Efficient Fractionation

We are now in a position to summarize the presented material and deduce the best conditions for fraction preparation. The collected data covering the conceivable cricumstances fairly completely the conclusions may be expected to have general validity. Of course, they hold true only for fractionation involving distribution over two partially miscible liquids in equilibrium systems. 

The generally recommended conditions (small fraction size, large volume ratio of the phases) are not necessarily the best. On the contrary, in most cases they must be expected to yield worse results than would be obtained under other, less obvious, circumstances. If the initial distributions have a positive skewness (peak at low molecular weight and long 

If the distribution of the parmt polymer is not known, as is usually the case, this extraction method is not quite reliable. If the distribution has a negative skewness or the polymer is contaminated with low-molecular-weight material, the result of extraction is liable tobeunsatishictory in that the extracted fraction may have a wider dbtribution than the initial polymer. 

An examination of all calculated cases presented here discloses that, whatever the nature of the system or the shape of the initial distribution, b (x) nearly always drops steeply in the rai e of large x-values. Evidently, a reliable preparative method must make use of this general feature. Hence, repeated extraction of the concentrated phase riiould yield the best results obtainable. By its very nature the procedure is highty suited for application in large-scale fractionation work. 

If in the course of such a process, the distribution of the fiaction in the concentrated phase has narrowed down, the descent of b x) becomes less evident and, eventually, vanishes nearly altogether. This is illustrated in Fig. 24. A further substantial sharpening of the fraction distribution calls for a very large number of separation steps. 

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The efficient fractionating columns in general use may be divided into two main groups —... 

Pure pyridine may be prepared from technical coal-tar pyridine in the following manner. The technical pyridine is first dried over solid sodium hydroxide, distilled through an efficient fractionating column, and the fraction, b.p. 114 116° collected. Four hundred ml. of the redistilled p)rridine are added to a reagent prepared by dissolving 340 g. of anhydrous zinc chloride in a mixture of 210 ml. of concentrated hydrochloric acid and 1 litre of absolute ethyl alcohol. A crystalline precipitate of an addition compound (probable composition 2C5H5N,ZnCl2,HCl ) separates and some heat is evolved. When cold, this is collected by suction filtration and washed with a little absolute ethyl alcohol. The yield is about 680 g. It is recrystaUised from absolute ethyl alcohol to a constant m.p. (151-8°). The base is liberated by the addition of excess of concentrated... 

The 40-45 per cent, hydrazine solution may be concentrated as follows. A mixture of 150 g. (144 ml.) of the solution and 230 ml. of xylene is distilled from a 500 ml. round-bottomed flask through a well-lagged Hempel (or other efficient fractionating) column fitted into a cork covered with tin foil. All the xylene passes over with about 85 ml. of water. Upon distillation of the residue, about 50 g. of 90-95 per cent, hydrazine hydrate (5) are obtained. 

Any other efficient fractionating column may be used, e.g., an all-glaas Dufton column—see Sections 11,15 and 11,17. 

Place 35 ml. of water in the separatory funnel and run it into the vigoroiisly stirred reaction mixture at such a rate that rapid refluxing occurs. Follow this by a cold solution of 15-5 ml. of concentrated sulphuric acid in 135 ml. of water. Two practically clear layers will now be present in the flask. Decant as much as possible of the ethereal layer A) into a 500 ml. round-bottomed flask. Transfer the remainder, including the aqueous layer, into a separatory funnel wash the residual solid with two 10 ml. portions of ether and combine these washings with the liquid in the separatory funnel. Separate the ethereal portion and combine it with (A). Distil off the ether through an efficient fraction-... 

Freshly distilled ethyl formate must be used. Commercial ethyl formate may be purified as follows. Allow the ethyl formate to stand for 1 hour with 16 per cent, of its weight of anhydrous potassium carbonate with occasional shaking. Decant the ester into a dry flask containing a little fresh anhydrous potassium carbonate and allow to stand for a further hour. Filter into a di flask and distil through an efficient fractionating column, and collect the fraction, b.p. 53-54° protect the receiver from atmospheric moisture. 

The lower pyridine layer contains most of the excess of thionyl chloride it may be recovered by distillation through an efficient fractionating column. 

The approximate dimensions of the packing are 25 cm. X 18-20 mm. Any other form of efficient fractionating column may bo used. 

Ethyl acetate. Use 58 g. (73-5 ml.) of absolute ethyl alcohol, 225 g. of glacial acetic acid and 3 g. of concentrated sulphuric acid. Reflux for 6-12 hours. Work up as for n-propyl acetate. B.p. 76- 77°. Yield 32 g. Much ethyl acetate is lost in the washing process. A better yield may be obtained, and most of the excess of acetic acid may be recovered, by distilhng the reaction mixture through an efficient fractionating column and proceeding as for methyl acetate. 

Ethyl n-butyrate. Use a mixture of 88 g. (92 ml.) of n-butyric acid, 23 g. (29 ml.) of ethanol and 9 g. (5 ml.) of concentrated sulphuric acid. Reflux for 14 hours. Pour into excess of water, wash several times with water, followed by saturated sodium bicarbonate solution until all the acid is removed, and finally with water. Dry with anhydrous magnesium sulphate, and distU. The ethyl n-but3rrate passes over at 119 5-120-5°, Yield 40 g. An improved yield can be obtained by distilhng the reaction mixture through an efficient fractionating column until the temperature rises to 125°, and purifying the crude ester as detailed above under methyl acetate. 

The benzyl chloride may also be isolated by distillation under atmospheric pressure. The material boiling between 165° and 185° is collected and redistilled the final product is collected at 178-182° (pure benzyl chloride has b.p. 179°). The resulting benzyl chloride is, however, of lower purity unless an efficient fractionating column is used. 

The ester and catalj st are usually employed in equimoleciilar amounts. With R =CjHs (phenyl propionate), the products are o- and p-propiophenol with R = CH3 (phenyl acetate), o- and p-hydroxyacetophenone are formed. The nature of the product is influenced by the structure of the ester, by the temperature, the solvent and the amount of aluminium chloride used generally, low reaction temperatures favour the formation of p-hydroxy ketones. It is usually possible to separate the two hydroxy ketones by fractional distillation under diminished pressure through an efficient fractionating column or by steam distillation the ortho compounds, being chelated, are more volatile in steam It may be mentioned that Clemmensen reduction (compare Section IV,6) of the hj droxy ketones affords an excellent route to the substituted phenols. 

Place 150 g. of benzoic acid, 150 g. (139 ml.) of acetic anhydride and 0-2 ml. of syrupy phosphoric acid in a 500 ml. bolt-head flask. Fit the latter with a two-holed stopper carrying a dropping funnel and an efficient fractionating column (compare Fig. 7/7, 61, 1) it is advisable to lag the latter with asbestos cloth. Set up the flask in an oil bath or in a fusible metal bath. Distil the mixture very slowly and at such a rate that the temperature of the vapour at the head of the column does... 

Tar. Before the development of gas chromatography (gc) and high pressure Hquid chromatography (hplc), the quantitative analyses of tar distillate oils involved tedious high efficiency fractionation and refractionation, followed by identification or estimation of individual components by ir or uv spectroscopy. In the 1990s, the main components of the distillate fractions of coal tars are deterrnined by gc and hplc (54). The analytical procedures included in the specifications for tar bulk products are given in the relevant Standardi2ation of Tar Products Tests Committee (STPTC) (33), ISO (55), and ASTM (35) standards. 

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