Purification of organic compounds

Organic compounds from nature or from a reaction are seldom pure. The method of purifying them depends on how the compound differs from its impurities:

• Sublimation: a solid that turns straight into vapour on heating (such as camphor or naphthalene) is separated from one that does not.
• Crystallisation: the impure solid is dissolved in a hot solvent in which it is less soluble when cold; on cooling, pure crystals form, and the impurities stay in solution. Fractional crystallisation separates two solutes with different solubilities, by repeated crystallisation.

Distillation separates liquids by their boiling points:

• Simple distillation: for liquids whose boiling points are very different, such as chloroform (334 K) and aniline (457 K).
• Fractional distillation: for liquids whose boiling points are close; a fractionating column gives many repeated evaporations and condensations. Crude oil is separated this way.
• Distillation under reduced pressure: for liquids that decompose at or below their normal boiling point. Lowering the pressure lowers the boiling point.
• Steam distillation: for liquids that are volatile in steam and do not mix with water, such as aniline. Steam is passed through the mixture, which boils when the two vapour pressures together equal atmospheric pressure, so it boils below 373 K and the compound is not damaged.

Differential extraction: a compound dissolved in water is shaken with an immiscible solvent (one that does not mix with water) in which it is more soluble. The compound moves into that solvent, which is then separated off and evaporated.

Chromatography separates the parts of a mixture by how strongly each is held by a stationary phase while a moving phase carries them along. Adsorption chromatography includes column and thin layer (TLC); partition chromatography includes paper chromatography. Each substance has its own retardation factor:

Rf = distance moved by the substance ÷ distance moved by the solvent.

The purity of the final compound is checked by its sharp melting point (for a solid) or boiling point (for a liquid).