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Tests for carbonyls, acids, phenols and arenes

Edexcel IAL Chemistry · WCH16 Practical Skills in Chemistry II

A2 organic analysis adds tests for carbonyl compounds and uses chemical clues to spot acids, phenols and benzene rings. The reactions are in the Unit 4 and 5 lessons; here you practise choosing and interpreting tests.

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The A2 tests

Reagent and conditions	Positive result	Shows
2,4-DNPH (Brady's reagent)	orange (yellow-orange) precipitate	C=O in an aldehyde or ketone
Tollens' reagent, warm in a water bath	silver mirror	aldehyde
Fehling's or Benedict's solution, warm	blue solution to red (red-brown) precipitate	aliphatic aldehyde
acidified potassium dichromate(VI), warm	orange to green	aldehyde, primary or secondary alcohol
iodine in sodium hydroxide, warm	pale yellow precipitate of CHI3 (antiseptic smell)	CH3CO- group, or CH3CH(OH)-
sodium hydrogencarbonate solution	effervescence (CO2)	carboxylic acid (phenols do not react)
bromine water	decolourised and a white precipitate	phenol (2,4,6-tribromophenol)
burn a small sample	smoky (sooty) yellow flame	high carbon to hydrogen ratio, such as a benzene ring
acidified potassium manganate(VII)	purple to colourless	C=C, aldehyde or alcohol (oxidisable groups)

Technique points

Warm Tollens' reagent and other organic mixtures in a water bath, not over a flame: many organic compounds are flammable.
To identify a particular carbonyl compound, filter and recrystallise the 2,4-DNPH precipitate, measure its melting temperature and compare it with data-book values.
To show a smoky flame, put a small amount of the solid on a combustion spoon or in an evaporating basin, ignite it with a lit splint in a fume cupboard and look at the flame.

Combining results

Use each test to include or exclude groups:

2,4-DNPH positive, Tollens' negative: ketone.
Fizzes with NaHCO3 and gives a white precipitate with bromine water: contains COOH and a phenol OH.
No change with acidified manganate(VII): no C=C, no aldehyde, no primary or secondary alcohol.
Smoky flame: probably aromatic.

Worked example

Solid P (C7H6O3) fizzes with NaHCO3, gives no precipitate with 2,4-DNPH, decolourises bromine water giving a white precipitate, and burns with a very smoky flame. Deduce what P is.

NaHCO3 effervescence: COOH group.
No 2,4-DNPH precipitate: no aldehyde or ketone C=O.
Bromine water decolourised with a white precipitate: a phenol (an alkene gives no precipitate).
Smoky flame: a benzene ring.
C7H6O3 = HOC6H4COOH.

Answer: A hydroxybenzoic acid, such as 2-hydroxybenzoic acid

Key idea: 2,4-DNPH: any C=O; Tollens' or Fehling's: aldehyde only; iodoform: CH3CO-; NaHCO3: COOH; bromine water with white precipitate: phenol; smoky flame: arene. Warm in a water bath.

Check yourself

Which reagent distinguishes propanal from propanone?
- 2,4-DNPH
- Bromine water
- Tollens' reagent
- Sodium hydrogencarbonate
Show answer
Tollens' reagent. Only the aldehyde reduces Tollens' reagent; both give a 2,4-DNPH precipitate.
Which compound gives a pale yellow precipitate when warmed with iodine and sodium hydroxide?
- Propanone
- Propanal
- Pentan-3-one
- Methanal
Show answer
Propanone. Propanone has a CH3CO- group.
Which test distinguishes benzoic acid from phenol?
- 2,4-DNPH: only phenol gives a precipitate
- Burning: only phenol gives a smoky flame
- Sodium hydrogencarbonate: only the carboxylic acid fizzes
- Tollens': only benzoic acid gives a mirror
Show answer
Sodium hydrogencarbonate: only the carboxylic acid fizzes. Phenol is too weak an acid to release CO2 from hydrogencarbonate.
A compound burns with a very smoky flame. What does this suggest?
- A carboxylic acid group
- A high carbon to hydrogen ratio, such as an aromatic ring
- A high oxygen content
- An aldehyde group
Show answer
A high carbon to hydrogen ratio, such as an aromatic ring. Incomplete combustion of carbon-rich compounds gives soot.
A compound gives an orange precipitate with 2,4-DNPH but no change with Fehling's solution. What is it?
- An aldehyde
- A carboxylic acid
- An alcohol
- A ketone
Show answer
A ketone. It has C=O but is not an aldehyde.
Why is a test tube containing Tollens' reagent and the sample warmed in a water bath rather than a Bunsen flame?
- Tollens' reagent needs to be boiled
- Organic compounds are often flammable, and a water bath gives gentle, even heating
- The silver mirror only forms below 0 °C
- Flames turn the solution blue
Show answer
Organic compounds are often flammable, and a water bath gives gentle, even heating. Gentle heating, no naked flame.
Explain how the 2,4-DNPH test can be used to identify which aldehyde or ketone a sample is.
Show answer
Filter and recrystallise the orange precipitate (the derivative), measure its melting temperature and compare it with data-book values.. Each derivative has a characteristic melting temperature.

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Last updated 8 November 2026. Spotted a mistake? Tell us.