Answer to Question #63284 in General Chemistry for Ian baldwin

Question

I attempted to synthesis triisopropyl borate using boron trioxide and 50% percent isopropyl alcohol in water.(1 mole of boron to 3 moles alcohol.) I combined the chemicals into a small pint jar and then realized I had no hygroscopic catalyst to complete the reaction.(a little late to realize this I know). Not wanting to dump the experiment I put it in a cupboard with no lid on it until I could find a catalyst. That was last week, I pulled it out yesterday to check on it and it has turned into a white crystal with a brown layer of what looks like elemental boron. A flame test on it came back negative for boron and it appears to be insoluble in water. Could you please tell me what I created? Thank you.

Answer 1

The question is very complicated, and we cannot provide only a possible explanation. Also we recommend you to perform the further analysis of your crystals (maybe NMR or some other methods) to find out what it really is.

In aqueous solution you have an equilibrium between the products of the reaction, in our case triisopropylborate, and the starting reagents. To obtain pure triisopropylborate you have to take away the water as you said. If alcohols are left on direct sunlight in the presence of air, they can form some amounts of peroxides which can serve as catalysts for radical polymerization. Also there is a possibility of very slow further oxidation to acetone, but such reactions normally need strong oxidizing agents, and I do not think that this oxidation happened when the mixture was simply left on air. But if we assume that the oxidation to acetone happened, then I have a solution to suggest. Acetone can be homopolymerized by the polyaldolic condensation reaction under the action of protic acids, superacids, or Lewis acids. And triisopropylborate is known to be a Lewis acid. So your crystal can be a polymer formed by such a polycondensation, and maybe contain some intermediate condensation products, and the brown layer on it can be the result of partial oxidation of polymer on air. But I say again, I am not sure if this suggestion is correct, and it is still very surprising that you did not found boron in the mixture after the reaction.