Boric Acid Esters: Green Flame from Borate Ions in Isopropyl Alcohol Solution

[Cesium]

I dissolved some boric acid into some isopropyl alcohol and when I burned the solution the flame was green. I am guessing that I am not getting triisopropyl borate and that the flame just burns green due to the presence of borate ions in solution. Adding sulfuric acid would create the ester.

Is this all correct?

 

[renerob]

The green color is due to the presence of boron and tells you nothing about the presence of borate ions or any other ions. It is unlikely that you formed any isopropyl borate without any cooking of the solution.

 

[Cesium]

I previosuly agreed with you but what you are saying directly refutes this:

http://81.207.88.128/science/chem/exps/borate_ester/index.html

 

[renerob]

I was unable to read the reference you cited, so I can't comment.

 

[Cesium]

"The methyl ester of boric acid

Take 1 ml of methanol. Add a spatula of solid boric acid to this. Most of the acid quickly dissolves in the methanol. Pour this solution on a clean glass or ceramic surface and quickly light again. This results in a beautiful green flame, which is much lighter than the pale blue flame of pure methanol."

"The nice thing about these borate esters is that they burn with a true green flame. Just like normal fire, but green instead of orange. This looks very cool and one has to see it oneself in order to fully appreciate."

 

[renerob]

Thank you for providing me the reference.
A burning test of this kind is not in any way capable to tell the difference between a mixture and a reaction product, The main reason is that it is a destructive test and thus does not allow you to say anything about the formation of new bonds. The way to follow the reaction is by studying the absorption infrared spectrum which will show the disappearance of the OH bonds and the formation of the B-O-C bonds. No doubt this has been done and probably published.
A quick check of the literature shows the following for the methyl analogue.
One gram of boric acid dissolves in 6 ml of boiling alcohol, which means that it does not react.
The tri ester can be made by the reaction of boric acid and methanol under certain conditions described in Schlesinger. J. Am. Chem. Soc. 75,213(1953).
Several other processes are described in six U. S. patents. I can give you the numbers,if you are interested, but what it indicates is that the simple mixing of boric acid and methanol does not give the esters, because patents would not be granted for such a simple mixing procedure.

 

[Cesium]

but what it indicates is that the simple mixing of boric acid and methanol does not give the esters

My piece of literature must have been flawed. H2SO4 is needed.

The main reason is that it is a destructive test and thus does not allow you to say anything about the formation of new bonds.

Ah thank you renerob. This makes sense.

 

[chem_tr]

I have done this reaction with borax, Na₂B₄O₇.11H₂O with sulfuric acid and methanol. I got a nice green color, since sulfuric acid destroys the mineral to give free boric acid, B(OH)₃, and also shifts the reaction to the ester side.

Hope this observation helps too.

 

[woodyanderson]

In your discussion on boron esters you stated "Several other processes are described in six U. S. patents. I can give you the numbers,if you are interested, but what it indicates is that the simple mixing of boric acid and methanol does not give the esters, because patents would not be granted for such a simple mixing procedure." What are those patents? I'm trying to remove oxygen from boron powers and think this might work