Organic chemistry employs solid-liquid, liquid-liquid, and acid-base extractions. The following applies to liquid-liquid extractions, which will be used in this course. It is very common for organic products synthesized in a reaction to be purified by liquid-liquid extraction.
A separatory funnel see picture is used for this process. In this procedure, the organic product is isolated from inorganic substances.
The organic product will be soluble in an organic solvent organic layer while the inorganic substances will be soluble in water aqueous layer.
The organic solvent used for extraction must meet a few criteria:. Common extraction solvents are diethyl ether and methylene chloride. Removal of water: Mohrig, pp. Thus, water must be removed before separating the organic product from the organic solvent or else the product will be contaminated with water. A drying agent must be used. There are a number of drying agents available to the organic chemist: we will be using sodium sulfate and magnesium sulfate in this course.
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 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 pK a.
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 solvent can be an organic compound such as acetone , toluene , ether, alcohol, benzene, etc.
The solvent can be polar or nonpolar and depending on the polarity while solutes dissolve in the solvent. Solutions of iodine in alcohol and solutions of iodine in carbon tetrachloride are examples of nonaqueous solutions. We can divide solutions into two groups as aqueous and nonaqueous depending on the solvent. The key difference between aqueous and nonaqueous solution is that the solvent of an aqueous solution is water, whereas, in nonaqueous solutions, the solvent is any substance other than water.
Aqueous solutions of sodium chloride, aqueous ammonia, etc. Basically, we can divide solutions into two groups as aqueous and nonaqueous depending on the solvent. If it is difficult to track where the water droplets go, also keep track of the volume of the layers: whichever layer increases with the addition of water is the aqueous layer. Consider relative volumes of aqueous and organic solvents, based on quantities used in the experiment.
If these were the quantities used in an experiment, the aqueous layer would have to be the lower layer as it is so much larger. Although unequivocal in this case, it is important to know that the odd shape of the separatory funnel may cause you to misjudge volumes.
A separatory funnel with equal volumes of aqueous and organic layers is shown in Figure 4.
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