Aniline Separation From Toluene
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Separation of MixturesThe identification of the compounds of a mixture involves, first, a separation into individual components and, second, the characterization of each of those components. It is very rarely possible to identify the constituents of a mixture without separation. The separation of the compounds in a mixture should be as nearly quantitative as possible in order to give some idea of the actual percentage of each component. Also, it is very important to carry out the separation in such a manner that each compound is obtained in a pure form. The method of separation chosen should be such that the compounds are obtained as they existed in the original mixture.1
Derivatives of the original compounds are not very useful unless they may be readily reconverted into the original compounds. In selecting and performing a separation procedure, one should be concerned with: 1. Will the sample survive the separation procedure? That is, are the components of the mixture stable under the conditions of the procedure? 2. Is this the easiest and most efficient way to carry out the separation? Stability of the sample under the conditions of the separation procedure may not be known until the separation is attempted.2
Compounds which are thermally unstable to the heat required for distillation at atmospheric pressure should be distilled at reduced pressure. Extractions and column chromatography do not involve heat and thus may be appropriate for samples which cannot be distilled. However, some samples decompose because of chemical reactions with acid or base in extractions or with chromatographic packing or support in column chromatography. A TLC test is a fast and useful check for sample durability under chromatographic conditions.
Preliminary Examination of Mixtures1. Note the physical state.Take advantage of existing separations. If a solid is suspended in a liquid, remove the solid by filtration and examine it separately. If two immiscible liquids are present, separate them and examine them separately.
2. Determine the solubility of the mixture in water.Classify the mixture according to Figure 5.1 and Table 5.1.
3. With liquid mixtures, evaporate 2 mL of the solution to dryness on a glass or porcelain crucible cover and not the presence or absence of a residue.Apply the ignition test to the residue or 0.1 g of the liquid or solid.4
4. In liquid samples, determine the presence of water by(a) determining the miscibility of the solution with ether, (b) using the anhydrous copper sulfate test, or (c) using the distillation test for water
Copper sulfate test: add a small sample of anhydrous CuSO4 to the liquid. If the solution turns blue, it is indicative of the presence of water; indicating the copper sulfate has absorbed the water. The distillation test is more reliable, and is carried out by placing 5 mL of the liquid and 5 mL of anhydrous toluene in the distilling flask of a distillation apparatus. Heat the mixture gently with a flame until distillation occurs. Collect 2 mL of the distillate. Add 5 mL of toluene to the distillate. The presence of two layers or distinct drops suspended in toluene indicates the presence of water. If the solution is 5only cloudy, traces of water are indicated.
5. If water is absent,Determine the presence of a volatile solvent by placing 1.0 mL of the mixture in a distilling flask in a simple distillation apparatus. Place the distilling flask in a beaker of water and heat the water to boiling. Any liquid that distills under these conditions is classified as a volatile solvent. Examine the distillate, which may be a mixture of readily volatile compounds, and the residue in the flask separately. It frequently happens that distillation of a water soluble mixture yields a volatile solvent and a water-insoluble residue. The separation of such a mixture is therefore carried out by removing all of the volatile solvent. The residue is then treated as a water insoluble mixture. If the residue after distillation is a water-soluble liquid, it is best not to remove the solvent at this stage because the separation is usually not quantitative. If, however, the residue after distillation is a water-insoluble solid and the removal of the solvent seems quantitative, then remove all of the volatile solvent and examine the distillate and the residue separately. If water is present, no such separation should be attempted.6
6. Determine the reaction of aqueous solution or suspension of the mixture to litmus and phenolphthalein. If the mixture is distinctly acidic, titrate I mL (of a known exact weight) of the solution in 2.5 mL of water or ethanol with a standardized 0.1 M sodium hydroxide solution to determine whether considerable amounts of free acid are present or whether the acidity is due to traces of acids formed by hydrolysis of esters. Perform the titration in an ice-cold solution, and take the first pink color of phenolphthalein as the end point. Obtain an IR spectrum of a mixture to reveal the presence of several carboxylic acid groups.
7. Acidify 2 mL of the mixture with 5% hydrochloric acid, and cool the solution. Note the evolution of a gas or the formation of a precipitate. Add 5% sodium hydroxide solution to the solution until the solution is basic and note the result. 8. Make 2 mL of the mixture basic with 5% sodium hydroxide solution. Note the separation of an oil or solid, the liberation of ammonia, and/or any color change. Heat the solution just to boiling and then cool. Compare the odor with that of the original mixture. The presence of esters is often indicated by a change in odor. Next, add 5% hydrochloric acid until the solution is acidic and note the result.8
9. In the case of water-insoluble mixture, perform an elemental analysis. If water or a large amount of a volatile solvent is present in a water-insoluble mixture, omit the elemental analysis of the mixture. If the water-soluble mixture is composed of solids, perform an elemental analysis. 10. If water is absent, cautiously determine the effect of the following classification reagents: (a) metallic sodium; (b) acetyl chloride 11. Determine the action of the following classification reagents on an aqueous solution or suspension of the original mixture: (a) bromine water; (b) potassium permanganate solution; (c) ferric chloride solution; (d) alcoholic silver nitrate solution; (e) Fuchsin-aldehyde reagent; and (f) 2,4-dinitrophenylhydrazine.9
DistillationThe short-path process allows distillation of materials such as lowmelting solids for which long exposure to elevated temperatures could be damaging.
A microscale distillation apparatus can be used to distill 0.5-2.0 mL of a sample.10
In order to improve the efficiency of a distillation, a column can be placed between the vessel to be heated and the condenser tube. Frequently, the column is a condenser filled with glass beads or steel wool to provide increased surface area and/or increase cooling surfaces. This vertical is not surrounded by a jacket of water as typically favored in the traditional condenser. 11
Consider a 50:50 mixture of cyclohexane (BP = 78oC) and toluene (BP = 111oC) Question: can cyclohexane be separated by heating the mixture to 78oC? In a mixture of cyclohexane and toluene, the vapor above it is not 100% cyclohexane! Roults law: vapor pressure of cyclohexane is equal to the product of the vapor pressure of pure cyclohexane and the mole fraction of cyclohexane in the liquid mixture. The total vapor pressure above the solution, PTot, is given by the sum of the partial12
Pc = P N c0 c 0
Pt = Pt N t
P = P +P Tot c t
pressures due to the cyclohexane and toluene.
Daltons law states that the mole fraction of cyclohexane in the vapor at a given temperature is equal to the partial pressure of cyclohexane at that temperature divided by the total pressure.
Pc Xc = total vspor pressureTherefore, to separate a mixture of cyclohexane and toluene, a series of fractions would be collected and each of these partially redistilled. If this fractional distillation were done enough times the two components could be separated.13
Efficiency of column is evaluated according to the height equivalent to a theoretical plate (HETP). The smaller the HETP, the more plates the column will have and the more efficient it will be.
Fractional DistillationA fractional distillation apparatus uses the vertical column in addition to the condenser. With this distillation apparatus, compounds with a difference in their boiling points of 5-10oC or more can be efficiently separated.
Frequently, the result of a more efficient distillation apparatus, is that the distilling compound remains on the vertical column for a longer time. To avoid heat loss, the column should be externally insulated with glass wool, cotton, or aluminum foil. An important aspect in distillation is the method of heating the distilling pot. For volatile liquids, a steam bath is used. Baths containing oils, or other involatile, inert substance can be used; such that liquids provide a very even method of heat application and can be used to higher temperatures (ca. 250-400oC). Heating mantles attached to Variacs can be used for heating application.16
Steam DistillationSteam distillation is a technique whereby a compound of relatively low volatility can be purified by co-distilling it with water. When a mixture of cyclohexane and toluene is distilled, the boiling point of these two miscible liquids is between the boiling points of each of pure components. By contrast, if a mixture of benzene (bp 80.1oC) and water (bp 100oC) (immiscible liquids) is distilled, the boiling point of the mixture will be below the boiling point of each pure component. This distillation occurs because both of the