Aldehyde vs Ketone Tests — Chemistry Mnemonic
Target: mnemonic for aldehyde ketone tests Tollens Fehling chemistry
Why is this hard to memorize?
Aldehydes and ketones are both carbonyl compounds (C=O) but differ in reactivity: aldehydes are more easily oxidized because they have a C-H bond adjacent to C=O. The key distinguishing tests are: Tollens test (silver mirror — positive for aldehydes), Fehling test (red precipitate — positive for aldehydes), Schiff test (pink colour — positive for aldehydes), and 2,4-DNP test (orange precipitate — positive for BOTH, confirms C=O). NEET and JEE test which reagent distinguishes what, and which specific aldehydes/ketones give exceptions.
Classic mnemonics you should know
"Tollens = Silver mirror. RCHO + 2[Ag(NH₃)₂]⁺ → RCOO⁻ + 2Ag↓(mirror) + 3NH₃"
Tollens reagent (ammoniacal silver nitrate, AgNO₃ + NH₃) is reduced by aldehydes to metallic silver, which deposits as a mirror on the test tube wall. Ketones give NO reaction. "Aldehydes give silver, ketones don't."
"Fehling = Red. RCHO + Cu²⁺(blue) → RCOO⁻ + Cu₂O↓(red)"
Fehling solution (Cu²⁺ in alkaline tartrate) is reduced by aldehydes to Cu₂O (brick-red precipitate). Blue solution → red precipitate = aldehyde present. Exception: aromatic aldehydes (PhCHO) don't reduce Fehling (steric + electronic reasons).
"2,4-DNP gives orange/yellow precipitate with ALL carbonyl compounds (aldehydes AND ketones). It confirms C=O exists but does NOT distinguish them."
2,4-dinitrophenylhydrazine reacts with C=O to form a hydrazone (orange/yellow precipitate). This is the screening test: positive = carbonyl present. Then use Tollens/Fehling to determine if it's aldehyde or ketone.
The complete list
- Tollens test: silver mirror (aldehydes only)
- Fehling test: red Cu₂O precipitate (aldehydes only)
- Schiff test: pink colour (aldehydes only)
- 2,4-DNP: orange precipitate (all C=O compounds)
- Benedict test: similar to Fehling (Cu²⁺ → Cu₂O)
- Iodoform test: CH₃CO- or CH₃CHOH- groups
- PhCHO does NOT reduce Fehling (exception)
- HCHO reduces Tollens but also Fehling
Frequently asked questions
What is the difference between Tollens test and Fehling test?
Both detect aldehydes by oxidizing them, but use different oxidizing agents: Tollens uses Ag⁺ (reduced to Ag metal → silver mirror). Fehling uses Cu²⁺ (reduced to Cu₂O → red precipitate). Key difference: Fehling fails with aromatic aldehydes (benzaldehyde gives no reaction), but Tollens works with all aldehydes. Use Tollens for a definitive test.
What is the iodoform test?
Treatment with I₂/NaOH: if the compound has a CH₃CO- group (methyl ketone like acetone) or CH₃CHOH- group (secondary alcohol like ethanol), it gives CHI₃ (yellow precipitate with antiseptic smell). Acetaldehyde (CH₃CHO) also gives positive iodoform test. This test detects a specific structural feature, not just aldehyde vs ketone.
Why does benzaldehyde not reduce Fehling solution?
Fehling solution works in strongly alkaline medium. Benzaldehyde undergoes Cannizzaro reaction in alkali (disproportionation to benzoic acid + benzyl alcohol) instead of being oxidized by Cu²⁺. Also, the aromatic ring makes the aldehyde group less reactive toward oxidation. Tollens test (mildly basic) still works with benzaldehyde because it doesn't trigger Cannizzaro.
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