Before providing students with the problem set, I spend about twenty minutes introducing extraction. This includes a discussion of how extractions are carried out experimentally (most students have probably encountered a separation funnel, although lately I have often had a few first-year students in the course). I also introduce distribution and partition coefficients and talk about how extraction is used for bulk separations of chemicals with similar properties.
1. Devise a way to separate the materials in the following sample by performing an extraction.
The sample consists of water with a complex mixture of trace levels of organic compounds. The compounds can be grouped into broad categories of organic acids, organic bases, and neutral organics. The desire is to have three solutions at the end, each in methylene chloride, one of which contains only the organic acids, the second contains only the organic bases, and the third contains only the neutrals.
Students are also given the following hint to aid in thinking about a solution to this problem.
Remember - Ions are more soluble in water than in organic solvents.
- Neutrals are more soluble in organic solvents than in water.
I point out how the separation of acids, bases and neutrals is a common bulk separation scheme that is often used in areas like the analysis of environmental samples. Groups are then allowed to think about the problem. Often the students think they can just add methylene chloride and extract the neutrals without extracting anything else. Groups often realize that the presence of acids and bases in the same sample then means that neutralization has likely occurred to some degree. At this point I prompt them to write down answers to the following questions.
What do organic acids and bases look like?
After a few minutes the students can identify carboxylic acid groups as acids and amine groups as bases. I then ask them to think about the nature of these chemicals as a function of pH, and more specifically at extremes of pH.
What would these groups look like at a pH of 1? What about at a pH of 14?
It should take the students just a few minutes to correctly draw each of the four cases. They should realize that the key point is that at a low pH amines are protonated and carry a positive charge while at a high pH carboxylic acids are deprotonated and carry a negative charge. At this point I ask them
Can you now devise a scheme for separating these organic molecules?
Give the students five minutes to come up with the scheme. I ask individual groups as I’m circulating through the class what acid and base they would use to adjust the pH, and they immediately respond with hydrochloric acid and sodium hydroxide. Once each group has an acceptable scheme I spent a few minutes going through a scheme at the board to show where each class of molecules is at each step. I also point out that the usual way this is done in practice involves a separation of the acidic compounds from the base/neutral components so only involves two solutions instead of three.
I then ask the students the following question.
Suppose you had metal ions in water. Can you think of a way to extract them into the organic phase?
After completing the equilibrium unit, students talking with their group usually recognize quickly that complexing the metal with a ligand to make a neutral complex will move the metal into the organic layer. I then talk a little about how we can selectively complex metals by varying the pH, so that in some cases it is possible to adjust the pH of the aqueous phase to extract one metal ion in the presence of others.
How would you get the metal ions back into the aqueous phase?
Students should all suggest decreasing the pH in order to protonate the ligand to drive the equilibrium back toward the uncomplexed species.
2. Devise a way to solubilize the organic anion shown below in the organic solvent of a two phase system in which the second phase is water. As a first step to this problem, show what might happen to this compound when added to such a two phase system.
What would happen to this molecule in the two phase system?
The groups often initially think that the species will enter the aqueous layer because it is an ion. I then ask them to consider the non-polar nature of the long carbon chain and where this group would prefer to reside in such a two-phase system. Groups usually then wonder if it is possible for the species to lie right at the interface of the system with the ionic end in the aqueous phase and carbon chain in the organic phase. I indicate that this is what would happen and then briefly talk about the formation of micelles if this salt were to be solely dissolved in water and a reverse micelle if dissolved in an organic phase.
Can you now think of a way to move it to the organic phase?
Students immediately propose lowering the pH as one method, and I challenge the students to think of others. Usually they are stumped by this and I ask them to think about the solubility properties of sodium relative to other possible cations. After a few minutes, I usually have to call the groups’ attention to me and discuss the concept of ion pairing and how the use of a lipophillic organic cation (e.g., quaternary amine with bulky aliphatic groups) would create an organic-soluble ion pair.