Jor
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trimethyl borate
I plan on making some really soon, I have already mixed 10g of boric acid with 40mL methyl alcohol (large excess, I know, I think this will speed
things up. Needed or not?), and added about 2mL of concentrated sulfuric acid (catalyst). I will start refluxing tomorrow, for about 1 hour. I will
collect the azeotrope of trimethyl borate and methanol, probably together with some of the excess methanol.
But..., how to I break the azeotrope, to isolate the pure trimethyl borate?
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kilowatt
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Maybe you could distill it with a non-polar solvent to break the azeotrope. There's also vacuum and pressure-swing distillation, or possibly zeolites
that will absorb methanol, followed by a post-azeotropic distillation.
[Edited on 5-7-2008 by kilowatt]
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Jor
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I only have a simple distillation-setup and no vaccuum, so i think your ideas can't be performed by me. Maybe the non-polar solvent destillation, but
I really don't know what solvent I should be using then.
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kilowatt
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You could try benzene or toluene. I would think TMB would have a higher affinity for those than methanol.
The mind cannot decide the truth; it can only find the truth.
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not_important
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You'll need fractional distillation to use any of the co-solvent methods.
One way around the problem is to add an excess of high activity boric oxide to methanol, so that all the methanol is converted to ester and all water
is taken up as boric acid, then distilling most but not all the liquid under reduced pressure to avoid dehydrating the boric acid.
Or you can add an excess of boric acid and H2SO4 to give the same result - all the MeOH is ester, all the water is grabbed by the H2SO4.
Or you can add a salt to the MeOH-B(OMe)3 azeotrope to salt out the ester/tie up all the alcohol - see attached.
Attachment: US3004058A1.pdf (445kB)
This file has been downloaded 1620 times
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Jor
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I don't have any benzene or toluene. However, I am getting 250ml of benzene soon, but as not_important stated, I will need to perform fractional
destillation, wich is not possible at the moment.
I am very interested in the last 2 methods not_important stated. I think I will use NaCl.
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S.C. Wack
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Properties of the Azeotrope.
b.p. 54° at 750 mm., 75.5% methyl borate by weight.
A. Preparation of Azeotrope
(1) From Methanol and Boric Oxide or Boric Acid.
Four moles of methanol was placed in a 5 L 3-neck round-bottom flask fitted with a mercury sealed stirrer and a reflux condenser, the use of which was
adequate because no attempt was made to separate the small excess of methanol from the azeotrope. Through the third neck, one mole of anhydrous boric
oxide (99.5% purity) was introduced in small quantities at such a rate that the mixture refluxed gently. The reaction mixture was heated for an hour
following completion of the addition. The reflux condenser was converted to a downward condenser and the material distilling up to 70° was collected
in a flask protected from atmospheric moisture. The yield was essentially quantitative according to the equation B2O3 + 4CH3OH →
[B(OCH3)3.CH3OH] + H3BO3. [3:1 mole ratio, 71.6% yield, 3.5:1, 89.9%, 4:1, 99.4%, 5:1, 100%.]
When the methanol-boric oxide ratio was less than 4:1, the residual material was gummy and difficult to handle. Presumably, methyl metaborate,
(CH3OBO)x, was present. When 12 moles of methanol instead of 4 were used per mole of boric oxide, 96.7% of the latter was converted to the azeotrope.
Similarly, from 124 g. (2 moles) of boric acid and 512 g. (16 moles) of methanol, a 92.6% yield of azeotrope was obtained.
From Borax and Methanol.
A 1 L round-bottom flask was attached to a column, 25 mm. i.d. by 75 cm. in length packed with 3/16" stainless steel helices and rated at 15
theoretical plates. The flask was charged with 0.5 mole of borax (Na2B4O7.10H2O), 16 moles of methanol and 1 mole of sulfuric acid. The reaction
mixture was slowly distilled. The major portion of the product was obtained as the azeotrope at 54°; a smaller fraction containing methyl borate was
obtained at 54-62°. The yield was calculated on the total borate contained in both fractions. Sulfuric acid-borax ratio: 2H2SO4:Na2B4O7.10H2O. [Yield
from 12 moles MeOH, 10 hour distillation: 87.8%. 16 moles MeOH, 9 hours: 92.9%.]
When the quantity of sulfuric acid was reduced to the ratio of 1 H2SO4:1Na2B4O7, the yields were identical, but the reaction was somewhat slower. In a
typical experiment, instead of 9 hours, a reaction time of 16 hours was required to obtain an ester yield of 92.4%. It should be pointed out, however,
that the reaction time is primarily dependent upon the efficiency and capacity of the column.
B. Separation of Azeotrope
(1) Sulfuric Acid Separation
[3 L of the azeotrope was extracted with conc. H2SO4, once with 60 ml., and then 9 X 10 ml.; 98% of the methanol and 14.5% of the ester was
extracted.]
Distillation of the extracted borate ester yielded a small first fraction at 54° which contained the remaining methanol and a second fraction of
practically pure methyl borate. (b.p. 67.5-68° at 750 mm.).
A mixture of 570 ml. (500 g.) of the azeotrope and 570 ml. of ligroin (b.p. 110°, previously washed with sulfuric acid) was treated with two portions
of 20 ml. of sulfuric acid. Only 4.8% of the ester was lost and 98.1% of the methanol was removed. Distillation of the solvent led to 91.2% yield of
ester boiling from 67.5-68.5°.
Lithium Chloride Separation.
The addition of 120 g. of anhydrous LiCl to 1000 g. of the azeotrope caused the mixture to separate into two layers. The lower one consisted of a
solution of LiCl in methanol plus some suspended salt and contained from 3.6 to 4.0% of the methyl borate present in the azeotrope. The upper layer
contained about 96% of the methyl borate of from 99.5 to 99.7% purity; the slight contamination was methanol.
The methanol was recovered from the lower layer by distillation. Below 90°, the small quantity of the methyl borate present distilled as the
azeotrope. At a pot temperature of from 95 to 115°, 96% of the methanol was recovered practically pure. At this stage the LiCl was obtained partly as
a powder and partly as readily powdered lumps.
By raising the temperature to 160°, the remaining 4% of the methanol could be recovered. When that was done, the LiCl was obtained as a hard cake
which, though usable, was inconvenient to handle. It is, therefore, recommended that heating the residue above 115° be avoided if the lithium
chloride is to be re-used in subsequent separations.
[The use of 30 g. NaCl per 135 g. azeotrope gave product of 98% purity; 15 g. MgCl2 98.2%; 46 g. ZnCl2 99.6%.]
JACS 75, 213 (1953)
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ScienceSquirrel
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Why do you want to make pure trimethyl borate?
Mixing methanol and boric acid produces a solution that is quite adequate for most purposes.
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not_important
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Not true if you are reacting it with many organometallics, where the acidic hydrogen of the alcohol would destroy it. For example http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=cv5...
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garage chemist
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Turning the boric acid into boric oxide firstly by fusion in a metal crucible will greatly increase the yield.
It's amazing to see just how much water vapor is emitted when melting boric acid into boric oxide! And this water will then later not be present in
the equilibrium with methanol.
Heating is done until foaming of the viscous melt stops (requires strong heating with a bunsen burner).
If simply left to cool down, the very hard solidified melt will be difficult to remove from the crucible- it's much better to cool the crucible from
the outside with water while still very hot, the melt will then shatter into many pieces while solidifying.
The boric oxide does not require any H2SO4 to react with the methanol.
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not_important
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Note that Inorganic Syntheses V II contains a procedure for producing porous, highly active boric oxide without fusion, but rather by placing boric
acid in a container, pulling a vacuum on it, and slowly raising the temperature to 200 C. This porous oxide, only slightly sintered at the most,
reacts energetically with water and very quickly with lower alcohols.
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Magpie
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I have made methyl borate according to the procedure shown in the following reference provided by solo in another thread:
"Procedures for Preparation of Methyl Borate," by
H.I.Schlesinger, Herbert C. Brown, Darwin L. Mayfield and James r. Gilbreath
JACS 1975, 213
An excerpt of this article has been provided above by S. C. Wack.
To get full utilization of the boron I used a 12:1 methanol:B2O3 mole ratio. The products were then distilled using simple distillation up to a
distillate temperature of 70C. I then attempted to break the azeotrope by saturating with NaCl according to this article. This was not successful as
2 phases did not appear. I then fractionally distilled this salt saturated product. This was apparently successful as my distillate came over at a
steady 54.6C which is the literature value for the bp of the azeotrope.
An earlier JACs article (1933) by Webster & Dennis, "Preparation and Purification of Methyl Borate and Ethyl Borate," says that the
azeotrope can be broken and a phase separation attained by adding con sulfuric acid.
My question to the forum is: Have any of you tried either one or both of these techniques for breaking the azeotrope and thereby isolating the methyl
borate. If so what were your results, and what are your recommendations?
[Edited on 24-5-2011 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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smuv
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What if you just allowed the product to stand over CaCl2? This should soak up the methanol, but not the ester.
"Titanium tetrachloride…You sly temptress." --Walter Bishop
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Magpie
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Quote: Originally posted by smuv  | | What if you just allowed the product to stand over CaCl2? This should soak up the methanol, but not the ester. |
CaCl2 is one of the salts tested by Schlesinger et al in the 1952 article I referenced. It did not perform as well as NaCl. Furthermore they
reported "Calcium chloride has previously been used for this purpose, but we did not find it satisfactory. The salt swells greatly in the process and
from the resulting large bulk of solid material the methyl borate is difficult to recover."
The single most important condition for a successful synthesis is good mixing - Nicodem
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not_important
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I know someone that used 4A molecular sieves to soak up the MeOH, the washed the used sieves with more MeOH to recover mechanically retained ester,
then used the wash for making the next batch of the ester.
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Magpie
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I thought of that just after going to bed last night, then forgot about it - thanks for the reminder! I do just happen to have some 4A sieves.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magpie
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I just ran an experiment on my methyl borate/methanol azeotrope in reference to isolating the ester. I set up 3 small test tubes, each containing 2
mL of the azeotrope (75.5% ester by wt) as follows:
1) added 0.5mL ice-cold con sulfuric acid & mixed
2) added non-iodized NaCl to saturation & mixed
3) added 1.5 mL of 4A molecular sieves
The results:
1) sulfuric acid: formed a 2-phase liquid
2) NaCl: did not separate into 2 phases
3) mole sieves: steadily releasing tiny air bubbles and sucking up liquid
Presumably I could get an isolated ester with methods 1 and 3. I realize that I would have to repeat the sulfuric acid wash many times for a clean
product. The sieves would be by far the easiest method.
When I have the cleaned ester I will fractionally distill it, taking the product coming off at 68.7C.
[Edited on 25-5-2011 by Magpie]
[Edited on 25-5-2011 by Magpie]
[Edited on 26-5-2011 by Magpie]
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magpie
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From internet research I found that 4A mole sieves should absorb 15% of their weight in methanol. So, I loaded up my methyl borate/methanol azeotrope
with 4A sieves and let it set overnight. Today I drained off the liquid and fractionally distilled it. It all came over at 54.6C, ie, it was all
azeotrope! For some reason, the 24.5% that was methanol was not selectively absorbed.
I will now try the 1933 sulfuric acid method of Webster & Dennis.
The single most important condition for a successful synthesis is good mixing - Nicodem
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Magpie
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Because of a significant loss of material when trying the 4A sieves I only had about 20mL of azeotrope left. This was washed with 05.mL, and 4X
0.2mL, successively, con sulfuric acid.
The washed liquid was then fractionally distilled. At first about 3 mL of azeotrope came off. This was promptly followed by about 7mL of methyl
borate at 68.2C. Literature value is 68.7C.
There is no point in trying to calculate a yield due to the large mechanical losses of various experiments. A visibly obvious source of loss is the
addition, generation, and unintentional ingress of H2O, even air humidity H2O, ie:
B(OCH3)3 + 3H2O ----> H3BO3 + 3CH3OH
The H3BO3 is visible as a white solid.
The single most important condition for a successful synthesis is good mixing - Nicodem
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