EXTRACTION
§ Extraction – is the process by which a solute is transferred from one phase to a new phase.
Extraction consists of three consecutive steps: (1) the mixing of the feed (starting mixture) with the extractant, (2) the mechanical separation of the two phases formed, and (3) the removal of the extractant from both phases and its recovery. After the mechanical separation, a solution of the extracted substance in the extractant (extract) and the residue of the starting solution (raffinate) or solid are obtained. The separation of the extracted substance from the extract and the concurrent recovery of the extractant are accomplished by distillation, evaporation, crystallization, or salting-out.
The advantages of extraction are low working temperatures, the feasibility of obtaining substances from dilute solutions, the possibility of separating mixtures consisting of components with similar boiling points and azeotropic mixtures, the possibility of combining extraction with other technological processes, such as rectification and crystallization, the simplicity of the equipment used, and the ease of automating the various steps. A shortcoming in many cases is the difficulty in completely removing the extractant from the extracted substances.
General Extraction Procedure:
Here’s everything you will need to perform an extraction!
Ensure that the stopcock to the separatory funnel is closed. As a safety measure, place a beaker underneath the funnel in case it leaks.
Here is a separatory funnel with a beaker under it before pouring a mixture into the separatory funnel.
(This is done just in case there is a leak in the sep. funnel OR as in this picture, the stopcock is accidentally left open.)
Place the solution to be extracted in the separatory funnel. As the organic solvent and water are not miscible with each other, you should be able to see the two layers (organic and aqueous layers) clearly. You should also have two beakers ready, one labeled “organic layer” and the other labeled “aqueous layer”. To remove all inorganic substances from the organic layer, shake the separatory funnel to increase the contact between these substances and the water. The proper way to hold a separatory funnel is to grasp the funnel so that the stopper is in the palm of one hand the stopcock is held with the other. This way leaks are prevented and any pressure built up inside the funnel will not pop the stopper off.
It is imperative to VENT the separatory funnel of any gas pressure. After a few shakes, hold the funnel upside down with the funnel stem pointed away from nearby people, and open the stopcock to release any pressure. Close the funnel and shake the funnel a little more vigorously and vent again as necessary. This “shake and vent” method can be repeated two or three times as needed. When finished, the funnel can be returned to the ring stand and the layers allowed to separate. To determine which layer is which, one can simply add distilled water to the funnel. Whichever layer increases in size must be the aqueous layer and the other is the organic layer. At this point the two layers can be separated into their respective beakers. To be safe, never throw out any removed layers until the desired product has been isolated! Once the extraction process is completed, drying agents caow be used and the product can be isolated from the organic solvent. Your instructor will give you specific instructions on drying your organic layer.
§ Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent.
Аaq « Аor
§ Process of dissolved substance transferring from one phase to another phase, which are immiscible or restrictedly miscible, is named liquid-liquid –partition or partition between two phase of liquids.
Partition law of Nernst – Shilov:
§ The relationship of dissolved substance concentration in both phases at constant temperature is constant and does not depend on concentration of the dissolved substance:
D = CAorg / CAaqu
§ D – the distribution ratio remains constant if there are no processes:
§ dissociation or association
§ polymerization or other transformations of the dissolved substance
§ The ratio of substance activity in one certain form in organic solvent phase to its activity in a water phase is named a distribution constant
§ The distribution ratio and constant are connected with substance solubility
Main concepts:
§ Extraction is process of transferring substance of a water phase in organic
§ Extraction reagent is reagent which with investigated substance forms compound which then is extracted
§ Extragent is organic solvent which is used for extraction
§ Latent solvent
§ Extract – is a substance made by extracting a part of a raw material, often by using a solvent such as ethanol or water. Extracts may be sold as tinctures or in powder form
§ Re-extraction is process of transferring substance of organic phase in water
§ Re-extragent
Conditions of a choice of solvent which is used as extragent:
1. Should not mix up with water.
2. Should be selective.
3. Should have the big capacity in relation to extractive.
4. The density of extragent should be difference from water density.
5. Should have the minimum viscosity.
6. Should be inexpensive.
7. Cannot be explosive.
Classification extraction processes:
§ Periodical extraction – is the process in which separatory funnel (which contain substance which extragent) is shaken
§ Continuous extraction
§ Countercurrent extraction
Schematic diagram of a Soxhlet extractor.
1: Stirrer bar/anti-bumping granules
2: Still pot (extraction pot) – still pot should not be overfilled and the volume of solvent in the still pot should be 3 to 4 times the volume of the soxhlet chamber.
3: Distillation path
4: Soxhlet Thimble
5: Extraction solid (residue solid)
6: Syphon arm inlet
7: Syphon arm outlet
8: Expansion adapter
9: Condenser
10: Cooling water in
11: Cooling water out
The main quantitative characteristics of extraction.
§ Factor or extraction efficiency
R = ν (A) / ν (A)o
where ν (A) – moles of solute in organic phase
ν (A)o – moles of solute initially present in water phase
§ Extraction efficiency for single extraction
§ Extraction efficiency:
for multiple (m) extraction
§ Separation coefficient (factor) of A, B ions equal relationship the distribution ratio this ions
Types of extraction systems
1. Halogenides with covalent linkage: HgCl2, HgJ2, SbJ3, AsBr3, GeCl4, element iodine etc.
2. Intracomplex salts: dithizonate, dithiocarbamates, oxyquinolines, oxyns, β-diketonate, and also di-(2-ethylhexyl)-phosphates actinoids, rare-earth and some other elements, etc.
3. Complex metal acids: HFeCl4, HІnBr4, HSbCl6, etc.
4. coordinatively not solvated (a) and coordinatively solvated (b) salts:
a) Salts tetraphenyl arsonium, tetraphenyl phosphonium etc.
b) The compounds which is formed at extraction uranyl nitrate and nitrate of thorium by tributyl phosphate from nitrate solutions.
5. heteropoly compounds of phosphorus, arsenic, silicon, vanadium, molybdenum, tungsten etc.
!!!! Most widely is used in extraction process intracomplex salts, complex metalo halogenide acids and coordinatively solvated (b) salts
Choosing a Solvent System
One important aspect when choosing a solvent system for extraction is to pick two immiscible solvents. Some common liquid/liquid extraction solvent pairs are water-dichloromethane, waterether, water-hexane. Notice that each combination includes water. Most extractions involve water because it is highly polar and immiscible with most organic solvents. In addition, the compound you are attempting to extract, must be soluble in the organic solvent, but insoluble in the water layer. An organic compound like benzene is simple to extract from water, because its solubility in water is very low. However, solvents like ethanol and methanol will not separate using liquid/liquid extraction techniques, because they are soluble in both organic solvents and water.
There are also practical concerns when choosing extraction solvents. As entioned previously, the two solvents must be immiscible. Cost, toxicity, flammability should be considered. The volatility of the organic solvent is important. Solvents with low boiling points like ether are often used to make isolating and drying the isolate material easier. If ether is used (bp =
Identifying the Layers
One common mistake when performing an extraction is to mix-up the layers and discard the wrong one. 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. The density of dichloromethane is 1.325 g/mL and water is 1.000 g/mL. Dichloromethane is more dense that water; therefore, dichloromethane will be the bottom layer and water will be the top layer. Table lists the densities of some extraction organic solvents.
(For a complete list of physical properties of some common organic solvents, please see the table located in the front of your laboratory notebook.) Although the density is the physical property that determines which layer is on top or bottom, a very concentrated solute dissolved in either layer can reverse the order. The best method to avoid making a mistake is a drop test. Add a few drops of water to the layer in question and watch the drop very carefully. If the layer is water, then the drop will mix with the solution. If the solvent is the mistaken organic layer, then the water drop will create a second layer. In general, this method can help determine the identity of the layer.
However, it is still best to keep ALL the layers until the extraction is complete and your product has been isolated.
The main organic reagents which use in extraction method
§ 8-oxyquinoline reacts with more than 50 elements
§ Acetylacetonate forms compound with more than 60 elements
§ Thionyl trifluoride acetone is used for excretion and separation actinoids.
§ dithizon is used for determination of Pd, Au, Hg, Ag, Cu, Bi, Pt, In, Zn, Cd, Co, etc.
!!! It is of great importance in the toxicological analysis.
§ Sodium diethyl dithiocarbamate reacts with several tens of elements
!!! It is of great importance in the toxicological analysis.
Microscale Extraction
For microscale separations, pipet layer separation is convenient and normally very little product loss is incurred. Since the two solvents are already in a reaction tube, instead of transferring the small volumes of solvent to another piece of glassware and ultimately losing product, the solvents can be mixed and separated directly from the reaction tubes. Use a Pasteur pipet to gently mix the layers. This can easily be accomplished by gently drawing the liquid up and down with the pipet. Do not simply swirl the tube. This mixing method will not allow the two layers to mix properly and decreasing the success of the extraction. Once the layers are thoroughly mixed, use the pipet to draw up the bottom layer as shown in Figure below.
The ether and water layers are now separated. Normally, two or more ether extractions would be completed to ensure the complete removal of the organic compound. Both the macroscale and microscale separations are typical examples of how liquid/liquid extraction can be used to separate water soluble inorganic materials from organic products. Finally, the ether or other organic solvent could then be evaporated, leaving the mixture of organic product with traces of starting material and by-products (often called the crude product). This can be purified by recrystallization or sublimation.
Emulsions
An emulsion is a suspension of tiny droplets of one solvent mixed in the other. Emulsions are common in extraction because proper mixing is essential. In Italian salad dressing, an emulsion is desired to keep the water and oil mixed. Additives are added to the dressing in order to keep the two normally immiscible solvents miscible. In a liquid/liquid or solid/liquid extraction however, an emulsion will lead to a poor separation. Gentle shaking and swirling the separatory funnel is the best technique to avoid emulsions. However, if an emulsions occurs, there are several simple methods to destroy it. The first is time. Over time the layers will eventually separate. With a severe emulsion, you may not have time during a three hour lab period to wait.
Another method is to add brine or salt water to the mixture. Since ether is less soluble in a highly ionic solution such as salt water, the ether and water will be forced to separate. This method works well with small emulsions. If you have a more difficult emulsion, separate the layers as much as possible and dry the organic layer with a drying agent. The water will be removed from the organic layer along with the drying agent. Subsequent extractions should proceed without further trouble.
Drying Agents
One significant problem with liquid/liquid extraction is that no solvent is COMPLETELY insoluble in another solvent. In practice, one additional step is usually carried out before evaporating the organic solvent: drying over anhydrous sodium sulfate or other drying agent.
Drying a liquid might seem like a peculiar concept, since we normally think of all liquids as being wet. Drying an organic liquid in the organic lab has a special meaning to chemists. It means to remove all traces of water. Even water and hexane are slightly soluble in each other. After separating the two solvents, residual water will remain in the hexane or ether organic layer. This will remain and stick to the solid product when we remove the more volatile solvent. Therefore, 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 although other drying agents are also available. All of the inorganic solids work by reacting with the water to form hydrates, which is their preferred form if water is available.
These compounds will associate or hydrate themselves with water. Table lists some common drying agents along with their speed, capacity, and hydration.
These drying agents do not dissolve in the solvent they are “drying”. They may change somewhat, for example, sodium sulfate will clump together as it reacts with water, but they will remain solids iormal extraction solvents. This makes them easy to remove by decantation (pouring off) of the liquid or by gravity filtration. Usually the organic solvent will go from cloudy to clear in the process of being “dried”. You should be careful to remove all of these solid drying agents before solvent evaporation or you might think they are your product. When you take a melting point and the product doesn’t melt by
It is relatively inexpensive and fast.
It is recommended that the drying agent you choose be in a granular form. After the drying agent has removed the residual water, it is easier to remove large granular particles. Drying a solvent however, is not an exact science. An excess of drying agent should be used to ensure that all the water is removed. If the water remains after the materials are collected, it could interfere with the analysis. Add drying agent until there are no longer clumps of drying agent stuck to the sides or bottom of the flask. The drying agents should be free floating in the beaker, like snow.
Do not be afraid to use too much. There are many other choices for drying agents including molecular sieves and sodium metal. There are benefits and disadvantages to each one. Sodium, for example, is an excellent drying agent, however it violently decomposes in water to create NaOH and H2 gas and may ignite spontaneously. Therefore it should be used with caution and only when removing very small amounts of water. Many times a particular drying agent will work better than others in a certain situation.
Acid/Base Extraction
There are also three special cases of liquid/liquid extraction that are extremely useful for isolating and purifying amines, carboxylic acids and phenols. All three of these functional groups can be interconverted from non-ionic organic-soluble forms to water-soluble ionic forms by changing the pH.
Solid/liquid or liquid/liquid extractions rely on the solubility of the solute to be extracted. In acid/base extraction, the molecule to be extracted is transformed so that we impose a new solubility on the molecule. One specific example is benzoic acid, an organic acid. Benzoic acid is soluble in most organic solvents including dichloromethane and ether. However, this acid can be easily deprotonated with base to give a charged ionic species that is readily soluble in water.
Solid/liquid or liquid/liquid extractions rely on the solubility of the solute to be extracted. In acid/base extraction, the molecule to be extracted is transformed so that we impose a new solubility on the molecule. One specific example is benzoic acid, an organic acid. Benzoic acid is soluble in most organic solvents including dichloromethane and ether. However, this acid can be easily deprotonated with base to give a charged ionic species that is readily soluble in water.
Acid/base extraction is one of the more difficult principles in organic chemistry to understand.
The most straight forward approach to understanding this subject is to create a flow chart (mentally or on paper) to follow which species has been created and where the molecule resides.
If you can imagine the molecule changing and moving to the appropriate layer, you will be able to complete the unknown separation very easily.
Figure is a detailed flow chart of the separation of a strong organic acid, a weak organic acid, an organic base, and a neutral component.
If you can follow the steps involved below, the unknown extraction in this chapter will be much easier to understand.
Whether you use acid/base, solid/liquid or liquid/liquid, extraction is a useful organic tool to separate a mixture of compounds. From the early drugs that were extracted from trees and plants to modern day pharmacology, extraction is still used to separate and purify organic molecules.
The following experiments demonstrate both acid/base extraction on a microscale and solid/liquid extraction on a macroscale.
