Hard-Soft Acid-Base HSAB theory

In order to clarify the different behavior of anion 2 and 3 (Scheme 4.10) toward DMC, various anions with different soft/hard character (aliphatic and aromatic amines, alcohoxydes, phenoxides, thiolates) were compared with regard to nucleophilic substitutions on DMC, using different reaction conditions. Results were in good agreement with the hard-soft acid-base (HSAB) theory. Accordingly, the high selectivity of monomethylation of CH2 acidic compounds and primary aromatic amines with DMC can be explained by two different subsequent reactions, which are due to the double electrophilic character of DMC. The first... 

Further examination of the results indicated that by invocation of Pearson s Hard-Soft Acid-Base (HSAB) theory (57), the results are consistent with experimental observation. According to Pearson s theory, which has been generalized to include nucleophiles (bases) and electrophiles (acids), interactions between hard reactants are proposed to be dependent on coulombic attraction. The combination of soft reactants, however, is thought to be due to overlap of the lowest unoccupied molecular orbital (LUMO) of the electrophile and the highest occupied molecular orbital (HOMO) of the nucleophile, the so-called frontier molecular orbitals. It was found that, compared to all other positions in the quinone methide, the alpha carbon had the greatest LUMO electron density. It appears, therefore, that the frontier molecular orbital interactions are overriding the unfavorable coulombic conditions. This interpretation also supports the preferential reaction of the sulfhydryl ion over the hydroxide ion in kraft pulping. In comparison to the hydroxide ion, the sulfhydryl is relatively soft, and in Pearson s theory, soft reactants will bond preferentially to soft reactants, while hard acids will favorably combine with hard bases. Since the alpha position is the softest in the entire molecule, as evidenced by the LUMO density, the softer sulfhydryl ion would be more likely to attack this position than the hydroxide. 

Br0nsted Equation, Hard-Soft Acid-Base (HSAB) Theory, and Factors Determining Nucleophilicity... 

While this theory was shaping up to account for ptaquiloside carcinogenicity, some inconsistencies appeared. Extensive examination of several alkylating agents revealed the existence of a correlation between the site of alkylation in DNA and the final effect [257]. Alkylation at the 06 of guanine, O2 and O4 of thymine, and O2 of cytosine in the DNA polymer could be correlated with procarcinogenicitv, whereas reactions at the nitrogen positions of the heterocycles resulted in enhanced levels of cytotoxicity. Application of the hard-soft acid-base HSAB theory [258,259] to alkylation of DNA nucleosides [260] provided support to this view. APT (14) may be classified as a semi-hard electrophile which is be prone to react with less hard N over harder O nucleophiles. If so,... 

The Hard-Soft-Add-Base (HSAB) theory was developed by Pearson in 1963. According to this theory, Lewis acids and Lewis bases are divided into two groups on one hand hard acids and bases, which are usually small, weakly polarizable species with highly localised charges, and on the other hand soft acids and bases which are large, polarizable species with delocalised charges. A selection of Lewis acids, ordered according to their hardness in aqueous solution is presented in Table 1.3. 

This chapter is intended to provide basic understanding and application of the effect of electric field on the reactivity descriptors. Section 25.2 will focus on the definitions of reactivity descriptors used to understand the chemical reactivity, along with the local hard-soft acid-base (HSAB) semiquantitative model for calculating interaction energy. In Section 25.3, we will discuss specifically the theory behind the effects of external electric field on reactivity descriptors. Some numerical results will be presented in Section 25.4. Along with that in Section 25.5, we would like to discuss the work describing the effect of other perturbation parameters. In Section 25.6, we would present our conclusions and prospects. 

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. 

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. 

HSAB hard-soft acid-base (theory)... 

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