Acid-base concepts classification

The Lux-Flood theory relates to oxide melts. Geologists have often used acid-base concepts for the empirical classification of igneous silicate rocks (Read, 1948). Silica is implicitly assumed to be responsible for acidity, and the silica content of a rock is used as a measure of its acid-base balance ... 

Classification of biologically important metal ions and ligands according to the hard-soft acid-base concept and their general characteristics... 

TABLE 2.1 Classification of Biologically Important Metal Ions and Ligands According to the Hard—Soft Acid—Base Concept and their General Characteristics... 

The above classification is generally in agreement with Pearson s acid-base concept. Protic solvents are hard in nature, and they solvate small anions with strong hydrogen bonds, whereas dipolar aprotic solvents have a soft character, and they interact more strongly with the large, polarizable anions. 

The acid-base concept, either in the limited interpretation of the Bronsted-Lowry theory or in the more general sense of the Lewis theory, is one of the most useful classification schemes in chemistry and a tool for systematizing the relationship between structure and reactivity. In this section we examine how this general concept can be applied to solid surfaces [29], From the most general (Lewis) point of view, a surface is classified as acidic (electron acceptor) or basic (electron donor) according to the direction of net electron transfer that results in the formation of new chemical bonds with an adsorbed molecule. 

Classification of Lewis acids and bases according to the hard and soft acid-base concept... 

The main drawback of these linear free-energy relationships is that they do not relate to reaction mechanisms. Curiously enough, like the original Brpnsted equation, the main objective appeared to be the correlation of rate data rather than the interpretation of reaction mechanisms. This deficiency was partly remedied in the concept (8) of hard-soft acid-base (HSAB), which was in effect a qualitative extension of the Swain-Edwards equation but was more powerful in the sense that different types of reaction were related to the hard-soft classification, and the concept therefore has a wide application in organic synthesis and inorganic equilibria. 

Some other contributions of organometallic compounds to fundamental research are (a) the detection of free alkyl radicals by the pyrolysis of lead alkyls (b) the classification of hydrocarbon acidity via organoalkali compounds (c) the study of Lewis acid-base interactions with Group III alkyls (d) the development of the concept of electron-deficient compounds by the study of metal alkyls (e) the discovery of stereospecific olefin polymerization and (f) the investigation of nucleophilic additions to unsaturated organic compounds via reactive metal alkyls. 

Concepts of acidity and basicity are, in practice, defined and evaluated by their utility. Since overly formd definitions can be restrictive the concepts of acidity evolve towaids more comprehensive definitions. For example the Lewis definition includes the Broensted definition simply regarding the proton as an electron acceptor. Because the interaction of Broensted acids and bases in solutions involves a common process, protic transfer, scales of acidity can be established, for example the Hammett [1] acidity function. For Lewis acid-base interaction there is no common process to provide a unique basis for comparisons of acid strength. Experimentally, the strength of a Lewis acid depends upon the particular Lewis base. The classification of acids and bases as hard or soft in the principle of hard and soft acids and bases (HSAB principle) clarifies the interactions of Lewis acids and bases [2a]. Strong interactions occur between hard acid and hard base, or between soft acid and soft base, hi the hard-hard interaction there is a considerable electrostatic contribution to bonding and in the soft-soft interaction there is a major covalent contribution to bonding. The use of density functional analysis has clarified the concepts of hardness and softness and an empirical ranking of Lewis acids, based on local hardness is, proposed [2c]. 

The ubiquitous nature of the similarity concept was well expressed by Rouvray, who stated that all issues of comparison, and thus of classification, are in fact related to similarity." It is well known that both procedures play a crucial role in chemistry. Aspects such as basicity and acidity are based on classification, the periodic system involves comparison and classification, and so on. As a consequence, similarity appears to be an important subject in chemistry. 

The concepts of acidity and basicity are used by scientists from all branches of theoretical, experimental, and engineering chemistry. They are fundamental in nature and are of unquestionable utility as a powerful classification tool. However, it is interesting to observe that the meaning of these two simple words may vary depending on the compounds or materials discussed, experimental conditions or methods of study, particular applications, state of matter or size of particles, and, in some instances, even the context in which the terms are used. Indeed, any discussion about acid-base properties is an assertion in comparative terms a debate about relativity. By definition, acidity and basicity are opposite properties (or principles) that manifest themselves only in relationship to each other. Acids are revealed and measured by bases the presence of bases is revealed and quantified by acids. 

It can be seen that these definitions are derived from experimental observation and are no more than classifications based on a set of properties shared by a group of substances. They are scientifically inadequate for the interpretation of results, which requires a definition based on concepts. Historically, the attempt to provide a model rather than a classification comes in the form of a search for imderlying universal principles. It seems that the alchemists recognized vague principles of acidity and alkalinity, and in the 17th century the iatrochemists made these the basis of chemical medicine. Disease was attributed to a predominance of one or other of these principles (Pattison Muir, 1883). 

The concept of hard and soft acids and bases ( HSAB ) should also be mentioned here. This is not a new theory of acids and bases but represents a useful classification of Lewis acids and bases from the point of view of their reactivity, as introduced by R. G. Pearson. 

This concept was introduced qualitatively in the late 1950s and early 1960s by Pearson, in the framework of his classification of Lewis acids and bases, leading to the introduction of the hard and soft acids and bases (HSAB) principle [19-21]. This principle states that hard acids prefer to bond to hard bases and soft acids to soft bases. In many contributions, the factor of 1/2 is omitted. The inverse of the hardness was introduced as the softness S=l/rj [22]. A third quantity, which can be expressed as a derivative with respect to the number of electrons is the Fukui function, was introduced by Parr and Yang [23,24] ... 

The concept of soft and hard acids and bases (7), which is in effect an extension of the Chatt-Ahrland classification (2) of A and B metals, is applied in the 1963 paper by Pearson (7) particularly to equilibria involving mainly inorganic systems. This paper follows an earlier discussion by Edwards and Pearson (3) of the Swain-Edwards equation (4) (1) for nucleophilic reactivity. 

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