Solubility and Drying Agents

Generally speaking, organic liquids and solids are insoluble in water. This is because organic compounds are not as polar as water. To dissolve, a substance must be able to replace the favorable interactions it has with molecules like itself in the pure state with comparably favorable interactions from the solvent. If the predominant interactions are non-polar and hydrophobic for a given organic compound, the organic molecules will not be able to find complimentary interactions in water which is polar and hydrophilic. Therefore, the organic compound will prefer to stay aggregated and segregated from the water. A good rule of thumb for predicting solubility in water is to assume that any molecule having six or more carbons and 0-1 functional group will be insoluble in water. Molecules having the described structural characteristics have physical properties that reflect the dominance of the hydrophobic hydrocarbon portion of the molecule.

When we predict that an organic compound is insoluble in water, that does not mean that one hundred percent of the compound will be insoluble in water. It means that enough of it will be insoluble so that two liquid layers or a significant precipitate will exist. Like most things, solubility is relative. So really, any time an organic compound comes in contact with water, some small amount of it will dissolve in the water and likewise, some small amount of water will end up in the organic. For an organic solid, the water can usually be removed by simply allowing the solid to air dry at atmospheric or some reduced pressure. Sometimes a low temperature oven can be employed to facilitate drying. The high vapor pressures of most liquids preclude the use of any of the described methods for the removal of water.

When an organic liquid has been exposed to water, a drying agent is frequently utilized. A drying agent is typically an insoluble, inorganic salt that hydrates upon exposure to water. Molecules that make hydrates have cavities in their molecular structure that will accommodate a certain number of water molecules. For example, if sodium sulfate (Na₂SO₄) is exposed to water it will form hydrates having the general formula Na₂SO₄ ^. nH₂O. Given enough time and enough drying agent, all the water in a contaminated organic layer can be incorporated into the inorganic salt. When the water is absorbed into the inorganic compound, the salt retains its solid state and can be removed by decanting or gravity filtration. The drying agent can be regenerated by heating it to a high temperature.

Listed below are a few drying agents that are commonly used by organic chemists. Organic liquids are considered to be wet if they contain water. Realize that the organic liquid will still be a liquid after it is dried.

Magnesium Sulfate: Magnesium sulfate is a great drying agent. It has a high capacity, is complete in its drying and is rapid. Capacity refers to how much water per gram the drying agent holds and complete means that drying equilibrium favors the hydrate. The only disadvantages to using magnesium sulfate is that it is normally available in a powder form and must be filtered out. More importantly, magnesium is a very strong Lewis acid and as such, is not inert to all functional groups. For example, epoxides are sensitive to magnesium.

Sodium Sulfate: Sodium Sulfate is the most widely used drying agent. It is very similar to magnesium sulfate in its capacity, but it is less complete (will leave more water in solution) and it is slower in terms of its rate. Sodium sulfate has the advantage in that it is less reactive and in granular form, is very easy to remove from liquids. The liquid can often be decanted off the drying agent without filtration.

Calcium Sulfate: Calcium sulfate is known by the trade name drierite. Calcium sulfate has a low capacity, but it is very complete and rapid. This means that you will have to use more of it to dry a solution. Calcium sulfate can be purchased in a chunky form. This form is very convenient to work with. It is also used as a drying agent in dessicators and to dry the air entering a water sensitive reaction. The drierite can be obtained in a dyed form. The dye functions as an indicator of how much water has been absorbed. The dyed form is not suitable for use as a drying agent for liquids. Why?

Calcium Chloride: Calcium chloride is very much like calcium sulfate except that it will also absorb methanol and ethanol.

Potassium Carbonate: Potassium carbonate is basic and as such, is generally used in basic media. It is of average capacity, completeness and rate.

Some Practical Advice

At this point it is important to reiterate that when you add a drying agent to a solution, it will just sit on the bottom of the container because it is an insoluble solid. The problem with drying agents is there is a tendency to add to much. So how much is enough? It is best to start conservatively. Add a spatula tip and then swirl. After the drying agent has settled, observe the solution. If the solution is transparent that is a good sign that you are close to the end point of addition. If the solution is cloudy, you probably need to add more in small increments. Once the solution begins to appear to be clear, study the drying agent itself. Does it look all clumpy or is it freely flowing? In other words, does it look the same as when you first added it? Drying agents clump and stick to the bottom of the vessel as they pick up water, so if the agent looks really clumpy and sticky, you need to add more. Continue to add drying agent until the newly added material appears unchanged and free flowing. Recognize the originally clumped material will never unclump.

Why be so cautious? Why not just dump a whole mess of drying agent in? I can assure you that it has been done before and hey, if some is good, more is better. This is definitely not the part of the procedure where you want to adopt a hedonistic philosophy. If you add excessive drying agent, you will lose a lot of your desired liquid upon isolation. Think about it, a finite amount of liquid will adhere to the surface of the drying agent. If you use copious quantities of drying agent, you will lose large amounts of the desired compound.