Editing Tollens' reagent (section)

===Qualitative organic analysis===
Once the presence of a [[carbonyl group]] has been identified using [[2,4-dinitrophenylhydrazine]] (also known as [[Brady's reagent]] or 2,4-DNPH or 2,4-DNP), Tollens' reagent can be used to distinguish [[ketone]] vs [[aldehyde]]. Tollens' reagent gives a negative test for most ketones, with [[acyloin|alpha-hydroxy ketones]] being one exception.

The test rests on the premise that aldehydes are more readily [[oxidation and reduction|oxidized]] compared with ketones; this is due to the carbonyl-containing carbon in aldehydes having attached hydrogen. The diamine silver(I) complex in the mixture is an [[oxidizing agent]] and is the essential reactant in Tollens' reagent. The test is generally carried out in a test tube in a warm water bath.

In a positive test, the diamine silver(I) complex oxidizes the aldehyde to a [[carboxylate]] ion and in the process is reduced to elemental silver and aqueous ammonia. The elemental silver precipitates out of solution, occasionally onto the inner surface of the reaction vessel, giving a characteristic "silver mirror". The carboxylate ion on acidification will give its corresponding [[carboxylic acid]]. The carboxylic acid is not directly formed in the first place as the reaction takes place under [[alkaline]] conditions. The ionic equations for the overall reaction are shown below; R refers to an [[alkyl]] group.<ref name="chemguide">{{cite web |url=http://www.chemguide.co.uk/organicprops/carbonyls/oxidation.html |title=Oxidation of aldehydes and ketones |website=chemguide.co.uk |access-date=31 January 2010}}</ref>

:<chem>2[Ag(NH3)2]+  +  R-CHO  + H2O   -> 2Ag(s)  +  4NH3  +  R-COOH  + 2H+ </chem>

Tollens' reagent can also be used to test for terminal [[alkyne]]s (<chem>R-C2H</chem>). A white precipitate of the [[metal acetylide|acetylide]] (<chem>AgC_2-R</chem>) is formed in this case.  Another test relies on reaction of the [[furfural]] with [[phloroglucinol]] to produce a colored compound with high molar absorptivity.<ref>{{cite journal|last1=Oshitna|first1=K.|last2=Tollens|first2=B.|title=Ueber Spectral-reactionen des Methylfurfurols|journal=Ber. Dtsch. Chem. Ges.|volume=34|page=1425|date=1901|issue=2 |doi=10.1002/cber.19010340212 |url=https://zenodo.org/record/1735046 }}</ref> It also gives a positive test with [[hydrazine]]s, [[hydrazone]]s, [[acyloin|α-hydroxy ketones]] and [[dicarbonyl|1,2-dicarbonyls]].

Both Tollens' reagent and [[Fehling's reagent]] give positive results with [[formic acid]].{{citation needed|date=December 2020}}