3.9. ORGANIC SYNTHESIS

What are the key tests to identify organic functional groups?

• Alkanes: No specific test.

• Alkenes:

  • Test: Decolorizes bromine water (orange → colorless).

• Halogenoalkanes:

  • Test: Form silver halide precipitate with AgNO₃ after acidification (AgCl = white, AgBr = cream, AgI = yellow).

• Alcohols (Primary, Secondary, Tertiary):

  • Test: Primary oxidized to aldehydes, secondary oxidized to ketones, tertiary cannot be oxidized.

• Aldehydes:

  • Test: Positive for Tollens’ (silver mirror) and Fehling’s solution (red precipitate).

• Ketones:

  • Test: Positive with 2,4-DNPH (orange precipitate).

• Carboxylic Acids:

  • Test: Reacts with carbonates to produce CO₂ gas (limewater turns cloudy).

• Esters:

  • Test: No specific test; identified by sweet fruity smell.

• Amines:

  • Test: Turns red litmus blue; turns universal indicator blue/purple.

• Nitriles:

  • Test: No official test.

What are the key oxidising and reducing agents in organic synthesis?

• Oxidising Agents:

  1. K₂Cr₂O₇/H₂SO₄:

     • Oxidizes primary alcohols → Aldehydes → Carboxylic acids (orange → green).

     • Secondary alcohols → Ketones (orange → green).

     • Tertiary alcohols → No oxidation.

  2. KMnO₄/H₂SO₄:

     • Oxidizes primary alcohols → Aldehydes → Carboxylic acids (purple → colorless).

     • Secondary alcohols → Ketones (purple → colorless).

     • Tertiary alcohols → No oxidation.

• Reducing Agents:

  1. NaBH₄:

     • Reduces aldehydes → Primary alcohols.

     • Ketones → Secondary alcohols.

     • Does not reduce carboxylic acids or alkenes.

  2. LiAlH₄:

     • Reduces carboxylic acids → Alcohols.

     • Aldehydes → Primary alcohols.

     • Ketones → Secondary alcohols.

What are the tests used to distinguish functional groups?

• Fehling’s Solution: Aldehyde → Red precipitate.

• Tollens’ Reagent: Aldehyde → Silver mirror.

• Bromine Water: Alkene → Orange → Colorless.

• 2,4-DNPH: Carbonyl compounds (aldehydes/ketones) → Orange precipitate.

• Iodoform Test: Methyl ketones (CH₃CO–) → Yellow precipitate.

• Silver Nitrate: Halogens → AgCl (white), AgBr (cream), AgI (yellow).

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