Purification & Characterisation of Organic Compounds
Syllabus Unit: Organic Chemistry
Why it matters
Chapter overview
Before an organic compound's structure can be determined, it must first be purified and its composition established. This topic covers the classical purification techniques (crystallisation, distillation, sublimation, chromatography), Lassaigne's qualitative tests for N/S/halogens, quantitative estimation methods that yield empirical and molecular formulas, and the modern spectroscopic techniques (IR, NMR, mass spectrometry) that have replaced slow classical degradation for structure elucidation.
1Methods of Purification
A newly synthesised organic compound is rarely pure — crystallisation, distillation, sublimation and chromatography each separate compounds by a different physical principle, chosen to match the impurity being removed.
Learning Objectives
- Explain crystallisation and describe how to choose an appropriate solvent for it.
- Distinguish simple, fractional, steam and vacuum distillation, and state when each is used.
- Explain sublimation as a purification method for solids that vaporise directly (naphthalene, camphor, iodine).
- Explain the principle of chromatography and distinguish column, TLC and paper chromatography.
Key Terms
A freshly synthesised organic compound is typically a mixture of the product, unreacted starting material, byproducts and solvent. Purification separates them by exploiting a difference in a physical property.
Crystallisation
The impure solid is dissolved in the minimum amount of hot solvent (chosen so the compound is sparingly soluble cold but freely soluble hot), filtered hot to remove insoluble impurities, and cooled slowly so the pure compound crystallises out selectively — the process can be repeated (recrystallisation) for higher purity.
Distillation types
- Simple distillation — separates a volatile liquid from a non-volatile or high-boiling solute.
- Fractional distillation — separates miscible liquids of close boiling points using a fractionating column.
- Steam distillation — co-distils a steam-volatile, water-immiscible compound (e.g. essential oils, aniline) with steam, so it boils well below 100°C.
- Vacuum distillation — lowers the boiling point by reducing pressure, for compounds that decompose near their normal boiling point.
Sublimation and chromatography
Solids that pass directly from solid to vapour (naphthalene, camphor, anthracene, iodine, NH4Cl) are purified by sublimation — heating to vaporise, then re-condensing, leaving non-subliming impurities behind. Chromatography separates components by differential adsorption or partition as a mobile phase moves past a stationary phase; in TLC, a spot's position is quantified by its Rf value.