Question

Discuss how a distillation works; also include the difference between a simple and a fractional distillation. Why is fractional distillation used in preparation of cyclohexene?

Answer 1

Distillation works by exploiting the different boiling temperatures of liquids. To separate two or more liquids by distillation, you first heat them in a flask. The more volatile liquid (the liquid with the lower boiling point) will typically evaporate first and the vapor will pass into a condensing column, where it can revert into a liquid (condense) on the cool glass where it trickles into a collection flask. Heating further will cause the less volatile liquids to evaporate and distill at higher temperatures. The two main kinds of distillation are simple distillation and fractional distillation, and both are used widely.

What is simple distillation?

The setup for a simple distillation is shown in Figure 1. A simple distillation apparatus consists of a boiling flask (round-bottom flask) attached to an adapter holding a thermometer (to determine the boiling temperature of the liquid). The adapter connects to a condenser into which cold water is constantly passed through. The condenser leads into a collection flask for the purified liquid.

What is fractional distillation?

Fractional distillation is essentially the same as simple distillation except that a fractionating column is placed between the boiling flask and the condenser. The fractionating column is usually filled with glass or plastic beads. These beads improve the separation between the liquids being distilled. The reason that fractional distillation gives better separation between the liquids is because the glass beads in the fractionating column provide "theoretical plates" on which the refluxing liquid can condense, re-evaporate, and condense again, essentially distilling the compound over and over. The more volatile liquids will tend to push towards the top of the fractionating column, while lower boiling liquids will stay towards the bottom, giving a better separation between the liquids. Of course, the more theoretical plates that you add to a column (the more surfaces or beads), the longer the distillation will take (typically), and the more energy required to keep reevaporating liquid in the fractionating column (this is more of a concern in industrial distillations than in an academic lab where energy cost is not a major cause for worry).

So, simple or fractional?

The choice of whether to use fractional distillation or simple distillation depends on the two liquids being separated. Typically, using simple distillation is preferrable because the apparatus is, well, simpler, and a simple distillation typically goes faster than a fractional distillation (and requires less energy). On the other hand, fractional distillation gives better separation between the liquids. The choice of whether to use simple or fractional distillation, then, depends usually on the difference in boiling temperatures between the two liquids. If there is a large difference in the boiling points (>70^oC)between the two liquids then simple distillation is probably the best option. On the other hand, if there is only a small temperature difference between the two liquids a fractional distillation is the preferrable option.

	Simple distillation	Fractional distillation
Advantages	simpler setup than fractional faster distillation times consumes less energy than fractional distillation	much better separation between liquids than simple distillation can more readily purify complex mixtures than simple distillation
Disadvantages	requires the liquids to have large boiling point differences (>70oC) gives poorer separation than fractional distillation only works well with relatively pure liquids	more complicated setup than simple distillation takes longer for liquids to distill consumes more energy than simple distillation
Best used for:	separating relatively pure liquids with large boiling differences or liquids with solid impurities	separating complex mixtures of liquids with smaller boiling point separations.