Tests for aldehydes and ketones: 2,4-DNPH, Tollens' reagent, Fehling's solution, the iodoform test and acidified dichromate, what each one shows, the equations, which test tells which pair apart, and a one-page summary table (PDF)

"Test for aldehydes and ketones" is a question that comes in two forms. In CBSE Class 12 it is "give a chemical test to distinguish between propanal and propanone", two marks, and the examiner wants the reagent, the observation and which compound gives it. In A-level it is a six-mark practical question that also wants the equation. This page gives every test once, with the observation, the equation and what a positive result proves, then the pairs examiners ask about, and a one-page summary table to print.

Download the summary table (PDF) (one page, every test with reagent, observation and what gives it; the common pairs on the back).

The order to think in

There are two questions and they are asked in order.

1. Is there a carbonyl group at all? 2,4-DNPH answers this. Aldehydes and ketones both give a yellow or orange precipitate; acids, esters and amides do not.
2. Aldehyde or ketone? An oxidising test answers this: Tollens', Fehling's or acidified dichromate. Aldehydes are oxidised; ketones are not.

A third question sometimes follows: is there a CH₃CO– group? The iodoform test answers that, for aldehydes, ketones and alcohols alike.

The summary table

2,4-DNPH: carbonyl or not

A few drops of the unknown are added to Brady's reagent. A yellow, orange or red-orange crystalline precipitate, a 2,4-dinitrophenylhydrazone, means a C=O group in an aldehyde or ketone. It is a condensation (addition-elimination) reaction; water is lost. The test does not tell the two apart. The mechanism, the equation and how the melting point of the purified derivative identifies the exact compound are in 2,4-DNPH and Brady's reagent.

Tollens' reagent: the silver mirror

Making it. Add a few drops of sodium hydroxide to silver nitrate solution: a brown precipitate of silver oxide forms. Add dilute ammonia drop by drop until the precipitate just dissolves. The colourless solution contains the diamminesilver(I) ion, [Ag(NH₃)₂]⁺. It must be made fresh and never stored.

The test. Add the unknown and warm in a water bath, not over a flame. An aldehyde reduces Ag⁺ to silver metal, which coats a clean tube as a mirror (in a less clean tube, a grey or black precipitate). The aldehyde is oxidised to the carboxylate ion.

RCHO + 2[Ag(NH₃)₂]⁺ + 3OH^– → RCOO^– + 2Ag + 4NH₃ + 2H₂O

Ketones give no change. All aldehydes give it, including benzaldehyde.

Fehling's solution: the brick-red precipitate

The two solutions. Fehling's A is aqueous copper(II) sulfate, blue. Fehling's B is sodium potassium tartrate (Rochelle salt) in sodium hydroxide, colourless. Equal volumes are mixed just before use to give a deep blue solution; the tartrate keeps Cu²⁺ in solution in the alkali.

The test. Warm with the unknown. An aliphatic aldehyde reduces Cu²⁺ to copper(I) oxide, a brick-red precipitate.

RCHO + 2Cu²⁺ + 5OH^– → RCOO^– + Cu₂O + 3H₂O

Ketones leave the solution blue. Aromatic aldehydes such as benzaldehyde also give no result, which is the fact CBSE questions are built on. Benedict's solution is the same test with citrate in place of tartrate.

Acidified potassium dichromate

Warm with potassium dichromate(VI) in dilute sulfuric acid. An aldehyde is oxidised to a carboxylic acid and the orange Cr₂O₇^2– is reduced to green Cr³⁺. A ketone leaves it orange.

RCHO + [O] → RCOOH

It is the least specific of the three, because primary and secondary alcohols also turn it green. Use it to tell an aldehyde from a ketone only when you already know the compound is a carbonyl.

The iodoform test

Warm with iodine in sodium hydroxide solution. A pale yellow precipitate of triiodomethane (iodoform), CHI₃, with an antiseptic smell, shows a CH₃CO– group attached to a carbon or a hydrogen atom.

RCOCH₃ + 3I₂ + 4NaOH → RCOONa + CHI₃ + 3NaI + 3H₂O

The alcohols are positive because the reagent first oxidises them to the corresponding CH₃CO– compound.

Which test tells which pair apart

The pattern for a two-mark answer is always the same three parts: reagent and conditions; what you see; which compound gives it and which does not. "Add Tollens' reagent and warm. Propanal gives a silver mirror; propanone gives no change." Twenty words, full marks.

Which board wants which

The mistakes that lose the marks

1. Writing "silver precipitate" for Tollens'. The observation is a silver mirror (or a grey-black precipitate if the tube is not clean).
2. Saying benzaldehyde gives Fehling's. It gives Tollens' but not Fehling's.
3. Using the iodoform test to tell ethanal from propanone. Both have CH₃CO–; both are positive. Use Tollens'.
4. Forgetting that ethanol gives iodoform. It is the standard test for ethanol against methanol.
5. "Orange to green" with no reagent named. The mark is for acidified potassium dichromate and the colour change together.
6. Heating Tollens' or Fehling's over a flame. Warm in a water bath; the organic compounds are flammable, and Tollens' must be freshly made and rinsed away after use.

Where to get help

A 1-on-1 online chemistry class with a tutor who holds a master's in chemistry and teaches CBSE Class 11 and 12, A-level, IB and AP organic chemistry: every test taught with the reason it works, the distinguishing pairs drilled as two-mark answers, and past-paper questions marked to the board's mark scheme. The chemistry page lists the boards; the free 30-minute first class is a real lesson on this week's topic. If the equations above are the hard part, balancing chemical equations step by step is the place to start.

Questions parents ask

See how we teach this, 1-on-1 online →