Hard-soft concept

A quantitative scale of reactivity for aromatic substrates (fused, heterocyclic, and substituted rings) has been devised, based on the hard-soft concept (p. 338). From MO theory, a quantity, called activation hardness, can be calculated for each position of an aromatic ring. The smaller the activation hardness, the faster the attack at that position hence the treatment predicts the most likely orientations for incoming groups. 

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

The equations 7, 8 and 9 fail to operate when the HOMO - LUMO energy gap becomes too small and do not consider the influence on chemical properties of other orbitals, besides the HOMO and LUMO s. Moreover, it is not possible to study the site selectivity of a chemical species considering only absolute hardness other than space-dependent (local) versions of hardness/softness concepts [5]. Thus in addition to the global definition of r) and S, the local hardness [4] and local softness [5] have been introduced as follows ... 

Lead (Pb +) presents a case that demonstrates limitations in the hard-soft concept. The first stability constant logarithms for lead and the halide ions, all at 25° C and 1.0 M ionic strength, appear in parentheses F(1.44) > Cl(0.90) < Br(l.lO) < 1(1.26). The increasing trend for the last three halides describes class (b) behavior, while the greatest value... 

Yajima et al. introduced the organosulfonic acid deprotecting procedure in 1974.t l MsOH or 1M TfOH in TFA was found to cleave the Bzl ester and the Z group smoothly. Thioan-isole,t l used as a scavenger, was found to accelerate acidolytic deprotection,the mechanism of which could be explained by the hard-soft concept as shown in Scheme... 

The hard -soft concept is, from a perspective of the metal ion, often recast in terms of two classes of metal ions as follows ... 

So far wc have been considering the global hardness and global softness for a system. However, clearly it is not possible to study the site selectivity of a chemical species unless one brings local versions of the hardness/softness concepts into play. The demand for local hardness (softness) [8-11, 30] prompted many workers to define local quantities which vary from place to place and which can be applied toward understanding the local properties of molecules or solids. 

Another characteristic property related to hardness and softness is electronegativity. Low electronegativity is connected with a soft character and high electronegativity with a hard character. This is illustrated in Figure 7.23. For the interaction of the molecules with a metal atom or metal ion only part of a molecule might be responsible. Therefore, it is possible to apply the hard/soft concept to special parts of the molecule. 

However, the sequence of C/E values is not in accordance with practical experience with the solvents. Drago s parameters and his equation also could not be used to describe Pearson s hard-soft concept. 

The utility of the hardness/softness concept derives from the HSAB principle, which states that soft bases react faster and form stronger bonds with soft acids, and hard bases react faster and form stronger bonds with hard acids. A vast amount of chemistry can be rationalized with this principle. 

The observation that silica facilitates reduction, while on aliunina reduction of Cr is more difficult, supports the idea of a support controlled redox behavior. Thus, the supports take actively part in the redox chemistry of Cr. This can be explained with the hardness-softness concepts, first introduced by Pearson the higher the Al-content of the support, the harder the support, the less polarizable or susceptible for electron fluctuations, tbe more difficult reduction [13,14]. 

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