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From my school lab manual for identification of organic compounds:

Take 2 ml of Tollens’ 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 give positive silver mirror test. Also, those which contain α,β-hydroxy ketone are known to react with Tollens’ reagent.

I have never heard of glycerol reacting with Tollens’ reagent. I found that on heating glycerol forms acrolein which contains aldehyde group:

$$\ce{(CH2OH)2CHOH → CH2=CHCHO + 2 H2O},$$

which I think is the reason, but I am not sure.

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    $\begingroup$ It is unlikely under Tollen's conditions that glycerol is converted into acrolein since both compounds are at the same oxidation level. Base is not going to effect the transformation. What ever aldehydic products are formed, they are at a higher oxidation level than acrolein. See: intechopen.com/chapters/66623 @Aniruddha Deb $\endgroup$
    – user55119
    Commented Oct 13, 2021 at 23:59
  • $\begingroup$ Could it happen due to tautomerism? Infact, there is a dynamic equilibrium between the enol (glicerol) and keton form (see: tgc.ac.in/pdf/study-material/chemistry/…) $\endgroup$
    – Pierluigi
    Commented Mar 25, 2022 at 7:50

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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 Tollens’ test) and this oxidized compound is giving a positive Tollens’ 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
    Commented Jan 23, 2020 at 9:05

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