Crystallisation from mechanisms

Sometimes the crude substance may contain an insoluble impurity, and on cooling the solution it may be difficult to judge how much of the solid matter is merely undissolved impurity and how much is solute which has subsequently crystallised from solution. To avoid this difficulty, the hot solution should be filtered, and should thus always be absolutely clear before cooling is attempted. Therefore filter the hot solution into a clean tube through a very small fluted filter-paper contained in a correspondingly small glass funnel, which should have had its stem cut off as that shown in Fig. 6, p. 12 (and for the same reason). Unless the upper part of the filter is cut awav to reduce its size to a minimum, a large proportion of the solution will remain held mechanically in the pores of the paper itself and only a few drops of clear filtrate will be obtained. 

Another example of this is the loss of acetic acid when delphinine is heated in hydrogen at 200-215°. Just as aconitine is so converted into pyraconitine so delphinine yields pyrodelphinine, C3 H4 0,N, m.p. 208-212°, and similarly a-oxodelphinine, C33H430j qN, under like treatment loses acetic acid and yields pyro-a-oxodelphinine, C3 H3gOgN, which crystallises from methyl alcohol in needles, m.p. 248-250°, after sintering at 238°. This, on hydrogenation, forms a hexahydro-derivative, m.p. 183-5°, presumably by saturation of the benzoyl radical, which therefore leaves unexplained the mechanism by which acetic acid is lost in this pyrolytic reaction (c/. pyropseudaconitine, p. 683). 

The size of particles may be increased from molecular dimensions by growing them by crystallisation from both solutions and melts as discussed in Chapter 15. Here, dissolving and recrystallising may provide a mechanism for controlling both particle size and shape. It may be noted, as also discussed in Chapter 15, that fine particles may also be condensed out from both vapours and gases. 

In the wet way selenides are best produced by allowing the solution of the metallic salt to drop slowly into a saturated aqueous solution of hydrogen selenide which is mechanically stirred. In this way the selenide is formed in the presence of excess of hydrogen selenide, no excess of metal ions being at anytime present in the solution.1 The alkali selenides may be obtained by the action of hydrogen selenide on the corresponding carbonates in aqueous solution in an atmosphere of nitrogen,2 followed by crystallisation from solution. 

Crude anthracene (about 40%) is mixed in a vessel fitted with a mechanical stirrer, with 1J times its weight of benzol or solvent naphtha (90% at 160°) this specification means that 90% by volume distils up to 160°. Sodium nitrite to one-tenth of the weight of crude anthracene taken is dissolved in 10 times its weight of water, and sufficient 10% sulphuric acid (7-2 gms. for each 1 gm. of 10% nitrite) to decompose this quantity of nitrite is added to the benzol—anthracene mixture and the temperature maintained at 25°. The nitrite solution is then run in at such a speed that no red fumes escape. When all the solution has been added the mixture is filtered at the pump. The filtrate consists of two layers, one of sodium sulphate solution and one of solvent naphtha, or benzol containing the impurities such as nitroso-carbazol. The purified anthracene on the filter is washed with benzol or solvent naphtha this latter on a large scale is used for the final treatment of a fresh lot of crude anthracene. The initial benzol or solvent naphtha, after separation from the aqueous solution, is recovered by distillation. The anthracene from this treatment will be about 80% pure. It may be purified to 95% by crystallising from heavy bases (pyridine, etc.), and is finally raised by sublimation and re-crystallisation from benzol to 98%. (For the estimation of purity, see p. 497.) (C. T., 23, 8, 21 D.R.P., 122852.)... 

Four-membered rings can be synthesised by [2 + 2] cycloadditions. However, thermal [2 + 2] cycloadditions occur only with difficulty they are not concerted but involve diradicals. Photochemical [2 + 2] reactions are common and although some of these may occur by a stepwise mechanism many are concerted. An example of a [2 + 2] reaction is the photodimerisation of cyclopent-2-enone. This compound, as the neat liquid, or in a variety of solvents, on irradiation with light of wavelength greater than 300 nm (the n - n excited state is involved) is converted to a mixture of the head-to-head (48) and head-to-tail (49) dimers, both having the cis,anti,cis stereochemistry as shown. It is believed that the reaction proceeds by attack of an n - n triplet excited species on a ground state molecule of the unsaturated ketone (Section 2.17.5, p. 106). In the reaction described (Expt 7.24) the cyclopent-2-enone is irradiated in methanol and the head-to-tail dimer further reacts with the solvent to form the di-acetal which conveniently crystallises from the reaction medium as the irradiation proceeds the other dimer (the minor product under these conditions) remains in solution. The di-acetal is converted to the diketone by treatment with the two-phase dilute hydrochloric acid-dichloromethane system. 

In examining a crystalline structure as revealed by diffraction experiments it is all too easy to view the crystal as a static entity and focus on what may be broadly termed attractive intermolecular interactions (dipole-dipole, hydrogen bonds, van der Waals etc., as detailed in Section 1.8) and neglect the actual mechanism by which a crystal is formed, i.e. the mechanism by which these interactions act to assemble the crystal from a non-equilibrium state in a super-saturated solution. However, it is very often nucleation phenomena that are ultimately responsible for the observed crystal structure and hence we were careful to draw a distinction between solution self-assembly and crystallisation at the beginning of this chapter. For example paracetamol, when crystallised from acetone solution gives the stable monoclinic crystal form I, but crystallisation from a molten sample in the absence of solvent... 

A mixture of 5-phthalimidopentyl bromide (Drake and Garman, J.Amer. Chem. Sec, 1949, 71, 2426) (296.0 g) and potassium p-nitrophenoxide (177.0 g) in ethanol (1.5 L) was stirred mechanically, boiled under reflux for 20 h, concentrated to half its volume, cooled and filtered. The product was washed with water, dried and crystallised from acetone, giving l-p-nitrophenoxy-5-phthalimidopentane, melting point 123°-124°C. 

Throughout the history of the synthesis of zeolite-like materials, there has been much debate about the location of the structure-forming activity [28,36,39,50]. At one extreme, there is the possibility of solution-mediated crystallisation. In this view, the reactants dissolve (to a greater or lesser extent) in the reaction mixture to afford active species, from which the product is formed as it crystallises from the solution. In the opposite view, the solution is seen as more or less inert with the product being formed within the gel phase by a process described as a solid state transformation . Whereas the solution-mediated route is well known in the science of crystallisation, the alternative is a somewhat shadowy phenomenon for which no chcmically-specific mechanism has ever been published. 

The previous section has highlighted the complex mechanisms involved in the deposition process associated with crystallisation from solution. For this reason adequate mathematical models for describing the process are difficult to formulate. Reviews of fouling by crystallisation have been made by Hasson [1981] and Bott [1988] that include some basic models. 

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