Sulfur compounds transfer constants

Transfer Constants to Monomers, Polymers, Catalysts and Initiators, Solvents and Additives, and Sulfur Compounds in Free Radical Polymerization+... 

TABLE 5. cant d Transfer Constants to Sulfur Compounds n/151... 

Because a single lump is used, the apparent kinetic rate constant (k pp) depends on the type of catalyst, mass and heat transfer, surface operating conditions, and the particular distribution of sulfur and metals in the feed. For instance, it has been found that during hydrotreating of diesel fractions, the sulfur compounds exhibit different reaction rates. Refractory compounds (e.g., those dibenzothiophenes with 4-, 6-, and 4-6 alkyl positions) have low values of the rate constant, while compounds without alkyl substituents in those positions (e.g., simpler molecules such as dibenzo-thiophene) exhibit high values of the rate constant (Ma et al., 1994). 

Involvement of AModo species in electrophilic C-iodinations needs to be considered since a number of imidazoles are known to form such compounds in basic medium. Charge-transfer complexes, too, are quite well known. They seem to be of the n -type through the unshared electron pair at N-3. Equilibrium constants for their formation are known to increase regularly in line with electron-donating powers of substituents (or vice versa). Some KCT values at 20°C (L M are imidazole (200), 1-methylimidazole (333), 1,2-dimethylimidazole (1165), 4-phenylimidazole (152), and 4,5-diphenylimidazole (141) (83BSB923). The charge-transfer complexes formed between iodine and imidazole-2-thiones appear to involve the sulfur atoms (88JA2586). 

Figure 16.5 Second-order rate constants [multiplied by (1 - aia) for phenols] for reactions of several compounds with 02 (left scale) as a function of pH. The abbreviations in parentheses indicate the reaction type ef = endo-peroxide formation (Eq. 16-14) er = ene reaction et = electron transfer so = sulfur oxidation. The scale on the right indicates the half-lives of the compounds in the epilimnion of Greifensee on a clear summer day (data from Scully and Hoigne, 1987).

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