of the solution with a few drops of pyridine. Note that a similar addition product is formed.
3. In a small distilling flask mix 10 ml of the solution with 12 ml of acetic anhydride. Reflux the mixture for 10 minutes, then distill slowly, collecting the distillate which comes over between 65 and 100°. It should be mainly ethyl acetate.
Wash the distillate with about 50 ml of 10 per cent sodium carbonate to free it from boron trifluoride and any acetic anhy­ride; collect the ethyl acetate with a separatory funnel, noting its smell and its volume. The yield would be practically theoretical, except that some of the ethyl acetate remains behind in the distilling flask as the very stable complex CH3C0002H6•BF3.
Make the following test with the aqueous solution of boron trifluoride: Test for fluoride ion by diluting with water, neutraliz­ing with sodium bicarbonate, making slightly acid with acetic acid, and adding calcium chloride solution.
A striking test of the catalytic power of boron trifluoride can be made if the materials are available. Condense some isobuty­lene in a beaker surrounded by dry ice, or simply put some pieces of dry ice in a beaker and pass in gaseous isobutylene; then add a few drops of isoprene. Stir the liquid mush and pass in boron trifluoride. Immediately a solid white mass of butyl rubber is formed.9

7 Inorganic Syntheses, 1, 21, and 2, 23.
F. J. Sowa and J. A. Nieuwland, J. Am. Chem. Soc., 58 (1936), 272.

8 Inorganic Syntheses, 2, 23.

9 Annual Reports of the Chemical Society, 1942; p 128



  • Guest
« Reply #1 on: July 12, 2004, 09:10:00 AM »
Aha, that actually looks like a very helpful procedure lugh  8)

ammonium bifluoride = almost free
boric acid = almost free
sulfuric acid = almost free
ether = almost free


  • Guest
> ammonium bifluoride = almost free And,...
« Reply #2 on: July 12, 2004, 03:16:00 PM »
> ammonium bifluoride = almost free

And, like almost all fluorides, a suspicious item due to its possible use in certain organophosphate products.

Needless to say, it will also fuck up your glass. And don't attempt anything like that unless you know exactly what you are doing there, BF3 isn't exactly harmless and neither are fluorides.


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Ammonium Fluoride
« Reply #3 on: July 13, 2004, 02:10:00 AM »
Ammonium Fluoride is poisonous, and can be made by interacting  ammonium hydroxide and hydrofluoric acid, with subsequent crystallization, both of which are both available in weak concentrations OTC in SWIL's area of the world  :P  Ammonium fluoride is heat sensitive, so any heat used must be very limited  ;)  Listed uses are in agriculture for improving sandy soils, as an antiseptic in brewing, etching glass, textile mordant and in wood preservations  :)  Note that the ammonium fluoride and boric oxide are processed in an iron dish, so etching shouldn't bee a problem if the directions are followed. Those without a high quality fume hood and a thorough knowledge of preparitive inorganic chemistry technique shouldn't consider attempting this  ;D


  • Guest
> available in weak concentrations OTC Wow,
« Reply #4 on: July 13, 2004, 11:11:00 AM »
> available in weak concentrations OTC

Wow, I wonder how they get away with selling HF in any concentrations to the general public?! The only kind of OTC source I have discovered so far for HF is some sort of paste (which also contains HNO3 and a lot of other shit) that is being used to clean stainless steel welds, and that product is only sold to businesses at places where the general public cannot easily buy. I don't think it's illegal to have that stuff, they just don't make it very easy to get.
Needless to say the common welder never uses gloves and fume hoods for this shit, they just complain it fucks up your skin and colours it yellow.  ::)
Yeah dude, I can totally relate. Maybe you should use another method of applying and cleaning it up than using the same old dirty wet rag for every weld you make.


  • Guest
look harder Osmium !
« Reply #5 on: July 13, 2004, 10:39:00 PM »
The EU provides freedom for manufacturers and trade (the capital so to say  ;D ). European law rules that everything what can be sold legal an a country of the EU can be sold legally in all other countries. (there are some harmonizing problems regarding pharmaceuitica though. Mainly between Bavaria and the Dutch  ;) ). But aside of drugs and weapons and such, all OTC stuff for sale in X has to be for sale in Y or european law is broken.
By some twists of historic development almost anything is somewhere OTC in Europe. Perborates and 98% H2SO4 in Italy, 30% H2O2 and perchlorethylene and more in CR, even in Germany something was allowed to sell to the public, ah! Yes! Tablesalt.  ;)
This way a german inhabitant who will get instantly shot if he dares to ask a pharmacist who is not known to him for 30 years or longer for pervesties like NaOH or conc. H2SO4, can buy now italian drain opener, the good US-style one. If he looks around a little bit.
Some people even are told to use ultramodernhighesttek methods to accomplish their evil plans: The e-mail the manufacturer for the name of the local distributor/shops which carry the product.

In short: You get HF in reasonable concentration and in a easy to separate form in any country of the EU. OTC. Not in the "Müller-Markt" though.

As told: Look harder! Use dictionaries.



  • Guest
Fluor in meine kostbaren reinen Körpersäft
« Reply #6 on: July 13, 2004, 11:05:00 PM »
A small amount of HF and fluoboric acid are easily procured.
Think glass.

And calcium fluoride is used in pottery, a lot of art supply houses have it in stock.

Of course if you ask pharmacies they will be nosy, they are always nosey. I have had more than one harassing me over a bottle of ether or even fucking boric acid.


  • Guest
« Reply #7 on: July 13, 2004, 11:21:00 PM »
Of course if you ask pharmacies they will be nosy, they are always nosey. I have had more than one harassing me over a bottle of ether or even fucking boric acid.

You must have hit some of the pharmacists belonging to the .1% group that is not addicted to his/her pharmaceuticals and is interested in his/her job more than in his/her income.


  • Guest
> This way a german inhabitant who will get
« Reply #8 on: July 14, 2004, 11:22:00 AM »
> This way a german inhabitant who will get instantly shot if he dares
> to ask a pharmacist who is not known to him for 30 years or longer for
> pervesties like NaOH or conc. H2SO4, can buy now italian drain opener

A not to be named bee confirmed that NaOH, H2SO4, ether, THF, 30% H2O2, 20% NH3, acetic acid, formic acid etc. are all available OTC in Germany, very reasonably priced (a fraction of the regular Merck/Fluka/Aldrich ripoff) in pretty good purity, and have been available for years. Without the buyer having to show ID, without signing anything, without stupid questions being asked. That person even convinced his local pharmacy to order stuff right out of the Merck catalogue, repeatedly.


  • Guest
« Reply #9 on: July 14, 2004, 07:48:00 PM »
And in other European countries it's the same.

The example that I gave of the chemo-fascist pharmacist is not common, but it happens.
I even read about one pharmacist that reported the sale of ammonium chloride to the authorities!

But of course there are plenty of pharmacists that just want your money, and will not have a problem at all ordering you some things from the big companies. Also, pharmacy supply companies carry some chemicals too, ask for their catalog  :) .

And it is not just pharmacies, there are many other places willing to do that for you, such as druggists  and art supply stores. It all comes down to knowing where to look.


  • Guest
« Reply #10 on: July 14, 2004, 07:58:00 PM »
If you find a pharmacist with a big, expensive Mercedes and with a green flashing aesculapius in front of his "store", you know you found the one...


  • Guest
This may help,
« Reply #11 on: July 17, 2004, 06:07:00 AM »