Experiment: tests for aldehydes and ketones
Beyond NCERT In the JEE Main and NEET syllabus; the Class 12 book has no section for it.
Quick answer
The 2,4-DNP test gives a yellow, orange or red precipitate with aldehydes and ketones. Schiff’s and mild oxidation tests help distinguish ordinary aldehydes from simple ketones, but Tollens’, Fehling’s and Benedict’s also respond to other reducing compounds; aromatic aldehydes normally fail Fehling’s test. Use fresh reagents and controls, and promptly deactivate Tollens’ residues under the laboratory’s procedure.
How do you test for aldehydes and ketones?
Tests for aldehydes and ketones
The 2,4-dinitrophenylhydrazine (2,4-DNP) test detects aldehydes and ketones by a yellow, orange or orange-red precipitate. Schiff’s and mild oxidation tests help distinguish ordinary aldehydes from simple ketones: aldehydes give a pink colour with Schiff’s reagent, and they are oxidised easily by the mild oxidising agents in Fehling’s, Benedict’s and Tollens’ reagents, which leave simple ketones unchanged (NCERT Laboratory Manual, Unit 8, pages 92–95, which says Schiff’s, Fehling’s and Tollens’ tests “are given by aldehydes only”, page 94). A positive metal-ion result is not unique to aldehydes, though: NCERT calls all monosaccharides, “whether aldose or ketose”, reducing sugars, which reduce Fehling’s solution and Tollens’ reagent (Unit 10, page 282), so a ketose such as fructose (Unit 10, page 286), an α-hydroxy ketone, responds too (reasoned). The 2,4-DNP test rests on this NCERT line:
2,4-DNP-derivatives are yellow, orange or red solids, useful for characterisation of aldehydes and ketones.
NCERT Chemistry Part II (Class 12), Section 8.4, page 238
The NCERT Class 12 textbook explains the chemistry behind the 2,4-DNP test (NCERT Section 8.4, page 238) and Tollens’ and Fehling’s tests (page 239), but it does not describe Schiff’s or Benedict’s test or the method. The method below follows the NCERT Laboratory Manual (Chemistry, Class 12), Unit 8, Experiment 8.1, part IV (pages 92–95).
In plain words: 2,4-DNP first tests for an aldehyde or ketone; further tests narrow the possibilities. A simple ketone such as propanone gives 2,4-DNP but not the other responses. A positive oxidation test means a reducing substance is present; confirm the functional group with the full pattern of results.
The 2,4-DNP test for the carbonyl group
- Theory. Derivatives of ammonia add to the C=O group, and the addition is followed by elimination of water, giving an unsaturated compound. Reaction rate depends on pH. For hydrazone formation, acid catalysis helps, while excessive protonation reduces the nucleophilicity of the nitrogen reagent: use the prepared reagent at its specified optimum pH (Lab Manual, page 92). With 2,4-dinitrophenylhydrazine, aldehydes and ketones give their 2,4-dinitrophenylhydrazones (page 92), which are yellow, orange or red solids (NCERT page 238). In full, with the water that is eliminated (reasoned; the Lab Manual’s scheme on page 92 leaves it out):
2,4-DNP test
R2C=O + H2N–NH–C6H3(NO2)2 → R2C=N–NH–C6H3(NO2)2 + H2O
- Procedure. Take 2–3 drops of a liquid compound, or dissolve a few crystals of a solid in 2–3 mL of alcohol, and add a few drops of an alcoholic solution of 2,4-dinitrophenylhydrazine. A yellow, orange or orange-red precipitate “confirms the presence of carbonyl group” (Lab Manual, page 94), that is, of an aldehyde or ketone: carboxylic acids, esters and amides also contain C=O but normally do not respond (reasoned from NCERT page 245, where resonance makes the carboxylic carbon less electrophilic than a carbonyl carbon). If none appears at room temperature, warm the mixture in a water bath for a few minutes and cool (Lab Manual, page 94).
Schiff’s, Tollens’ and Fehling’s tests for aldehydes
Ordinary aldehydes are easily oxidised, while simple ketones require stronger conditions; mild oxidising reagents therefore help distinguish them (Lab Manual, page 92; NCERT page 239). Schiff’s test is different: it is a colour reaction of aldehydes with a decolourised dye, not an oxidation (Lab Manual, page 93; reasoned).
Schiff’s test
- Theory. Schiff’s reagent is an aqueous solution of the dye p-rosaniline hydrochloride, decolourised with sodium sulphite or SO2 gas. Aldehydes give a pink colour with it; ketones do not (Lab Manual, page 93).
- Procedure. Take 3–4 drops of the liquid compound, or a few crystals dissolved in alcohol, and add 2–3 drops of Schiff’s reagent. Appearance of a pink colour “indicates the presence of an aldehyde” (Lab Manual, page 94). Our advice: read the colour promptly, under the specified test conditions.
Tollens’ test
- Theory. Tollens’ reagent is an alkaline solution of silver ions complexed with ammonia. An aldehyde reduces the silver ions to silver metal, which coats the glass as a mirror (Lab Manual, page 93; NCERT page 239). The Lab Manual writes the product as an ammonium salt. The following textbook ionic form better represents the alkaline medium, with carboxylate and ammonia:
Tollens’ test
RCHO + 2[Ag(NH3)2]+ + 3OH− → RCOO− + 2Ag + 2H2O + 4NH3
- Procedure. (i) To 1 mL of freshly prepared, about 2% silver nitrate solution add 1–2 drops of NaOH solution and shake: dark brown silver oxide precipitates. Dissolve it by adding ammonium hydroxide solution drop by drop. (ii) Add an aqueous or alcoholic solution of the compound. (iii) Heat in a water bath for about 5 minutes. A layer of silver that shines like a mirror “indicates the presence of an aldehyde” (Lab Manual, page 95). Strictly, a mirror or a dark deposit of metallic silver shows a reducing group (reasoned; see the caution above).
- NCERT slip (Laboratory Manual, pages 93–95): the Lab Manual spells the reagent “Tollen’s”. It is named after Bernhard Tollens (NCERT textbook, page 239), so write Tollens’ reagent and Tollens’ test.
Fehling’s test
- Theory. Fehling’s solution A is aqueous copper sulphate; solution B is an alkaline solution of sodium potassium tartrate (Rochelle salt). They are mixed in equal amounts just before use, because the mixture deteriorates on keeping, while A and B are stable apart. The tartrate complexes the Cu2+ ions, keeping their concentration below that at which copper(II) hydroxide would precipitate (Lab Manual, page 93). An aldehyde reduces Cu2+ to red copper(I) oxide:
Fehling’s test
RCHO + 2Cu2+ + 5OH− → RCOO− + Cu2O + 3H2O
- Procedure. To about 1 mL each of Fehling’s solutions A and B add 2–3 drops of the liquid compound, or about 2 mL of a solution of the solid in water or alcohol, and heat in a water bath for about 2 minutes. A brick red precipitate of copper(I) oxide “indicates the presence of an aldehyde” (Lab Manual, page 94; the NCERT textbook, page 239, calls it reddish brown); strictly, of a reducing group (reasoned).
- Aromatic aldehydes do not give Fehling’s test (Lab Manual, pages 93–94; NCERT page 239).
- NCERT slip (Laboratory Manual, page 93; NCERT textbook, page 239): both books print “tartarate”. The salt of tartaric acid is a tartrate, as in sodium potassium tartrate, so write “tartrate” in an exam.
Benedict’s test
- Theory. Benedict’s solution is a single alkaline solution of copper sulphate and sodium citrate, more stable than Fehling’s reagent; Cu2+ is reduced to Cu+ in the same way (Lab Manual, page 93).
- Procedure. Add 5 drops of the liquid compound, or of a solution of the solid in water or alcohol, to 2 mL of Benedict’s reagent and keep the tube in a boiling water bath for 5 minutes. An orange-red precipitate “indicates the presence of an aldehyde” (Lab Manual, page 95); strictly, of a reducing group, reducing sugars included (reasoned).
Results at a glance
| Test | Positive result | Shows |
|---|---|---|
| 2,4-DNP | yellow, orange or orange-red precipitate | carbonyl group (Lab Manual): aldehyde or ketone |
| Schiff’s | pink colour | aldehyde |
| Fehling’s | brick red precipitate | aliphatic aldehyde (Lab Manual); strictly, reducing group |
| Benedict’s | orange-red precipitate | aldehyde (Lab Manual); strictly, reducing group, sugars included |
| Tollens’ | silver mirror | aldehyde (Lab Manual); strictly, reducing group |
Precautions
- Always use freshly prepared reagents (Lab Manual, page 95).
- Do not heat the reaction mixture directly on a flame; use a water bath (Lab Manual, page 95).
- After the test, the Lab Manual destroys the silver mirror by adding dilute nitric acid and drains off the solution with excess of water (page 95). Beyond the Lab Manual: aged Tollens’ reagent can form highly explosive silver nitride, so prepare it immediately before use, never store it or let residues dry, and destroy leftovers with dilute acid (Wikipedia, “Tollens’ reagent”); collect silver waste as your laboratory directs.
- Also beyond the Lab Manual: dry DNPH is friction- and shock-sensitive and is supplied wet for that reason (Wikipedia, “2,4-Dinitrophenylhydrazine”), so use the supplied wet DNPH or the prepared reagent, and never dry or scrape the solid.
- Use a reagent blank and a known positive control. Poor solubility, contaminated glassware (including acetone residue) or deteriorated reagent can mislead; a negative test is only meaningful when the controls work.
- Handle silver nitrate and ammonia solution with care: the Lab Manual marks both as corrosive, and ammonia gas as flammable and toxic (page 94).
Solved examples
Example 1: Propanal or propanone?
Two colourless liquids both give an orange precipitate with 2,4-DNP. One gives a silver mirror with Tollens’ reagent. Which is propanal and which propanone?
Answer: Both have a carbonyl group, hence the 2,4-DNP result. The one that gives the silver mirror is the aldehyde, propanal, CH3CH2CHO; propanone, CH3COCH3, a ketone, does not reduce Tollens’ reagent (reasoned from Lab Manual, pages 93–95; compare NCERT Exercise 8.13 (i), page 256).
Example 2: Copper(I) oxide from an aldehyde
At most, what mass of copper(I) oxide can 0.58 g of propanal give with excess Fehling’s reagent? (C = 12, H = 1, O = 16, Cu = 63.5.)
Answer: M(CH3CH2CHO) = 36 + 6 + 16 = 58 g mol−1, so 0.58 g is 0.010 mol. By the equation, 1 mol of aldehyde gives 1 mol of Cu2O, M = 127 + 16 = 143 g mol−1: 0.010 × 143 = 1.43 g (reasoned from the Lab Manual’s equation, page 93).
Common mistakes to avoid
Common mistake
Using the 2,4-DNP test to tell an aldehyde from a ketone
Both give a 2,4-dinitrophenylhydrazone. Use Schiff’s, Fehling’s or Tollens’ test, which the Lab Manual says only aldehydes give (page 94), with their limitations: benzaldehyde fails Fehling’s, and reducing sugars or some α-hydroxy ketones can reduce the metal-ion reagents.
Common mistake
Expecting benzaldehyde to give Fehling’s test
Aromatic aldehydes do not give Fehling’s test. Benzaldehyde does give Tollens’ test.
Quick revision summary
- 2,4-DNP: yellow, orange or orange-red precipitate with aldehydes and ketones (a 2,4-dinitrophenylhydrazone).
- Schiff’s gives pink with ordinary aldehydes. In the Lab Manual, Fehling’s, Benedict’s and Tollens’ indicate an aldehyde; strictly they are reducing-group tests, not aldehyde-only proofs, and aromatic aldehydes normally fail Fehling’s.
- Fehling’s solutions A (CuSO4) and B (alkaline Rochelle salt) are mixed just before use; Tollens’ reagent is made fresh from AgNO3, NaOH and ammonia.
- Use a water bath, controls and fresh reagents. Destroy the silver mirror with dilute HNO3 (Lab Manual), or follow the laboratory’s silver-waste procedure; keep DNPH reagent wet.
Formula sheet
- RC=O + HN–NH–CH(NO) → RC=N–NH–CH(NO) + HO
- RCHO + 2[Ag(NH)] + 3OH → RCOO + 2Ag + 2HO + 4NH
- RCHO + 2Cu + 5OH → RCOO + CuO + 3HO
Questions students ask
What is the difference between Tollens’ test and Fehling’s test?
Tollens’ and Fehling’s tests both detect aldehydes by oxidising them to the carboxylate ion, but Tollens’ reagent holds silver ions complexed with ammonia and shows a positive result as a silver mirror on the glass, while Fehling’s holds copper ions complexed with tartrate and shows it as a brick red precipitate (Lab Manual, pages 93–95). Both are made up just before use, Tollens’ from freshly prepared silver nitrate solution and Fehling’s by mixing equal amounts of solutions A and B (Lab Manual, pages 93 and 95). Both are warmed in a water bath, about 5 minutes for Tollens’ test and about 2 minutes for Fehling’s (Lab Manual, pages 94–95).
What is Brady’s test, and is it the same as the 2,4-DNP test?
Yes. Brady’s reagent is an acidified alcoholic solution of 2,4-dinitrophenylhydrazine. Aldehydes and ketones form hydrazone precipitates; carboxylic acids, esters and amides normally do not under these conditions. Colour may vary with conjugation but does not uniquely identify a compound. In the general equation, each R can be a carbon group and one or both may be H, allowing aldehydes as well as ketones.
Why must Tollens’ reagent be freshly prepared?
Ammoniacal silver reagent can form explosive silver-containing residues if left standing or allowed to dry. Prepare it just before use and arrange prompt instructor-supervised deactivation and waste handling; do not store leftover mixed reagent. This is why both the NCERT manual and university demonstration guidance insist on fresh reagent and immediate cleanup. A deposit of silver metal may look dark rather than forming a perfect mirror.
Do any ketones give Tollens’ or Fehling’s test?
Simple ketones such as propanone give neither Tollens’ nor Fehling’s test (Lab Manual, page 93; NCERT page 239), but α-hydroxy ketones, with an –OH on the carbon next to C=O, give both, a point beyond the NCERT text: they are reducing compounds; in suitable structures, alkaline enediol rearrangement can produce an aldehyde that is then oxidised. Other α-hydroxy ketones can oxidise to diketones, so aldehyde tautomerisation is not the universal explanation (Wikipedia, “Tollens’ reagent” and “Fehling’s solution”). Hydroxyacetone, CH3COCH2OH, is one such ketone (reasoned example). So a positive result shows an easily oxidised group, usually –CHO, rather than proving an aldehyde (reasoned).
Where are these tests taught?
NCERT Class 12 Chemistry Part II, Unit 8, explains 2,4-DNP derivatives on page 238 and Tollens’ and Fehling’s tests on page 239. The Laboratory Manual, Unit 8, Experiment 8.1, pages 92–95, provides the methods. Consult the official syllabus for your examination year for current practical coverage.
Practice questions
Try each question first, then open the answer to check your working.
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Q1. Why are Fehling’s solutions A and B kept separately and mixed only before the test?
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The mixed reagent deteriorates on keeping, while solutions A and B are quite stable on their own (Lab Manual, page 93).
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Q2. A compound gives an orange precipitate with 2,4-DNP but no colour with Schiff’s reagent and no mirror with Tollens’ reagent. What does it contain?
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A ketonic carbonyl group: 2,4-DNP shows a carbonyl group, and the negative Schiff’s and Tollens’ tests rule out an aldehyde (reasoned from Lab Manual, pages 93–95). This assumes fresh reagents, adequate sample solubility and successful positive controls.