Chapter 5. Chemical Extraction

Transcription

1 Chapter 5. Chemical Extraction 1. Solid-liquid extraction 2. Liquid-liquid extraction 1 Introduction - Extraction is a physical process by which a compound is transferred from one phase to another : - Water soluble components in the tea leaves or in the coffee beans are transferred from a solid phase in the leaves or beans into a liquid phase the boiling water. This process is known as solid-liquid extraction. - Liquid-liquid extraction is a basic operation that should be measured in the organic chemistry laboratory : 2

2 Soxhlet Extraction - If a component in a solid mixture has a low solubility a Soxhlet extraction canbeused to isolate it. - The solid mixture is placed in a porous thimble which is repeatedly washed with fresh condensing solvent in a specialized piece of glassware know as Soxhlet extractor Condenser Syphon Tube Compound to be extracted Extraction Thimble Insoluble material Vapor By-pass Tube 3 Soxhlet Extraction

3 Soxhlet Extraction - The condensing solvent falls into the chamber in which the thimble is placed - Eventually the chamber fills and the liquid siphons back into the boiling flask - The process is repeated and the solid is eventually extracted into the boiling flask ilp.net/labtechniques/soxhletextractionvideo.htm Compound to be extracted Insoluble material 5 Chapter 5. Chemical Extraction 1. Solid-liquid extraction 2. Liquid-liquid extraction 6

4 Introduction - Liquid-liquid extraction is a basic operation that should be measured in the organic chemistry laboratory : - By liquid-liquid extraction we can isolate single components from a mixture. The physical process that rules liquid-liquid extraction is known as solvent-solvent partitioning or the distribution of solutes between pair of solvents(immiscible solvent). 7 Introduction - Solvents such as diethyl ether, tertbutyl methyl ether, methylene chloride and toluene have very limited solubility in water. The solubility of water in such solvents is also very low. We say that these solvents are immiscible with water. - Two layer system is obtained : (lower layer) dense solvent (upper layer) less dense solvent Toluene (d=0.867) Water (d=1.000) - The organic solvent layer is called the organic layer or organic phase and the water layer the aqueous layer or aqueous phase. - Inorganic and water soluble materials will stay in the aqueous(water) layer and more organic molecules will remain in the organic layer 8

5 Immiscibility & Density of sovlent - The densities of the solvents will predict which solvent is the top or bottom layer. - In general, the density of nonhalogenated organic solvents are less than 1.0 g/ml and halogenated solvents are greater than 1.0 g/ml. - One common solvent pair is dichloromethane and water(aqueous layer). Common extraction solvents listed by density. (ex. diethyl ether, methyl tert butyl ether) (=Methylene chloride) (=trichloromethane) 9 Solvent-Solvent Partitioning Immiscible Miscible 10

6 Solvent-Solvent Partitioning d=1.000g/ml d=1.492g/ml Organic phase Aqueous phase Solvent Nonpolar solvent Water Solute Nonpolar solutes dissolve here Ionic and polar solutes dissolve here Density The organic phase does not always float on top. Chloroform, for example, sinks below the aqueous phase. Water is usually denser than other solvents, but some organic solvents are even denser. 11 Solvent-Solvent Partitioning - Lets recall the golden rule of solubility: like dissolves like; Highly polar and ionic compounds are readily soluble in water but have very low solubility in most organic solvents, which are polar than water. - On the other hand organic compounds of medium and low polarity are more soluble in organic solvents than water. - This selective solubility behavior can be advantageously used to separate compounds by liquid-liquid extraction. 12

7 Solvent-Solvent Partitioning - ex1. Due to their high solubility in water and low solubility in organic solvents, very polar solutes such as ionic compounds and sugars, display very low partition coefficients between most organic solvents and water. Therefore these solutes remain in the water phase even after several extractions with organic solvents. - ex2. Conversely organic compounds of medium and low polarity for example, cholesterol, DDT(Dichloro Diphenyl Trichloroethane), and aspirin have larger partition coefficients and thus are easily extracted into organic solvents. 13 Criteria for selecting an extracting solvent The organic solvent used for extraction must meet a few criteria: - Should readily dissolve substance to be extracted. - Should not react with the substance to be extracted. - Should not react with or be miscible with water (the usual second solvent). - Should have a low boiling point so it can be easily removed from the product. - Should be reasonably safe to work with and relatively inexpensive. Common extraction solvents are diethyl ether and methylene chloride. 14

8 Criteria for selecting an extracting solvent 15 Nernst Distribution Law - In the simplest extraction case, the distribution ratio is constant in accordance with the classical Nernst distribution law; Nobel Prize in Chemistry At constant temperature, a solute distributes itself between two immiscible solvents only in a particular ratio. 16

9 Distribution Ratio (Partition Coefficient) - Lets suppose that we add a solute A to a mixture of water and toluene, shake the system thoroughly to attain equilibrium, and allow it to settle. - Solute A will be present in both layers but we expect to find it in a larger amount in the solvents is directed by the Distribution coefficients of A between the solvents. - The Distribution ratio is the ratio of the total analytical concentration of a solute in the extract (regardless of its chemical form) to its total analytical concentration in the other phase The phrase Partition coefficient" is now considered obsolete by IUPAC, and Distribution ratio" is more appropriate terms that should be used 17 Distribution Ratio - The distribution of a solute between two immiscible solvents can be described by the Distribution Ratio D. D = [solute] 1 [solute] 2 where, [solute] represents the stoichiometric or formal concentration of a solute and the subscripts 1 and 2 refer to the two phases. - Since in most cases, two-phase system is of analytical interest, an organic(org) solvent and aqueous(aq) are involved, D will be understood to be; - ex. D = [solute] Org [solute] Aq D Toluene/Water = [A Toluene ] / [A Water ], 18

10 Distribution Ratio - If the ionized species has an appreciable solubility only in the aqueous phase, then the distribution coefficient, D, can be defined as D = [solute u ] Org [solute u ] Aq + [solute i ] Aq where, [solute] Org : the conc. of the unionized species in organic phase [solute] Aq : the conc. of the unionized species in aqueous phase [solute] Aq : the conc. of the ionized species in aqueous phase 19 % Extraction - The more commonly used term for expressing the extraction efficiency by analytical chemist is the percent extraction E, which is related to D as where V represent solvent volume - The percent extraction may be seen to vary with the volume ratio of the two phases as well as with D. - At extreme values of D, E becomes less sensitive to changes in D. For example, at a phase volume ratio of unity(v aq =V org ), for D values from 500 to 1000, the value of E changes only from 99.5 to 99.9%. 20

11 Extraction procedure The total capacity of the separatory funnel should be at least twice the volume of the solution - Stopper the funnel and invert it. Slowly open the stopcock to release any built up pressure, then close the stopcock. - Gently shake the separatory funnel to allow intimate mixing of the solutions (Caution: When shaken, the mixture may develop pressure; be sure to vent it periodically). - Clamp the separatory funnel to a retort stand and allow the mixture to separate into two layers. - Remove the stopper and collect the layer containing the compound what we want to obtain. Vacuum rotary evaporation 21 Extraction Techniques: Layer identification - One of the most common and frustrating mistakes made by beginners to liquid-liquid extraction is to keep the wrong phase and discard the one with the desired product. - In principle the identity of the layers can be predicted by the density of the solvents. - However a high solute concentration may change the density of the solution to the point that the layer with the less-dense solvent becomes the denser and lower layer. ex. Saturated sodium chloride solution : 1.2 g/ml Conc. Sulfuric acid : 1.8g/ml 22

12 Extraction Techniques: Layer identification - If there is any doubt concerning the identity of the layers the following test should be performed: Place about 1 ml of water in a test tube and add a few drops of the upper layer with a Pasteur pipet. If the drops dissolve in the water, it means they come from the aqueous phase. If the drops are immiscible with the water in the test tube, they belong to the organic phase. 23 Extraction Techniques: Multiple extraction - Since the distribution coefficient is a ratio, unless K is very large, not all of a solute will reside in the organic layer in a single extraction. - Usually two, three, or four extractions of the aqueous layer with an organic solvent are carried out in sequence in order to remove as much of the desired product from the aqueous layer as possible. - The effectiveness of multiple small volume extractions : if one extraction can recover 90% of the compound. A second extraction with the same solvent may be able to pull out 90% of the remaining material. Effectively 99% of the compound was recovered with two extractions. - Many smaller extractions are more efficient than one large extraction. This phenomenon can be proved mathematically, but in short follows the equation: 24

13 Extraction Techniques: Multiple extraction - Many smaller extractions are more efficient than one large extraction : Fraction of material extracted by solvent B is where, n : the number of extractions performed, K : the distribution coefficient, V A, V B : the volume of solvent A and B 25 Extraction Techniques: Multiple extraction Example Prob. Give at 20 o C only 0.24 g of an organic acid A dissolves in 100 ml of water, but 2.70 g of the same acid dissolves in 100 ml of ether. a) calculate the value of partition coefficient. : b) calculate the percentage of extraction if 0.12 g of acid extracted in 100 ml of aqueous solution: 92 % c) calculate the volume of ether required to extract 85% of a 3.00 g of acid A in 100 ml solution : 51 ml d) calculate the total amount of acid extracted by a double extraction of 50 ml ether in each extraction in part ( c). First extraction (first 50 ml ether) : X o1 = 2.57 g acid extracted in first extraction X w1 = 3.00 g 2.57 g = 0.43g acid remained in aqueous solution after first extraction Second extraction (second 50 ml ether) X o2 = 0.38 g acid extracted in first extraction X o (total) = X o1 + X o2 = 2.57 g g = 2.93 g %E = (2.93 /3.00) x100 = 97.7 % 26

14 Breaking Emulsions - After shaking the two phases may fail to separate sharply and may instead form a suspension of one liquid in the other. This is called emulsion and it is unwelcome in the laboratory. - In order to break the emulsion and get a clean separation between layers, Swirl the funnel very gently. If the emulsion persists add a small amount of a saturated solution of sodium chloride in water: The sodium chloride increases the ionic strength of the water phase and decreases its miscibility with the organic solvent. This process is called salting out and may help break the emulsion. - Swirl the flask occasionally over a period of 5-10 minutes and then gravity-filter the system using flutted filter paper. Rinse the magnesium sulfate on the filter with a few milliliters of the same organic solvent. Discard the magnesium sulfate. The solution should be clear now. - Another way of breaking the emulsions is to centrifuge the layers for a few minutes. Glass tubes are preffered for this operation because some organic solvents may dissolve plastic centrifuge tubes. 27 Microscale Liquid liquid Extraction - When small volumes ( ml) are to be extracted, a conical vial or a screw capped test tube are the glass ware of choice. Conical vials are available in different sizes (0.5-5mL), test tubes are suitable when the volume to be extracted is between 2 and 10 ml. - The solution of the desired compound and the extracting solvent are placed in the vial, the total volume should not be larger than three-fourths of the maximum capacity of the vial. - The vial is tightly capped and shaken vigorously for a few seconds. The pressure is released by carefully unscrewing the cap. This process is repeated four to six times. - The lower layer is withdrawn with the aid of a Pasteur pipet and placed in a clean test tube. In doing this operation squeeze the rubber bulb before immersing the Pasteur pipet in the extraction vial. This will prevent air bubbles from disrupting the interface between the phases. - Very volatile solvents such as diethyl ether and methylene chloride have a tendency to squirt out the tip of the pipet. One way is to saturate the inside of the pipet with the solvents vapors by drawing fresh solvent 28

15 Drying the Organic Layer - One significant problem with liquid/liquid extraction is that no solvent is COMPLETELY insoluble in another solvent. - After contact with an aqueous phase any organic solvent will be saturated with water, and this poses a problem in the recovery of the solute from the organic layer: evaporation of the solvent renders a product mixed with water. - Also, many reactions themselves are performed in an aqueous solution. This water must be removed before the required compound can be properly characterized. - To avoid these wet residues water should be removed before evaporation. Two methods of drying solutions are commonly used: saturated aqueous sodium chloride and solid drying agents. 29 Drying the Organic Layer: Saturated aqueous sodium chloride - The bulk of the water can often be removed by shaking or "washing" the organic layer with saturated aqueous sodium chloride (otherwise known as brine). - The salt water works to pull the water from the organic layer to the water layer: the concentrated salt solution wants to become more dilute and salts have a stronger attraction to water than to organic solvents. 30

16 Drying the Organic Layer Saturated aqueous sodium chloride - To dry your organic product by this method, place the organic solution (methylene chloride, d=1.3) in a separatory funnel. - Add an amount of saturated aqueous sodium chloride(d=1.2 g/ml), less than or equal to the amount of organic solution you have. - Stopper the funnel and shake as in an extraction. Allow the layers to separate. - Drain off the lower layer: in this case, this is the organic layer and the layer you want to save. Dispose of the aqueous layer in the aqueous waste carboy. 31 Solid drying agents Drying the Organic Layer - Final traces of water are removed by treating the organic solution with a drying agent. A drying agent is an inorganic salt which readily takes up water to become hydrated: drying over anhydrous sodium sulfate or other drying agent. - chemists remove the water from the organic layer by adding an insoluble inorganic solid to the solution which will absorb the water, thus drying it. - Granular anhydrous sodium sulfate is the drying agent most often used: all of the inorganic solids work by reacting with the water to form hydrates, if water is available. ethyl acetate was dried with a little Na 2 SO 4 (the solid at the bottom). Calcium chloride Magnesium sulfate Sodium sulfate 32

17 Drying the Organic Layer Solid drying agents Because these compounds will deliquesce (absorb moisture from the air), you MUST cap bottles of drying agents between uses! Otherwise, they will end up looking like this! Hydrated complexes for some common drying agents. 33 Drying the Organic Layer Solid drying agents - Add a small amount (but enough to ensure that all the water is removed) of the solid drying agent directly to the organic solution. - Swirl the solution and Observe the drying agent; if it is all clumped together, add more. - The pictures below show how a solution with drying agent looks when it is clumped and when it is free-flowing. - In most cases, drying is as complete as it will get in 20 minutes. When drying is complete, you need to remove the dried organic solution from the drying agent. 34

18 Drying the Organic Layer Solid drying agents - If the powder is quite fine (as when using magnesium sulfate) or if the volume is large, gravity filtration is the method of choice. - If the drying agent is of larger particle size (as when using sodium sulfate or calcium chloride), decanting is the method of choice. - An alternative to decanting is removing the liquid from the drying agent simply by drawing it off with a Pasteur pipet. decanting 35 Drying the Organic Layer Solid drying agents - There are other choices for drying agents including molecular sieves and sodium metal. There are benefits and disadvantages to each one. Sodium: A very powerful drying agent, removes traces of water from solvents such as toluene, benzene and diethyl ether and aliphatic amines before distillation. It reacts violently with water. It should be handled with extreme care by trained people only. Molecular sieves: Small spheres made of porous aluminosilicates. The average pore size is 3 and 4Å respectively. 36