Dibenzothiophenes formation from biphenyls

Most researchers have found pseudo-first-order behavior for the various steps, and so it is possible to match theoretical curves with data to obtain the best rate constant values. Unfortunately, in most instances, too few data points were obtained to generate a unique theoretical fit. It is absolutely imperative that data be obtained for at least four conversion levels that are well spaced in the conversion matrix and extend to over 95% conversion. The partially hydrogenated dibenzothiophene intermediates are most often never detected as their desulfurization rates are extremely high (fcD, and kn2). The cyclohexylbenzenes and bicyclohexyls can arise from two different routes, and the concentrations of their precursors (biphenyl and cyclohexyl-biphenyl, respectively) pass through maximum values that can easily be calculated from the relative values of the formation and conversion rate constants. However, unique values for these relative rates can only be predicted if data are available well prior to and well beyond the times of maximum concentrations for these intermediates, because minor experimental errors can confuse curve-fitting optimization. 

In the laboratory PCDTs have been prepared by reactions between PCBs, sulfur and aluminum chloride and by direct chlorination of dibenzothiophene [28-30]. The temperature strongly affects the formation of TeCDTs from tetra-chlorobiphenyls and sulfur. Temperatures used were 120,160,200, and 240°C. Reaction times of 5-50 h were tried. The chlorinated biphenyls (50-200 mg) with sulfur (20-100 mg) and A1C13 (a few mg) were heated in closed tubes in an oven constructed for this purpose. After cooling the mixtures were extracted with toluene. More chlorine cleaving occurred at higher temperatures. In the case of a mixture of 3,4,3, 4 -tetrachlorobiphenyl and 2,3,3, 4 -tetrachlorobi-... 

Poliak et al. claimed that the reaction of biphenyl 32 with excess chlorosulfonic acid (six equivalents) at 18 °C affords dibenzothiophene-5,5-dioxide-3,7-disulfonyl chloride 34, but more recent attempts to reproduce this result were unsuccessful and only the 4,4 -disulfonyl chloride 33 (80%) was isolated. However, the cyclized product 34 was obtained in good yield (72%) when biphenyl 32 was heated with a large excess of chlorosulfonic acid (20 equivalents) at 150 °C (4 hours) (Equation 11). The reaction presumably involves the formation of the intermediate biphenyl-2,4,4 -trisulfonyl chloride, 35 which subsequently cyclizes with loss of hydrogen chloride to give the dibenzothiophene dioxide 34. Further study of the action of chlorosulfonic acid on biphenyl 32 showed that the optimum yield of the 4-sulfonyl chloride (43%) was derived from treatment of 32 with the reagent (1.5 equivalents) in thionyl chloride as solvent at 0 °C (1 week). The best yield (89%) of the 4,4 -disulfonyl chloride 33 was also achieved by treatment of the hydrocarbon 32 with chlorosulfonic acid (three equivalents) in thionyl chloride at room temperature (1 week). 

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