One of the uses of the Tollens' reagent is to confirm the presence of a terminal triple bond in a compound. Does acetylene give a positive Tollens' test? If not, why is this so?
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$\begingroup$ chemistry.stackexchange.com/questions/33854/… $\endgroup$– MithoronCommented Jan 10, 2017 at 20:56
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2 Answers
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Tollen's reagent (ammoniacal silver nitrate) reacts with terminal acetylenes to form the silver acetylide which precipitates out of solution
$$\ce{RC#CH + AgNO3 + NH4OH -> RC#CAg v + NH4NO3 + H2O}$$
In the case of acetylene itself, since both ends of the molecule are (if you will) terminal acetylenes, a bis-silver acetylide salt is formed and precipitates out of solution (see page 3, item 6b in a PDF from www.sakshieducation.com/ or via the Internet Archive)
$$\ce{HC#CH + 2AgNO3 + 2NH4OH -> AgC#CAg v + 2NH4NO3 + 2H2O}$$
Tollen's reagent is not very stable, therefore it needs to be freshly prepared before use. The reagent has a number of other uses in chemistry (test for aldehydes and α-hydroxy ketones), anatomical staining, and silver mirroring. This Wikipedia article provides a nice, concise overview.
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5$\begingroup$ So this means that alkynes don't reduce the Tollen's reagent, as the aldehydes do, right? $\endgroup$ Commented May 15, 2017 at 13:21
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4$\begingroup$ @ReeshabhRanjan Yes, that's correct. $\endgroup$– ronCommented May 15, 2017 at 14:39
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$\begingroup$ Actually, you can skip a couple of steps and react silver nitrate solution directly with acetylene to get silver acetylide. Also, be careful, silver acetylide is a sensitive explosive. $\endgroup$ Commented Apr 5, 2018 at 7:16
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Terminal hydrogens in acetylene are acidic and are easily removed to form acetylide ions, and in this case, elementary silver will be displaced from the ammoniacal silver nitrate and will form silver mirror on the surface of the reacting vessel.
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$\begingroup$ The article linked in the first answer specifically rules out formation of silver mirror, the bis silver acetylide that precipitates is white. $\endgroup$ Commented Apr 4, 2018 at 22:03