Identification of Functional Groups by Test-Tube Reactions

Test for Alkenes (Bromine Water Test)

Reagent: Bromine water (Br₂(aq))

Observation:
Alkene present: Orange to colourless
No alkene: No change

Reaction:
CH₂=CH₂ + Br₂ → CH₂BrCH₂Br

Test for Alcohols

Reagent: Acidified potassium dichromate (K₂Cr₂O₇/H₂SO₄)

Primary and Secondary Alcohols:
Observation: Orange → Green (oxidation occurs)

Tertiary Alcohols:
No visible change (not oxidised).

Test for Aldehydes (Fehling’s and Tollens’ Tests)

Fehling’s Solution Test:

Reagent: Fehling’s solution (Cu²⁺ ions)

Observation:
Aldehyde: Forms a brick-red precipitate (Cu₂O).
Ketone: No change.

Tollens’ Silver Mirror Test:

Reagent: Tollens’ reagent ([Ag(NH₃)₂]⁺)

Observation:
Aldehyde: Forms a silver mirror (Ag⁰).
Ketone: No change.

Test for Carboxylic Acids

Reagent: Sodium carbonate (Na₂CO₃) or Sodium hydrogen carbonate (NaHCO₃)

Observation:
Carboxylic acid present: Effervescence (CO₂ gas produced)
No acid: No visible change.

Reaction:
CH₃COOH + Na₂CO₃ → CH₃COONa + CO₂ + H₂O

Summary Table: Functional Group Tests