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Extraction

Extraction is a common technique used in organic chemistry to isolate a target compound. In the extraction process, a solute is transferred from one phase to another to separate it from unreacted starting materials or impurities. Extraction is also used to facilitate the isolation of a solute from a reaction solvent that is difficult to remove by evaporation, such as a solvent with a high boiling point. Generally, there are three types of extractions. First, in solid-liquid extraction, the solute is transferred from a solid phase to a liquid phase. In liquid-liquid extraction, a solute is transferred from one liquid to another. In acid-base extraction, a solute is transformed into an ionic compound and transferred from an organic phase to an aqueous phase. A common example of extraction is the brewing coffee or tea. The solid phase, which contains the caffeine, plant flavors, and odors, is extracted by the hot water into the liquid phase.

Liquid-Liquid Extraction

A liquid-liquid extraction either transfers an organic compound that is dissolved in an aqueous phase to an organic solvent, or it is used to transfer unreacted reactants, salts, and other water-soluble impurities to the aqueous phase while leaving the organic compound of interest in the organic phase. Immiscible liquids are liquids that never form a homogenous solution, even when thoroughly mixed. Instead, immiscible liquids separate into different phases, like oil and water.

A liquid-liquid extraction transfers an organic compound that is dissolved in an aqueous phase to an organic solvent. To perform a liquid-liquid extraction, first, the aqueous solution containing the solute is added to a separatory funnel. Then, a non-water-soluble organic solvent is added to the separatory funnel. When the contents of the funnel are mixed well, the organic compound partitions into the organic phase based on its higher solubility in the organic phase than the aqueous phase.

Since the two solvents are immiscible, the two liquids form discrete layers, with the dense liquid on the bottom and the less dense liquid on the top. Once the two phases settle back into two layers, they are separated by opening the stopcock at the bottom of the separatory funnel and allowing one layer to flow out. The liquid that had the solute removed is called the raffinate, while the liquid that gained the solute is called the extract.

The organic compound is partitioned between the two layers based on its solubility in each phase. Equilibrium is reached when the chemical potential of the solute is the same in the two phases. The partition coefficient for a solute, K, is the ratio of the concentration of the sample in the organic layer divided by the concentration in the aqueous phase. The partition coefficient is a constant dependent on both the solute and the pair of solvents used in the extraction. The partition coefficient is an expression of preference of the solute for each of the two solvents. Solutes with a large partition coefficient have a higher tendency to be extracted into the organic solvent layer. Solutes with small partition coefficients prefer to transition into the aqueous phase.

Determining which solvent pair to use for a liquid-liquid extraction is a vital step. Consider the following when choosing which solvents to use: First, the solute must be more soluble in the solvent than in water. Therefore, knowing the partition coefficient of the solute in a potential solvent pair is necessary. Second, the solvent-pair must be immiscible in water and not form a homogeneous solution when mixed. Third, the solvents must be inert and not react with the solute. The solvent should also be volatile so that it can be removed from the solute easily. The water-immiscible organic solvent generally possesses a non-polar or low polarity.

It is also critical to know the densities of the solvents to determine the identities of the top and bottom layers. Most organic liquids have a lower density than water, with the exception of chlorinated organic solvents, and will settle to the bottom of the separatory funnel.

Acid-Base Extraction

Acid-base extraction is a type of liquid-liquid extraction that separates organic compounds based on their acid-base properties. If a solute is an acid or base, its charge changes as the pH is changed. Generally, most organic compounds are neutral, and therefore more soluble in organic solvents than they are in water. However, if the organic compound becomes ionic, then it becomes more soluble in water. This is useful in extracting an organic acid or base compound from an organic phase to an aqueous phase.

Acid-base extraction harnesses this property by transforming the solute into its water-soluble salt form, thereby changing its solubility. The solubility of the organic compound and its salt must be dramatically different in order for the technique to be effective.

For example, consider a mixture containing an organic carboxylic acid, an amine, and a neutral compound. Carboxylic acids consisting of six carbons or more are insoluble in water and entirely soluble in organic solvents. However, their conjugate bases (an ionic compound) are water-soluble and insoluble in organic solvents. An amine consisting of at least seven carbons is insoluble in water but soluble in organic solvents. The conjugate acid of that amine (an ionic compound) is water-soluble and insoluble in organic solvents.

When reacted with a base, the carboxylic acid is neutralized to its salt form. The other compounds in the mixture remain neutral. Once the carboxylic acid is transformed into a salt, it will partition to the aqueous phase, while the neutral compounds remain in the organic phase.

Acid-base extraction is also used to separate two weak acids or two weak bases with a significant difference in their pKa. In the case of the acids, the relatively stronger acid, having a small pKa value, is neutralized to a salt using a weak base. The weak base does not efficiently react with the weaker acid, and only the stronger acid is transformed to a salt. Then, the salt is extracted into the aqueous phase during extraction. The process follows similarly for weak bases, where the relatively stronger base is neutralized to a salt using a weak acid.

References

  1. Harris, D.C. (2015). Quantitative Chemical Analysis. New York, NY: W.H. Freeman and Company.

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