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I did an experiment in my school lab in which we had to identify the given organic compound. In my Lab Manual, it's written:

Take 2 ml of Tollen's Reagent and add a few drops of organic sample. Heat the contents on boiling water bath for 5 minutes. Silver Mirror is formed. The given organic compound is Glycerol.

As far as I know, the compounds with aldehyde groups in them give silver mirror. Also those which contain $\alpha$,$\beta$-hydroxy ketone are known to react with Tollen's Reagent. I have never heard of Glycerol reacting with it. I searched but couldn't find this thread anywhere. Though I found that on heating Glycerol, it gives us Acrolein which does contain aldehyde group, which I think is the reason but I am not sure.

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Your guess of formation of Acrolein is a pretty good one, as indicated in this answer, but a boiling water bath will probably not give you the 160°C temperature you are looking for to convert glycerol to acrolein

This question is most similar to yours and has a pretty good answer by bon. The answer says:

However, from personal experience, gentle heating of a primary alcohol with Tollens' will cause a small amount of oxidation and result in a fine black precipitate.

It is most likely that Glycerol is undergoing a small amount of oxidation to any of it's oxidation products (most probably glyceraldehyde, since that gives tollen's test) and this oxidized compound is giving a positive tollen's test.

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  • $\begingroup$ Bon says that he is not sure, it's only his personal experience. Also I do wonder why wouldn't secondary and tertiary alcohols undergo oxidation when heated with ammoniacal silver nitrate. $\endgroup$ – Vedansh Agrawal Jan 23 at 9:05

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