Test for functional groups

Alkane

• Test: Add bromine water to the sample. Shake.

• Positive result: no change.

Alkene

• Test: Add bromine water to the sample. Shake.

• Positive result: Alkene decolourises bromine water rapidly

Aldehyde

• Test 1: Tollen's reagent (active reagent: diamminesilver(I) complex). 

• Positive result: silver mirror is formed. 

• Test 2: Fehling's solution (active reagent: Cu(II) ions complexed to tartrate in NaOH). Warm in water bath.

• Positive result: orange-red precipitate is formed. 

Positive test for aldehyde: silver mirror with Tollen's reagent & orange-red ppt with Fehling's solution

Carbonyl group (C=O) or Ketone

• Test: Brady's reagent (active reagent: 2,4-DNP or 2,4-DNPH)

• Positive result: a yellow or red precipitate (dinitrophenylhydrazone) is formed depending on the type of ketone. Aliphatic ketone will produce a yellow precipitate while an aromatic ketone will produce a red precipitate.

Positive test for ketone; yellow/red ppt with Brady's reagent

Ethanal or ketone containing methylcarbonyl (CH3C=O) group

• Test: Triiodomethane (iodoform)

• Positive test: yellow precipitate of triiodomethane is formed

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