Chemical Tests for Functional Groups

Chemical tests (also called wet tests or bench tests) allow you to identify functional groups in unknown organic compounds using simple laboratory reagents. While spectroscopic methods are more powerful, chemical tests remain important for quick identification and are frequently tested at A-Level. This lesson covers the key tests, reagents, observations, and equations you need to know.

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Test for Alkenes (C=C Double Bond)

Reagent: Bromine water (Br₂(aq), orange/yellow).

Method: Add bromine water to the sample and shake.

Positive result: Bromine water is decolourised (orange to colourless).

Equation (e.g., ethene): CH₂=CH₂ + Br₂ → CH₂BrCHBr (1,2-dibromoethane)

Mechanism: Electrophilic addition.

Exam Tip: Benzene does NOT decolourise bromine water (despite having C=C in the Kekulé model) because the delocalised π system is too stable. Only alkenes react. This is a key distinction.

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Tests for Alcohols

Classification: Primary, Secondary, or Tertiary

Reagent: Acidified potassium dichromate(VI), K₂Cr₂O₇/H₂SO₄ (orange).

Alcohol Type	Observation	Product
Primary (R–CH₂OH)	Orange → green (on warming)	Aldehyde (then carboxylic acid if excess oxidant)
Secondary (R₂CHOH)	Orange → green (on warming)	Ketone
Tertiary (R₃COH)	No colour change (stays orange)	No reaction (resistant to oxidation)

The colour change from orange to green is due to the reduction of Cr₂O₇²⁻ (orange, Cr⁶⁺) to Cr³⁺ (green).

Exam Tip: To distinguish a primary alcohol from a secondary alcohol using oxidation alone, you need to test the product. A primary alcohol gives an aldehyde (which gives a positive result with Tollens’ or Fehling’s), while a secondary alcohol gives a ketone (which does not).

Test for the –OH Group (General)

Reagent: Phosphorus pentachloride (PCl₅).

Observation: Steamy/misty fumes of HCl gas are produced.

Equation: ROH + PCl₅ → RCl + POCl₃ + HCl

This test works for all alcohols (including tertiary) and also for carboxylic acids.

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Test for Aldehydes

Tollens’ Reagent (Silver Mirror Test)

Reagent: Tollens’ reagent — ammoniacal silver nitrate solution, [Ag(NH₃)₂]⁺(aq).

Preparation: Add a few drops of dilute NaOH to AgNO₃(aq) to form a brown precipitate of Ag₂O, then add dilute ammonia solution dropwise until the precipitate just dissolves, forming the colourless diamminesilver(I) complex.

Method: Add Tollens’ reagent to the sample in a clean glass test tube and warm gently in a hot water bath.

Positive result (aldehyde): A silver mirror forms on the inside of the test tube (or a black/grey precipitate of silver if the tube is not perfectly clean).

Negative result (ketone): No silver mirror. Solution remains colourless.

Equation: RCHO + 2[Ag(NH₃)₂]⁺ + 2OH⁻ → RCOO⁻ + 2Ag(s) + 4NH₃ + H₂O

The aldehyde is oxidised to a carboxylate ion; Ag⁺ is reduced to Ag.

Fehling’s Solution (or Benedict’s Solution)

Reagent: Fehling’s solution — an alkaline solution of copper(II) complexed with tartrate ions (blue).

Method: Add Fehling’s solution to the sample and heat.

Positive result (aldehyde): Blue solution produces a brick-red precipitate of Cu₂O.

Negative result (ketone): Solution remains blue.

Equation: RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O(s) + 3H₂O

The aldehyde is oxidised; Cu²⁺ (blue) is reduced to Cu₂O (brick-red).

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Test for Carbonyl Compounds (Aldehydes AND Ketones)

Brady’s Reagent (2,4-Dinitrophenylhydrazine, 2,4-DNPH)

Reagent: 2,4-dinitrophenylhydrazine dissolved in methanol and sulfuric acid.

Method: Add Brady’s reagent to the sample.

Positive result: An orange/yellow precipitate forms (a 2,4-dinitrophenylhydrazone).

This test is positive for both aldehydes and ketones (any compound with a C=O that is an aldehyde or ketone, not esters or carboxylic acids).

Use: To confirm the presence of a carbonyl group. The precipitate can be purified by recrystallisation and its melting point measured. Comparison with known melting points of 2,4-DNPH derivatives can identify the specific aldehyde or ketone.

Exam Tip: 2,4-DNPH tells you a C=O is present (aldehyde or ketone) but does NOT distinguish between them. To distinguish, follow up with Tollens’ or Fehling’s test (positive for aldehyde only).

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Test for Carboxylic Acids

Sodium Carbonate or Sodium Hydrogencarbonate

Reagent: Na₂CO₃(aq) or NaHCO₃(aq).

Method: Add the sample to the carbonate/hydrogencarbonate solution.

Positive result: Effervescence (bubbles of CO₂ gas produced). The gas turns limewater milky.