Fehling solution & Tollen’s reagent test
Free resources –A/AS level (3)
Fehling solution
Fehling solution is an alkaline solution of CuSO4 containing some
Rochelle salt i.e. sodium potassium tartarate as the complexing agent.
Fehling solution A : Copper sulphate solution
Fehling solution B: An alkaline solution of Rochlle salt.
Preparation of Fehling solution
It is prepared by adding Fehling solution B into Fehling solution A until ppt of
Cu(OH)2 first formed just re-dissolve to form a deep blue solution.
O
R
C
O
+ 2Cu(OH)2 + NaOH
H
R
C
+ Cu2O
+ 3H2O
Na
Fehling solution
Red ppt
Aldehyde on warming with Fehling solution gives red precipitate of
copper oxide (Cu2O).
NOTE:
1) Methanal has two hydrogen atoms (HCHO) attached to the
carbonyl group, both of which are oxidisable. Therefore,
methanal is more powerful reducer than other aldehydes like
CH3CHO,CH3CH2CHO.
2) Methanal can reduce Cu(II) to Cu(0), while, while other
aldehydes reduce it only to Cu(I).
www.tutoronline.co.in
Page: 1
Fehling solution & Tollen’s reagent test
Free resources –A/AS level (3)
3) Aromatic aldehydes however do not reduce Fehling solution.
Therefore reduction can be chosen to differentiate between
aliphatic and aromatic aldehydes.
Tollens reagent
Tollens reagent is an ammonical solution of silver nitrate.
Preparation of Tollens reagent
Tollens reagent prepared by adding ammonium hydroxide solution (NH4OH)
to silver nitrate solution(AgNO3) till the precipitate of Ag2O first formed just
re-dissolves.
AgNO3 + NH4OH
AgOH + NH4NO3
AgOH + 2NH4OH
[Ag(NH3)2]OH + 2H2O
O
O
R
+
C
-
R
+ 2[Ag(NH3)2] OH
+ 2Ag + 3NH3
C
+ H2O
ONH4
H
Silver mirror
OR
O
O
R
+
C
+ 2Ag +H2O
H
R
+ 2Ag
C
+ 2H+
OH
Silver mirror
Aldehyde on warming with Tollens reagent forms a silver mirror on
the walls of the container.
NOTE:
Both aliphatic and aromatic aldehydes are reduced by tollens reagent.
www.tutoronline.co.in
Page: 2
Fehling solution & Tollen’s reagent test
Free resources –A/AS level (3)
Cannizaro’s Reaction
Aldehydes which do not have α- hydrogen atom (e.g.
formaldehyde/methanal) when treated with concentrated sodium
hydroxide (NaOH) solution, undergoes disproportionation reaction
i.e. self oxidation reduction reaction.
In the presence aqueous NaOH, molecules of HCHO simultaneously
oxidize and reduce one another.
2HCHO
+
NaOH
HCOONa + CH3OH
OR
O
H
C
H
O
H
+ H
O
50% NaOH
C
H
H
C
OH + H
C
OH
H
From reduction
of methnal
From oxidation
of methnal
Other aldehydes do not give this reaction because in the presence of
sodium hydroxide they polymerises.
Benzaldehyde gives this reaction since it doesn’t have α- hydrogen
atom.
O
O
C
+
H
H
C
50% NaOH
O
H
C
OH
H
Phenylmethanol
www.tutoronline.co.in
+
OH
C
Benzoic acid
Page: 3