Acid-Base Extraction: Separating Compounds via Solvent-Solvent Extraction and pH Control, Lecture notes of Chemistry

Download Acid-Base Extraction: Separating Compounds via Solvent-Solvent Extraction and pH Control and more Lecture notes Chemistry in PDF only on Docsity! Acid-Base Extraction. Extraction involves dissolving a compound or compounds either (1) from a solid into a solvent or (2) from a solution into another solvent. A familiar example of the first case is making a cup of tea or coffee - the soluble flavor and odor chemicals and caffeine are extracted from the solid tea leaves or ground coffee beans into hot water (the solvent). Insoluble plant material is left behind in the tea bag or coffee filter. An example of the second case is an experiment that is often done in an introductory organic lab - an organic solvent is used to extract the caffeine from an aqueous tea or coffee solution, leaving the more water-soluble compounds behind in the aqueous solution. This in fact is how decaffeineated tea and coffee are sometimes made. An acid-base extraction, this week's experiment, is a modification of the second case above; that is, it is a solvent-solvent extraction. Before looking at what makes it an acid-base extraction, first consider solvent-solvent extractions in general. A requirement of a solvent-solvent extraction is that the two solvents be mostly insoluble in one another. That is, a mixture of the two solvents will separate into two layers. It is important to note that the upper layer is that which is less dense. For example, a mixture of tert-butyl methyl ether and water will separate into two layers, with the ether layer (density = 0.74) being on top of the water layer (density = 1.0). Note that in almost every case, one of the solvents is water or an aqueous solution. The other solvent is an organic solvent that is mostly insoluble in water. Examples include tert-butyl methyl ether, hexane, and dichloromethane. The organic solvent must also be volatile (easily-evaporated) so it can be easily removed by evaporation at the end. As an example of a separation that could be accomplished by using a solvent-solvent extraction, consider a mixture of two solid compounds, naphthalene and glucose. O HO OH HO HO OH ß-D-GlucopyranoseNaphthalene - low polarity - high ether solubility - low water solubility - high polarity/H-bonding - low ether solubility - high water solubility If the mixture of naphthalene and glucose is dissolved in tert-butyl methyl ether and mixed well with water, the glucose will mostly dissolve in the lower water phase (phase = layer) and the naphthalene will mostly dissolve in the upper ether phase. The two phases can then be physically separated using a pipet and placed into two separate tubes. The water and ether can then be evaporated to yield the separated solid compounds. This is the gist of an extraction. An acid-base extraction operates on the same principle, but can provide a further level of fine tuning. If one or more of the compounds in the mixture to be separated is acidic or basic, the solubilities of these acidic and basic components can be manipulated to our advantage by applying simple acid-base reactions. Using such a manipulation, an acidic or basic compound that may be ether soluble and water insoluble can be changed to be ether insoluble and water soluble by carrying out an acid-base reaction. The solubilities of acidic and basic compounds can thus be changed at will. Once the layers are separated as before, the neutral acid or base can be regenerated by yet another acid-base reaction. An example of such a manipulation of solubilities is shown below for a carboxylic acid. O O H O O Na O O H NaOH / HOH HCl / HOH carboxylic acid - high solubility in ether - low solubility in water carboxylic acid - high solubility in ether - low solubility in water salt of carboxylic acid - low solubility in ether - high solubility in water In the experiment done in this lab, a mixture of a carboxylic acid (stronger acid), a phenol (weaker acid), and a neutral compound will be separated by acid-base extractions. The separated compounds will be purified by recrystallization and identified by melting points. A general scheme for the separation is given below. The mixture is dissolved in ether and mixed thoroughly with aqueous sodium bicarbonate (weaker base). After the layers settle, they are separated and placed into different tubes. The ether layer is then extracted with sodium hydroxide (stronger base), the layers separated, and each layer worked up separately. Three separate solutions result, one ether and two aqueous. Dissolved water is removed from the ether solution using a drying procedure and the ether is evaporated to yield the solid neutral compound. This can then be recrystallized to yield pure solid neutral compound. The basic aqueous solutions are neutralized with concentrated HCl to yield the carboxylic acid and phenol, which, because of their water insolubility, precipitate out. These can be collected, recrystallized and allowed to dry to yield pure carboxylic acid and pure phenol. Note that the extractions must be done in the order shown, that is, first with sodium bicarbonate and then with sodium hydroxide. (What would happen if the sodium hydroxide extraction were done first?)