boiling point separation

Methods of Purification of Organic Compounds

Sublimation

What is Sublimation?

Sublimation is the process in which a solid directly changes into vapour without becoming liquid.

Example:

  • Ammonium chloride
  • Iodine
  • Camphor
  • Naphthalene

Why does it happen?

Some solids have:

  • Weak intermolecular forces
  • High vapour pressure even at lower temperatures

So, when heated, they directly escape into vapour form.

Sublimation as a Separation Technique

It is used to purify substances:

✔ Sublimable substance → turns into vapour
✔ Non-sublimable impurities → remain as solid

Then vapour is cooled → forms pure solid again.

Simple Setup

  1. Heat the mixture
  2. Sublimable substance vaporizes
  3. Vapour condenses on a cold surface
  4. Pure solid is collected

Example Use

Separating:

  • Ammonium chloride + sand

Ammonium chloride sublimates, sand stays behind.

Crystallisation

What is Crystallisation?

A purification technique used to purify solid organic compounds based on difference in solubility.

Principle

  • Substance is less soluble at low temperature
  • Substance is more soluble at high temperature

So:

  • Dissolve at high temp
  • Cool → pure crystals form

Steps

  1. Dissolve impure solid in hot solvent
  2. Make a saturated solution
  3. Filter (if needed, remove insoluble impurities)
  4. Cool the solution
  5. Pure crystals separate out
  6. Filter and dry crystals

Important Terms

  • Mother liquor → remaining liquid after crystallisation
  • Activated charcoal → removes colored impurities

🔹 Examples

Example 1: Benzoic Acid purification

  • Impure benzoic acid dissolved in hot water
  • On cooling → pure crystals of benzoic acid form

When repeated crystallisation is needed?

When

Simple Distillation (large boiling point difference)

When used:

  • Difference in b.p. > 25–30°C
  • Or liquid + non-volatile impurity

Examples:

Example 1:

  • Water + salt
    Water distils, salt remains

Example 2:

  • Chloroform (334 K) + Aniline (457 K)
    Easily separated

Example 3:

  • Alcohol + sugar solution
    Alcohol vaporises, sugar stays

Example 4:

  • Acetone + water
    Acetone (low b.p.) comes first

Fractional Distillation (close boiling points)

When used:

  • Difference in b.p. < 25°C

Examples:

Example 1:

  • Ethanol (78°C) + Water (100°C)
    Cannot separate by simple distillation

Example 2:

  • Benzene (80°C) + Toluene (110°C)
    Need fractionating column

Example 3:

  • Hexane + Heptane
    Very close boiling points

Example 4 (Industrial):

  • Crude oil refining
    Petrol, diesel, kerosene separated

3. Steam Distillation (heat-sensitive substances)

When used:

  • Substance:
    • Immiscible with water
    • Volatile with steam
    • Decomposes at high temperature

Examples:

Example 1:

  • Extraction of essential oils
    From plants (e.g., clove oil, eucalyptus oil)

Example 2:

  • Aniline purification
    High b.p. but steam volatile

Example 3:

  • Nitrobenzene
    Separated using steam

Example 4:

  • Rose oil extraction (attar)
    Used in perfumes

Quick Comparison (Very Important)

MethodConditionExample
Simple DistillationLarge b.p. differenceWater + salt
Fractional DistillationSmall b.p. differenceEthanol + water
Steam DistillationHeat-sensitive, steam volatileEssential oils

Distillation Under Reduced Pressure (Vacuum Distillation)

Statement:

Distillation under reduced pressure → Glycerol + spent-lye

Why this method is used?

Some liquids:

  • Have very high boiling points
  • Decompose before boiling

Example: Glycerol

Principle

When pressure is reduced:
➡ Boiling point decreases

So, substance can distil at lower temperature without decomposition

Application in given case

Mixture:

  • Glycerol (high b.p., heat sensitive)
  • Spent-lye (impurities)

What happens?

  • Pressure ↓
  • Glycerol boils at lower temp
  • Vapours collected → pure glycerol
  • Impurities remain

Why not simple distillation?

❌ Glycerol decomposes at high temperature
✔ So vacuum distillation is required

🔹 More Examples

Example 1:

  • Glycerol purification (most common)

Example 2:

  • High boiling oils
  • Example 3:
  • Fatty acids

Example 4:

  • Petroleum residues

Quick Summary

MethodUsed for
Simple distillationLow b.p. liquids
Fractional distillationClose b.p. liquids
Steam distillationHeat-sensitive, steam volatile
Reduced pressure distillationHigh b.p., decomposing liquids

Differential Extraction

When an organic compound is present in an aqueous medium, it is separated by shaking it with an organic solvent in which it is more soluble than in water. The organic solvent and aqueous solution should be immiscible so that they form two distinct layers which can be separated using a separating funnel. The organic solvent is later removed by distillation or evaporation to obtain the compound.

Concept: Differential (Liquid–Liquid) Extraction

Principle

Based on difference in solubility of a compound in two immiscible liquids

Usually:

  • Water (aqueous layer)
  • Organic solvent (like ether, benzene)

Key Conditions

✔ Two liquids must be immiscible
✔ Compound should be more soluble in organic solvent

Process (Separating Funnel)

  1. Take mixture in separating funnel
  2. Add organic solvent
  3. Shake well
  4. Allow layers to separate

Two layers form:

  • Upper layer (usually organic solvent)
  • Lower layer (water)
  1. Separate layers
  2. Evaporate solvent → get pure compound

Example

Example 1:

  • Iodine in water + carbon tetrachloride (CCl₄)
    → Iodine moves to organic layer

Example 2:

  • Benzoic acid from water using ether

Continuous Extraction

When used?

  • When compound is slightly soluble in solvent

Why?

  • Single extraction inefficient
  • Repeated extraction increases yield

Same solvent reused again and again

Advantages

  • Efficient separation
  • Better recovery of compound
  • Widely used in organic chemistry labs

Chromatography

What is Chromatography?

A separation technique where components of a mixture are separated based on their different affinities between two phases.

Principle

Based on distribution of components between two phases:

  1. Stationary phase → fixed (solid or liquid)
  2. Mobile phase → moving (liquid or gas)

Different substances move at different speeds → separation occurs

Types of Chromatography

1. Paper Chromatography

  • Stationary phase → paper
  • Mobile phase → solvent

Used for:

  • Ink separation
  • Plant pigments

2. Thin Layer Chromatography (TLC)

  • Stationary phase → silica gel layer
  • Faster and more accurate than paper chromatography

3. Column Chromatography

  • Column filled with adsorbent (silica/alumina)
  • Used for larger scale separation

4. Gas Chromatography (GC)

  • Mobile phase → gas
  • Used for volatile compounds

How Separation Happens?

  • Component with more attraction to stationary phase → moves slowly
  • Component with more attraction to mobile phase → moves faster

Example

Ink Separation

Black ink → separates into different colors on paper

Plant Pigments

Chlorophyll, carotene separated using chromatography

Important Term: Rf Value

Rf=Distance travelled by substanceDistance travelled by solventRf = \frac{\text{Distance travelled by substance}}{\text{Distance travelled by solvent}}

Uses

✔ Separation of mixtures
✔ Purification
✔ Checking purity
✔ Drug testing
✔ Food analysis