Hard, soft-acid base theory

Although sometimes referred to as the hard-soft acid-base theory, it is actually a principle that relates to many types of chemical interactions. It provides a good explanation of why HF is a weak acid. If H+ might potentially interact with either H20 or F, the situation with regard to the preferred bonding mode could be shown as follows ... 

In accord with the hard-soft acid-base theory, it is not unexpected that benzimi-dazol-2-ones are alkylated on both the N1- and N3 atoms under mildly basic Iiquiddiquid two-phase conditions [90]. (V-AIkylation of 1-isopropenylbenzimi-dazol-2-ones, followed by treatment with acid, provides a viable route to mono-alkylated derivatives (Scheme 5.21). 

The utilization of Pearson s hard-soft acid-base theory to interpret the reactions of lignin has also been described in work from the Soviet Union by Zarubin and Kirysun (59). Beyond this specifically related paper, researchers in the Soviet Union have been quite active in the application of numerical methods to lignin-related problems (60,61). 

These observations have been interpreted in terms of the hard-soft acid-base theory (77CJC4112), in which the salts of the harder cations, such as the Li+ ion, lead to C-alkylation, whilst the salts of the soft cations, such as the quaternary ammonium salts, are TV-alkylated. This interpretation is particularly relevant in understanding the reactivity of the heteroaryl-magnesium salts. The Mg2+ ion is a harder cation than the Li+ ion and, with the more strongly associated Grignard compounds, C-alkylation predominates. Generally, the pyrrole... 

The properties of dimethyl carbonate, (MeO)2CO, as an ambident electrophile have been investigated by analysis of the products of its reaction with various nucleophiles having different hard-soft character. Results were in good agreement with the Hard-Soft Acid-Base theory, hard nucleophiles attacking the hard C=0 group and soft nucleophiles the soft Me group (Scheme ll).37... 

Woerpel and Nevarez demonstrated the synthetic potential of silaaziridines by selective insertion reactions (Scheme 7.52).123 Silver-catalyzed aldehyde insertion into the Si-N bond of 169b produced the N,0-cyclic acetal 180 as the cis isomer. In contrast to this process, insertion of tert-butyl isocyanide occurred into the weaker C-Si bond to afford imine 181. The authors rationalized the chemoselectivity for these two processes on the basis of Pearson s hard-soft acid-base theory 124-126 the more ionic Si-N bond reacted with harder benzaldehyde electrophile, whereas the more covalent Si-C bond reacted with the softer isocyanide. 

The appearance of SnF+ ions is also observed in the mass spectra of (p- or m-CF3C6FLt)4Sn and it becomes the major ion in the spectra of the perfluorinated (CgFs Sn species. This fluorine migration has been rationalized in terms of hard/soft acid base theory (HSAB) by considering Sn2+ as a hard acid that favors fluoride transfer55. 

The large differences seen in Cu(II) adsorption capacity for thiol- and amino-modified silicas does not occur for Pb(II) adsorption (Figure 7). While it is tempting to argue these differences in adsorption behavior based on hard-soft acid-base theory the hardness of Cu2+ and Pb2+ are very similar, both generally categorized as intermediate cases.18 It is more likely that Pb(II) adsorption behavior under the conditions of this study is dictated largely by solution behavior and speciation of Pb2+ in aqueous solution at pH 5. Indeed it was found that these solutions were unstable and prone to precipitation slowly over time under these conditions. Further work will be needed as a function of pH, surface functionality, and metal ion to determine the role of various factors on adsorption behavior of these systems. 

Central to the stability and chemistry of complexes formed by mixed donor ligands are two key concepts of coordination chemistry. The first is the chelate effect, which applies to all polydentate ligands, and reflects the increase in stability of a type of complex as monodentate donor molecnles are replaced by polydentates with donors linked by chelate rings. The hard-soft acid-base theory is particnlarly relevant to mixed donor ligands where donors of distinctly different character may bind to a central metal ion. The like prefers like concept means hard nonpolarizable donor atoms (N and O, for example) bond preferentially to hard nonpolarizable metal... 

Our approach was outlined in the framework of the Hard-Soft Acid-Base theory (HSAB, Ref. 90). In a short definition, the HSAB theory states that hard nucleophiles prefer to react with hard electrophiles and soft nucleophiles prefer to react with soft electrophiles. 

Formation constants for many metal complexes have been compiled by Ramunas Motekaitis and Art Martell, and these as well as techniques for measuring them in the laboratory will be covered in Chapters 3 and 8. One can, however, predict the relative stability of a desired complex based on simple bonding theories. Crystal field theory, as well as the Irving-Williams series and Pearson s hard-soft-acid-base theory (see the next section) enable us to predict what might happen in solution. 

Acid-Base Chemistry (Hard-Soft-Acid-Base Theory)... 

This text uses several concepts and tools not present in most undergraduate organic chemistry texts to aid in understanding the most difficult sections of the course. Hard-soft acid-base theory is used to guide decisions and to explain and predict the dual reactivity of many species. Energy diagrams and surfaces are presented so that students have a physical model to help with the more complex decisions. 

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