Organic Chemistry and Ethereal Extraction Solvents - Lab Report Example

Chapter 7: Extraction is one of the oldest chemical operations known to humankind. The preparation of a cup of coffee or tea involves the extractionof flavor and odor components from dried vegetable matter with hot water. Aqueous extracts of bay leaves, stick cinnamon, peppercorns, and cloves, along with alcoholic extracts of vanilla and almond, are used as food flavoring. For the past 150 years or so, organic chemists have extracted, isolated, purified, and characterized the myriad compounds produced by plants that for centuries have been used as drugs and perfumes-substances such as quinine from cinchona bark, morphine from the opium poppy, cocaine from coca leaves, and menthol from peppermint oil. The extraction of compounds from these natural products is an example of solid/ liquid extraction-the solid being the natural product and the liquid being the solvent into which the compounds are extracted. In research, a Soxhlet extractor is often used for solid/liquid extraction. Although solid/liquid extraction is the most common technique for brewing beverages and isolating compounds from natural products, liquid/liquid extraction is a very common method used in the organic laboratory, specifically when isolating reaction products. Reactions are typically homogeneous liquid mixtures and can therefore be extracted with either an organic or aqueous solvent. Organic reactions often yield a number of by products-some inorganic and some organic. Also, because some organic reactions do not go to 100% completion, a small amount of starting material is present at the end of the reaction. When a reaction is complete, it is necessary to do a workup, that is, separate and purify the desired product from the mixture of byproducts and residual starting material. Liquid/liquid extraction is a common separation step in this workup, which is then followed by purification of the product. There are two types of liquid/liquid extractions: neutral and acid/base. The experiments in this chapter demonstrate solid/liquid extraction and the two types of liquid/liquid extraction. Organic products are often separated from inorganic substances in a reaction mixture by liquid/liquid extraction with an organic solvent. For example, in the synthesis of 1-bromobutane, 1-butanol, also a liquid, is heated with an aqueous solution of sodium bromide and sulfuric acid to produce the product and sodium sulfate. 2 CH3CH2CH2CH2OH +2NaBr +H2S042CH3CH2CH2CH2Br +2H20 +Na2S04 The 1-bromobutane is isolated from the reaction mixture by extraction with t-butyl methyl ether, an organic solvent in which 1-bromobutane is soluble and in which water and sodium sulfate are insoluble. The extraction is accomplished by simply adding t-butyl methyl ether to the aqueous mixture and shaking it. Two layers will result: an organic layer and an aqueous layer. The t-butyl methyl ether is less dense than water and floats on top; it is easily removed/drained away from the water layer and evaporated to leave the bromo product free of inorganic substances, which reside in the aqueous layer. Partition Coefficient The extraction of a compound such as 1-butanol, which is slightly soluble in water as well as very soluble in ether, is an equilibrium process governed by the solubilities of the alcohol in the two solvents. The ratio of the solubilities is known as the distribution coeeficient, also called the partition coefficient (k), and is an equilibirium constant with a certain value for a given substance, pair of solvents, and temperature The concentration of the solute in each solvent can be well correlated with the solubility of the solute in the pure solvent, a figure that is readily found in solubility tables in reference books. For substance C K = concentration of C in t-butyl methyl ether Concentration of C in water >solubility of C in t-butyl methyl ether (g/100mL) Solubility of C in water (g/100mL) Consider compound A that dissolves in t-butyl methyl ether to the extent of 12 g/100mLand dissolves in water to the extent of 6g/100mL. K = 12g/100mL t-butyl methyl ether =2 6g/100mL water IF a solution of 6 g of A in 100mL of water is shaken with 100mL of t-butyl methyl ether, then K = x g of A/100mL t-butyl methyl ether 6-x g of A/100 mL water From which X=4.0 g of A in the ether layer 6-x = 2.0 g of A left in the water layer It is, however, more efficient to extract the 100 mL of aqueous solution twice with 50mL portion so f t-butyl methyl ether, we can calculate that 1.5 g of A will be in the ether layer, leaving 1.5 g in the water layer. So two extractions with 50mL portions of ether will extract 3.0 g +1.5 g =4.5 g of A, whereas one extraction with a 100 mL portion of t-butyl methyl ether removes only 4.0 g of A. Three extractions with 33-mL portions of t-butyl methyl ether would extract 4.7 g. Obviously, there is a point at which the increased amount of A extracted does not repay the effort of multiple extractions, but remember that several small scale extractions are more effective than one large-extraction. Properties of Extraction Solvents Liquid/liquid extraction involves two layers: the organic layer and the aqueous layer. The solvent used for extraction should possess many properties, including the following: It should readily dissolve the substance to be extracted at room temperature It should have a low boiling point so that it can be removed readily It should not react with the solute or the other solvent It should not be highly flammable or toxic It should be relatively inexpensive In addition, it should not be miscible with water (the usual second phase). No solvent meets every criterion, but several come close. Some common liquid/liquid extraction solvent pairs are water-ether, water dichloromethane, and water-hexane. Notice that each combination includes water because most organic compounds are immiscible in water and therefore can be separated from inorganic compounds. Organic solvents such as methanol and ethanol are not good extraction solvents because they are soluble in water. Solvent Density (g/mL) Hexane .695 Diethyl ether .708 t-Butyl methyl ether .740 Toluene .867 Water 1.000 Dichloromethane 1.325 Cloroform 1.492 Identifying the layers One common mistake when performing an extraction is to misidentify the layers and discard the wrong one. It is good practice to save all layers until the desired product is in hand. The density of the solvents will predict the identities of the top and bottom layers In general, the densities of nonhalogenated organic solvents are less than 1.0 g/mL and those of halogenated solvents are greater than 1.0 g/mL. Table above lists the densities of common solvents used in extraction. Although density is the physical properties that determines which layer is on top or on bottom, a very concentrated amount of a solute dissolved in either layer can reverse the order. The best method to avoid a misidentification is to perform a drop test. Add a few drops of water to the layer in question and watch the drop carefully. If the layer is water, then the drop will mix with the solution. If the solvent is the organic layer, then the water drop will create a second layer. Ethereal Extraction Solvents In the past, diethyl ether was the most common solvent for extraction in the laboratory. It has high solvent power for hydrocarbons and oxygen-containing compounds. It is highly volatile (bp 34.6 degrees C) and is therefore easily removed from an extract. However, diethyl ether has two big disadvantages: it is highly flammable and poses a great fire threat, and it easily forms peroxides. The reaction of diethyl ether with air is catalyzed by light. The resulting peroxides are higher boiling than the ether and are left as a residue when the ether evaporates. If the residue is heated it will explode because ether peroxides are treacherously high explosives. In recent years, a new solvent has come on the scene-tert-butyl methyl ether. Tert-butyl methyl ether, called methyl tert-butyl ether (MTBE) in industry, has many advantages over diethyl ether as an extraction solvent. Most important, it does not easily form peroxides, so it can be stored for much longer periods than diethyl ether. And, in the United States, it is less than two-thirds the price of diethyl ether. It is slightly less volatile (bp 55 degrees C), so it does not pose the same fire threat as diethyl ether, although one must be as careful in handling this solvent as in handling any other highly volatile, flammable substance. The explosion limits for t-butyl methyl ether mixed with air are much narrower than for diethyl ether, the toxicity is less (it is not a carcinogen), the solvent power is the same, and the ignition temperature is higher (224 degrees C versus 180 degrees C) The weight percent solubility of diethyl ether dissolved in water is 7.2%, whereas that of t-butyl methyl ether is 4.8%. The solubility of water in diethyl ether is 1.2%, while in t-butyl methyl ether it is 1.5%. Unlike diethyl ether, t-butyl methyl ether forms an zoetrope with water (4% water) that boils at 52.6 degrees C. This means that evaporation of any t-butyl methyl ether solution that is saturated with water should leave no water residue, unlike diethyl ether. The low price and ready availability of t-butyl methyl ether came about because it replaced tetraethyl lead as the antiknock additive for high-octane gasoline and as a fuel oxygenate, which helps reduce air pollution, but its water solubility has allowed it to contaminate drinking water supplies in states where leaking underground fuel storage tanks are not well regulated. Consequently, it is being replaced with the much more expensive ethanol. In this text, t-buytl methyl ether is strongly suggested wherever diethyl ether formery would have been used in an extraction. It will not, however, work as the only solvent in the Grignard reaction, probably because of steric hindrance. So whenever the word ether appears in this text as an extraction solvent, it is suggested that t-buytl methyl ether be used and not diethyl ether. Mixing and Separating the layers For microscale separations, mixing and separating the layers with a pipette normally incurs very little products loss. Because the two solvents are typically in a reaction tube for microscale extraction, the two layers can be mixed by drawing u and rapidly exelling them with a pipette. Then the layers are allowed to separate, and the bottom layer is separated by drawing it up into a pipette and transferring it to a different container, For macroscale separations, a separatory funnel is used to mix and separate the organic and aqueous layers. In macroscale experiments, a frequently used method of working up a reaction mixture is to dilute the mixture with water and extract it with an organic solvent, such as ether, in a separatory funnel. When the stoppered funnel is shaken to distribute the components between the immiscible solvents t-butyl methyl ether and water, pressure always develops through volatilization of ether from the heat of the hands, and liberation of a gas (CO2) (in acid/base extractions) can increase the pressure. Consequently, the funnel is grasped so that the stopper is held in place by one hand and the stopcock by the other. After a brief shake or two, the funnel is held in the inverted position shown, and the stopcock is opened cautiously (with the funnel stem pointed away from nearby persons) to release pressure. The mixture can then be shaken more vigorously, with pressure released as necessary. When equilibration is judged to be complete, the slight, constant terminal pressure due to ether is released, the stopper is rinsed with a few drops of ether delivered by a Pasteur pipette, and the layers are allowed to separate. The organic reaction product is distributed wholly or largely into the upper ether layer, whereas inorganic salts, acids, and bases pass into the water layer, which can be drawn off. If the reaction was conducted in alcohol or some other water-soluble solvents, the bulk of the solvent is removed in the water layer, and the remainder can be eliminated in two or three washings with 1-2 volumes of water conducted with the techniques used in the first equilibration. The separatory funnel should be supported in a ring stand. Before adding a liquid to the separatory funnel, check the stopcock. If it is glass, see that it is properly greased, bearing in mind that too much grease will clog the hole in the stopcock and also contaminate the extract. If the stopcock is Teflon, see that it is adjusted to a tight fit in the bore. Store the separatory funnel with the Teflon stopcock loosened to prevent sticking. Because Teflon has a much larger temperature coefficient of expansion than glass, a stuck stopcock can be loosened by cooling the stopcock in ice or dry ice. Do not store liquids in the separatory funnel; they often leak or cause the stopper or stopcock to freeze. To have sufficient room for mixing the layers, fill the separatory funnel no more than three0fourths full. Withdraw the lower layer from the separatory funnel through the stopcock and pour the upper layer out through the neck. All too often the inexperienced chemist discards the wrong layer when using a separatory funnel. Through incomplete neutralization, a desired component may still remain in the aqueous layer, or the densities of the layers may change. Cautious workers save all layers until the desired product has been isolated. The organic layer is not always the top layer. If in doubt, perform a drop test by adding a few drops of each to water in a test tube. Date _______________________ Name ___________________________________ EXPERIMENT IV ISOLATION OF CAFFEINE FROM COLA SYRUP Ch. 7. Extraction (ES #7 Extraction of Caffeine from Cola Syrup, ES #9 Caffeine Salicylate p. 165-166, 169-170; also read Technique of Liquid/Liquid Extraction, p. 142-147 and ES #6 Extraction of Caffeine from Tea, p. 163-164, for general discussion and procedure detail) Pre-lab Write up Physical Constants Caffeine mp ___________ Slb. H2O 25oC __________ 100oC __________ Dichloromethane bp ___________ density ___________ Salicylic acid mp ___________ Caffeine Salicylate mp ___________ Structures (As in the handout in Course Documents for the course on BlackBoard) Caffeine Salicylic Acid Caffeine Salicylate Dichloromethane Procedure Outline (No sublimation) Date _______________________ Name ___________________________________ EXPERIMENT IV ISOLATION OF CAFFEINE FROM COLA SYRUP Experimental Report ES #7 Extraction of Caffeine from Cola Syrup Amount Observations Cola Syrup ____________ __________________________________ Extraction Observations (upper/lower layer, color, clarity etc) Organic Layer Aqueous Layer __________________________________ __________________________________ Drying of the Extract Drying Agent Used _______________________ Observations of the Drying Process: Caffeine from Extraction Wt of F Flask _________ Wt of F Flask+Caffeine _________ Wt of Caffeine __________ Observations: ES #9 Caffeine Salicylate Amount Borrowed, if any Amount Used Caffeine ____________ ____________ Amount used Observations Salicylic Acid ____________ ________________________ Separation of Caffeine Salicylate by Crystallization Describe briefly but clearly how you carried out the crystallization. Amount Observations Caffeine Salicylate * ____________ ________________________ Observed mp ____________ * weight of the product handed in after taking a sample for mp Percent Yield Calculations Show limiting reactant and theoretical yield calculations as well as percent yield calculations. Cleaning Up EXPERIMENT IV ISOLATION OF CAFFEINE FROM COLA SYRUP Post-lab Questions (9pts) 1. During extraction, if in doubt, how can one identify which layer is the aqueous layer and which layer is the organic layer Just check the density of the organic layer. If the density of the organic solvent (phase) is higher than 1 (density of pure water), then the organic layer will be below the aqueous layer. But, if the organic solvent has a density < 1, it will be above the aqueous layer. Also note: that if any salts are present in the water, the density will vary. One should also consider this valid point. 2. One way to purify caffeine is sublimation. Describe this technique briefly. Sublimation of an element or compound is a transition from the solid to gas phase with no intermediate liquid stage. Sublimation is a phase transition that occurs at temperatures and pressures below the triple point. At normal pressures, most chemical compounds and elements possess three different states at different temperatures. In these cases the transition from the solid to the gaseous state requires an intermediate liquid state. However, for some elements or substances at some pressures the material may transition directly from solid to the gaseous state. The pressure referred to here is the vapor pressure of the substance, not the total pressure of the entire system. 3. Look at the structure of caffeine and caffeine salicylate (p.169) Why is the particular nitrogen that became protonated more basic (its lone pair of e's more easily available) than the two nitrogens in the six membered ring (the nonbonded electron pair of the nitrogen in the five membered ring is needed for the aromaticity of that ring). In the above reaction, all other nitrogen (except with + sign) are in conjugation with double bond or double bond having oxygen at one end. In the case, where two nitrogen is present in six member ring lone pair of electron remains attracted towards double bond having oxygen at one end. Here due to higher electronegativity of oxygen, shared electron is found closer to it. This makes the other end, electron deficient. Now as electron pair of nitrogen in vicinity is attracted towards electron deficient end of double bond. Similarly in case of nitrogen present in five member ring electron pair are utilized in ring formation (i.e. 4n+2 roles) and structure stabilization. In the above case, one nitrogen is left out with free electron pair and become basic in nature which is subsequently utilized in bond formation with OH (Acid) of salicylic acid. 4. Use Aldrich Catalog Handbook of Fine Chemicals or any other source to find out the classification of caffeine, dichloromethane, salicylic acid, and its derivative acetylsalicylic acid (aspirin) in terms of "Toxic Effects". All the four compounds are non-toxic. Aldrich labels as Xn, means harmful. Note: All the four can be toxic if taken in large quantities or overdosed. The consequences, if overdosed are also mentioned below (this might be an extra information). Caffeine: Not Toxic (Only harmful, Xn), Toxic, if overdosed. Dichloromethane (HPLC Grade): Not Toxic (Only harmful, Xn) Dichloromethane is the least toxic of the simple chlorohydrocarbons, but it is not without its health risks as its high volatility makes it an acute inhalation hazard. Dichloromethane is also metabolised by the body to carbon monoxide potentially leading to carbon monoxide poisoning. Prolonged skin contact can result in the dichloromethane dissolving some of the fatty tissues in skin, resulting in skin irritation or chemical burns. It may be carcinogenic, as it has been linked to cancer of the lungs, liver, and pancreas in laboratory animals. Dichloromethane crosses the placenta. Fetal toxicity in women who are exposed to it during pregnancy however has not been proven. In animal experiments it was fetotoxic at doses that were maternally toxic but no teratogenic effects were seen. In many countries products containing dichloromethane must carry labels warning of its health risks. Salicylic acid: Not Toxic (Only harmful, Xn), toxic if taken into large quantities. Acetylsalicylic acid: Not Toxic (Only harmful, Xn) The toxic dose of aspirin is generally considered greater than 150mg per kg of body mass. Moderate toxicity occurs at doses up to 300mg/kg, severe toxicity occurs between 300 to 500mg/kg, and a potentially lethal dose is greater than 500mg/kg.[50] This is the equivalent of many dozens of the common 325mg tablets, depending on body weight. However children cannot tolerate as much aspirin per unit body weight as adults can. Read More