Functional Group Identification

HKDSE Tip

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  Bromine test (addition):

  Always specify the condition. Bromine in organic solvent (CH₂Cl₂ or CCl₄) decolourises rapidly in the dark for alkenes (C=C).
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  2,4-DNP:

  Identifies carbonyl groups only (aldehyde or ketone), but cannot distinguish between them.
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  Tollens' test:

  A redox reaction — the aldehyde is oxidised while Ag⁺ is reduced to form the silver mirror.
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  Acidified K₂Cr₂O₇:

  Can distinguish 1°/2° alcohols from 3° (3° shows no reaction).
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  Bicarbonate test:

  Use sodium hydrogencarbonate ($NaHC{O}_3$) to uniquely identify carboxylic acids — effervescence of $C{O}_2$ gas that turns limewater milky.