Editing Tollens' reagent

{{short description|Chemical reagent used to distinguish between aldehydes and ketones}}
{{Use dmy dates|date=November 2022}}
[[File:Tollens.JPG|thumb|upright=1.0|Tollens' test for aldehyde: left side positive (silver mirror), right side negative]]
[[File:Diamminesilver(I)-3D-balls.png|thumb|right|200px|[[Ball-and-stick model]] of the diamminesilver(I) complex]]

'''Tollens' reagent''' (chemical formula <chem>Ag(NH3)2OH</chem>) is a chemical [[reagent]] used to distinguish between [[aldehyde]]s and [[ketone]]s along with some [[alpha-hydroxy ketone]]s which can [[tautomerize]] into aldehydes. The reagent consists of a solution of [[silver nitrate]], [[ammonium hydroxide]] and some [[sodium hydroxide]] (to maintain a basic [[pH]] of the reagent solution). It was named after its discoverer, the German chemist [[Bernhard Tollens]].<ref>{{cite journal|last1=Tollens|first1=B.|title=Ueber ammon-alkalische Silberlösung als Reagens auf Aldehyd|journal=Berichte der Deutschen Chemischen Gesellschaft|date=1882|volume=15|issue=2|pages=1635–1639|url=https://zenodo.org/record/1910987|trans-title=On an ammonical alkaline silver solution as a reagent for aldehydes|language=de|doi=10.1002/cber.18820150243}}</ref> A positive test with Tollens' reagent is indicated by the precipitation of elemental silver, often producing a characteristic "[[Silvering|silver mirror]]" on the inner surface of the reaction vessel.

==Laboratory preparation==
This reagent is not commercially available due to its short [[shelf life]], so it must be freshly prepared in the laboratory. One common preparation involves two steps. First a few drops of dilute [[sodium hydroxide]] are added to some aqueous 0.1&nbsp;[[molar (unit)|M]] [[silver nitrate]]. The <chem>OH-</chem> ions convert the [[transition metal aquo complex|silver aquo complex]] form into [[silver oxide|silver(I) oxide]], <chem>Ag2O</chem>, which precipitates from the solution as a brown solid: 
:<chem>2AgNO3 + 2NaOH   -> Ag2O(s)  +  2NaNO3 +  H2O</chem>

In the next step, sufficient [[aqueous ammonia]] is added to dissolve the brown silver(I) oxide. The resulting solution contains the [Ag(NH<sub>3</sub>)<sub>2</sub>]<sup>+</sup> complexes in the mixture, which is the main component of Tollens' reagent. Sodium hydroxide is reformed:
:<chem>Ag2O(s) + 4NH3 + 2NaNO3 + H2O -> 2[Ag(NH3)2]NO3 + 2NaOH </chem>

Alternatively, [[aqueous ammonia]] can be added directly to silver nitrate solution.<ref name = webpath/> At first, ammonia will induce formation of solid silver oxide, but with additional ammonia, this solid precipitate dissolves to give a clear solution of diamminesilver(I) [[coordination complex]], <chem>[Ag(NH3)2]+</chem>. Filtering the reagent before use helps to prevent false-positive results.

==Uses==

===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}}

===Staining===
In [[anatomic pathology]], ammonical silver nitrate is used in the [[Histological techniques|Fontana–Masson stain]], which is a [[silver stain]] technique used to detect [[melanin]], [[argentaffin]], and [[lipofuscin]] in tissue sections. Melanin and the other chromaffins reduce the silver nitrate to metallic silver.<ref name = webpath>Webpath website http://library.med.utah.edu/WebPath/HISTHTML/MANUALS/FONTANA.PDF. Retrieved 4 February 2009</ref>

===In silver mirroring===
Tollens' reagent is also used to apply a silver mirror to glassware; for example the inside of an insulated vacuum flask.  The underlying chemical process is called '''silver mirror reaction'''.  The reducing agent is [[glucose]] (an aldehyde) for such applications.  Clean glassware is required for a high quality mirror. To increase the speed of deposition, the glass surface may be pre-treated with [[tin(II) chloride]] stabilised in [[hydrochloric acid]] solution.<ref>{{cite book|last=Hart|first=M.|title=Manual of scientific glassblowing|year=1992|publisher=British Society of Scientific Glassblowers|location=St. Helens, Merseyside [England]|isbn=0-9518216-0-1}}</ref>

For applications requiring the highest optical quality, such as in [[reflecting telescope|telescope mirrors]], the use of [[tin(II) chloride]] is problematic, since it creates nanoscale roughness and reduces the reflectivity.<ref>
{{cite journal
 | last1 = N. Chitvoranund1
 | last2 = S. Jiemsirilers
 | last3 = D.P. Kashima
 | title = Effects of surface treatments on adhesion of silver film on glass substrate fabricated by electroless plating
 | journal = Journal of the Australian Ceramic Society
 | volume = 49
 | pages = 62–69
 | date = 2013 }}</ref><ref name="silvering_mirrors"/>
Methods to produce telescope mirrors include additional additives to increase adhesion and film resilience, such as in Martin's method, which includes [[tartaric acid]] and [[ethanol]].<ref name="silvering_mirrors">{{cite journal|last1 = C. Heber D.|title = Methods of Silvering Mirrors|journal = Publications of the Astronomical Society of the Pacific|volume = 23|issue = 135|pages = 15–19|date = Feb 1911|doi=10.1086/122040|bibcode=1911PASP...23...13C| url = https://zenodo.org/record/1431273|doi-access = free|hdl = 2027/mdp.39015018047608|hdl-access = free}}</ref>

==Safety==
Aged reagent can be destroyed with dilute acid to prevent the formation of the highly explosive [[silver nitride]].<ref name="Vogel 7th">{{cite book
|author1=Svehla, G. |author2=Vogel, Arthur Anton |title=Vogel's Qualitative Inorganic Analysis
|publisher=Longman
|location=New York
|year=1996
|isbn=0-582-21866-7
}}</ref>

== See also ==
* [[Benedict's reagent]]
* [[Walden reductor]] (opposite use involving metallic silver)

==References==
{{Reflist}}

==External links==
* [https://web.archive.org/web/20050329000040/http://www.chemistry.wustl.edu/~courses/genchem/Labs/RedoxIdentity/silver.htm Video of experimental process involving Tollens' reagent]
* [http://www.wiu.edu/users/mftkv/Chemistry102/oxidationaldehydes.html Tollens' reagent on www.wiu.edu]
* [http://jialigao.org/teaching/silv2.jpg Univ. of Minnesota Organic Chemistry Class Demo] {{Webarchive|url=https://web.archive.org/web/20160303195839/http://jialigao.org/teaching/silv2.jpg |date=3 March 2016 }} [http://jialigao.org/teaching/silv3.jpg Result] {{Webarchive|url=https://web.archive.org/web/20160303190235/http://jialigao.org/teaching/silv3.jpg |date=3 March 2016 }}

{{silver compounds}}
{{Analytical reagents}}

[[fr:Réaction de Tollens]]
[[pl: Próba Tollensa]]

[[Category:Silver compounds]]
[[Category:Oxidizing agents]]
[[Category:Coordination complexes]]
[[Category:Chemical tests]]
[[Category:Analytical reagents]]
[[Category:Ammine complexes]]