Sciencemadness Discussion Board

Boroglycerin, an obsolete boric acid ester

Ioxoi - 9-3-2009 at 10:19

I was searching for info on ethylene glycol ester of boric acid and I came across this old page from a medical book from Google books:

It describes something called boroglycerin that apparently was once used as an antiseptic, and is simply made by fusing equimolar amounts of glycerol and boric acid and boiling off water.

I was searching for boric acid esters of nonvolatile alcohols because I read on another forum that boric acid esters of volatile alcohols (ex. trimethyl borate) could likely be made indirectly, by first making a boric acid ester of a nonvolatile alcohol (they stated ethylene glycol) and then mixing this ester with an equimolar amount of methanol, adding a bit of acid catalyst, and boiling away the trimethyl borate leaving behind the nonvolatile glycerol. The advantage of this setup, if it really works, is that little residual methanol comes away in the distillate, as opposed to the typical 70% azeotrope (supposedly; it did not seem like the poster had actually tested it to see if it works). Whether this works with boroglycerin, I have no idea.

Right now I'm making some boroglycerin, as per the limited instructions from the medical book. It probably has little lab utility, but I figured it would be interesting to characterize it, as there is no information on its properties other than it being a "soft white mass." Here's what I'm doing:

1. 46g glycerol and 31g boric acid were weighed out, added to a 250mL beaker and mixed to give a toothpaste-like glop. It might have been because the glycerin had been stored in the cold basement, but the mixture seemed to get frigid cold.

2. Heated mixture on a hotplate with spoon stirring. Eventually the mix became completely clear, and steam came off. Some bubbling.

3. Continued boiling and stirring. Theoretically it should lose 27g of water. After losing 18g it was getting a little thicker and started releasing a faint sweet smell, maybe because of too high a temperature. After 23g lost it was faintly yellowish (oxidation?).

4. I managed to boil off 24g of (presumably) water, and let it cool. As it cooled it gradually became thicker and thicker.

I'm waiting for it to cool down right now, will post what happens in a little while (suspense suspense).

garage chemist - 9-3-2009 at 10:39

Interesting. Tell us how it goes. We need more of that kind of experimentation here.
But won't it form some kind of polyester with the trifunctional boric acid and glycerin? Perhaps try more than the stochiometric amount of glycerin.

If your target is trimethyl borate with substantially lower than azeotropic methanol content, have you tried turning your boric acid into boric anhydride?
Melting down boric acid (in a metal can, as it attacks glass) releases large amounts of steam and turns it into a glass consisting of HBO2 and B2O3 (completely dehydrating it to B2O3 takes more than that, like very slow heating in a stream of dried air to avoid melting). Heat until the glass stops foaming (red incandescence, easily reached with a tin can and strong bunsen burner), let it cool just a bit and then quench the can with water from the outside.
This makes the glass shatter into many pieces, making removal easy. Otherwise it is very difficult to get out of the can- it's very hard and tough.

This glass rapidly dissolves in boiling methanol (no catalyst neede) giving much more trimethyl borate than plain boric acid.
If substantially less than the theoretical amount of methanol is used, theoretically, almost all of the methanol should be converted into the ester.

Ioxoi - 9-3-2009 at 11:28

@ GarageChemist: I figured that if I will try to make trimethyl borate I'd probably use B2O3, thanks for the tip to shatter the B2O3 glass with cold on the outside of the can to make it easier to get out. Right now I'm just trying to make boroglycerin and see what it does.

The mixture has cooled down and, far from being a "soft mass," it has become a brittle clear glass (hmm, an organic glass). Wrenching the spoon out of the beaker broke out some shards of the material. Taking a piece between the fingers and snapping it, it reminds me of polystyrene (brittle) and it's a tiny bit sticky. Whether it's a polymer or actually composed of a single esterified boric acid and glycerol molecule is far beyond my "what does it look like, what does it feel like" analysis :).

It would be interesting to see what kind of solvents it can dissolve in. Maybe if it's a polymer it will turn a solvent like acetone into a honey-like form? I have no idea what I will do with it now that I have it. Incidentally, I'm not even sure I know how to get the stuff out of my flask, other than melting it again. Argh!

Again, this stuff probably has no real use, but I thought I'd post the results in the Misc section for you guys anyways. Maybe you guys can figure out something cool to do with it. I wonder what it looks like when it's powdered and mixed with KClO3 and burned? Green flare? Dessicant?

Distilling with methanol: Not really worth it

Ioxoi - 9-3-2009 at 13:28

I took ~6g of the boroglycerin chunks and placed them in a test tube along with a little over 2mL methanol, a drop of H2SO4, stoppered it and ran a bit of glass tubing out the end. After that I heated the mix and swirled until all the solid had dissolved. Then I clamped it to a stand and used a flame to distill the mix.

I got about 1.5mL of a clear distillate. About 2/3 of the way through the heating, the solution in the test tube first became purple-brownish and then became a thick black syrup, probably decomposing glycerin. The distillate, when smelled, produced a burning sensation in my nose (probably acrolein or something from the overheated glycerin). The liquid burned with a nice green flame, so it's probably mostly trimethyl borate.

In short, distilling alcohol borates with boroglycerin is basically useless in comparison to the usual way.

Also, because the solid boroglycerin is slightly tacky, it's probably impossible to grind to powder, so if one were to make a pyro mix with it, it'd probably have to be melted in, or incorporated in with solvent, which is a hassle.

Now I have to figure out what to do with the other 40g of boroglycerin I have left over. Any ideas?

kclo4 - 9-3-2009 at 14:46

I looked, doesn't look like it has many uses besides being an antiseptic heh I guess just keep it until you can find a use for it. :P

chemoleo - 9-3-2009 at 14:58

Very nice, it's an idea I had thought of trying out as well...
Garage chemist, I suppose the idea is that it could form a sort of covalent cage around the boron atom. If this is sterically possible, without weird bond angles and bonddistances, then glyceroborate, as a monomeric ester, should be much in favour of forming than a polymer (due to the chelate effect or entropy if you like). Otherwise you'll get the polymer.
Loxoi, does the white sticky mass burn with a green flame? Just put some on a spatula and try... I doubt a simple mixture of anhydrous glycerol and boric acid/oxide would readily burn...

not_important - 9-3-2009 at 21:53

A better way to make B2O3 is to dehydrate B(OH)3 under a vacuum, slowly raising the temperature from room to about 180 C or so. This gives a very active slightly fritted form of B2O3, easy to powder.

Your mixture was glassy and not "a soft mass" because you left out the second step in the Principles of Pharmacy instructions, mixing (in you case) 50 grams of glycerol with the hot dehydrated mass.

I'd not use H2SO4 as a catalyst, it will cause the glycerol to decompose, as you saw. Instead use a slight excess, just a couple of mole-percent, of B(OH)3 so that there will be a percent or two of (RO)2BOH groups, which are fairly acidic when formed with glycerol and some other polyols.

To get above the B(OMe)3-MeOH azeotrope you'll likely have to use a very large excess of the boroglycerin, enough so that one mole goes off as B(OMe)3 while the rest are left as diesters of boric acid and glycerol - just one -OH on the boron and glycerol. That form is somewhat energetically favored, possibly enough so that it would drive the reaction enough to consume all the methanol. An equal molar amount would still exchange alcohols, resulting in the azeotrope being distilled off.

nitric - 15-4-2009 at 08:24

i note that ive read in literature that H2SO4 and B(OH)3 make B2O3 in glass form if heated strongly, then that is allowed t cool and i belive that B2O3 crystalizes out(corection)and can be decanted.

pacifyer - 13-1-2012 at 10:14

Hi all. I am also interested in making methyl borate. I did once several hundred grams, using the dehydration of H3BO3 to B2O3, then treatment with MeOH, to give methyl borate. The B2O3 is hard, and I had to break one flask to recover it... Perhaps the SAME flask could be used in the whole preparation, and hopefully the sintered anhydride would react with MeOH, form a softer H3BO3, and this way one flask less would go to the heaven of glassware... :o
Other two problems here: first, the yields suck, and a MeOH-Borate azeotrope is formed. The yield is low because 2 H3BO3 react to form 1 B2O3, with the reacts with 3 MeOH, to give H3BO3 (yes, half of your precious B2O3 is wasted just sucking water), and 1 Me3BO3. Then, the other waste is in H2SO4, which is needed to remove the MeOH from the azeotrope. A lot of heat develops, and some more borate is lost...
I think several things could be improved in this preparation...I started with like half a kilo of H3BO3 and got less than 400 mL of Me3BO3... :(