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The mechanism of the reaction of the Tollens reagent

Posted on 21. December, 2011.

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The Tollens test for aldehydes has been used for over 100 years but no reason has been given for adding sodium hydroxide to the silver nitrate before the solution is cleared with ammonia. In this paper, a mechanism is proposed which explains why the addition of alkali makes the test much more sensitive.

Bernhard Tollens introduced his silver test for aldehydes when he was working on carbohydrates in the late ninteenth century. This test involves the reduction of ammoniacal silver nitrate by an aldehyde to give a silver mirror. If a few drops of sodium hydroxide are initially added to the silver nitrate, the test becomes much more sensitive though no explanation has been given for this. There are two possible reasons. The higher pH could increase the yield of the reaction and/or it could increase the rate of reaction.
The addition of alkali makes the Tollens test much  more sensitive because the rate is much faster. The key feature seems to be the formation of the anion of the gem diol. The ease of formation of this anion varies with the aldehyde and correlates well with how rapidly they respond to Tollens test.
William E. Benet, Gabriella S. Lewis, Louise Z. Yang and D. E. Peter Hughes*
Westminster School, 17 Dean’s Yard, London SW1P 3PB, UK


Journal of Chemical Research DECEMBER 2011, Volume 35,  pp 675–677
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DOI: 10.3184/174751911X13206824040536