Thiourea, formation constants with

The second problem involves the measurement of pKa values for carbonyl and thiocarbonyl derivatives. Grieg and Johnson (157) have pointed out that the measurement of pKa values for very weak bases (11) is an inaccurate and arbitrary process. Of particular difficulty for our purposes is the fact that different carbonyl derivatives may require different acidity functions. As a result of this situation, no attempt was made to make correlations of pKa data for carbonyl and thiocarbonyl derivatives with eq. (2). Because accurate pKa values can be measured for imines, these values were correlated with eq. (2), although the conformational problem remains. The imine sets were first studied by Charton and Charton (73), who correlated them with eq. (2). No correlations of data for carbonyl or thiocarbonyl derivatives with eq. (2) are extant in the literature. Bhaskar, Gosavi, and Rao (158) have reported that AG values for complex formation of substituted thioureas with iodine are a linear function of the Taft a values. Drago, Wenz, and Carlson (159) have reported similar results for complex formation between iodine and substituted amides. Oloffson (160) has reported a linear relationship between -AH for the complex of substituted N,N-dimethylamides with SbCls and the ffj constants. 

An important property of the S-nitroso thiourea derivatives is the ability to effect electrophilic nitrosation of any of the conventional nucleophilic centres. This is manifest kinet-ically by the catalysis of nitrous acid nitrosation effected by added thiourea (equation 29). The situation is completely analogous to the catalysis of the same reactions by added halide ion or thiocyanate ion. The catalytic efficiency of thiourea depends on both the equilibrium constant Xxno for the formation of the intermediate and also its rate constant k with typically a secondary amine65. Since Xxno is known (5000 dm6 mol-2), it is easy to obtain... 

Re—O—Re linkage.The complexing of rhenium(iv) with thiourea (L) has been studied in HCl solutions in the presence of tin(ii) by a cation-exchange method, and equilibrium stability constants Pj—P4 for [ReOL4] -t- nCl" [ReOCl L4 -I- nh determined.The complexing of rhenium(iv) with thiocyanate ions in HCl solutions in the presence of SnClj has also been investigated and the formation of a 1 1 Re -SCN" complex confirmed. ° ... 

Due to the absorption of UV radiation by the reactant nucleophiles and the solvent in the zone of interest, application of classical spectrophotometric methods to follow the mechanism of formation of imidazo[4,5-f]-[l,2,5]thiadiazoles 51 from reaction of the parent thiadiazoles 1,1-dioxides 89 and ureas/thioureas was restricted. Consequently, details of the mechanism and equilibrium constants were established by CV <2004JP01091, 2003JP0220>. It was found that the reaction proceeded by a stepwise mechanism with an intramolecular second step as outlined in Scheme 4. In solution, this reaction was reversible, and the equilibrium could be shifted to the side of the reactants by a temperature increase. For example, a 3.54mM solution of 51b in DMF, with a 0.1 M NaC104 as supporting electrolyte, presented a featureless CV between 0.9 and —2.8 V which corroborated with the... 

NMR data was found to be consistent with the formation of predominantly 1 1 complexes of the receptors and anions. Stability constants were determined by Cram s extraction method in water-saturated chloroform at 30 °C [40]. AfWties for both chloride and bromide anions increased through the series 27a-d, reflecting the increase in acidity of the NH groups due to the electron-withdrawing aryl substituents and the change from urea to thiourea in 27d. In the case of chloride the association constant for the unsubstituted derivative 27a was calculated to be 1.62 x 10 M with the... 

Studies of oxidation reactions of differing substrate types have been described. The nature of oxidant species in HCIO4 and H2SO4 media has been examined by investigation of catalysis of the Ce -Hg reaction. It is suggested that at 2.OM-HCIO4 the iridium(iv) is hydrolysed with protonation constant of 0.4 for [Ir(H20)s0H] +. Most kinetic studies, however, refer to reaction of the hexachloro and hexabromo ions. The oxidation of thiourea (tu), iV,iV -dimethyl-thiourea (dmtu), and 2-imidazolidinethione (it) follows a rate law second order in [substrate] and first in [Ir "]. The rate of oxidation follows the reactivity trend established previously for aquo-metal ions. The mechanism proposed involves rapid pre-equilibria followed by disulphide radical formation,... 

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