Tests for carbonyl compounds
Introduced| English | Meaning |
|---|---|
| iodoform/ˈaɪədɒfɔːm/ | iodoform |
| precipitate/prɪˈsɪpɪteɪt/ | An insoluble solid formed when substances in solution react. |
| Fehling/ˈfeɪlɪŋ/ | Fehling |
| Tollens/ˈtəʊlənz/ | Tollens |
| silver mirror/ˈsɪlvə ˈmɪrə/ | silver mirror |
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.
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
2,4-DNPH gives an orange precipitate with any carbonyl (aldehyde or ketone).
Match each carbonyl test to its result.
Difficulty 2/5 · 140 coins + a small bonus on first completion today
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:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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:
Difficulty 1/5 · 100 coins + a small bonus on first completion today
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