Hardness physico-chemical concepts

It has been discussed in Section 2 that concepts such as electronegativity and hardness could explain important aspects of chemical reactions and could be related to different physico-chemical properties. Density functional theory has been found to provide a rigorous theoretical background for electronegativity, hardness, and related concepts. 

Perhaps little did Ralph Pearson realize when he proposed the hardness concept that it would encompass such a multitude of physico-chemical problems and that it would spawn so many new concepts. The Maximum Hardness Principle and the HSAB Principle, if they prove out, should be cornerstones for understanding molecular structure and molecular reactivity. The complex of ideas related to hardness and softness deserve extensive further application and careful further theoretical study. 

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. 

The concept of hard and soft acids and bases (HSAB) is usually related to the physico-chemical properties of organic compounds and is not much recognized in synthetic organic chemistiy [15, 16]. There is also the less often used concept of hard and soft nucleophiles and electrophiles [17]. The fundamental distinction between these two concepts, i.e., between basicity and nucleophiUcity on the one hand and acidity and electrophilicity on the other, lies in that fact that basicity and acidity are thermodynamic properties, whereas nucleophiUcity and electrophUicity are kinetic terms. 

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