How to Purify by Fractional Distillation at Atmospheric Pressure

How it works: A fractional distillation is used when separating mixtures of liquids whose boiling points are similar (separated by less than 70 ^oC). In a fractional distillation, a mixture of liquids is boiled and the resulting vapors travel up a glass tube called a “fractionating column” and separate. The fractionating column is placed between the flask containing the mixture and the “Y” adaptor and improves the separation between the liquids being distilled. Fractional distillation leads to a better separation than simple distillation because the glass beads in the fractionating column provide "theoretical plates" on which the vapors can condense and then re-evaporate, and re-condense, essentially distilling the compound many times over. One theoretical plate is equivalent to one vaporization-condensation cycle, which is equivalent to one simple distillation. The more volatile liquids will gradually move towards the top of the fractionating column, while higher boiling liquids will stay towards the bottom, giving a better separation between the liquids. The vapor eventually reaches the condenser, where it is cooled and then drips in to the collection vessel. The column used is typically a Vigreux-type (such as Chemglass’ CG-1231 series) and the optimal length is a function of the proximity of the boiling point of the desired material to the impurity. Keep in mind, the longer the Vigreux, the lower the recovery.

A procedure to carry out a fractional distillation is provided below.

1. To a round bottom flask equipped with a stir bar, add the organic mixture to be purified.
2. Set up a fractional distillation apparatus as shown below. In some cases these units can be purchases as an “all-in-one” piece such as Chemglass’ CG-1246.

   

   Things to Note:

   • Position of the thermometer should be at the intersection of the “Y” piece.
   • The flask and collection flask should both be secured with clamps to the back of the fume hood.
   • Cold water should travel from bottom to top of the condenser.
   • It is usually a good idea to keep the system under nitrogen to avoid decomposition or reactions with oxygen (especially aldehydes) since the material will be heated.
   • Keck clamps should be placed on joints to avoid pieces from disconnecting.
   • If you are distilling a very low-boiling compound (around 25 ^oC), you can place the receiving flask in an ice bath.

3. Use a heating mantle/block or oil bath to heat the organic mixture to the appropriate temperature.
   • You should start by setting the temperature to distill the lowest-boiling compound first, to aid in separation.
   • Note the ring of condensate rising slowly up the fractionating column.
   • The rise should be gradual, which ensures proper separation of the component mixture across the “theoretical plates”
   • If the ring of condensate stops rising, increase the temperature slightly.

4. Distillation will commence once the flask reaches the desired temperature and the vapor begins to condenses in the liquid-cooled jacketed part of the distillation piece.
   • After the first few drops of liquid start to fall down the condenser, the temperature should remain nearly constant.
   • To obtain the optimal distillation, the distillation rate must be constant and relatively slow.
   • This is the temperature that should be recorded as the boiling point of the compound.
   • If the distillation is occurring very slowly, wrap the column with aluminum foil or glass wool to protect it from cold drafts.
   • Once the lower-boiling material is finished being distilled, you should observe the temperature of the distillate drop significantly (ideally back to around 25 oC).

5. At this time, you can switch to a new, clean receiving flask and increase the temperature of the heating element to distill the next lowest-boiling compound.
   • After the first few drops of liquid start to fall down the condenser, the temperature should remain nearly constant.
   • This is the temperature that should be recorded as the boiling point of the compound.
   • If the distillation is occurring very slowly, wrap the column with aluminum foil or glass wool to protect it from cold drafts.
   • Once this material is finished being distilled, you should observe the temperature of the distillate drop significantly again.

6. Repeat this process until you obtain your desired compound.
7. Once the distillation is complete, transfer the pure organic compound in to a clean, labeled vial for safekeeping.