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H.L. Heys and boron

Dear Reader,

Page 105 of his book deals with glycerol (propane-1,2,3-triol) the book has the text

Dissolve a salt spoon of borax in half a test-tube of water and add one or two drops of phenolphthalein solution. The liquid will turn a rose-red colour. Now make a weak solution of glycerine by dissolving one drop of glycerine in an inch of water in another test-tube. Add the glycerine solution, drop by drop, to the borax solution until the red colour disappears. If now the tube is warmed the red colour will reappear, only to vanish once more when the tube is cooled. This can be repeated as often as desired.

A similar effect is obtained by using a sugar solution in place of glycerine (see p. 122).

In common with the rest of his book Heys (sorry I do not know his first name) does not explain the chemistry behind this experiment. I will explain some of the chemistry, firstly phenolphthalein or even phenolphthalein is an acid/base indicator which is red when alkaline and colourless when in acid. So it is reasonable to assume that some sort of pH changes are occurring in this experiment.

Boric acid is a very weak acid, we can regard it as a reaction of trihydroxyl borane with a hydroxide anion to form a borate anion. If a polyalcohol is used which can chelate to the boron then the consumption of hydroxide anions by the trihydroxyl borane is favoured. The chelate effect makes the complex of B(OH)3 and HO more stable thus the pKa of the boric acid changes.

I looked in the Cambridge database and I found a nice example of a polyalcohol which binds with three hydroxyl groups to the boron. This polyalcohol is scyllo-Inositol which is a slightly exotic sugar. It chelates to two borons to form a nice looking dianion. The sodium salt was characterized by X-ray crystallography by C.T.Grainger and the results were published in Acta Crystallogr.,Sect.B:Struct.Crystallogr.Cryst.Chem., 1981, 37, 563.

Complex of boron and scyllo-inositol

Complex of boron and scyllo-inositol

You might ask “why is chelation important”, the answer is that it strongly favours the binding of two things. I would like to ask you to do a thought experiment. Hold a biro in one hand, if I want to take the pen from you I have to release one hand from it. If you were to hold the biro in both hands at once then to release it I must free it from both of your hands at the same moment. It does not matter if I release one hand from the pen if you are holding it in the other. If I release one hand at a time, while I am dealing with the second hand then the first hand is likely to reattach. This explains the difference between a monodentate and a bidentate (bites with one or two) ligand.

If you were to hold the biro using both hands and your teeth at the same time then you would be a tridentate ligand for the biro. Now to release the pen I must open your mouth and get both hands to release all at the same time. I will assume that you understand that the tridentate way of holding the biro is a even harder to defeat than the bidentate method of holding the pen.

With some luck we will discuss boron chemistry and the book by Heys again soon.

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