1. Tollen's Test
Definition: Tollen’s test is a qualitative chemical test used to detect the presence of aldehydes.
Reagent: Tollen’s reagent is an ammoniacal silver nitrate solution — \ce{[Ag(NH3)2]^+}.
Function: Acts as an oxidizing agent. Silver ions get reduced to metallic silver.
Used For: Detection of aldehydes (they give positive test) and distinguishing them from ketones (which usually give negative test).
Reaction:
\ce{RCHO + 2[Ag(NH3)2]^+ + 3OH^- -> RCOO^- + 2Ag (s) + 4NH3 + 2H2O}
Observation: Formation of a silver mirror on the inner wall of the test tube.
2. Fehling’s Test
Definition: Fehling’s test is used to detect reducing sugars and aliphatic aldehydes.
Reagent: Fehling’s solution = Fehling A (\ce{CuSO4}) + Fehling B (alkaline tartrate solution).
Function: Alkaline \ce{Cu^{2+}} ions oxidize aldehydes.
Used For: Detection of aliphatic aldehydes and reducing sugars like glucose.
Reaction:
\ce{RCHO + 2Cu^{2+} + 5OH^- -> RCOO^- + Cu2O (s) + 3H2O}
Observation: Formation of a brick red precipitate of \ce{Cu2O}.
3. Benedict’s Test
Definition: Benedict’s test detects reducing sugars (like glucose and fructose) and aldehydes.
Reagent: Benedict’s reagent = \ce{CuSO4} + sodium citrate + \ce{Na2CO3}.
Function: Alkaline \ce{Cu^{2+}} ions oxidize aldehydes or reducing sugars.
Used For: Identification of reducing sugars and some aldehydes.
Reaction:
\ce{Reducing\ Sugar + Cu^{2+} -> Cu2O (s) + oxidized\ sugar}
Observation: Formation of a colored precipitate (green, yellow, or red) depending on amount of sugar.
4. Haloform Test
Definition: Haloform test detects compounds containing the group \ce{-COCH3} or compounds that can be oxidized to it.
Reagent: A halogen (like \ce{I2}) + base (like \ce{NaOH}).
Function: The methyl ketone group is halogenated and then hydrolyzed to give haloform.
Used For: Detection of methyl ketones and compounds like ethanol (oxidizable to acetaldehyde).
Reaction:
\ce{RCOCH3 + 3I2 + 4OH^- -> RCOO^- + CHI3 (yellow ppt) + 3I^- + 3H2O}
Observation: Formation of a yellow precipitate of iodoform (\ce{CHI3}), with antiseptic smell.
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