Distinction Tests — Comparison of Tollens', Fehling's, and 2,4-DNP — Notes

Comparison Table

Feature	2,4-DNP Test	Tollens' Test	Fehling's Test	Iodoform Test
Reagent	2,4-Dinitrophenylhydrazine	Ammoniacal AgNO3	Alkaline Cu2+(tartrate)	I2 / NaOH
Observation (positive)	Orange/yellow precipitate	Silver mirror (Ag deposited)	Brick-red ppt (Cu2O)	Yellow ppt (CHI3)
Works for	Both aldehydes AND ketones	Aldehydes ONLY	Aldehydes ONLY	CH3CO-R, CH3CHO, EtOH, iPrOH
Detects	Presence of C=O	Reducing aldehyde	Reducing aldehyde	Methyl ketone / precursor
Ketones react?	Yes	No	No	Only methyl ketones

Testing Hierarchy (NEET Logic)

First apply 2,4-DNP — if negative, no C=O present (not aldehyde or ketone)
If positive, apply Tollens' — if positive, it's an aldehyde
If Tollens' negative, it's a ketone
Apply iodoform — if positive, it's a methyl ketone (or methyl precursor)

Balanced Reactions

Tollens' test: $\text{RCHO} + 2[\text{Ag(NH}_3)_2]^+ + 2\text{OH}^- \rightarrow \text{RCOO}^- + 2\text{Ag} \downarrow + 4\text{NH}_3 + \text{H}_2\text{O}$

Fehling's test: $\text{RCHO} + 2\text{Cu}^{2+} \text{(tartrate)} + \text{OH}^- \rightarrow \text{RCOOH} + \text{Cu}_2\text{O} \downarrow \text{(red)}$

Iodoform: $\text{CH}_3\text{COR} + 3\text{I}_2 + 3\text{NaOH} \rightarrow \text{CHI}_3 \downarrow + \text{RCOONa} + 3\text{NaI} + 2\text{H}_2\text{O}$