How to Purify by Distillation

About

Distillation is a process of separating the component substances from a liquid mixture by selective evaporation and condensation. It is one of the most common laboratory techniques used by chemists for the purification and identification of organic liquids. Because different compounds often have different boiling points, the components will separate from a mixture when the mixture is distilled. Three different methods can be employed for purifying organic compounds. The optimal method to use will depend on the properties of the mixture:

A simple distillation at atmospheric pressure is used when the organic liquid is:

• Low-boiling ( <150^oC in general). Heating organic compounds above this temperature often leads to decomposition and can be problematic.
• Relatively pure (containing no more than 10% liquid contaminants).
• Has a non-volatile component, for example, a solid contaminant such as a polymer.
• Is contaminated by a liquid with a boiling point that differs by at least 70 ^oC.

A fractional distillation at atmospheric pressure is used when separating mixtures of liquids with boiling points separated by less than 70 ^oC.

A reduced pressure distillation under vacuum is used when:

• The boiling point of the compound (or solvent) is too high (> 150 ^oC under atmospheric pressure) to distill the compound (or the solvent) without significant decomposition.
• The compound decomposes upon heating at atmospheric pressure.

General Notes

• Use small glassware, for two reasons.
  1. It minimizes the volume occupied by vapor, increasing efficiency
  2. It maximizes recovery, since less compound will be lost to the walls of the vessel and distillation head.
• Have extra receiving flasks on hand (better to collect more fractions if you are unsure!)
• Polar compounds typically have higher boiling points than nonpolar ones.
• Be wary of azeotropes. Solvents that form azeotropes do not always have similar polarities (e.g. cyclohexane and water!). See this handy PDF for some common azeotropes to get an idea of how diverse these can be. Although difficult to predict, azeotropes are usually characterized by broader, depressed boiling points. Analysis of fractions will reveal whether or not this is occurring. Sometimes, by intentionally adding an additional solvent, one can disrupt the azeotrope.
• Fluorination nearly always lowers the boiling point of structurally related compounds.
• In most cases, distillation of amides/ureas, carboxylic acids, and polyaromatics (which tend to sublime) should be avoided. Distillation is not always possible, and other methods work far better.
• Nearly all of the compounds you will make yourself will have a boiling point greater than 150 ^oC, or will decompose upon heating. For these purifications, the vacuum distillation method will be required.

Rules of Thumb

• Unless distilling large quantities of materials (>250 g), it is best to use 14/20 type glassware.
• The thermal bath should be set 20-30 ^oC higher than the boiling point of the material being distilled.
• Hydrocarbons, aldehydes, ethers, thiols, and silanes are the easiest to distill (although if wet with any non-negligible quantity of water, these tend to become nightmares due to foaming)
• Alcohols typically require higher temperatures for distillation (30-50 oC above the boiling point), due to high heats of vaporization.
• When the boiling point of the compound (or solvent) is > 150 oC under atmospheric pressure, carry out a reduced pressure (vacuum) distillation.
• The typical vacuum distillation is conducted at 0.1 mmHg. For best results, carry out your distillation at a pressure where your compound boils between 45^oC and 180^oC.
• Temperatures exceeding 180^oC distillations can be performed, but this then becomes an issue of compound stability and safe heating practices (certain oil baths cannot be used at temperatures exceeding 200^oC without risk of fire!).

See also:

• How to Purify by Simple Distillation at Atmospheric Pressure
• How to Purify by Fractional Distillation at Atmospheric Pressure
• How to Purify by Distillation at Reduced Pressures (Vacuum Distillation)