Purifying Organic Compounds: Simple Methods

After extracting or making organic compounds, we must purify them! 🧪 Here’s how:

1. Sublimation 😌

Idea: Some solids turn directly into vapor when heated (skipping liquid phase!).
Use: Separates sublimable compounds (like camphor) from non-sublimable gunk.

2. Crystallization 💎

How it works:

  1. Dissolve impure solid in hot solvent (sparingly soluble when cold!).
  2. Cool solution → pure crystals form!
  3. Filter out crystals. Repeat if needed.
Pro tip: Use charcoal to remove colored impurities!

3. Distillation 🔥

For: Separating volatile liquids from non-volatiles OR liquids with different boiling points.

  • Simple Distillation:
    Boil mixture → collect vapors of lower-boiling liquid first (e.g., chloroform b.p. 334K vs. aniline b.p. 457K).
    Simple distillation setup
  • Fractional Distillation:
    For liquids with close boiling points. Uses a fractionating column with many “plates” to separate vapors.
    Fractional distillation
  • Reduced-Pressure Distillation:
    For compounds that decompose at their normal boiling point. Lower pressure = lower boiling point!
  • Steam Distillation:
    For steam-volatile, water-insoluble compounds (e.g., aniline).
    💡 Boils when: \( p = p_1 + p_2 \) (p1 = vapor pressure of compound, p2 = water’s vapor pressure).

4. Differential Extraction ⚗️

For: Organic compounds in water.
Steps:

  1. Shake aqueous mix with organic solvent (e.g., ether).
  2. Separate layers using a funnel.
  3. Evaporate solvent → pure compound!
Continuous extraction: Used for compounds with low solubility.

5. Chromatography 🌈

Goal: Separate mixtures, purify compounds, check purity.
Types:

  • Adsorption Chromatography:
    • Column Chromatography: Mixture poured over adsorbent (silica/alumina) column → separated by eluent flow.
      Column chromatography
    • Thin Layer Chromatography (TLC): Spot mixture on adsorbent-coated plate → place in jar with solvent → measure \( R_f \) value!
      \[ R_f = \frac{\text{Distance moved by substance (x)}}{\text{Distance moved by solvent (y)}} \]
      TLC setup
      TLC result
  • Partition Chromatography:
    • Paper Chromatography: Spot mixture on chromatography paper → solvent rises via capillary action → separates components!
      Paper chromatography

Testing Purity ✅

Pure compounds have sharp melting/boiling points. Modern methods use chromatography/spectroscopy.

Finding Elements in Organic Compounds 🔍

Carbon & Hydrogen

Heat compound with CuO → look for:

  • CO2 (turns limewater milky): \(\ce{C + 2CuO -> 2Cu + CO2}\)
  • H2O (turns anhydrous CuSO4 blue): \(\ce{2H + CuO -> Cu + H2O}\)

Nitrogen, Sulfur, Halogens, Phosphorus

Lassaigne’s Test: Fuse compound with sodium metal → test fusion extract:

  • Nitrogen: Boil extract with FeSO4 → acidify → Prussian blue color confirms N.
  • Sulfur: Add sodium nitroprusside → violet color OR lead acetate → black ppt.
  • Halogens: Acidify extract → add AgNO3 →
    White ppt. (Cl–), pale yellow (Br–), yellow (I–).

NEET Must-Knows! 🚀

  1. Distillation Types: Fractional vs. steam vs. reduced-pressure (when to use each).
  2. Chromatography: Calculate \( R_f \) values in TLC & understand separation principles.
  3. Lassaigne’s Test: Reactions for N, S, halogen detection (colors/precipitates).
  4. Crystallization: Why repeated crystallization? For impurities with similar solubility.
  5. Sublimation: Real-world examples (e.g., purifying iodine or camphor).

Keep practicing — you’ve got this! 💪