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Free for 24 hours - The mechanism of the reaction of the Tollens reagent

Posted on 7. March, 2018.

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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 overall reaction may be written as in the main image above.

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Authors: William E. Benet, Gabriella S. Lewis, Louise Z. Yang and D. E. Peter Hughes*
Westminster School, 17 Dean’s Yard, London SW1P 3PB, UK

Keywords: Tollens reagent, aldehyde, gem diol, free radical

Published in Journal of Chemical Research, Volume 35, Number 12, December 2011, pp. 675-677.