Sulfur: reactivity

   Sulfur is a reactive element. It burns in air with a blue flame to give SO₂, and reacts with F₂, Cl₂ and Br₂ (equation 1.1).F or the syntheses of other halides and oxides.

                              (1.1)

     Sulfur does not react directly with I₂, but in the presence of SbF₅, the salt [S₇I][SbF₆]  is produced; the cation [S₇I]⁺‏ possesses structure 1.1. When treated with hot aqueous alkali, sulfur forms a mixture of polysulfides, [S_x]^2- , and polythionates( 1.2), while oxidizing agents convert it to H₂SO₄.

(1.1)                                        (1.2)

   Saturated hydrocarbons are dehydrogenated when heated with sulfur, and further reaction with alkenes occurs. An application of this reaction is in the vulcanization of rubber, in which soft rubber is toughened by cross-linking of the polyisoprene chains, making it suitable for use in, for example,  tyres. The reactions of sulfur with CO or [CN]^- yield OCS (1.3) or the thiocyanate ion (1.4), while treatment with sulfites gives thiosulfates (equation 1.2).

(1.3)                                                     (1.4)

                   (1.2)

    The oxidation of S₈ by AsF₅ or SbF₅ in liquid SO₂ yields salts containing the cations [S₄]²⁺‏, [S₈]²⁺‏ (Figure 1.1a) and [S₁₉]²⁺‏. In reaction 1.3, AsF₅ acts as an oxidizing agent and a fluoride acceptor (equation 1.4).

Fig. 1.1 (a) Schematic representation of the structure of [S₈]²⁺‏. (b) The change in conformation of the ring during oxidation of S8 to [S₈]^2‏+. (c) Structural parameters for [S₈]²⁺‏ from the [AsF₆]^-  salt. (d) One resonance structure that accounts for the transannular interaction in [S₈]^2‏+. (e) Schematic representation of the structure of [S₁₉]²⁺‏; the rings are puckered.

          (1.3)

                           (1.4)

   Two-electron oxidation of S₈ results in a change in ring conformation (Figure 1.1a). The red [S₈]^2‏+ cation was originally reported as being blue, but the blue colour is now known to arise from the presence of radical impurities such as [S₅]‏.  In S₈, all the S^_S bond lengths are equal (206 pm) and the distance between two S atoms across the ring from one another is greater than the sum of the van der Waals radii (r_v = 185 pm). The structure of the [AsF₆]^- salt of [S₈]^2‏+ has been determined and Figure 1.1c illustrates (i) a variation in S^_S bond distances around the ring and (ii) cross-ring S^_S separations that are smaller than the sum of the van der Waals radii, i.e. [S₈]^2‏+‏ exhibits transannular interactions. The most important transannular interaction corresponds to the shortest S…..S contact and Figure 1.1d shows a resonance structure that describes an appropriate bonding contribution. The [S₄]^2+‏ cation is square (S^_S = 198pm) with delocalized bonding. In [S₁₉]^2‏+ (Figure 1.1e), two 7-membered, puckered rings are connected by a five-atom chain. The positive charge can be considered to be localized on the two 3-coordinate S centres.

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