Topic 20 · page 2 of 15
Tests for functional groups
Each test-tube reaction is a question with a yes or no answer. A positive result shows one functional group, a negative result rules it out, and together the answers narrow a formula down to one structure.
Read first: Formulae from combustion and composition
One test, one group 20.1
A formula such as C₃H₆O fits several compounds. A functional-group test is a quick way to choose between them. The test gives an observation, and the observation lets you deduce a group. Write both.
| Reagent | Positive result | Group shown |
|---|---|---|
| bromine water, shaken | orange to colourless | C=C (alkene) |
| bromine water with phenol | orange to colourless and a white precipitate | phenol |
| PCl₅, on a dry sample | steamy white fumes of HCl | –OH (alcohol or carboxylic acid) |
| Na₂CO₃(aq) or NaHCO₃(aq) | fizzing; the gas turns limewater milky | carboxylic acid, –COOH |
| acidified K₂Cr₂O₇, warmed | orange to green | primary or secondary alcohol, or an aldehyde |
| 2,4-dinitrophenylhydrazine | orange or yellow precipitate | C=O of an aldehyde or ketone |
| Tollens' reagent, warmed | silver mirror | aldehyde |
| Fehling's or Benedict's solution, warmed | blue solution to red-brown precipitate | aldehyde |
| iodine and NaOH(aq), warmed | pale yellow precipitate of CHI₃ | CH₃CO– or CH₃CH(OH)– |
| NaOH(aq) warmed, then excess HNO₃, then AgNO₃(aq) | white, cream or yellow precipitate | C–Cl, C–Br or C–I |
The tests for halogenoalkanes first hydrolyse the compound to free the halide ion. The nitric acid removes hydroxide ions, which would otherwise give a brown precipitate of silver oxide and spoil the result.
Negative results are evidence 20.1
A test that gives no change is as useful as one that does. Compare these two.
- A C=O compound that gives an orange precipitate with 2,4-dinitrophenylhydrazine but no silver mirror is a ketone.
- An alcohol that gives steamy fumes with PCl₅ but leaves dichromate orange is neither primary nor secondary. It is a tertiary alcohol.
Worked exampleChoosing between isomers
Illustrative data. Compound W has the molecular formula C₄H₈O. It gives an orange precipitate with 2,4-dinitrophenylhydrazine, no silver mirror with Tollens' reagent, and a pale yellow precipitate with iodine and sodium hydroxide. Identify W.
- The orange precipitate shows a C=O, so W is an aldehyde or ketone.
- No silver mirror: W is not an aldehyde, so it is a ketone.
- The yellow precipitate (CHI₃) shows a CH₃CO– group.
- The only ketone with four carbons is butanone, CH₃COCH₂CH₃, and it does contain CH₃CO–. The aldehydes butanal and 2-methylpropanal are ruled out by step 2.
Worked exampleA tertiary alcohol
Illustrative data. Compound V, C₄H₁₀O, gives steamy white fumes with PCl₅ and no colour change with acidified dichromate on warming. Identify V.
- The fumes show an –OH group, so V is an alcohol (an ether would not react).
- No change with dichromate rules out primary and secondary alcohols (and aldehydes).
- A C₄H₁₀O alcohol with no H on the carbon that carries the –OH is 2-methylpropan-2-ol, (CH₃)₃COH.
Reading several results together 20.1
Real questions give you three or four results at once. Treat each as one fact about the compound and combine them.
| Result | What it tells you |
|---|---|
| decolourises bromine water | C=C present |
| fizzes with Na₂CO₃ | carboxylic acid present |
| orange to green with dichromate | oxidisable: primary or secondary alcohol, or aldehyde |
| PCl₅ gives fumes | –OH present |
| silver mirror | aldehyde |
| no reaction with any reagent above | consider an ether, ester, ketone without CH₃CO–, or an alkane |
Worked exampleThree results, one structure
Illustrative data. Compound U, C₃H₆O, decolourises bromine water, gives steamy fumes with PCl₅ and turns acidified dichromate from orange to green. Identify U.
- Bromine water decolourised: C=C is present.
- Fumes with PCl₅: an –OH group is present.
- Orange to green: the –OH is on a primary or secondary carbon.
- C₃H₆O with a C=C and a primary –OH is prop-2-en-1-ol, CH₂=CHCH₂OH. One molecule gives both the alkene and the alcohol result.
Check yourself
- Propanal and propanone both have the formula C₃H₆O. Describe what you would see when each is warmed with Tollens' reagent, and when each is treated with 2,4-dinitrophenylhydrazine.
- Name one reagent that distinguishes ethanoic acid from ethanol, and give the observation for each.
- Describe how to show that 1-bromobutane contains a bromine atom. Give the observation and the ionic equation.
- Propan-1-ol and propan-2-ol both turn dichromate green. Which test separates them, and what is the result for each?
- Compound T has the formula C₃H₆O₂. It fizzes with sodium carbonate solution and gives steamy fumes with PCl₅. Identify T and give the equation for the reaction with sodium carbonate.
- Explain why the sample must be dry for the PCl₅ test, and why nitric acid is added before silver nitrate in the halogenoalkane test.
Answers cover these until you have tried the questions
- Tollens': propanal gives a silver mirror; propanone gives no change. 2,4-dinitrophenylhydrazine: both give an orange (or yellow) precipitate, because both contain C=O.
- Sodium carbonate solution: ethanoic acid fizzes, and the gas turns limewater milky; ethanol gives no fizzing. (PCl₅ is not suitable, because both contain –OH.)
- Warm with NaOH(aq) to hydrolyse the compound. Cool and add excess dilute nitric acid, then silver nitrate solution. A cream precipitate of silver bromide forms.
- Add iodine and sodium hydroxide solution and warm. Propan-2-ol, CH₃CH(OH)CH₃, gives a pale yellow precipitate of CHI₃. Propan-1-ol gives no precipitate.
- Fizzing shows a carboxylic acid, and the –OH of the acid also explains the PCl₅ result. C₃H₆O₂ is propanoic acid, CH₃CH₂COOH.
- Water also contains –OH and gives steamy fumes with PCl₅, which would give a false positive. Nitric acid reacts with the excess hydroxide ions, which would otherwise form a brown precipitate of silver oxide and hide the colour of the silver halide.