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14 Ralf Steudel · Bodo Eckert 2. 3. 4Cp2TiS5þ4SO2Cl2 ! S20þ4SO2þ4Cp2TiCl2 ð26Þ The probable reaction mechanism of this multistep reaction is given in Scheme 1 above (see the section “Preparation of S15”). The product was ob- tained as pale-yellow crystals in 8% yield, sometimes still containing traces of S10 which can be removed by recrystallization from CS2. The largest sul- fur ring detected by HPLC in this reaction mixture is S30 which has however not been isolated yet [14]. From liquid sulfur: small amounts of S20 have been isolated from quenched sulfur melts by extraction and fractional crystallization; see above under “Preparation of S12, S18 and S20 from liquid sulfur”. Preparation of S20 from S2Cl2 and potassium iodide: dichlorodisulfane, dis- solved in CS2, reacts at 20 C with aqueous potassium iodide to a mixture of even-membered sulfur rings; see Eq. (21). 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 pure form in the yields given in parentheses [11]. Sulfur-rich diiodosulfanes SnI2 are probably intermediates in this reaction which eliminate I2 intramolecularly with ring closure to S20. 2.1.2 Allotropes Consisting of Long Sulfur Chains (Polymeric Sulfur: Sm , Sy and Sw) Those forms of elemental sulfur which are insoluble even in carbon disulfide at 20 C have been termed as polymeric sulfur. These materials consist of chain-like macromolecules but the additional presence of large rings Sn (n>50) is very likely. In other words, polymeric sulfur is a mixture of chains of differing lengths and rings of differing sizes rather than a pure com- pound. The nature of the chain-terminating endgroups is unknown. In some cases crystalline phases have been obtained and the molecular structures were determined by X-ray crystallography. These phases are known as Sw1 and Sw2 and consist of helical chains (catenapolysulfur); they will be dis- cussed later. Otherwise, polymeric sulfur is often termed as m-sulfur or Sm but there is no principal difference between Sm and Sw. Polymeric sulfur is a component of liquid sulfur at all temperatures after the chemical equilibrium has been established which takes about 10 h at 120 C and correspondingly less at higher temperatures [19, 43, 48]. The polymer can be isolated by quenching the melt and extracting the soluble ring molecules with CS2 at 20 C. The polymer content of the melt increases from 1% at 135 C to a maximum of 45€10% at 250–300 C (different au- thors give differing maximum polymer concentrations) [49]. The quenching can be achieved by pouring the sulfur melt into water or, better, into liquid nitrogen [19, 43] as well as by blowing a thin stream of liquid sulfur by a jetstream of cold air against a sheet of aluminum on which the melt solidifies immediately as a thin film [50]. After quenching and extraction the poly- meric sulfur is initially amorphous but tends to convert to a microcrystal- line structure and, more slowly, to a-S8 on storage at room temperature.PDF Image | Topics in Current Chemistry
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