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Solid Sulfur Allotropes 11 cooled to 78 C for 20 h whereupon a mixture of much S8 (large yellow crystals) and little S12·CS2 (small, almost colorless crystals) crystallizes out. The latter can be separated by rapid flotation in CS2 yielding pure S12·CS2 in 0.2% yield based on the initial amount of elemental sulfur [42, 43]. On prolonged standing in air the crystals of S12·CS2 loose their solvent and con- vert to a powder of S12, single crystals of which can be obtained by recrys- tallization from hot benzene or toluene resulting in pale-yellow needle-like crystals of m.p. 146–148 C. The above CS2 solution “A” from which S12·CS2 and most of the S8 has crys- tallized out is used for the preparation of S7, S18, and S20 as follows. Stirring of solution “A” at 78 C after addition of some finely ground glass powder (or S7 seed crystals) for about 2 h results in the precipitation of finely pow- dered sulfur which is isolated by removing the solution by means of an im- mersion filter frit. The residue is extracted three times with small amounts of toluene leaving an orange residue “B”. S7 crystallizes from the toluene so- lution on cooling to 78 C and may be recrystallized from CS2. Yield: 0.7% based on the initial amount of elemental sulfur [44]. The amorphous orange residue “B” consists of a mixture of sulfur rings Sx with x possibly ranging up to 50 or more. The mean molecular mass corre- sponds to an average value of x=25. The rings up to x=28 have been detect- ed chromatographically by HPLC. Sx is stable only in CS2 solution; on standing of a concentrated solution at 20 C for 2–3 days small crystals of endo-S18 (intense yellow orthorhombic plates) and S20 (pale-yellow rods) precipitate. This crystal mixture can be separated by flotation in a CHCl3/ CHBr3 mixture since the density of endo-S18 is slightly higher than that of S20 (see below, Table 22). Yields: 0.02% endo-S18, 0.01% S20 [42, 43]. 4. Preparation of S12, S18, and S20 from S2Cl2 and potassium iodide: dichlorodisulfane, dissolved in CS2, reacts at 20 C with aqueous potassium iodide to a mixture of even-membered sulfur rings: nS2Cl2þ2nKl ! S2nþnI2þ2nKCl ð21Þ The main product is S6 (36%; see above) but by a sequence of precipitation and extraction procedures S12 (1–2%), endo-S18 (0.4%) and S20 (0.4%) have been prepared in a pure form in the yields given in parentheses [11]. 2.1.1.9 Preparation of S13 To prepare S13 by the ligand transfer reaction requires first the synthesis of the chain-like dichlorooctasulfane which is best achieved by carefully con- trolled chlorination of cyclo-S8 with elemental chlorine in a CS2/CCl4 mixture at 0–20 C: S8þCl2 ! S8Cl2 ð22ÞPDF Image | Topics in Current Chemistry
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