Pearson concept group

In the reaction of O-Iithium-N,N-bis(silyl)hydF0xylaimde with trimethylchloiostannan N,0-bis(ter/-butyldimethylsilyl)-N-(trimethylstannyl)hydroxyIamine was fonned (Eq. 5). In contrast to the expected N,N-bis(siIyI)-0-stannylhydroxylamme the reaction product was N,0-bis(silyl)-N-stannylhydroxylamine. The migration of the stannyl group from the oxygen to the nitrogen atom can be explained with the Pearson concept. A solid-state structure was elucidated by X-ray diffraction method (Fig. 6). 

The Lewis acid/base complex is formed via an overlap between a doubly occupied orbital of the donor D and vacant orbital of the acceptor A. This acid/base approach was extended by Pearson who divided Lewis acids and bases into two groups, hard and soft, according to their electronegativity and polarizability (principle of hard and soft acids and bases (HSAB concept). Hard acids (e.g., H, Lf, Na, BF3, AICI3, hydrogen-bond donors HX) and hard bases (e.g., F", CL, HO, RO, H2O, ROH, R2O,... 

The application of the HSAB concept to solutions leads to the rule that hard solutes dissolve in hard solvents and soft solutes dissolve in soft solvents (Pearson, 1987). For example, benzene is considered a very soft solvent since it contains only a basic function. Contrary to benzene, water is a very hard solvent, with respect to both its basic and acidic properties, ft is the ideal solvent for hard bases and hard acids. The hardness of water is reduced by the introduction of alkyl substituents in proportion to the size of the alkyl group. In alcohols, therefore, softer solutes become soluble. 

A similar picture holds for other nucleophiles. As a consequence, there might seem little hope for a nucleophile-based reactivity relationship. Indeed this has been implicitly recognized in the popularity of Pearson s concept of hard and soft acids and bases, which provides a qualitative rationalization of, for example, the similar orders of reactivities of halide ions as both nucleophiles and leaving groups in (Sn2) substitution reactions, without attempting a quantitative analysis. Surprisingly, however, despite the failure of rate-equilibrium relationships, correlations between reactivities of nucleophiles, that is, comparisons of rates of reactions for one carbocation with those of another, are strikingly successful. In other words, correlations exist between rate constants and rate constants where correlations between rate and equilibrium constants fail. 

Combine and extend the concept of the generalized physical properties approach and Guggenheim s method to the case of an irreversible second-order reaction. Use your analysis to solve a problem adapted from one composed by J. W. Moore and R. G. Pearson Kinetics and Mechanisms, 3rd ed., p. 78, Wiley, New York, 1981]. Note that most of the data were recorded at time t and at time f -f 110 s and that stoichiometric quantities of reactants were used in the experiment. You may find it useful to group your time terms such that they appear in the form (f -I- A) V — f K, where A, is the measured property at time t = f -I- A and A is the measured property at time t. S. W. Tobey [/. Chem. Educ., 39, 473 (1962)] studied the rate of hydrolysis of phenyl-m-nitrobenzoate in a 50% aqueous dioxane solution at 25°C. 

The phenomenon can also be interpreted qualitatively according to Pearson as hard-hard and soft-soft interactions. One can similarly make use of the Ahrland concept [Ah 66] of the interaction of reaction partners belonging to groups A and B,... 

Under the same conditions, silver thiophenolate aflFords mostly 2-nitro-5-phenylthio-l,3,4-thiadiazole (160), together with minor amounts of (158) and (159). The observations indicate a dependence of the leaving-group potential upon the cation associated with the anionic nucleophile an interpretation has been proposed in terms of Pearson s acid-base concept. 

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