Tests for carbonyl compounds
Tests for carbonyl compounds
- One test detects any carbonyl.
- Two more tell an aldehyde from a ketone.
- And the iodoform test spots a particular group.
Detecting any carbonyl
- Add 2,4-DNPH reagent.
- An orange precipitate confirms the compound is an aldehyde or a ketone.
Practice
An orange precipitate with 2,4-DNPH shows the compound is:
2,4-DNPH gives an orange precipitate with any carbonyl (aldehyde or ketone).
Aldehyde vs ketone
Aldehydes are easily oxidised; ketones are not:
| Test | Aldehyde | Ketone |
|---|---|---|
| Fehling's (blue) | brick-red precipitate | no change |
| Tollens' (colourless) | silver mirror | no change |
Practice
A silver mirror with Tollens' reagent indicates:
Aldehydes are oxidised and reduce Tollens' to silver (a mirror); ketones give no change.
Practice
Fehling's solution turns from blue to a brick-red precipitate with:
An aldehyde reduces Fehling's to a brick-red precipitate; a ketone does not react.
The iodoform test
- A compound with the $\text{CH}_3\text{CO}-$ group, warmed with alkaline aqueous iodine, gives a pale-yellow precipitate of CHI₃ (tri-iodomethane).
Practice
A pale-yellow precipitate of CHI₃ in the iodoform test shows the compound has the:
The iodoform (CHI₃) test is positive for compounds containing the CH₃CO– group (and CH₃CH(OH)–).
You've got it
Key idea
- 2,4-DNPH → orange precipitate = any carbonyl (aldehyde or ketone)
- Fehling's (brick-red) and Tollens' (silver mirror) are positive for an aldehyde, not a ketone
- iodoform (yellow CHI₃) tests for the $\text{CH}_3\text{CO}-$ group