Crystallisation spontaneous

Di-(1,2-0-cyclohexylidene-a-D-xj/o-pentodialdofuranose-5-hydrate)-5,5 3, 5-dianhydride. Add a solution of 14.3 g (0.061 mol) of sodium metaperiodate in 220 ml of water dropwise to a well-stirred solution of 17.4 g (0.067 mol) of 1,2-O-cyclohexylidene-a-D-glucofuranose (Expt 5.115) in 50 ml of water (1)- Stir for a further 30 minutes and remove the water at a temperature below 50 °C by evaporation under reduced pressure. Extract the solid residue with three 75 ml portions of dichloromethane and dry the combined extracts over magnesium sulphate. Filter and evaporate to give a residue which crystallises spontaneously. After recrystallisation from acetone the dimer has m.p. 182— 183 °C, the yield is 7.9 g (51%). 

The degree of supercooling to which liquid phosphorus can be subjected without solidification also depends on its previous history. If the cooling be slow, pure liquid phosphorus may be kept for days at 18° C. If the liquid be heated to 100° C. and suddenly cooled to ordinary temperatures it crystallises spontaneously in a few seconds without inoculation.1 2- 3... 

An important feature of transient supersaturation is its duration or, in other words, the rates of solute nucleation and crystal growth. Some of the factors that influence these rates have already been discussed for the case of pure water. For crystallisation from solutions, two additional factors are the viscosity of the residual freeze-concentrated liquid phase and probably also the conflgurational complexity of the crystal structures of the solutes involved. Thus, for NaCl, a degree of supersaturation of up to 6 M, reached at ca. —25°C, seems reasonable, at which temperature the salt is likely to crystallise spontaneously. For a typical cooling rate of 10°C min the duration of supersaturation would then be ca. 5 min, with the NaCl concentration transient reaching 6 M ... 

Then filter off the solid azoxybenzene at the pump, wash it thoroughly with water, and drain well. Recrystallise from a minimum of m ethylated spirit, allowing the hot solution to cool spontaneously (with occasional stirring) until crystallisation starts, and then cool in ice-water. If crystallisation is delayed, seed the solution with a trace of the crude product if on the other hand the azoxybenzene separates at first as an emulsion, add methylated spirit, drop by drop, with stirring until the solution is clear, and then allow the cooling to proceed as before. The... 

Finally, add an excess of concentrated hydrochloric acid slowly with stirring to the alkaline filtrate remaining from the original reaction product. As the solution becomes acid, the sulphonyl-aniline separates as a thick sticky syrup which, when stirred, rapidly crystallises. Cool the mixture in ice-water if necessary, and then filter off the solid product at the pump, wash well with water, and drain. Recrystallise from a mixture of 2 volumes of ethanol and i volume of water to prevent the sulphonyl-aniline from separating as an emulsion, allow the hot solution to cool spontaneously (with occasional stirring) until crystallisation starts, and... 

B) Methiodi s. Members of Classes (i), (ii) and (iv) combine wdth methyl iodide (some very vigorously) to form quaternary methiodides. It is best to add the amine to an excess of methyl iodide dissolved in about twice its volume of methanol, allow any spontaneous reaction to subside, and then boil under reflux for 30 minutes (extend to 1 hour for Class (iv) except pyridine and quinoline). The methiodide may crystallise when the reaction-mixture cools if not, evaporate the latter to small bulk or to dryness, and recrystallise, (M.ps., pp. 553-554 )... 

Orientation. Most articles made of HDPE, including film, fiber, pipes, and injection-molded articles, exhibit some degree of molecular and crystal orientation (21). In some cases, orientation develops spontaneously for example, during melt flow into a mold and its subsequent crystallisation. When blown HDPE film and fiber are manufactured, orientation can be introduced dehberately by stretching. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

In 1979 the bieyclic diol exo-2,ejco-6-dihydroxy-2,6-dimethylbicyclo[3.3.1]nonane (i) was prepared and observed to co-crystallise with various solvents, including ethyl acetate, chloroform, toluene, dioxane, and acetone. A crystal structure determination of the ethyl acetate compound revealed the occurrence of a helical canal host structure, containing ethyl acetate as guest (with 3 1 diol ethyl acetate stoichiometry), and that spontaneous resolution had occurred on crystallisation of the multimolecular inclusion compound 6>. 

An ethereal solution of some 100 g of the crude nitrile was allowed to spontaneously evaporate and crystallise. The crystalline slurry so produced exploded violently without warning. Previously such material had been found not to be shock-sensitive to hammer blows, but dry recrystallised material was very shock-sensitive. Traces of free hydrogen azide could have been present, and a metal spatula had been used to stir the slurry, so metal azides could have been formed. See Other CYANO COMPOUNDS, ORGANIC AZIDES... 

The very high friction-sensitivity, particularly of large crystals, and brisance on explosion are to be expected from the thermodynamic properties of the salt. Its great sensitivity, even under water, renders it unsuitable as a practical detonator [1], Spontaneous explosions dining interciystalline transformations have been observed, or on crystallisation from hot water [2], A safe method of preparing solutions in aqueous THF for synthetic purposes is available [3],... 

In a (stereo-speeifie) rubber spontaneous crystallisation occurs under strain here also small regions in whieh ehain parts lie parallel, aet as nuclei.These nuelei, which improve the strength eonsiderably, do, however, not grow out into a continuous phase they disappear upon stress release. 

Crystallisation by Evaporation.—This method is employed when the substance is so easily soluble in all solvents (hot and cold) that it will only separate after partial evaporation. The solvent is allowed to evaporate spontaneously in the air or in a desiccator if in the latter the evaporation is greatly hastened by using a suitable absorbent as well as evacuating the desiccator. The type of vessel employed depends on the volatility of the solvent obviously the conical flask already recommended for crystallisation by cooling is not suitable for spontaneous evaporation, while a beaker or shallow crystallising dish is. When the latter type of vessel is used, crusts often form on the sides above the surface of the liquid. Such crusts seldom consist of pure substance, and they should be carefully removed with a spatula before attempting to filter off the crystals. 

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