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10 Ralf Steudel · Bodo Eckert rine at 0–20 C in a CS2/CCl4 mixture; see Eq. (7). In the solid state the S11 molecules are of approximate C2 symmetry [37, 38]. 2.1.1.8 Preparation of S12 Thermodynamically, S12 is the second most stable sulfur ring after S8. There- fore, S12 is formed in many chemical reactions in which elemental sulfur is a product. In addition, S12 is a component of liquid sulfur at all temperatures. The same holds for S18 and S20 which are often formed together with S12: 1. The preparation of S12 from titanocene pentasulfide and SCl2 has been de- scribed above under “Preparation of S6”: 2Cp2TiS5þ2SCl2 ! S12þCp2TiCl2 ð18Þ 2. Preparation of S12 from S2Cl2 and a polysulfane mixture H2Sx: sulfanes H2Sn and dichlorosulfanes SnCl2 react with each other with elimination of HCl forming new S-S bonds. Since pure sulfanes with more than two sulfur atoms are difficult to prepare, this synthesis uses a mixture of sulfanes, called “crude sulfane oil”, which can easily be prepared from aqueous sodi- um polysulfide and concentrated hydrochloric acid at 0 C [39, 40]: Na2S4þ2HCl ! H2S4þ2NaCl ð19Þ Since the aqueous sodium polysulfide contains already several polysulfide anions in equilibrium and since the acidification results in some intercon- version reactions, a sulfane mixture H2Sx is obtained rather than pure H2S4. This mixture nevertheless reacts in dry CS2/Et2O mixture at 20 C with dichlorodisulfane, besides other products, to S12 which has been isolated in 4% yield by extraction with CS2 and fractional crystallization [41]: 2S2Cl2þ2H2S4 ! S12þ4HCl ð20Þ Evidently, this reaction proceeds in several steps with H-S6-Cl and H-S12-Cl as likely intermediates. 3. Preparation of S12, S18 and S20 from liquid sulfur: liquid sulfur after equili- bration contains sulfur homocycles of all sizes [19] and some of these can be isolated by quenching, extraction, fractional precipitation and crystalli- zation depending on their differing solubilities. Commercial elemental sul- fur (several hundred gram) is heated electrically to about 200 C for 5– 10 min or longer and is then allowed to cool to 140–160 C within ca. 15 min. As soon as the melt has become less viscous, it is poured in as thin a stream as possible into liquid nitrogen in order to quench the equilibri- um. The boiling nitrogen ruptures the melt into small pieces resulting in a yellow powder. The liquid nitrogen is decanted off this powder which is then extracted with CS2 at 20 C (solution “A”). A small amount of polymer- ic sulfur remains undissolved and is filtered off. The yellow solution isPDF Image | Topics in Current Chemistry
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