Revision: Purification and Characterisation of Organic Compounds JEE Main Purification and Characterisation of Organic Compounds

The change of state from solid to gas directly is called sublimation.

The change of a solid substance directly into a gas or vapour without first changing into a liquid is called sublimation.

OR

The change from solid state to vapour state without passing through the liquid state is called sublimation, and the substance is said to sublime.

Distillation : Distillation is the method of getting a pure liquid from a solution by evaporating and then condensing the vapours.

The process in which liquid is converted into its vapor phase at its boiling point and the vapor is then condensed back to liquid on cooling is known as distillation.

Stationary phase is a solid or a liquid supported on a solid which remains fixed in a place and on which different solutes are adsorbed to a different extent.

• Organic compounds obtained from natural sources or prepared in the laboratory are usually impure.
• Purification is required to obtain compounds in a pure form for the study of physical and chemical properties.
• Impurities may be solids, liquids, or gases mixed with the compound.
• Different purification methods are used based on physical properties like solubility, boiling point, and volatility.
• The choice of method depends on the nature of the compound and impurity.

• Based on difference in solubility of the compound and impurities at different temperatures.
• Compound more soluble in hot solvent → dissolved at high temperature → cooled → pure crystals form → impurities remain in solution (mother liquor).
• Common solvents: water, alcohol, ether, chloroform, acetone, benzene, petroleum ether.
• Example: Benzoic acid (slightly soluble in cold water, completely soluble in hot water) → separated from naphthalene.
• Fractional crystallisation: Used when two compounds have different solubilities in the same solvent → less soluble crystallises first on cooling (e.g., K₂CO₃ and KCl).

• Used for separation of a mixture of two or more miscible liquids with close boiling points (difference < 25°C).
• Uses a fractionating column for repeated vaporisation and condensation.
• Distillate collected in fractions at different temperatures.
• Example: Acetone (b.p. 330 K) and methyl alcohol (b.p. 338 K); benzene and toluene.
• Cannot be used to separate azeotropic mixtures (constant boiling point mixtures).

Applicable for separation/purification of organic compounds (solids or liquids) that are:

1. Insoluble in water
2. Volatile in steam
3. Have vapour pressure 10–15 mm Hg at 373 K
4. Contain non-volatile impurities

Organic compound is passed with steam → distils at a temperature below its normal boiling point.

Example: Aniline (b.p. 457 K) purified by steam distillation at 371.5 K.

Other examples: nitrobenzene, bromobenzene, o-nitrophenol, salicylaldehyde, o-hydroxyacetophenone, essential oils, turpentine oil.

• Introduced by Tswett in 1906.
• Based on the difference in rates at which different components of a mixture move through a porous medium (stationary phase) under the influence of a solvent or gas (mobile or moving phase).
• The technique separates a mixture by distributing its components between one stationary phase and one mobile phase.
• Mobile phase determines the Rᶠ or R factor — dependent on the solubility of components in the given eluent.

Based on the fact that different compounds are adsorbed to different degrees on an adsorbent.

When a mobile phase (solvent) moves over the stationary phase, the components of the mixture move by different distances depending on their adsorption tendencies.

(i) Column Chromatography

• Adsorbent (alumina Al₂O₃, silica SiO₂, Fuller's earth) is packed as a column in a long, burette-like tube → acts as the stationary phase.
• Mixture dissolved in solvent → poured on top of column.
• The most strongly adsorbed component remains at the top.
• Solvent (eluent) is passed → components elute at different rates.

(ii) Thin Layer Chromatography (TLC)

• Separation of a mixture of substances over a thin layer of adsorbent coated on a glass plate.
• A glass plate is placed in a closed jar containing eluant.
• As the eluent rises, components move to different heights depending on their degree of adsorption → separation occurs.

Based on differences in the tendencies of substances to distribute or partition between the stationary phase and mobile phase.

Paper Chromatography

• A type of partition chromatography applied as a drop on paper.
• Paper is suspended in a suitable solvent or mixture → solvent rises by capillary action.
• Paper selectively retains different components according to their partition in the two phases.
• Colourless compounds are visualised using a ninhydrin spray (which forms coloured products).

Key terms:

• Stationary phase: The phase that is stationary with respect to the components of the mixture. In column chromatography, the stationary phase adsorbs components at different heights.
• Mobile phase (eluent): The phase that moves along with the components of the mixture.

Rᶠ Factor (Retardation Factor):

\[R_f=\frac{\text{Migration distance of substance}}{\text{Migration distance of solvent front}}\]

• Used to identify the product separated using paper or thin-layer chromatography.
• Rᶠ value is constant for a given compound under given conditions.