Hard electrophiles, definition

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. 

An attempt has been made to analyse whether the electrophilicity index is a reliable descriptor of the kinetic behaviour. Relative experimental rates of Friedel-Crafts benzylation, acetylation, and benzoylation reactions were found to correlate well with the corresponding calculated electrophilicity values. In the case of chlorination of various substituted ethylenes and nitration of toluene and chlorobenzene, the correlation was generally poor but somewhat better in the case of the experimental and the calculated activation energies for selected Markovnikov and anti-Markovnikov addition reactions. Reaction electrophilicity, local electrophilicity, and activation hardness were used together to provide a transparent picture of reaction rates and also the orientation of aromatic electrophilic substitution reactions. Ambiguity in the definition of the electrophilicity was highlighted.15... 

Model Definition. The HSAB model classifies Lewis acids (electrophiles) and bases (nucleophiles) as either "hard" or "soft." Hard acids and bases are relatively small, and exhibit low polarizability and a comparatively low tendency to form covalent bonds. Soft acids and bases have the opposite characteristics (24). Stated simply, the model postulates that hard acids react most readily with hard bases, and soft acids react most readily with soft bases (26). 

It soon became clear that these two classes should be called hard and soft electrophiles, respectively. Since the terms nucleophile and electrophile refer to rates of reaction, by definition, the acid-base reaction involved is... 

As discussed in the Introduction, Huckel s rule is an electronic criterion for aromaticity, and is based on the configuration of the rr electrons. Another characteristic of aromatic compounds is a relatively large HOMO-LUMO gap, which indicates the absence of high-energy, reactive electrons, in agreement with the reduced reactivity of aromatic compounds to electrophilic reagents. This facet of electronic configuration can be expressed in terms of hardness (see p. 96 for the definition of hardness in terms of DFT theory). ... 

Geerlings De Proft, 2008 Cardenas et al., 2009 Senet, 1996). One may obtain a condensed-to-atom variant and also for the electrophilic, nucleophilic and radical attacks in the usual way. Moreover, the inverse of tu(r,r ) may generate a hierarchy of nucleophilicity kernel. Unlike the previous formulations, the overall treatment here is general and analytic with hardly any bearing on the explicit form of E(N). The traditional operational definition of local softness and hardness contain the same potential information and they should be interpreted as the local abundance or concentration of their corresponding global properties. 

We present the main results in Table 4.14 where the basic electronegativity and chemical hardness calculated by finite difference approximations in terms of IP and EA definitions are considered (Lackner Zweig, 1983). This is based on two positive arguments they are based on the Parr s DFT groimd state parabola method that is consistent with definition (4.234) of global electrophilicity electronegativity imder Mulliken... 

Conceptual density functional theory has been quite successful in providing quantitative definitions for popular qualitative chemical concepts like electronegativity , hardness and electrophilicity . It has also been found to be useful in providing firm theoretical bases for the associated electronic structure principles. Various global and local reactivity descriptors " have played an important role in analyzing bonding, reactivity, stability, interactions and aromaticity in a variety of many-electron systems as well as a host of their physico-chemical properties. 

Robert G. Parr and his students have developed the field of conceptual DFT which is a theory of chemical reactivity within a broad density based quantum mechanical framework. Strong theoretical foundations and quantitative definitions of various popular qualitative chemical concepts like electronegativity hardness and electrophilicity been provided within conceptual DFT in addition to plausible rationales for the associated electronic structure principles. 

In the brief guidelines given above for what makes a good nucleophile and electrophile, we touched on the energy and accessibility of the electrophilic and nucleophilic orbitals. This brings us to another related concept, that of "hard" and "soft" acids and bases. In this definition, the acids and bases are best viewed as being of the Lewis type. Here we examine the "hardness" and "softness" of the acid and base to predict reactivity. In this analysis, the character of a nucleophile or electrophile is most often correlated with the polarizability of the species hard reactants are non-polarizable, whereas soft reactants are polarizable. The... 

Eq. 5.30 is a general relationship for the interactions of electrophiles and nucleophiles, and is not restricted to definitions and discussions of hard and soft acids and bases. It tells us that the relative nucleophilicity of several Lewis bases will depend upon which electrophile is used, because the c s and yS values will change for each different electrophile. Similarly, the relative electrophilicities of several Lewis acids will depend upon what nucleophile is used. We will see exactly such results when we explore quantitative scales for various nucleophiles and electrophiles, where the scales are highly dependent upon the particular reaction that is chosen to analyze relative reactivities (see Chapter 8). Eq. 5.30 nicely explains the reactivity trends for soft acids and bases. It predicts that the Eoveriap will be best for Lewis acids and bases that have electrophilic and nuclephilic orbitals of roughly the same energy, which is the cases for the soft acids and bases of Table 5.8. 

The chemical potential, chemical hardness and softness and reactivity indices have been used by a number of workers to assess a priori the reactivity of chemical species from their intrinsic electronic properties. The concept of electrophilicity has been known for several decades, although there has not been a rigorous definition of it until recently, Parr et al. [39] proposed a definition did they inspired by the experimental findings of Maynard et al. [40]. The revolution begins, with this simple index which has the ability to connect the major facets of chemical sciences. 

TABLE 3.67 The Values of Electrophilicity for CIPAHs of Table 3.63 Computed Using Definition (3.168) Applied To the Electronegativity and Chemical Hardness Values From Tables 3.60 and 3.61 (Putz Putz, 2013b)... 

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