Fractionation methods crystallization, fractional

Method 1. Arrange the flask containing the reaction mixture for steam distillation as in Fig. II, 40, 1. Proceed with the steam distillation until crystals of p-dibromobenzene appear in the condenser. Change the receiver and continue with the distillation until all the p-dibromobenzeiie has passed over from time to time run out the water from the condenser so that the crystals melt and run down into the receiver. Reject the residue in the flask. Transfer the first distillate to a separatory funnel, wash it with a httle water, and dry the lower layer with a little anhydrous magnesium sulphate or anhydrous calcium chloride filter. Distil slowly from a small distilling flask use a wire gauze or an air bath (Fig. II, 5, 3). Collect the fraction which passes over at 150-170° pour the residue (R), while it is still hot, into a small beaker or porcelain basin for the isolation of p-dibromobenzene. Redistil the fraction of b.p. 150-170° and collect the bromobenzene at 154-157° (3). The yield is 60 g. 

Classical methods for separation and purifica tion include fractional distillation of liquids and re crystallization of solids and these two methods are routinely included in the early portions of laboratory courses in organic chemistry Because they are capa ble of being adapted to work on a large scale frac tional distillation and recrystallization are the preferred methods for purifying organic substances in the pharmaceutical and chemical industries... 

In order to carry out an experimental study of the kinetics of crystallization, it is first necessary to be able to measure the fraction d of polymer crystallized. While this is necessary, it is not sufficient we must also be able to follow changes in the fraction of crystallinity with time. So far in this chapter we have said nothing about the experimental aspects of determining 6. We shall now briefly rectify this situation by citing some of the methods for determining 6. It must be remembered that not all of these techniques will be suitable for kinetic studies. 

For preparative purposes batch fractionation is often employed. Although fractional crystallization may be included in a list of batch fractionation methods, we shall consider only those methods based on the phase separation of polymer solutions fractional precipitation and coacervate extraction. The general principles for these methods were presented in the last section. In this section we shall develop these ideas more fully with the objective of obtaining a more narrow distribution of molecular weights from a polydisperse system. Note that the final product of fractionation still contains a distribution of chain lengths however, the ratio M /M is smaller than for the unfractionated sample. 

Fluorozirconate Crystallization. Repeated dissolution and fractional crystallization of potassium hexafluorozirconate was the method first used to separate hafnium and zirconium (15), potassium fluorohafnate solubility being higher. This process is used in the Prinieprovsky Chemical Plant in Dnieprodzerzhinsk, Ukraine, to produce hafnium-free zirconium. Hafnium-enriched (about 6%) zirconium hydrous oxide is precipitated from the first-stage mother Hquors, and redissolved in acid to feed ion-exchange columns to obtain pure hafnium (10). 

A fourth ahoy separation technique is fractional crystallization. If shica is co-reduced with alumina, nearly pure shicon and an aluminum shicon eutectic can be obtained by fractional crystallization. Tin can be removed to low levels in aluminum by fractional crystallization and a carbothermic reduction process using tin to ahoy the aluminum produced, fohowed by fractional crystallization and sodium treatment to obtain pure aluminum, has been developed (25). This method looked very promising in the laboratory, but has not been tested on an industrial scale. 

Laue Method for Macromolecule X-Ray Diffraction. As indicated above it is possible to determine the stmctures of macromolecules from x-ray diffraction however, it normally takes a relatively long period of data collection time (even at synchrotrons) to collect all of the data. A new technique, the Laue method, can be used to collect all of the data in a fraction of a second. Instead of using monochromated x-rays, a wide spectmm of incident x-rays is used. In this case, all of the reflections that ate diffracted on to an area detector are recorded at just one setting of the detector and the crystal. By collecting many complete data sets over a short period of time, the Laue method can be used to foUow the reaction of an enzyme with its substrate. This technique caimot be used with conventional x-ray sources. 

Chlorine and bromine add to benzene in the absence of oxygen and presence of light to yield hexachloro- [27154-44-5] and hexabromocyclohexane [30105-41-0] CgHgBr. Technical benzene hexachloride is produced by either batch or continuous methods at 15—25°C in glass reactors. Five stereoisomers are produced in the reaction and these are separated by fractional crystallization. The gamma isomer (BHC), which composes 12—14% of the reaction product, was formerly used as an insecticide. Benzene hexachloride [608-73-17, C HgCl, is converted into hexachlorobenzene [118-74-17, C Clg, upon reaction with ferric chloride in chlorobenzene solution. 

Boiling the solution speeds the conversion of intermediate hypobromites and bromites to bromate. The less soluble bromate can be separated from the hahde by fractional crystallization. A method that is often more economical is the oxidation of bromides into bromates by hypochlorites in aqueous solution. This can be done by passing chlorine into an alkaline bromide solution (75) ... 

Cocoa butter substitutes and equivalents differ greatly with respect to their method of manufacture, source of fats, and functionaHty they are produced by several physical and chemical processes (17,18). Cocoa butter substitutes are produced from lauric acid fats such as coconut, palm, and palm kernel oils by fractionation and hydrogenation from domestic fats such as soy, com, and cotton seed oils by selective hydrogenation or from palm kernel stearines by fractionation. Cocoa butter equivalents can be produced from palm kernel oil and other specialty fats such as shea and ilHpe by fractional crystallization from glycerol and selected fatty acids by direct chemical synthesis or from edible beef tallow by acetone crystallization. 

Another method of fractional crystallization, in which advantage is taken of different ciystallization rates, is sometimes used. Thus, a solution saturated with borax and potassium chloride will, in the absence of borax seed ciystals, precipitate only potassium chloride on rapid coohng. The borax remains behind as a supersaturated solution, and the potassium chloride crystals can be removed before the slower borax crystalhzation starts. 

Separation of chloroformic fraction of raw material by a polyamide sorbent with the following individual fractions elution and crystallization allowed to study lignans of burdock with IR-, mass- and NMR spectroscopy. In result, aixtiin, arctigenin, lappaols were identified. Sepai ation of essential oils was provided by the method two of State Phaiiuacopeias of the Ukraine. Essential oils were analyzed by the means of gas chromatography. 

The starting material is moderately soluble in hot chloroform, while 2-hydroxyisophthalic acid is quite insoluble. Fractional crystallization from water, an alternative method suggested for the separation of starting material, has been found by the submitters to be unsuccessful. 

Although the traditional method of separating the diastereomeric compounds generated in a resolution procedure is fractional crystallization, chromatographic procedures are now common and convenient. Diastereomeric compounds exhibit different adsorption... 

Potassium chloride is crystallized from sea bitterns containing chlorides of potassium, sodium and calcium by ammoniation (Jagadesh etai, 1992). This process is less energy intensive and more efficient than by fractional crystallization by evaporation, as the ammonia used is recovered by distillation. Crystallization produces a better quality product in terms of both size and purity than by other methods. 

Yttrium and lanthanum are both obtained from lanthanide minerals and the method of extraction depends on the particular mineral involved. Digestions with hydrochloric acid, sulfuric acid, or caustic soda are all used to extract the mixture of metal salts. Prior to the Second World War the separation of these mixtures was effected by fractional crystallizations, sometimes numbered in their thousands. However, during the period 1940-45 the main interest in separating these elements was in order to purify and characterize them more fully. The realization that they are also major constituents of the products of nuclear fission effected a dramatic sharpening of interest in the USA. As a result, ion-exchange techniques were developed and, together with selective complexation and solvent extraction, these have now completely supplanted the older methods of separation (p. 1228). In cases where the free metals are required, reduction of the trifluorides with metallic calcium can be used. 

Because p-xylene is the most valuable isomer for producing synthetic fibers, it is usually recovered from the xylene mixture. Fractional crystallization used to be the method for separating the isomers, but the yield was only 60%. Currently, industry uses continuous liquid-phase adsorption separation processes.The overall yield of p-xylene is increased... 

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