Trimethyl Borate is the ester of Methanol and Boric Acid and can easily be made by reacting them both in an esterification reaction. It is widely known due to its beautiful green flame which will be demonstrated in this post.
Trimethyl Borate, when pure, is a useful reagent in Organic Chemistry. For instance, it reacts with Sodium Hydride to produce the versatile reducing agent Sodium Borohydride.
It should be noted that the goal of this post is to produce a Trimethyl Borate solution in Methanol which is usually not pure enough to use as a reagent. Pure Trimethyl Borate can be made through other routes and that may be covered in a future post.
- Boric Acid;
- Sulfuric Acid 98%.
The reaction itself isn’t particularly dangerous. The final product is to be burned so regular fire safety should be used. Burning Trimethyl Borate produces fumes that should be avoided. Gloves and safety googles are recommended.
- Methanol isn’t particularly toxic unless one mistakes it for Ethanol and has a sip. Ironically, one of the antidotes for Methanol poisoning is simply drinking Ethanol;
- Boric Acid is mostly benign;
- 98% Sulfuric Acid is highly corrosive.
Start by weighting 15 g of Boric Acid.
Next, measure around 100 mL of Methanol.
Prepare to mount a Reflux Apparatus.
To the refluxing flask add both reagents.
Pipette 2 mL of 98% Sulfuric Acid and add it, slowly, to the mixture.
Finish assembling the Reflux Apparatus and start heating.
Due to the low boiling points of Methanol, Trimethyl Borate and the azeotrope of those two, the mixture reaches a reflux quite fast.
The mixture should be allowed to reflux for 30 minutes. After 30 minutes of reflux the heating is stopped and everything is allowed to cool.
Here is the final product. Notice that only a few milligrams of Boric Acid are left.
The first set of demonstrations are of burning Trimethyl Borate. Its characteristic green flame is quite remarkable.
Take 1: Trimethyl Borate is burned in a dish.
Take 2: Trimethyl Borate is burned on aluminium. Every time the flame starts to go out some more is shot trough a pipette.
Take 3: A crude “sort-of” feedback apparatus is mounted. The tube has too much ester which clogs the tubing.
Take 4: Same as take 3 but with less liquid.
The second set of demonstrations involves burning a small portion of vapor in a flask.
Boric Acid and Methanol react as follows:
3CH3OH + H3BO3 ↔ (CH3)3BO3 + 3H2O
This is an example of an esterification reaction (albeit the acid isn’t organic). Thus, it is an equilibrium. To achieve equilibrium faster Sulfuric Acid is added as a catalyst and the mixture is refluxed (both steps are common procedure in esterification reactions). In small batches, Sulfuric Acid also acts as a dehydrating agent by absorbing some of the water, hence pushing the equilibrium to the right.
Methanol – 32,04 g/mol – 40,45 mL/mol
Boric Acid – 61,83 g/mol
Trimethyl Borate – 103,91 g/mol –
Water – 18,02 g/mol
Fifteen grams of Boric Acid are used in this procedure. As such, the stoichiometric amount of Methanol is 29,2 mL. However a large excess of Methanol is used. There are two main reasons for this:
- Having a large excess of Methanol shifts the equilibrium to the right forcing the production of more ester and consuming all visible Boric Acid.
- The large excess of Methanol serves as the solvent for the ester. Theoretically, even if all the Boric Acid reacted, only 27 mL of Trimethyl Borate would be produced. Having large excess of Methanol is particularly useful because it greatly enlarges the volume of the “green-burning liquid”.
Note 1: I have attempted this reaction by using stoichiometric amounts of both chemicals. The yeld is terrible as most of the Boric Acid remains unreacted. The liquid formed burns in the same manner in which the liquid formed in this procedure burns. The vapor, however, burns with a deeper green color.
Note 2: The amount of time the mixture is allowed to reflux isn’t critical. I have obtained positive results with only 15 minutes. However, 30 minutes ensures everything reacts as it should.
Note 3: If the recommended 100 mL of Methanol are used, the vapor from the resulting solution may, sometimes, burn with a green bluish tone. This is because of the large excess of Methanol that concentrates in the vapor phase. If this effect isn’t desirable, a little bit less of Methanol should be used instead.
I am a huge fan of colored flames so those demonstrations were quite fun to make. As always, feel free to comment.