THE PURIFICATION OF COMMON ORGANIC SOLVENTS

Reasonably pure solvents are required for many organic reactions and for recrystaUisations methods for obtaining these from commercial products will accordingly be described. Frequently, the pure solvent (e.g., the analytical reagent) can be purchased, but the cost is usually iiigh, particularly if comparatively large volumes are required. Furthermore, it is excellent practice for the student to purify inexpensive commercial solvents. 

Absolute diethyl ether. The chief impurities in commercial ether (sp. gr. 0- 720) are water, ethyl alcohol, and, in samples which have been exposed to the air and light for some time, ethyl peroxide. The presence of peroxides may be detected either by the liberation of iodine (brown colouration or blue colouration with starch solution) when a small sample is shaken with an equal volume of 2 per cent, potassium iodide solution and a few drops of dilute hydrochloric acid, or by carrying out the perchromio acid test of inorganic analysis with potassium dichromate solution acidified with dilute sulphuric acid. The peroxides may be removed by shaking with a concentrated solution of a ferrous salt, say, 6-10 g. of ferrous salt (s 10-20 ml. of the prepared concentrated solution) to 1 litre of ether. The concentrated solution of ferrous salt is prepared either from 60 g. of crystallised ferrous sulphate, 6 ml. of concentrated sulphuric acid and 110 ml. of water or from 100 g. of crystallised ferrous chloride, 42 ml. of concentrated hydiochloric acid and 85 ml. of water. Peroxides may also be removed by shaking with an aqueous solution of sodium sulphite (for the removal with stannous chloride, see Section VI,12). 

In practice, it is best to purify a quantity, say one Winchester quart bottle, of technical 0 720 ether to cover the requirements of a group of students. The Winchester quart of ether is divided into two approximately equal volumes, and each is shaken vigorously in a large separatory funnel with 10-20 ml. of the above ferrous solution diluted with 100 ml. of water. The latter is removed, the ether transferred to the Winchester bottle, and 150-200 g. of anhydrous calcium chloride is added. The mixture is allowed to stand for at least 24 hours with occasional shaking. Both the water and the alcohol present are thus largely removed. The ether is then filtered through a large fluted filter paper into another clean dry Winchester bottle (CAUTION all flames in the vicinity must be 

Attention is directed to the fact that ether is highly inflammable and also extremely volatile (b.p. 35°), and great care should be taken that there is no naked flame in the vicinity of the liquid (see Section 11,14). Under no circumstances should ether be distilled over a bare flame, but always from a steam bath or an electrically-heated water bath (Fig.//, 5,1), and with a highly efficient double surface condenser. In the author s laboratory a special lead-covered bench is set aside for distillations with ether and other inflammable solvents. The author s ether still consists of an electrically-heated water bath (Fig. 11, 5, 1), fitted with the usual concentric copper rings two 10-inch double surface condensers (Davies type) are suitably supported on stands with heavy iron bases, and a bent adaptor is fitted to the second condenser furthermost from the water bath. The flask containing the ethereal solution is supported on the water bath, a short fractionating column or a simple bent still head is fitted into the neck of the flask, and the stUl head is connected to the condensers by a cork the recovered ether is collected in a vessel of appropriate size. 

Di-teo-propyl ether. The commercial product usually contains appreciable quantities of peroxides these should be removed by treatment with an acidified solution of a ferrous salt or with a solution of sodium sulphite (see under Diethyl ether). The ether is then dried with anhydrous calcium chloride and distilled. Pure di-iao-propyl ether has b.p. 68-5°/760 mm. 

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Both polymers 10 and 11 are soluble in common organic solvents, melt without decomposition, and can be drawn into the fibers. Molecular weights of the polymers 10 and 11, determined by gel permeation chromatography with tetrahydrofuran as the eluant after purification by reprecipitation from benzene-ethanol, showed a broad monomodal molecular weight distribution. The degree of polymerization depends on particle size of sodium metal. Polymers with molecular weights of 23,000-34,000 are always obtained, if fine sodium particles are used. 

It should also be stressed here that many of these complexes are neutral and therefore relatively soluble in common organic solvents, an important issue for their purification and crystallization. Among all these paramagnetic complexes, only a fraction has been investigated for their magnetic properties in the solid state,... 

As was pointed out in Section 2.18, the crude products of most organic reactions are multicomponent mixtures, and a convenient initial isolation procedure, for the first stages of both the separation of such mixtures and of the purification of the components, may involve solvent extraction processes. The general cases which are discussed below to illustrate the technique of solvent extraction are selected to cover many of the commonly met systems. The student is recommended to refer to the comments in Section 2.18 on the necessity of assessing the chemical and physical nature of the components of a particular reaction mixture with regard to their solubilities in solvents, and to their acidic, basic or neutral characteristics. 

This last sentence brings us to the trade-off for the high stability of the decaphenylmetailocenes The solubility decreases dramatically to the extent that these compounds can be considered almost insoluble in all common organic solvents. From the space-filling plot in Fig. 3 one can imagine that the tightly knit, spherelike particle minimizes van der Waals interactions with solvent molecules. Therefore, purification becomes a... 

Note The palladium II) chloride adduct is practically insoluble in common organic solvents, but chloride abstraction with AgOTf yields a cationic complex that is soluble in acetonitrile - the method is recommended for the purification of these compounds [126],... 

The most commonly encountered of these are dimethyl sulphoxide (DMSO) and N,N-dimethylformamide (DMF). With these solvents it is often possible to remove most if not all by adding the quenched reaction mixture to a relatively large volume of water, and extracting this several times with ether. The combined organics are then washed with more water. Any remaining DMF or DMSO will need to be removed in further purification of the crude product. 

The purification of the protected peptide intermediates is an important aspect of the CSPPS strategy to ensure homogeneous molecular species, free from single-residue deletion peptides and other impurities. However, a prerequisite for any chromatographic purification is adequate solubility of the material to be purified in a solvent compatible with the procedure. Protected peptides exhibit unpredictable, but generally poor, solubility in water and in most of the commonly used organic solvents, which makes them difficult to purify and causes some of the most serious problems in CSPPS. 

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