Tollens reagent

Tollens reagent or silver mirror test 

Aldehydes are easily oxidised to carboxylic acids but ketones are not. Aldehydes
are oxidised not only by the same reagents, which oxidizes primary and secondary alcohols
(like acidified KMnO₄, K₂Cr₂O₇) but also by mild oxidising agents like Tollen’s reagent, Fehling’s Detail Explanation  and Benedict’s Detail Explanation . Aldehydes are very easily oxidised and thus are very powerful reducing agents.

Tollen’s reagent contains diamminesilver(I) ion, which is obtained through ammoniacal AgNO₃ Detail Explanation . Tollen’s reagent oxidises aldehydes to acid salt and they reduces to free silver in the form of silver mirror.       

RCHO + + 3OH⁻  →   +  2Ag↓ + 4NH₃ + 2H₂O

  Colourless     Silver                          Detail Explanation     mirror

Tollen’s reagent is useful in differentiating aldehydes from ketones because ketones do not react with them. Tollen’s reagent is a mild and selective oxidising agent, attacking only aldehydic group, keeping other groups untouched. Unsaturated aldehydes can be converted to unsaturated acid using Tollen’s reagent.

Bernhard Tollens (1841-1918) was a Professor of Chemistry at the University of Gottingen, Germany.

Aldehydes are easily oxidised, thus they also reduce Fehling’s Detail Explanation  (an alkaline Detail Explanation containing a complex of copper tartarate) to red cuprous oxide. Aldehydes also reduce Benedict’s Detail Explanation  (an alkaline Detail Explanation containing a complex of copper citrate) to red precipitate of cuprous oxide.

Fehling reagent comprises of two Detail Explanation s, Fehling Detail Explanation  A and Fehling Detail Explanation B. Fehling Detail Explanation A is aqueous copper sulphate and Fehling Detail Explanation  B is alkaline sodium potassium tartarate (Rochelle salt). These two Detail Explanation s are mixed in equal amounts before test.

R−CHO  + 2CuO  →  RCOO⁻  + Cu₂O↓  (Fehling’s and Benedict’s test)

            (Red)

Ketones are not easily oxidised, thus they do not reduce Fehling’s Detail Explanation  or Tollen’s reagent. But α−hydroxy ketones (compounds containing the unit ) readily reduce The compounds which respond to the test with Fehling’s Detail Explanation  and ammoniacal silver nitrate are given below in the tabulated manner.   

List of Molecules which give postive Tollens Test 

Aldehydes restore the magenta colour of the Schiff’s reagent (rosaniline hydrochloride is dissolved in H2O and SO2 is passed till the magenta colour is decolourised). Ketones do not restore the colour of Schiff’s reagent except acetone, which restores the colour very slowly.

Oxidation of ketones requires breaking of carbon−carbon bonds, which requires vigorous conditions. Cleavage involves the double bond of the enol form and wherever the structure permits, occurs on both sides of carbonyl group. Thus, in general, ketones on oxidation give a mixture of carboxylic acids.

3−Hexanone gives a mixture of carboxylic acids because it can form two enols, CH3CH=C(OH)CH2CH2CH3, which on cleavage gives CH3CO2H & CH3CH2CH2CO2H and CH3CH2C(OH)=CHCH2CH3, which on oxidation gives 2 moles of CH₃CH₂CO₂H.

Methyl ketones can be conveniently oxidised by hypohalite in the haloform reaction. Hypohalite is a selective oxidising agent, used for detecting methyl ketones as well as it do not attack carbon−carbon double bonds.

For example: