TheBear - 26-5-2005 at 12:16
Experiments with boric acid esters.
After reading about theses at this board we gave it a try at school. We distilled over the methanol/methyl borate azeotrope consisting of 70% B(OCH3)3
+30% methanol (bp 52-54°).
Observing that the ester rapidly hydrolyzes with water I draw a possible reaction mechanism:
[img]http://www.sciencemadness.org/scipics/B(OCH3)3_hydrolyzis.gif[/img]
I suggested this reaction mechanism to my chemistry teacher who saw no apparent flaws in it. Well most would be satisfied with that, but if I ask one
question 5 automaticly follows:
What if I would subsitute water with another molecule with a free pair of electrons which can also donate hydrogen... Ammonia?
Again I guess a possible (?) reaction mechanism:
[img]http://www.sciencemadness.org/scipics/B(OCH3)3_ammonia.gif[/img]
I have trouble naming the final compound B(NH2)3, perhaps: triamine borine
After searching for the compound there was almost no information to be found except for this:
http://www.ca.sandia.gov/HiTempThermo/db/data/P71T.dat
It seems that the molecule exists atleast. That was enough for us to try to synthesize this according to the reaction mechanisms proposed with ammonia
gas. So we bubbled dry ammonia gas through a testtube with our trimethyl borate/methanol azeotrope until the washbottle connected after the bubbler
gave alkaline reaction (BTB was used as an indicator in it).
What we observed: The trimethyl borate/methanol azeotrope dissolved or reacted with a fair ammount of NH3 (g) before it went through (and colored our
BTB solution blue). A very small amount of white percipitate was formed. My theory is that it's BO3(NH4)3 formed by the small amount of boric
acid that was most likely present in the solution (the boric acid may have formed when the mixture of trimethyl borate and methanol was exposed to the
atmosphere or any watervapour that might have escaped our CaO filled washbottle.
So now here are my questions:
1. Am I just babbling or is any of the theories that I've worked out correct?
2. Do anyone have any physical properties of B(NH2)3 at hand (no one of my highschool teachers could find any information about the compound)?
3. Could we have made B(NH2)3? How would one make an analysis of it?
I was thinking that dry B(NH2)3 would react with water releasing ammonia, but how would one free our now ammonia saturated solution from the gas and
verifying this, I was thinking about bubbling HCl (g) but that might just leave me with an HCl(g) saturated solution.
4. Could the white precipitate be ammoniumborate ( BO3(NH4)3 ) ?
Nerro - 26-5-2005 at 13:42
Wouldnt ammonia and trimethylborate form a stable complex? B(CH3O)3.NH3
Pyridinium - 26-5-2005 at 19:49
Looks like methanol makes a fine leaving group in either case... at first glance I can't find anything wrong w/ your structures either, but now
you've got me thinking.
That last one in your diagram looks like dimethoxyaminoboron or aminedimethoxyboron (?) ... not too sure / am a bit rusty with naming conventions.
Any naming experts here?
The B(NH2)3 compound I'd guess to be called triaminoboron, boron triamine, or maybe triaminoborane (?)
There are a lot of oddball compounds you can prepare with common reagents, yet they don't appear in any normal textbooks.
There should be a book on just that!
By the way, look up:
Aubrey, D., and Lappert, M.
J. Chem. Soc., 2927 (1959). "Cyclic Organic Boron Compounds, Part IV: B-Amino- and B-alkoxy-borazoles and their precursors the
tris(primary amino)borons and (primary amino)boron alkoxides."
(emphasis mine)
There we go... I figured your compound has to be floating around in the literature somewhere...
[Edited on 27-5-2005 by Pyridinium]
Nicodem - 27-5-2005 at 00:58
Sorry I did not take any time to search an answer for your question, but I just wanted to add that B(NH2)3 is very unlikely to be a stabile compound
as it can easily eliminate NH3.
Given the electronic character of boron compounds (electron pair acceptor - Lewis acid) and the oposite character of the amine group this is highly
likely. In fact even boric acid eliminates H2O easily (forming metaboric acid HBO2 at >170°C) even though the OH group is less prone to donate
electron pair in comparison to the NH2 group.
So I think B(NH2)3 would "deaminate" at room temperature forming first H2N-B=NH, but since I never heard about such a compound, further
decomposition might be possible too.
Or more probably the reaction itself would form some not fully methanolysed imidate-like intermediates, such as MeO-B=NH.
In short, I don't know, just wanted to say that B(NH2)3 is an unlikely product.
Pyridinium - 29-5-2005 at 15:10
I've done some reading... B(NH2)3 was prepared as early as 1902, using KNH2 and BF3 in liquid ammonia.
B(NH2)3 has another way to form using BCl3 and an N,N-dialkyl amine, but not sure what the steps are. The references were to sources I couldn't
locate.
They call the compound tris-aminoboron.
Also, Nerro was right. (MeO)3B will form a complex with NH3, having the formula (MeO)3B:NH3. The source I read says that trimethyl borate will, but
higher alkyl borates won't complex with ammonia because of back coordination between the alkoxy oxygens and the boron.
A good starting source is _The Organic Chemistry of Boron_ by W. Gerrard, 1961. If you could get the references it talks about, you'd find the
properties of tris-aminoboron.
As Nicodem said... not sure how stable the triaminoboron compound would be.
[Edited on 29-5-2005 by Pyridinium]
TheBear - 30-5-2005 at 08:53
Thank you all for all the replies! I have been searching in vain for the first reference you mentioned Pyridinium. Since I'm not an university
student yet I can't access journals. I will look for the book you mentioned.
The complexing explains why the solution was able to dissolve as much NH3 as it did. The white precipitate was most probably CaO from the drying tube
(the CaO was in the form of dusting powder).
ADP - 19-9-2005 at 17:27
I'm sorry for the dead topic resurrection here, but my ques. doesn't warrant a new thread. What are the proper equations for the borate
ester syntheses of methyl and ethyl borate?
So far, I have:
Trimethyl borate: (Boron trimethoxide)
H3BO3 + CH3OH (H2SO4) --> B(OCH3)3 + ?
MP: -29°C
BP: 69°C
Triethyl borate: (Boron triethoxide)
H3BO3 + CH3CH2OH (H2SO4) --> B(C2H5O)3 + ?
MP: -85 °C
BP: 118 °C.
I'm not quite sure what else is formed here in this reaction, would one help me finish it please? Also is there such an ester as trisopropryl
borate?
The_Davster - 19-9-2005 at 18:17
Water is produced in both situations. Isopropyl alcohol should work.
B(OH)3 +3ROH -(H2SO4)-> B(OR)3 + 3H2O
It would appear you are planning to synth some of this ester, so I will say that yields are improved if one refluxes the boric acid in alcohol for a
few minutes past the point at which the boric acid has all reacted. Then replaces the reflux collumn with a distillation setup. This is opposed to
the simple distilling of boric acid and alcohol together(as this is what I have seen here) as alcohol distills over first, reducing yields if an
excess is not used.
On a semi-related topic, my vial of the ethyl ester shows more signs of decomosition everytime I remove some, what I believe to be boric acid
precipitates, the ester must be hydrolyzing simply from the water in the air.
[Edited on 20-9-2005 by rogue chemist]
[Edited on 20-9-2005 by rogue chemist]
Boron chemistry
armo - 21-9-2005 at 05:33
Hi all,
Herbert C. Brown won a Nobel Prize on boron chemistry. He wrote some papers on how to make boron alkoxides. I´m not sure about tris aminoborane. All
papers were published during 1940´s decade, mostly in JOC.
I have some papers dealing with this subject, if it is necessary I can dig it from my refs cabinet.
Armo
The_Davster - 14-1-2006 at 13:49
Does anyone have any info on the percentages of B(OC2H5)3 and ethanol in the azeotrope? All I know is that it distilled at ~81C on my thermometer.
I searched google and all(there are quite a few) the threads on boric esters with no avail.
TheBear - 14-1-2006 at 14:01
I don't remember the exact figure, I recall it as being ~10% triethyl borate and 90 % ethyl alcohol, a lot less than the B(OCH3)3 * CH3OH azeotrope
being 70 % / 30 %.
S.C. Wack - 14-1-2006 at 16:40
bp 76.6 70% EtOH. (K-O)
Magpie - 15-1-2006 at 15:28
This inquiry is addressed to TheBear:
I am amazed that you are doing this level of inquiry in high school, and even more amazed that your teacher performed such an experiment in
class. Does this kind of thing occur very often? Are you in an accelerated chemistry class?
bio2 - 17-1-2006 at 00:42
.........I am amazed that you are doing this level of inquiry in high school..........
Unfortunately the dumbing down of America has taken it's toll and kids don't even take 12 years of math and english anymore.
In the late 60's we were making all sorts of stuff in the public school chem lab that is only done at university level now days.
A good teacher helps, mine was the inventor of the polaris solid fuel propellant. The retired gentleman taught high school chemistry for fun and he
was letting the better students order chemicals for the next years projects in 10th grade. Intro chem was 9th grade and was more advanced than some Jr
college stuff I've seen.
TheBear - 18-1-2006 at 12:57
To Magpie, and very off-topic:
Well, I'm not in (wasn't in) any accelerated chemistry class or anything. And the experiment wasn't carried out in class I'm afraid 
.
I read about the reaction on this board and asked my chemistry teacher if I and some of my friends could give the synthesis a go after ordinary school
time. (We later tried a boron synthesis which could have gone really bad and a couple of other synths).
Thanks to this board, and everyone of you who are making this board what it is by posting inspiring posts, I've become... ummm.. I've developed an
extraordinary interest in chemistry so to say. Me and my friends (mostly me I guess) had some heroic plans on starting a "science club" at the school
(perhaps I should've been born overseas in the late 40's?). Although making a couple of demos for some 11th year students, we were unable to get even
one of them interested enough to give it a try. I was rather striken down by that experience. Actually I have yet to meet anyone passionate about
chemistry. Hopefully there'll be some equal minded when I start studying at the collage/univeristy (in Sweden it's the same place).
And that is what makes this site so special. This is really the perfect place to get inspiration.
[Edited on 18-1-2006 by TheBear]
CyrusGrey - 15-3-2008 at 19:07
I will post this here instead of posting it in a new thread. I hope you dont mind me necromancing this thread.
I placed 73g of methanol (about 75mL) into a flask with 55g of boric acid. I refluxed this for about 10 minutes while I prepared the rest of the
apperatus. I then rearranged my apperatus to distill the mixture. I distilled about 50-55 mL from this, stopping when the mixture began to look crusty
and crumbly.
The distillate is perfectly clear and has a sweet smell, not like alcohol at all. I lit 4 mL of this on fire. It burned for about 30 seconds, leaving
a white residue.
[Edited on 16-3-2008 by CyrusGrey]
