Sulphur atoms, reaction + olefins

Further investigations into the reactisulphur atoms with olefins to yield thiirans have established absolute rate constants and a n ative temperature dependence on the rate constants for the reaction of sulphur in its state, obtained by flash i otolysis of carbon oxysulphide, with... 

With halogenated olefins and alkynes, the same competitive procedures have given calculated log k values as reported in Table 31. While these may all be self-consistent, there are no data from absolute rate studies. It is unfortunate that sulphur atom chemistry has not attracted other workers with different ideas and techniques, so that fresh view-points and alternative experimental procedures could be brought to bear on the reaction. This is not to impugn the results of Gunning and Strausz in any way, but simply to say that new approaches would add vitality to the area. 

The increasing reactivity with alkyl substitution in the acetylenes is very pronounced something in excess of a factor of 100 in relative rates at 298°K. This, it may be observed, is substantially greater than the factor of 20 between the reactivities of ethylene and 2-butene. Such large relative reactivity differences are rare in free radical reactions, and one realizes that the triplet sulphur atom must be very selective in its reaction with different olefins. A comparison of the behaviour of the triplet state atoms of oxygen, sulphur and selenium will be deferred until the end of the discussion of this whole group of atom additions. 

Clearly, more research needs to be done. For instance, there is a real need for more detailed product studies. The initial products with oxygen and sulphur atoms are the epoxides and episulphides. With selenium and tellurium the cyclic adducts are apparently only transient, so there is no accumulation of initial product to serve as a measure of the reaction rate. Callear and Tyerman [123] measure the selenium—olefin reaction rate by photometric methods, in which the appearance of bands attributed to an alkyl selenide are observed. These products are not stable, and decay according to... 

Some of the earliest experiments which demonstrated that this theory was oversimplistic were carried out by Meyer and Hohenem-ser (1935) using cyclohexene as a model alkene. However their conclusions, which involved the saturation of two double bonds per sulphur atom combined and the release and subsequent recombination of H2S, involved a reaction scheme built round the small fraction of the reaction products that they were able to identify. That their conclusions were erroneous was eventually demonstrated by Bloomfield et al. in 1946 but this should not be allowed to detract from their pioneering efforts with model olefins. 

Theoretical.—Molecular orbital calculations have been reported for the thiiranium ions derived from ethylene, propylene, isobutene, and butadiene. The most stable system involves a p-configuration of the valence electrons of sulphur. Differences in the extension of the lowest unoccupied MO s on the ethylene carbon atoms were said to be important in determining the regio-specificity of ring-opening addition reactions with chloride ion. Theoretical calculations on C2H4SH indicate that a bridged thiiranium structure would be more stable as an intermediate than an open structure in electrophilic additions of RSX to olefins. The barrier to pyramidal inversion at the sulphur atom was calculated to be 78.11 kcal mol . 

Only fluorine, atomic oxygen and FgO have higher redox potentials. The gas oxidises moist sulphur to jH2S04, raises silver(I) compounds to the 2 state and converts olefinic compounds to ozonides. The reaction 2O3 -> SOg, which is catalysed by many metals and metal oxides, is exothermic and rapid above 200°. Gaseous ozone is deeper blue than oxygen it condenses at — 112° to a dark blue liquid which freezes at —193° to a dark purple solid. Surprisingly, the liquid is not completely miscible with liquid oxygen. 

Certainly the very substantial spread of relative reactivities that was observed for the reaction of much the same olefins with oxygen, sulphur and bromine atoms, discussed in an earlier section, is not observed with nitrogen atoms. 

A number of interesting reactions of the dimer (66) with nucleophiles have been described. Addition of sodium methoxide in methanol yields dimethoxy-ether (71), by replacement of the olefinic fluorine atom and of an allylic fluorine, which in turn yields keten (72) by reaction with sulphur trioxide at 30—40 Diverse... 

It is apparent that substitution of fluorine for hydrogen initially causes a reduction in reactivity towards the electrophilic oxy n atom, but tetra-fluoroethylene is anomalous. A further study has indicated that substitution by trifluoromethyl has a strong deactivating effect compared with methyl, which has an activating effect. A study of the reactions of nearly thermal i F atoms, produced by F(n,2n) F and moderated by collisions with an excess of sulphur hexafluoride, with fluoro-oleflns (modes of addition were identified by scavenging the radicals produced with hydrogen iodide) has indicated that F atoms react preferentially with less-fluorinated olefins, and at the less-fluorinated end of a particular olefin. ... 

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