Practical Organic Chemistry
The detective work of the laboratory — a systematic set of colour, smell and precipitate tests that reveal an unknown compound's functional group and identity
- How a preliminary examination — state, colour, odour and the ignition test — narrows down an unknown.
- How to detect unsaturation with bromine water and Baeyer's reagent.
- The laboratory tests for alcohols and phenols, including the Lucas test.
- The tests for aldehydes and ketones — 2,4-DNP, Tollens', Fehling's and the iodoform test.
- The tests for carboxylic acids and amines (carbylamine, Hinsberg).
- The classic distinguishing tests and a systematic scheme for naming an unknown.
The Detective's Method
The whole of organic chemistry now comes together at the bench. Faced with an unknown compound, the chemist works like a detective: gather clues from how it looks and burns, find which elements it holds (Chapter 33), then run targeted tests for each functional group. Every test in this chapter is simply a reaction you already know — oxidation, addition, acid–base — chosen because it produces an unmistakable colour, smell, gas or precipitate.
Preliminary Examination
Before any reagent, simple observation pays. Physical state and colour hint at the class; a characteristic odour (the fruity smell of an ester, the sharp smell of an acid, the fishy smell of an amine) is a strong clue. The ignition (flame) test is especially useful.
Detecting Unsaturation
A carbon–carbon double or triple bond is revealed by two reagents that it consumes.
| Reagent | Observation | Reaction type |
|---|---|---|
| Bromine in \(\ce{CCl4}\) (or bromine water) | reddish-brown colour disappears | addition across the multiple bond |
| Baeyer's reagent (cold dilute alkaline \(\ce{KMnO4}\)) | purple fades; brown \(\ce{MnO2}\) appears | oxidation to a vicinal diol |
Alcohols & Phenols
Both carry an \(\ce{-OH}\) group, but their chemistry differs sharply: phenols are weakly acidic and give strong colours with iron(III), while alcohols do not.
| Group | Test | Positive result |
|---|---|---|
| Alcohol | ceric ammonium nitrate | yellow reagent turns red/wine |
| Alcohol | ester test (acid + conc. \(\ce{H2SO4}\)) | fruity-smelling ester |
| 1°/2°/3° alcohol | Lucas reagent (conc. \(\ce{HCl}\)+\(\ce{ZnCl2}\)) | turbidity: 3° at once, 2° in ~5 min, 1° none (cold) |
| Phenol | neutral \(\ce{FeCl3}\) | violet / blue / green colour |
| Phenol | bromine water | white precipitate (2,4,6-tribromophenol) |
The test forms an insoluble alkyl chloride (the turbidity) by an \(S_N1\) route, whose rate tracks carbocation stability. A tertiary alcohol reacts instantly, a secondary one in a few minutes, and a primary one not at all in the cold — so the time to cloudiness names the class.
Aldehydes & Ketones
The carbonyl group \(\ce{>C=O}\) is found first, then aldehyde is told from ketone by tests that exploit the aldehyde's easy oxidation.
| Test | Positive result | Detects |
|---|---|---|
| 2,4-DNP (Brady's reagent) | orange / yellow precipitate | any carbonyl (aldehyde & ketone) |
| Tollens' reagent | bright silver mirror | aldehydes only |
| Fehling's solution | brick-red \(\ce{Cu2O}\) precipitate | aliphatic aldehydes |
| Schiff's reagent | pink/magenta colour | aldehydes |
| Iodoform (\(\ce{I2}\)+\(\ce{NaOH}\)) | yellow \(\ce{CHI3}\) precipitate | methyl ketones & \(\ce{CH3CH(OH)-}\) |
Carboxylic Acids
Carboxylic acids are the strongest common organic acids and announce themselves clearly: they turn blue litmus red and, decisively, liberate carbon dioxide from sodium hydrogen carbonate.
Amines
Amines are basic and often fishy-smelling. The most useful bench tests both identify an amine and tell its class.
| Test | Result | Identifies |
|---|---|---|
| Carbylamine (\(\ce{CHCl3}\)+alc. \(\ce{KOH}\)) | extremely foul (isocyanide) smell | 1° amines only (aliphatic & aromatic) |
| Diazotisation + coupling | orange-red azo dye | aromatic 1° amines |
| Hinsberg's (benzenesulphonyl chloride) | see below | distinguishes 1°, 2°, 3° |
A 1° amine forms a sulphonamide with one acidic N–H, so it dissolves in \(\ce{KOH}\). A 2° amine forms a sulphonamide with no N–H, which stays insoluble in \(\ce{KOH}\). A 3° amine has no N–H to react and gives no product at all. Solubility in alkali therefore reads off the class.
Classic Distinguishing Tests
Examiners love the question "how would you distinguish A from B?" The trick is to pick one reagent that gives a clear positive with one and nothing with the other.
| Distinguish | Reagent | Who responds |
|---|---|---|
| Ethanol vs phenol | neutral \(\ce{FeCl3}\) | phenol → violet; ethanol → none |
| Phenol vs benzoic acid | \(\ce{NaHCO3}\) | benzoic acid → \(\ce{CO2}\); phenol → none |
| Aldehyde vs ketone | Tollens' / Fehling's | aldehyde → mirror / red ppt |
| Ethanal vs propanal | iodoform | ethanal → yellow ppt |
| 1° vs 2° vs 3° alcohol | Lucas reagent | turbidity timing differs |
| Formic vs acetic acid | Tollens' reagent | formic acid → silver mirror (it reduces) |
A Systematic Scheme
Put together, the tests follow a logical order — broad clues first, specific confirmation last — so that each step narrows the possibilities.
Putting It to Work
Problem. An unknown burns with a sooty, luminous yellow flame. What does this suggest?
Solution. A high C:H ratio burns incompletely:
Problem. A liquid with an \(\ce{-OH}\) group gives a violet colour with neutral \(\ce{FeCl3}\). Which is it?
Solution. Only phenols give the iron(III) colour:
Problem. Two carbonyl compounds both give a 2,4-DNP precipitate. One also gives a silver mirror. Identify each.
Solution. Tollens' responds only to the aldehyde:
Problem. Both dissolve in \(\ce{NaOH}\). One reagent tells them apart — which, and how?
Solution. Only the stronger acid frees \(\ce{CO2}\) from bicarbonate:
Problem. Which of ethanol and propan-1-ol gives a positive iodoform test, and why?
Solution. Only ethanol carries the \(\ce{CH3CH(OH)-}\) unit:
Problem. In Hinsberg's test an amine forms a product insoluble in \(\ce{KOH}\). Which class is it?
Solution. An insoluble sulphonamide has no acidic N–H:
Chapter Summary
State, colour, odour and the ignition test (sooty = aromatic) narrow the field first.
Bromine water and Baeyer's reagent are decolourised by C=C / C≡C bonds.
Phenol → violet with FeCl₃; alcohols → Lucas timing gives 1°/2°/3°.
2,4-DNP finds the carbonyl; Tollens'/Fehling's pick out the aldehyde; iodoform the CH₃CO– unit.
NaHCO₃ fizz = acid (not phenol); carbylamine = 1° amine; Hinsberg names the class.
Observe → ignite → detect elements → find the group → confirm the identity.
Problems
For each item, name the reagent, the expected observation, and the reaction behind it. Difficulty rises down the list.
- What does a sooty flame indicate, and why does it occur?
- Name two tests for unsaturation and the change observed in each.
- How would you distinguish ethanol from phenol?
- Explain the Lucas test and how it tells 1°, 2° and 3° alcohols apart.
- Which test detects all carbonyls, and which singles out aldehydes?
- Explain the iodoform test and list two compounds that give it positively.
- How do you distinguish a carboxylic acid from a phenol in one test?
- Describe the carbylamine test and state which amines respond.
- Explain Hinsberg's test and the result for each class of amine.
- How would you distinguish formic acid from acetic acid?
- An unknown is neutral, gives a silver mirror and a yellow iodoform precipitate. Suggest its identity.
- Outline the systematic scheme for identifying an unknown organic compound.