We study the principle that like dissolves like and partition equilibrium between two phases. This is the foundation of extraction procedures in organic chemistry.
Basic Knowledge
The basic principle of dissolution is matching polarity. Water (polar) dissolves NaCl and sugar, while hexane (nonpolar) dissolves lipids and oils. When a solute is added to two mutually immiscible phases, the partition coefficient K = corganic/cwater becomes constant. This principle is used in extraction with a separatory funnel.
Polar solvents（water, methanol, ethanol, DMSO）
Nonpolar solvents（hexane, benzene, toluene）
Partition coefficient K（K = corganic/cwater）
Extraction efficiency（many small portions > one portion）
Fraction remaining（(Vwater/(Vwater+KVorganic))n）
Chromatography（continuous separation based on partitioning）
Further Study (Principles and Applications)
It can be mathematically proven that extraction with multiple small portions gives a higher recovery than one extraction with the same total volume. All of the following are applications of partitioning: workup after organic reactions, isolation of natural products, pharmacokinetics of drugs (logP), and the separation principle of HPLC.
pH partitioning is also an important technique: acidic solutes move into the aqueous phase in basic water, while basic solutes move into the aqueous phase in acidic water.
- “Like dissolves like”
- Water and oil form two phases
- The partition coefficient K is constant at a fixed temperature
- Extraction should use many small portions
- Fraction remaining (Vwater/(Vwater+KVorganic))n
- How to use a separatory funnel
- logP = lipid affinity of a drug
Points of Caution (Common Confusions and Frequent Mistakes)
① Check the definition of K in your textbook (organic/water is common). ② Extraction with 50 mL×2 is more efficient than extraction with 100 mL once. ③ In pH partitioning, changing between acidic and basic conditions reverses the phase into which the solute moves. ④ CHCl3 and CH2Cl2 are denser than water and form the lower layer.
Practice
- A 1.0 g solute with K=4 is dissolved in 100 mL of water and extracted once with 100 mL of organic solvent. How many grams move into the organic phase?
- Under the same conditions, find the total amount transferred when extracting twice with 50 mL portions, and compare it with one extraction.
- When salicylic acid is extracted from water/ether with a basic aqueous solution, into which phase does it move? State the reason.