Acid—base theory

In the main introduction to this chapter the importance of acid-base interactions in determining the interfacial chemistry between fillers and matrices in particulate composites was emphasised. In this section, a summary of the more commonly used acid-base theories will be presented. 

Lewis [5] was the first to describe acids and bases in terms of their electron accepting and electron donating properties. Mulliken [6] further refined the understanding of the acid base interactions for which he was awarded the Nobel Prize for Chemistry. His quantum mechanical approach introduced the concept of two contributions, an electrostatic and a covalent, to the total acid-base interaction. Pearson [7] introduced the concept of hard and soft acids and bases, the HSAB principle, based on the relative contributions from the covalent (soft) interaction and the electrostatic (hard) interaction. In his mathematical treatment he defined the absolute hardness of any acid or base in terms of its ionisation potential and electron affinity. Pearson s is probably the most robust approach, but the approaches in most common use are those developed by Gutmann [8] and Drago [9], who separately developed equations and methods to quantify the acid or basic strength of compounds, from which their heats of interaction could be calculated. 

The classical theory of electrolytic dissociation put forward by Arrhenius is successful in explaining to a large extent the behaviour of weak acids and 

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Because of the mentioned leveling effect of the solvent (or excess acid itself acting as such) the acidity cannot exceed that of its conjugate acid. In the case of water the limiting acidity is that of HsO. Proton-ated water, H30 (hydronium ion), was first postulated in 1907, and its preeminent role in acid-catalyzed reactions in aqueous media was first realized in the acid-base theory of Bronsted and Lowry. Direct experimental evidence for the hydronium ion in solution and in the... 

Two acid-base theories are used in organic chemistry today the Br0nsted theory and the Lewis theory. These theories are quite compatible and are used for different purposes. ... 

For monographs on acids and bases, see Stewart, R. The Proton Applications to Organic Chemistry, Academic Press NY, 1985, Bell, R.P. The Proton in Chemistry, 2nd ed. Cornell University Press Ithaca, NY, 1973-, Finston, H.L. Rychtman, A.C. A New View of Current Acid-Base Theories, Wiley NY, 1982. 

For a monograph on Lewis acid-base theory, see Jensen, W.B. The Lewis Acid-Base Concept Wiley NY, 1980. For a discussion of the definitions of Lewis acid and base, see Jensen, W.B. Chem. Rev, 1978, 78, 1. 

This theory was a milestone in the development of acid-base concepts it was the first to define acids and bases in terms other than that of a reaction between them and the first to give quantitative descriptions. However, the theory of Arrhenius is far more narrow than both its predecessors and its successors and, indeed, it is the most restrictive of all acid-base theories. 

Lux (1939) developed an acid-base theory for oxide melts where the oxide ion plays an analogous but opposite role to that of the hydrogen ion in the Bronsted theory. A base is an oxide donor and an acid is an oxide acceptor (Lux, 1939 Flood Forland, 1947a,b Flood, Forland Roald, 1947) ... 

The Usanovich theory is the most general of all acid-base theories. According to Usanovich (1939) any process leading to the formation of a salt is an acid-base reaction. The so-called positive-negative definition of Usanovich runs as follows. 

From this discussion it can be seen that there is no ideal acid-base theory for AB cements and a pragmatic approach has to be adopted. Since the matrix is a salt, an AB cement can be defined quite simply as the product of the reaction of a powder and liquid component to yield a salt-like gel. The Bronsted-Lowry theory suffices to define all the bases and the protonic acids, and the Lewis theory to define the aprotic acids. The subject of acid-base balance in aluminosilicate glasses is covered by the Lux-Flood theory. 

Finston, H. L. Rychtman, A. C. (1982). A New View of Current Acid-Base Theories. New York John Wiley Sons. 

Luder, W. F. (1948). Contemporary acid-base theory. Journal of Chemical Education, 25, 555-8. 

The photodecomposition and thermodecomposition of nitromethane have been extensively studied as model systems in combustion, explosion and atmosphere pollution processes[l]. On another hand, nitromethane was selected as a model solvent in experiments aimed at examining non hydrogen-bonded solvent effects in a general acid-base theory of organic molecules [2.3]. This selection is based on the electronic and structural characteristics of nitromethane that has a high dielectric constant, and at the same time cannot form hydrogen bonds with solute molecules. 

The acid dissociation of neutral molecules is such a highly endothermic reaction that the acid dissociation of nitromethane can hardly take place. The results of the calculations presented here provide a theoretical support for nitromethane as an ideal model of aprotic solvent in the acid-base theory of organic molecules. 

This equation corresponds to today s general convention of expressing base strength also be means of pKa, where K is considered in the sense of the Bronsted acid-base theory as a protolysis constant of the following protolytic reactions for acids ... 

From our previous treatment of the Arrhenius, Bransted and Lewis acid-base theories, the importance of the choice between the divergent solvent types clearly appeared if we now confine ourselves to solvents to which the proton theory in general is applicable, this leads to a classification of eight classes as already proposed by Bronsted35,36 (Table 4.3). 

In 1923, the same year that Bransted and Lowry came up with their idea of what acids and bases were, an American chemist named Gilbert Newton Lewis began to work on his own acid-base theory. Lewis defined acid as any substance that accepted an electron pair. A base, on the other hand, is any substance that donates an electron pair. 

B The Lewis Definition of Acids and Bases 1. Lewis acid-base theory in 1923 proposed by G. N. Lewis (1875-1946 Ph. D. Harvard, 1899 professor, Massachusetts Institute of Technology, 1905-1912 professor, University of California, Berkeley, 1912-1946). 

Acid-base behavior according to the Lewis theory has many of the same aspects as does acid-base theory according to the Bronsted-Lowry theory. 

Although sometimes referred to as the hard-soft acid-base theory, it is actually a principle that relates to many types of chemical interactions. It provides a good explanation of why HF is a weak acid. If H+ might potentially interact with either H20 or F, the situation with regard to the preferred bonding mode could be shown as follows ... 

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