Tests for carbonyl compounds
| English | Chinese | Pinyin |
|---|---|---|
| iodoform | 碘仿 | diǎn fǎng |
| precipitate | 沉淀 | chén diàn |
| Fehling | 斐林 | fěi lín |
| Tollens | 托伦斯 | tuō lún sī |
| silver mirror | 银镜 | yín jìng |
Telling carbonyls apart
- One test detects any carbonyl.
- Two more tell an aldehyde from a ketone.
- And the iodoform 碘仿 test spots a particular group.
Carbonyl test lab
Match observations to aldehydes and ketones.
Tollens' reagent gives a silver ______ with an aldehyde but not a ketone.
Aldehydes are oxidised; ketones are not.
Detecting any carbonyl
- Add 2,4-DNPH reagent.
- An orange precipitate 沉淀 confirms the compound is an aldehyde or a ketone.
Fehling's 斐林 and Tollens 托伦斯' tests tell an aldehyde from a ketone
An orange precipitate with 2,4-DNPH shows the compound is:
2,4-DNPH gives an orange precipitate with any carbonyl (aldehyde or ketone).
Match each carbonyl test to its result.
Each item links the term to its correct meaning.
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 银镜 | no change |
Nucleophilic addition of HCN to a carbonyl group
A silver mirror with Tollens' reagent indicates:
Aldehydes are oxidised and reduce Tollens' to silver (a mirror); ketones give no change.
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).
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
- 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