Common reaction devices for organic experiments _ Reflux

Extraction and drying _ Solvent

Original title: Extraction and drying Extraction Extraction, also known as solvent extraction or liquid-liquid extraction (to distinguish it from solid-liquid extraction, that is, leaching), also known as extraction (commonly used in the petroleum refining industry), is a mass transfer separation process in which a liquid extractant is used to treat an immiscible two-component or multi-component solution to achieve component separation, and is a widely used unit operation. Based on the principle of similar compatibility, there are two ways of extraction: 1. Liquid-liquid extraction, which uses a selected solvent to separate a certain component in the liquid mixture. The solvent must be immiscible with the extracted mixture liquid, have selective solubility, and must have good thermal and chemical stability, and have low toxicity and corrosiveness. For example, benzene is used to separate phenols from coal tar; organic solvents are used to separate olefins from petroleum fractions; Br2. In CCl4 extraction water. Two Solid-liquid extraction, also known as leaching, uses solvents to separate components from solid mixtures, such as water to extract sugars from beets, alcohol to extract soybean oil from soybeans to increase oil production, and water to extract active ingredients from traditional Chinese medicines to make fluid extracts, which is called “leaching” or “leaching”. Although extraction is often used in chemical experiments, its operation does not result in a change (or chemical reaction) in the chemical composition of the material being extracted, so the extraction operation is a physical process. Extraction is one of the means used in organic chemistry laboratories to purify and purify compounds. The desired compound can be extracted from a solid or liquid mixture by extraction. Liquid-liquid extraction, which is commonly used in organic synthesis, is introduced here. Liquid-liquid extraction is a method of extracting a solute from a solution composed of one solvent and another solvent by using the different solubility of the solute in immiscible solvents. Principle A compound is transferred from one solvent to another by using the difference of solubility or distribution coefficient of the compound in two immiscible (or slightly soluble) solvents. After repeated extraction, most of the compounds are extracted. The partition law is the main basis of the theory of extraction method. Substances have different solubility to different solvents. At the same time, in two immiscible solvents, when a soluble substance is added, it can be dissolved in the two solvents respectively. Experiments show that at a certain temperature, when the compound does not decompose, electrolyze, associate and solvate with the two solvents, the ratio of the compound in the two liquid layers is a constant value. This is true regardless of the amount of substance added. It is a physical change. Expressed by the formula. CA/CB=K CA and CB respectively represent the amount and concentration of a compound in two immiscible solvents. K is a constant called the “partition coefficient”. Organic compounds are generally more soluble in organic solvents than in water. Extraction of a compound dissolved in water with an organic solvent is a typical example of extraction. During extraction, if a certain amount of electrolyte (such as sodium chloride) is added to the aqueous solution, the “salting-out effect” can be used to reduce the solubility of organic matter and extraction solvent in the aqueous solution, and the extraction effect can often be improved. In order to completely extract the desired compound from the solution, it is usually not enough to extract once, and the extraction must be repeated several times. The remaining amount of the compound after extraction can be calculated by using the relationship of the partition law. Let V be the volume of the original solution Wo is the total amount of the compound before extraction W1 is the remaining amount of the compound after one extraction W2 is the residual amount of the compound after the second extraction W3 is the remaining amount of the compound after n times of extraction S is the volume of the extraction solution Expand the full text After one extraction, the concentration of the compound in the original solution is w1/V, while the concentration of the compound in the extraction solvent is (w0-w1)/S, and the ratio of the two is equal to K. After simplification, the following formula is obtained: w1= [KV/(KV+S)]w0 Similarly, after the second extraction, there is w2= [KV/(KV+S)]w1 Therefore, after n extractions: wn=w0[KV/(KV+S)]n When a certain amount of solvent is used, it is desirable that the remaining amount in the water is as small as possible. The above formula KV/ (KV + S) is always less than 1, so the larger n is, the smaller wn is. That is to say, it is better to divide the solvent into several times for multiple extractions than to use the whole amount of solvent for one extraction. It should be noted, however, that the above formula applies to solvents that are nearly immiscible with water, such as benzene, carbon tetrachloride, and the like. The above formula is only approximate for solvents such as ether, which have a small amount of mutual solubility with water. However, the expected results can be qualitatively indicated. Application Compared with other methods for separating solution components, the extraction method has the advantages of normal temperature operation, energy saving, no solid or gas involved, and convenient operation. Extraction is generally advantageous in the following cases: 1, the boiling points of the components of the feed liquid are similar, and even an azeotrope is formed, which is not easy to work for rectification, such as the separation of alkanes and aromatics in petroleum fractions and the dephenolization of coal tar; 2. For the separation of low-concentration high-boiling components,nutsche filter dryer, the energy consumption of rectification is very high, such as the dehydration of dilute acetic acid; for the separation of various ions, such as the separation and purification of mineral leaching solution, if chemicals are added for fractional precipitation, not only the separation quality is poor, but also the filtration operation is needed, and the loss is also large; 3. Separation of unstable substances (such as heat-sensitive substances), such as the preparation of penicillin from fermentation broth. The application of extraction is still under development. Most of the elements in the periodic table can be extracted and separated by extraction. The selection and development of extractants, the determination of process and operating conditions, and the design and calculation of flowsheets and equipment are all subjects for the development of extraction operations. Precautions for extraction: 1. The single extraction solvent is not miscible with water ? Low solubility or insolubility in water; cannot be extracted separately with methanol, ethanol, THF, etc.; cannot be extracted with high boiling point solvent; [THF and saturated salt water are immiscible, and can be used for extraction under special circumstances; Similarly, for some substances with good water solubility, you can try to saturate the aqueous solution with sodium chloride first, and then extract it with acetonitrile. The disadvantage of this method is that acetonitrile will extract some sodium chloride. 2. Fully know the specific gravity of the extraction solvent ? Know which layer is the organic phase; the specific gravity of carbon tetrachloride, chloroform and dichloromethane > water > toluene, benzene, petroleum ether, diethyl ether, methyl tert-butyl ether and ethyl acetate. Do not throw away the product. Neither the aqueous phase nor the organic phase can be discarded until the product is determined to be available; 3. If there is a large amount of solid precipitation during post-treatment It is better to filter first and then extract in layers; 4. When the product is emulsified in an aqueous phase,wiped film distillation, ? Saturated aqueous sodium chloride solution is usually added and allowed to stand to promote layering; sometimes diatomite filtration can also demulsify; or centrifugal pump separation on the emulsion layer. 5. A small number of times when extracting However, the ratio of extraction solvent to aqueous solution should not be too small, usually controlled between 0.2 and 1; 6. Remember to deflate when extracting. ? Avoid flushing; 7. For substances that are highly soluble in water In general, mixed solvent such as DCM/MeOH (10/1), ethyl acetate and methanol is used for extraction. (Or other small molecular alcohol) or THF; CHCl3/i-Proh (3/1). In addition, n-butanol is a good choice. N-butanol itself is insoluble in water and has the common characteristics of small molecular alcohols and large molecular alcohols. Method for drying substances The process of removing small amounts of water and organic solvents from solids, liquids, or gases is called drying. Many chemical experiments must be carried out under anhydrous conditions, which requires that the raw materials, solvents and instruments used should be dried, and the moisture in the air should be prevented from entering the reactor during the experiment, otherwise the quality and yield of the product will be affected. Organic compounds must be dried before distillation to prevent heat from hydrolyzing certain compounds or forming azeotropic mixtures with water. To determine the physical constants of compounds, to carry out qualitative and quantitative analysis of compounds, and to analyze and determine the structures of compounds by using chromatography, ultraviolet spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy, mass spectrometry and other methods, the compounds must be completely in a dry state in order to obtain the correct results. Basic principles Drying methods are divided into physical methods and chemical methods. Physical methods include natural drying, drying, vacuum drying, fractionation, azeotropic distillation and adsorption. In addition, ion exchange resins and molecular sieves are also commonly used for dehydration drying. Ion exchange resin is a kind of high molecular polymer which is insoluble in water, acid, alkali and organic matter. Molecular sieves are a variety of aluminosilicate crystals. Because they have many gaps or holes inside, they can absorb water molecules. After heating, it can release water molecules, so it can be used repeatedly. The chemical method is to dehydrate the water with a desiccant. Desiccants can be divided into two types according to their dehydration: the first type can reversibly form hydrates with water, such as calcium chloride, magnesium sulfate, sodium sulfate, etc.; the second type can form new compounds after reacting with water, such as metal sodium, phosphorus pentoxide, etc. The first type of desiccant is widely used in the laboratory. Drying of liquids (1) dewater by fractional distillation or azeotropic mixture Liquid organics that do not form azeotropic mixtures with water, such as mixtures of methanol and water, can be completely separated by fractional distillation because of the large difference in boiling points. Sometimes, another organic substance can be added to the organic substance to be dried by taking advantage of the property that some organic substances and water form an azeotrope, and the water is gradually brought out during distillation by taking advantage of the property that the organic substance and water form the lowest azeotrope, so as to achieve the purpose of drying. For example, one of the industrial methods to prepare anhydrous ethanol is to add benzene to 95% ethanol, decarboxylation after extraction ,wiped film distillation, and use the azeotropic mixture of ethanol, water and benzene to take water out by azeotropic distillation. (2) Dewatering with desiccant ① Selection of desiccant When drying a liquid, the desiccant is usually thrown directly into it. Therefore, it is required that the desiccant cannot chemically react with the liquid to be dried and cannot be dissolved in the liquid to be dried. For example, alkaline desiccants cannot be used for acidic substances. Some desiccants can form complexes with some substances to be dried. For example, calcium chloride is easy to form complexes with alcohol, amine and some aldehydes and ketones; strong alkaline desiccants such as calcium oxide and sodium hydroxide can catalyze the condensation and oxidation reactions of some aldehydes and ketones, resulting in hydrolysis reactions of esters, so they cannot be used to dry these substances. When selecting the desiccant, the water absorption capacity and drying performance of the desiccant should also be considered. Water absorption capacity is the amount of water absorbed by a unit mass of desiccant. The greater the water absorption capacity, the more water the desiccant absorbs. For example, sodium sulfate can form 10 hydrates of crystal water at most, and its water absorption capacity is 1.25; calcium chloride can form 6 hydrates of crystal water at most, and its water absorption capacity is 0.97, so the water absorption capacity of sodium sulfate is large. Drying efficiency refers to the degree of dryness of the liquid being dried when equilibrium is reached. Hydrate forming inorganic salt desiccant is usually expressed by the vapor pressure of crystal water after water absorption. It is an equilibrium process for desiccant to absorb water to form hydrate. When different hydrates are formed, there are different vapor pressures. The greater the vapor pressure is, the worse the drying effect is. When drying compounds that contain more water and are not easy to dry, a desiccant with a larger water absorption capacity is often used to remove most of the water, and then a desiccant with a stronger drying efficiency is used to remove the residual trace water. Attention should also be paid to the drying speed and price of the desiccant. See Table 2-1 for the performance and application scope of common desiccants: Table 2.1 Performance and Application Range of Common Desiccants Desiccant Water Absorption Water Absorption Capacity Drying Efficiency Drying Speed Application Range The formation of CaCl 2 · nH 2O n = 1, 2, 4, 60.97 (calculated by CaCl 2 · 6H 2O) is moderately fast, but the surface is covered by a thin layer of liquid after water absorption, so it is better to leave it for a longer time. It can form complexes with alcohols, phenols, amides and some aldehydes and ketones. Industrial products may contain calcium hydroxide and should not be used to dry acids. Magnesium sulfate forms MgSO4.nH2O n = 1,2,4,5,6,71.05 (calculated by MgSO4.7H2O), which is weak and fast neutral. It has a wide range of applications. It can replace CaCl2 and can be used to dry esters, aldehydes, ketones, nitriles, amides and other compounds that can not be dried by CaCl2. Sodium sulfate Na2SO4 · 10H2O 1.25 is weakly slowly neutral and is generally used for the initial drying of organic liquids. Calcium sulfate 2CaSO4 · H2O 0.06 is strongly neutral and is often combined with magnesium sulfate (sodium sulfate) for final drying. Potassium carbonate K 2CO 3 · 1/2H 2O 0.2 is weak and weakly alkaline. It is used to dry basic compounds such as alcohols, esters, ketones, amines and heterocycles. It cannot be used to dry acids, phenols and other acidic compounds. Potassium (sodium) hydroxide is soluble in water — moderately fast and strongly alkaline. It is used to dry alkaline compounds such as amines and heterocycles. It cannot be used to dry alcohols, esters, aldehydes, ketones, acids, phenols, etc. Sodium metal Na + H 2O = NaOH + 1/2H 2 — strong, fast and limited to trace moisture in dry ethers and hydrocarbons. When used, cut into small pieces or pressed into sodium wire for use. Calcium oxide CaO + H 2 O = Ca (OH) 2-Strong and faster Suitable for drying lower alcohols. Phosphorus pentoxide P 2O 5 + 3H 2O = 2H 3PO 4 is strong and fast, but the surface is covered by viscous slurry after water absorption, which is inconvenient to operate. Suitable for drying trace amounts of moisture in ethers, hydrocarbons, halogenated hydrocarbons, nitriles, etc. Not suitable for alcohols, acids, amines, ketones, etc. The molecular sieve physical adsorption is about 0.25 strong and fast, and is suitable for drying various organic compounds. ② Dosage of desiccant The amount of desiccant can be estimated based on its water absorption capacity and the solubility of water in the liquid. In general, the actual amount of desiccant is greater than the theoretical value because it is difficult to completely separate the water during extraction or washing. In addition, for polar substances and liquid compounds containing hydrophilic groups, an excess of desiccant is required. However, the amount of desiccant should not be too much, because the surface of desiccant will adsorb some products, resulting in the loss of products. It is difficult to specify the amount of desiccant due to the varying moisture content in the liquid, the mass of the desiccant, the particle size, and the temperature at which it is dried, and the fact that the desiccant may also absorb some by-products. In general, about 0.5 to 1 G of desiccant is required for every 10 mL of liquid. ③ Experimental operation The moisture of the liquid to be dried shall be separated as completely as possible. If there are some floccules during liquid separation, it is better to discard them. Put the liquid into a dry conical flask, add a small amount of desiccant first, plug the stopper, shake the conical flask, if the desiccant has been attached to the bottom of the bottle and stuck together, it means that the amount of desiccant is not enough, add some more until the loose desiccant particles are clearly seen in the bottle, and then leave it for a period of time (at least half an hour). Sometimes, a small amount of water will appear after adding the desiccant. At this time, the water layer should be carefully absorbed with a dropper before adding a new desiccant. It should be noted that the change from turbidity to clarity of a liquid after the addition of a desiccant does not indicate that the liquid has dried. Transparency is related to the solubility of the compound in water. Blocky desiccant, such as calcium chloride, should be crushed into soybean-sized particles when used. If the particles are too large, the surface area will be small and the water absorption will be slow; if the particles are too small, more liquid will be absorbed and it will be difficult to filter and separate. The process of desiccant absorbing water to form hydrate is a reversible process, and the hydrate will lose water at high temperature, which will reduce the drying performance. Therefore, the desiccant must be filtered off before distillation. Certain desiccants (e.g. Sodium metal, phosphorus pentoxide) react with water to form relatively stable products, which can sometimes be distilled directly without filtration. Drying of solids The drying of solid substances is mainly to remove a small amount of low-boiling solvents remaining on the solid, such as water, ethanol, ether, acetone and benzene. (1) Natural drying Natural drying is suitable for solids that are stable in air, do not decompose, and do not absorb moisture. When drying, place the substance to be dried on a dry and clean watch glass or other vessel, spread it thinly, cover it with filter paper, and let it dry slowly in the air. (2) Drying Compounds that are thermally stable, difficult to sublimate and have a high melting point can be dried in a constant temperature oven or an infrared drying oven. The heating temperature should be well controlled to prevent the sample from yellowing, melting or even decomposing and carbonizing. Solvent-containing solids may melt at temperatures below their melting point. When drying, it should be turned frequently to prevent caking. (3) drying by a dryer Solids which are easy to decompose or sublimate can not be dried by heating, but can be dried in a dryer. There are two common dryers. ① For ordinary dryer (Fig. 2.2), the plane between the cover and the cylinder body is frosted, and the frosted part is coated with lubricating oil to seal it. A porou porcelain plate is arranged in that jar, a desiccant is place below the porcelain plate, and a watch glass for holding a sample to be dried is placed above the porou porcelain plate. ② Vacuum dryer (Fig. 2.3), its drying efficiency is better than ordinary dryer. There is a glass piston on the vacuum dryer, which is used for vacuum pumping. The lower end of the piston is hook-shaped, and the mouth is upward, so as to prevent the solid from being scattered due to too fast air inflow when it is opened to the atmosphere. The desiccant used shall be selected according to the solvent contained in the sample. For example, phosphorus pentoxide can absorb water; quicklime can absorb water or acid; paraffin flakes can absorb ether, chloroform, carbon tetrachloride and benzene, etc. Sometimes two kinds of desiccants are placed in the dryer at the same time, such as concentrated sulfuric acid at the bottom, and the vessel containing sodium hydroxide is placed on the porcelain plate, which can absorb both water and acid, and the effect is better. Fig. 2.2 Ordinary Dryer Fig. 2.3 Vacuum Dryer Drying of gas Gases produced in the laboratory often contain acid mist, moisture, and other impurities. If the experiment needs to purify and dry the gas, the absorbent and desiccant used should be selected according to the nature of different gases and the types of impurities contained in the gas. Generally, acid mist can be removed by water, moisture can be removed by concentrated sulfuric acid, anhydrous calcium chloride, etc., and other impurities should also be treated separately according to specific conditions. The purification and drying of the gas is carried out in the gas washing bottle (Fig. 2.4) and the drying tower (Fig. 2.5). The liquid treatment agent (such as water, sulfuric acid, etc.) is contained in a gas washing bottle. There is a porous plate at the bottom of the gas washing bottle. The glass tube for introducing gas is inserted into the bottom of the bottle. The gas is well dispersed in the liquid through the porous plate, which increases the contact area between the two phases. If the laboratory does not have a gas wash bottle,rotary vacuum evaporator, a conical flask with two stoppers may be used instead. A drying tube or a drying tower is adopted when the solid is used for purifying the gas. Sodium hydroxide, anhydrous calcium chloride and other solid particles shall be filled in the pipe or tower according to the specific requirements. The filling shall be uniform and the particles shall not be too fine to avoid blockage. The filling method is shown in Figure 2.5. 2.4 Diagram of gas bottle washing 2.5 Drying Tube and Drying Tower Suitable drying of common gases: The content of this article comes from the Internet, and the copyright belongs to the original author. Return to Sohu to see more Responsible Editor:. toptiontech.com

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Common reaction devices for organic experiments _ Reflux