MCAT Organic Chemistry Review
Separations and Purifications
Extraction requires two solvents that are immiscible in order to separate the product. But what happens when the product itself is a liquid that is soluble in the solvent? This is where distillation comes in handy. Distillation takes advantage of differences in boiling point to separate two liquids by evaporation and condensation. The liquid with the lower boiling point will vaporize first, and the vapors will rise up the distillation column to condense in a water-cooled condenser. This condensate then drips down into a vessel. The end product is called the distillate. The heating temperature is kept low so that the liquid with the higher boiling point will not be able to boil and therefore will remain liquid in the initial container. This is the process that is used to make liquor at adistillery. Because ethanol boils at a lower temperature than water, we can use distillation to make beverages with high ethanol contents.
Simple distillation, as the name indicates, is the least complex version of distillation. It proceeds precisely as described above. This technique should only be used to separate liquids that boil below 150°C and have at least a 25°C difference in boiling points. These restrictions prevent the temperature from becoming so high that the compounds degrade and provide a large enough difference in boiling points that the second compound won”t accidentally boil off into the distillate. The apparatus for this technique consists of a distilling flask containing the combined liquid solution, a distillation column consisting of a thermometer and a condenser, and a receiving flask to collect the distillate. The setup is the same as that shown in Figure 12.3, except that there will be no vacuum adaptor.
Figure 12.3. Vacuum Distillation The initial solution is placed in the heated distilling flask, where the components of the solution with the lowest boiling points will vaporize first. The vapor then condenses in the water-cooled condenser, and this distillate drips into the receiving flask.
We use vacuum distillation whenever we want to distill a liquid with a boiling point over 150°C. By using a vacuum, we lower the pressure, thereby decreasing the temperature that the liquid must reach in order to boil. This allows us to distill compounds with higher boiling points at lower temperatures so that we do not have to worry about degrading the product. The apparatus for vacuum distillation is shown in Figure 12.3.
Remember from Chapter 7 of MCAT General Chemistry Review that liquids boil when their vapor pressure equals ambient pressure. In vacuum distillation, we lower the ambient pressure so that the liquid can boil at lower temperatures.
To separate two liquids with similar boiling points (less than 25°C apart), we use fractional distillation. In this technique, a fractionation column connects the distillation flask to the condenser, as shown in Figure 12.4. A fractionation columnis a column in which the surface area is increased by the inclusion of inert objects like glass beads or steel wool. As the vapor rises up the column, it condenses on these surfaces and refluxes back down until rising heat causes it to evaporate again, only to condense again higher in the column. Each time the condensate evaporates, the vapor consists of a higher proportion of the compound with the lower boiling point. By the time the top of the column is reached, only the desired product drips down to the receiving flask.
Figure 12.4. Fractional Distillation With increased surface area in the distillation column, the distillate has more places to condense on its way up the column. This allows for more refined separation of liquids with fairly close boiling points.
MCAT Concept Check 12.2:
Before you move on, assess your understanding of the material with these questions.
1. Distillation separates compounds based on what property?
2. If we are given a solution of ether, with a boiling point of 308 K, and methylene chloride, with a boiling point of 313 K, which type of distillation should be used to separate them?
3. If we are given a solution of bromobenzene, with a boiling point of 156°C, and camphor, with a boiling point of 204°C, which type of distillation should be used to separate them?