Question
You are given an unknown organic compound. Describe the chemical tests to identify whether it contains an alcohol, aldehyde, ketone, carboxylic acid, or amine functional group.
(NEET and CBSE 12 ask identification tests almost every year)
Solution — Step by Step
Add the compound to NaHCO₃ solution. If brisk effervescence (CO₂ gas) occurs, a carboxylic acid is present.
Also turns blue litmus red. Do this test first — if positive, no need to check further.
Tollen’s test (silver mirror): Warm with ammoniacal AgNO₃. Aldehydes give a silver mirror on the test tube. Ketones do not react.
Fehling’s test: Warm with Fehling’s solution (alkaline CuSO₄ + tartrate). Aldehydes give a red precipitate of Cu₂O. Ketones give no reaction.
Both tests work because aldehydes are easily oxidised, ketones are not.
Add a small piece of sodium metal to the compound. If hydrogen gas evolves (with a “pop”), an —OH group is present.
To distinguish primary, secondary, and tertiary alcohols, use the Lucas test with ZnCl₂ + conc. HCl: 3° reacts immediately (turbidity), 2° reacts in 5 minutes, 1° reacts only on heating.
Carbylamine test (isocyanide test): Heat with CHCl₃ + alcoholic KOH. Primary amines give a foul-smelling isocyanide.
Hinsberg test distinguishes 1°, 2°, and 3° amines using benzenesulphonyl chloride.
flowchart TD
A[Unknown organic compound] --> B{NaHCO₃ test: effervescence?}
B -->|Yes| C[Carboxylic Acid]
B -->|No| D{Tollen's test: silver mirror?}
D -->|Yes| E[Aldehyde]
D -->|No| F{Na metal: H₂ evolved?}
F -->|Yes| G[Alcohol]
F -->|No| H{2,4-DNP: yellow ppt?}
H -->|Yes| I[Ketone]
H -->|No| J{Carbylamine test: foul smell?}
J -->|Yes| K[Primary Amine]
J -->|No| L[Other functional group]Why This Works
Each test exploits a specific chemical property of the functional group. Carboxylic acids are acidic enough to decompose NaHCO₃. Aldehydes are reducing agents (they reduce Ag⁺ to Ag and Cu²⁺ to Cu⁺). Alcohols have an active hydrogen that reacts with Na metal. Amines are basic and undergo nucleophilic reactions.
The 2,4-DNP (Brady’s) test confirms the presence of a carbonyl group (C=O) in both aldehydes and ketones — a yellow/orange precipitate forms. Then Tollen’s test distinguishes between the two.
Alternative Method
For board exams, use this testing sequence: acid → aldehyde → ketone → alcohol → amine. Test from most reactive to least reactive functional groups. This way, a positive test at any stage gives you the answer without needing to do all tests.
Common Mistake
Students use Tollen’s test on ketones and when they get no silver mirror, they wrongly conclude the compound is “not a carbonyl compound.” A negative Tollen’s test only rules out aldehydes and reducing sugars — it does not rule out ketones. Always follow up with the 2,4-DNP test (which is positive for both aldehydes and ketones) to confirm the presence of a C=O group.