Distillation is the primary method for purifying liquid organic compounds. It exploits differences in vapor pressure between components to achieve separation.
Simple Distillation
Simple distillation is used when the boiling point difference between components is >50 °C. The liquid is heated in a round-bottom flask, the vapor passes through a condenser, and the condensed liquid (distillate) is collected in a receiving flask.
The glassware setup (from bottom to top): heating mantle or oil bath → round-bottom flask (containing the liquid) → distillation head with thermometer → condenser (water-cooled) → vacuum adapter → receiving flask.
The thermometer bulb should be positioned at the branch of the distillation head to measure the vapor temperature accurately. Add boiling chips or a stir bar to prevent bumping. Heat gently — too fast reduces separation efficiency.
Fractional Distillation
For mixtures with boiling points closer than 50 °C, a fractionating column is inserted between the flask and the distillation head. The column provides multiple theoretical plates through repeated condensation and re-vaporization.
The column can be packed with glass beads, Raschig rings, or a Vigreux column (a glass tube with internal indentations). The more plates, the better the separation — a 30 cm Vigreux column provides approximately 3–5 theoretical plates.
The temperature rise during distillation should be slow. Collect the first fraction when the thermometer stabilizes, then switch to a fresh receiver as the temperature rises to the next plateau. Fractions with overlapping temperature ranges may still contain mixtures.
Steam Distillation
Steam distillation separates compounds that are immiscible with water and volatile with steam. The combined vapor pressure of water + the compound equals atmospheric pressure at a temperature below the boiling point of either pure component. This allows distillation of high-boiling organics without thermal decomposition.
Steam is generated in a separate flask and passed through the sample flask. The steam carries the organic compound vapor, and both condense in the condenser. The distillate forms two layers (water + organic) that are separated in a separatory funnel.
Vacuum Distillation
For compounds that decompose at their atmospheric boiling point, vacuum distillation lowers the boiling point by reducing the pressure. A vacuum pump or water aspirator creates the reduced pressure, and a manometer measures the vacuum.
The boiling point decreases approximately logarithmically with pressure. As a rule of thumb, reducing the pressure to 15–20 mmHg lowers the boiling point by 100–150 °C. Use a vacuum adapter with a take-off that allows the collection of multiple fractions without breaking the vacuum.
Safety: use only round-bottom flasks rated for vacuum. Never use Erlenmeyer or flat-bottom flasks. Wear a blast shield. Add boiling chips before applying heat.
Rotary Evaporation
The rotary evaporator (rotovap) is an essential tool for removing solvents after a reaction. A motor rotates the flask (typically 50–200 rpm) in a heated water bath while the system is under vacuum. The rotation creates a thin film of solvent on the flask wall, increasing the surface area and accelerating evaporation. The evaporated solvent is condensed in an ice- or dry-ice-cooled condenser and collected in a receiving flask.
