neutrino - 30-9-2004 at 16:11
I am planning on making an ester of boric acid with an alcohol and am concerned about the yield. There is an equilibrium between esterification and
hydrolysis which is usually shifted to the reactants' side by removing water. Vogel says to remove the water by fractional distillation of the
water formed with the alcohol used as a reactant, but this would require massive amounts of very dry alcohol, something I don't have access to.
So, I was thinking: what about adding an inert desiccant to the reaction mix? I was thinking that MgSO<sub>4</sub> might work, as the
equilibrium
3XOH<sub>(l)</sub> + H<sub>3</sub>BO<sub>3(aq)</sub> <--->
3H<sub>2</sub>O<sub>(s)</sub> + X<sub>3</sub>BO<sub>3(l)</sub>*
should be shifted strongly to the right, the water being solid because of its entrapment as a hydrate. I doubt that the low solubility of
MgSO<sub>4</sub> 7H<sub>2</sub>O will affect this much, but I’m not sure. I’m new to organic chemistry, so any feedback is
much appreciated.
*XOH is an alcohol, aq = dissolved in alcohol
[Edited on 18-1-2005 by chemoleo]
Marvin - 30-9-2004 at 22:19
Borate esters are more toxic than you'd expect from borates.
Needed to get that out of the way first.
Eqiuilibrium depends on the alcohol, methyl alcohol will aparently form the borate almost quantitifly, though I havnt checked this.
Sulphuric acid will force the reaction furthur towards stubborn esters, you could also think about dehydrating the boric acid first to the oxide,
which avoids the production of water and is a powerful dehydrating agent itself. A general method that wouldnt work very well in this case:- if the
alcohol is very large you can make use of ester interchange. Form the methyl ester and dry, and then mix this with the desired alcohol, a little
catalyctic acid and reflux off the unwanted methyl alcohol. Here there is only 4 or 5 degreees difference between the methyl ester and the alcohol
though.
neutrino - 1-10-2004 at 02:26
I thought about the oxide, but realized that it will still make some pesky water. Are you sure about the toxicity? I looked up triethyl borate and got
a 2 (moderate) for toxicity.
Well, I finally tried this last night with EtOH. I used the following proportions: 48mL EtOh (commercial, probably azeotropic), 50g boric acid, 4g
magnesium sulfate, and 1mL of battery for good measure. I used a slightly less EtOH than was needed, as I figure it should be easier to purify the
product with no alcohol left over instead of leaving a small amount unreacted EtOH and needing a fractionation column (also unavailable) Instead of
being left with a small amount of precipitate (the MgSO<sub>4</sub>
, I
have something very plentifully. Either the boric acid reacted with the sulfate to form sulfuric acid and magnesium borate (not likely), the reaction
is really slow (possible), or boric acid is insoluble in triethyl borate (also possible). I will just have to wait a while and see what happens.
Edit: spelling
[Edited on 1-10-2004 by neutrino]
incitatus - 18-1-2005 at 08:50
if you could start from boron trichloride it would be a lot easier
sparkgap - 18-1-2005 at 09:13
Yeah, it might be easier.
Too damn bad BCl3 is a corrosive gas that I believe won't be easy to acquire.
I was going to go at length about reactivity with water, but I guess you guys knew that already.
Just my two cents worth...
chemoleo - 18-1-2005 at 09:44
If you searched, there are already threads on boric acid esters... with practical experiences.
I don't quite understand what the problem is, you shift the equilibrium towards the right side by distilling the boric acid ester off. This is
the idea behind the destillation here, which is to remove the products in an equilibrium. Thus, don't expect all the boric acid to react, even if
heated, without destillation of the product.
For destillation, even the simplest setup can be used - seriously. If you are careful about it (temps), you will get it mostly free of the unreacted
alcohols. To get rid of the water in the distillate later, just dry the ester over fused CaCl2 or whatever.
Azeotropic distillation to remove water
phanchem - 24-1-2005 at 21:30
Azeotropic distillation to remove water produced in the reaction of boric acid and the target alcohol works very well, as I remember. Just choose
something which azeotropes with water--xylene is a good one---and the reaction goes to completion.
There used to be tables of low-boiling azeotropes in the Chemical Rubber Handbook, but it's been about 15 years since I've looked them up.
Good luck, phanchem
water/hydrocarbon azeotropes
bio2 - 24-1-2005 at 22:59
This should be helpful!
Binary azeotropes containing water
System
BP of azeotrope,
Deg C
Composition,wt %
Water
Other
component
Pentane 34.6 1.4 98.6
Hexane 61.6 5.6 94.4
Heptane 79.2 12.9 87.1
Benzene 69.25 8.83 91.17
Toluene 84.1 13.5 86.5
Ethylbenzene 92.0 33.0 67.0
m-Xylene 92 35.8 64.2