Posted by: Mark Foreman | January 12, 2011

Esters of main group acids

Now I hope that you are aware that an acid can be converted into an ester, an ester is formed when an acidic (Bronsted sense) hydrogen in an acid is replaced with an alkyl or aryl group. The esters are an important class of compound, typically they are found in fruit. All the fruits which I can think of contain esters for example countless esters have been observed in apples. I will not bore you by listing them all.

Another type of ester which is important are the esters of inorganic acids, it is possible to make esters of acids such as sulphuric, nitric and phosphoric acid. Also esters of chromic acid have been isolated and are thought to be important intermediates in the oxidation of alcohols by chromic acid (Jones’s reagent).

One compound which I was thinking about recently is a funny one which bucks the trend a little. Boric acid is a funny acid in one regard, as it is B(OH)3 while its conjugate base is B(OH)4-.  Boric acid works as a lewis acid, it reacts in aqueous solution with hydroxyl anions thus removing them from the equilibrium. Boric acid is a very weak acid, so weak that it will not change litmus paper red.

One interesting thing is that boric acid will react with polyalcohols such as ethylene glycol, sugars and polyvinyl alcohol (school glue) to form compounds with new B-O-C linkages. These compounds include things where one diol has reacted with the boric acid which can be regarded as esters of boric acid.

The real fun is that a second diol can react to give an ion such as [B(OCH2CH2O)2]-. As the chelate effect strongly favours this reaction more than the reaction of water or a monoalcohol with boric acid the boric acid will be much more acidic when the polyalcohol is present. Also this reaction cross links polyvinyl alcohol. By adding borax {sodium borate NaB(OH)4} to school glue a think slime can be formed, this increase in viscosity is due to the crosslinking of the polymer chains.

I will be adding a film soon of what happens when I add borax to school glue. Keep tuned.

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Responses

  1. What will be the reaction of Ethylene glycol with boric acid ??????

    • If ethylene glycol was to be reacted with boric acid it could form [1,3,2]dioxaborolan-2-ol which is a cyclic ester formed by reacting both alcohol groups in the glycol with the boric acid. It is important to note that the esters of inorganic acids tend to have very different molecular strucutres to the esters of carboxylic acids


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