The process of synthesizing and isolating an organic compound often results in an organic compound or solution contaminated with traces of water. For instance, in aqueous extractions some water will be transferred into the organic phase because of the partial miscibility of the organic phase and water. Also, many reactions themselves are performed in an aqueous solution. This water must be removed before the required compound can be properly characterized.
In the Organic Chemistry Teaching labs at CU, the following two methods of drying solutions are commonly used:
These two methods are described below.
The bulk of the water can often be removed by shaking or "washing" the organic layer with saturated aqueous sodium chloride. The salt water works to pull the water from the organic layer to the water layer. This is because the concentrated salt solution wants to become more dilute and because salts have a stronger attraction to water than to organic solvents.
Note: sometimes a saturated aqueous solution of sodium chloride is called brine.
Procedure
Place the organic solution in a separatory funnel. The organic solvent might be methylene chloride, diethyl ether, hexanes, etc., as long as it is not, of course, water.
Add an amount of saturated aqueous sodium chloride, less than or equal to the amount of organic solution you have.
Stopper the funnel and shake as in an extraction.
Allow the layers to separate. The rules as to which layer is on top are the same as for extraction. Since there is a lot of salt dissolved in the water, the density of the saturated aqueous sodium chloride solution is 1.2 g/mL.
Drain off the lower layer. In this case, this is the organic layer and the layer you want to save. Dispose of the aqueous layer in the aqueous waste carboy. (In some cases, for instance if you are using diethyl ether, the upper layer will be the desired organic layer.)
Final traces of water are removed by treating the organic solution with a drying agent. A drying agent is an inorganic salt which readily takes up water to become hydrated. Several such salts are used routinely in the organic chemistry teaching labs:
drying agent capacity speed applications calcium chloride, CaCl2 high medium used for hydrocarbons calcium sulfate, CaSO4 (Drierite) low fast generally useful magnesium sulfate, MgSO4 high fast not used for very acid-sensitive compounds potassium carbonate, K2CO3 medium medium not for acidic compounds sodium sulfate, Na2SO4 high slow generally useful
Of the five drying agents in the above table, magnesium sulfate is a fine powder and the rest are of a larger particle size. Calcium chloride, magnesium sulfate, and sodium sulfate are the three most commonly used agents. Click on the picture below to see what the drying agents look like.
Pictures of these drying agents:
Note: you MUST cap bottles of drying agents between uses! Here is what a drying agent looks like if it is left exposed to the air.
When the Lab Manual states: “Dry over . . . ,” it refers to the process below. Usually, you will perform a wash with saturated sodium chloride solution to remove the bulk of the water before treating with an inorganic salt.
Procedure
Add a small amount of the solid drying agent directly to the organic solution. Swirl the solution. Observe the drying agent; if it is all clumped together, add more. Click on the picture link at the right to see how a solution with drying agent looks when it is clumped and when it is free-flowing. Continue swirling and observing the solution for 5-15 minutes, adding more drying agent only until a fresh addition no longer forms clumps
Each drying agent will have a slightly different appearance when “clumped” and practice will make you better at judging whether or not the inorganic salt is wet or dry. There is no set “rule” as to how much drying agent needs to be added. The amount required depends on the amount of water in the solvent solution which you are drying, and this amount varies from experiment to experiment. Use as much as it takes to dry the solution. In most cases, drying is as complete as it will get in 20 minutes. When drying is complete, you need to remove the dried organic solution from the drying agent. There are several methods by which you can do this:
You can filter the solution by gravity filtration. If the powder is quite fine (as when using magnesium sulfate) or if the volume is large, gravity filtration is the method of choice. If the drying agent is of larger particle size (as when using sodium sulfate or calcium chloride, decanting is the method of choice. An alternative to decanting is remove the liquid from the drying agent simply by drawing it off with a Pasteur pipet. Squeeze the bulb of the pipet, carefully place it flush with the bottom of the flask, and slowly draw liquid into the pipet, leaving the desiccant behind. Squirt the liquid into a fresh, clean flask.
Quiz: What is WRONG with this picture?
 |
 |
