The carbonyl group
Both aldehydes and ketones contain the carbonyl group, C=O. The difference is where it sits. In an aldehyde the carbonyl is at the end of the chain, so the carbon also carries a hydrogen (–CHO). In a ketone it is in the middle, bonded to two carbon groups (R–CO–R). That single positional difference drives all their contrasting chemistry.
The C=O bond is polar (oxygen δ−, carbon δ+), so the carbonyl carbon is attacked by nucleophiles — nucleophilic addition is their characteristic reaction.
Oxidation splits them apart
The key distinction: aldehydes are easily oxidised to carboxylic acids (that terminal C–H is available to be removed), while ketones resist oxidation (no such hydrogen). This is exactly why a primary alcohol oxidises all the way to an acid but a secondary alcohol stops at a ketone.
That difference gives two classic tests to tell an aldehyde from a ketone.
The distinguishing tests
Both are mild oxidising agents that only the aldehyde reduces:
| Test | Aldehyde | Ketone |
|---|---|---|
| Tollens' reagent (ammoniacal silver nitrate) | silver mirror forms | no change |
| Fehling's / Benedict's solution | blue → brick-red precipitate | stays blue |
| Acidified dichromate | orange → green (oxidised) | no change |
Reduction and uses
Both can be reduced (the reverse of the alcohol oxidation): an aldehyde back to a primary alcohol, a ketone to a secondary alcohol. In use, methanal (formaldehyde) makes resins and preservatives, and propanone (acetone) is a common solvent (nail-polish remover).
One test, clear answer
Tollens' reagent gives a silver mirror ONLY with an aldehyde. It is the quick, memorable way to distinguish an aldehyde from a ketone — the aldehyde reduces the silver ions to silver metal.
A worked example
You have propanal (an aldehyde) and propanone (a ketone), both colourless liquids. How do you tell them apart with Tollens' reagent?
- Add Tollens' reagent (ammoniacal silver nitrate) to each and warm gently.Tollens' is a mild oxidising agent that only aldehydes reduce.
- Propanal (aldehyde) is oxidised and reduces the silver ions, depositing a silver mirror on the tube.Aldehydes are easily oxidised; the Ag⁺ is reduced to Ag metal.
- Propanone (ketone) does not react — no mirror.Ketones resist oxidation, so they cannot reduce the silver.
The one that forms a silver mirror is the aldehyde (propanal); the one that does nothing is the ketone (propanone).
Nucleophilic addition
The carbonyl group C=O is polar — the carbon is δ+ and the oxygen δ− — so its characteristic reaction is nucleophilic addition: a nucleophile attacks the carbon while the double bond opens onto the oxygen. Adding hydrogen cyanide (HCN), for example, gives a hydroxynitrile, lengthening the chain by one carbon.
Carbonyls are also readily reduced (e.g. with sodium borohydride, NaBH₄): an aldehyde becomes a primary alcohol and a ketone a secondary alcohol — exactly reversing the oxidation that made them.
Identifying carbonyls
A carbonyl compound is first confirmed with 2,4-dinitrophenylhydrazine (2,4-DNPH, Brady's reagent), which gives an orange precipitate with any aldehyde or ketone. Then you tell the two apart by oxidation: only an aldehyde reduces Tollens' reagent (a silver mirror) or Fehling's solution (a brick-red precipitate); a ketone does neither.
The iodoform (triiodomethane) test adds one more clue: a yellow precipitate with warm alkaline iodine indicates a methyl ketone (or ethanal), a group with the CH₃CO– arrangement.
| Test | Aldehyde | Ketone |
|---|---|---|
| 2,4-DNPH | orange precipitate | orange precipitate |
| Tollens' reagent | silver mirror | no change |
| Fehling's solution | brick-red precipitate | no change |
| Iodoform (if methyl carbonyl) | yellow ppt (ethanal only) | yellow ppt (methyl ketones) |