Generalized theory of acids and bases

A more general theory of acids and bases was devised independently by Johannes Br0n sted (Denmark) and Thomas M Lowry (England) m 1923 In the Brpnsted-Lowry approach an acid is a proton donor, and a base is a proton acceptor The reaction that occurs between an acid and a base is proton transfer... 

Thus, by definition, electrophiles are electron-pair acceptors and nucleophiles are electron-pair donors. These definitions correspond closely to definitions used in the generalized theory of acids and bases proposed by G. N. Lewis (1923). According to Lewis, an acid is any substance that can accept an electron pair, and a base is any substance that can donate an electron pair to form a covalent bond. Therefore acids must be electrophiles and bases must be nucleophiles. For example, the methyl cation may be regarded as a Lewis acid, or an electrophile, because it accepts electrons from reagents such as chloride ion or methanol. In turn, because chloride ion and methanol donate electrons to the methyl cation they are classified as Lewis bases, or nucleophiles ... 

The Arrhenius theory accounts for the properties of many common acids and bases, but it has important limitations. For one thing, the Arrhenius theory is restricted to aqueous solutions for another, it doesn t account for the basicity of substances like ammonia (NH3) that don t contain OH groups. In 1923, a more general theory of acids and bases was proposed independently by the Danish chemist Johannes Bronsted and the English chemist Thomas Lowry. According to the Bronsted-Lowry theory, an acid is any substance (molecule or ion) that can transfer a proton (H + ion) to another substance, and a base is any substance that can accept a proton. In short, acids are proton donors, bases are proton acceptors, and acid-base reactions are proton-transfer reactions ... 

Usanovic acid-base theory — A general theory of acids and bases taking into consideration electron exchange processes (redox reactions) was proposed in 1939 by Usanovic. His definition is a symmetrical one and includes all concepts discussed above, i.e., an acid is defined as a substance which is able to liberate protons or other cations (cation donator) or to take up anions or electrons (anion acceptor, electron acceptor). A base is defined as a substance which is able to release anions or electrons (anion donator, electron donator) or to take up protons or other cations (cation acceptor). According to that theory, all chemical reactions (excluding reactions between radicals resulting in covalent bindings) can be considered as acid-base reactions. 

Acids are defined classically as substances which liberate hydrogen ions in aqueous solutions, while bases are substances which dissociate in water with the formation of hydroxyl ions. This formulation of the concept of acids and bases had developed chiefly on the basis of acidic and basic properties displayed in aqueous solutions. When an attempt was made to establish a more general theory of acids and bases, the old formulation appeared in many ways to be one-sided and incomplete. 

A still more general theory of acids and bases than the proton donor-acceptor theory was introduced by G. N. Lewis. He called a base anything that has available an unshared pair of electrons such as NHg, H... 

Second, as originally proposed by G. N. Lewis in his generalized theory of acids and bases (Chapter 5), any substance that can donate an electron pair to form a covalent bond is a base. Thus, if chloride ion (CT) or water (H2O ) acting as nucleophiles donate electrons to, for example, the methyl cation (CHs", a Lewis acid) so as to form a bond, they are also Lewis bases. Indeed, any proton acceptor (Brpnsted base. Chapter 5) can be classified as a nucleophile ... 

An even more general theory of acids and bases was given by the American chemist G. N. Lewis in 1923. In this theory, an acid is an electron acceptor and a base is an electron donor. This is a more general theory than the Br0nsted-Lowry theory, because it allows the acid-base classification to be applied to reactions in which neither H (aq) nor OH (aq) play a role, or even to reactions in which there is no solvent. For example, the following are acid-base reactions in the Lewis theory... 

This more general view of acids and bases is named the Bronsted-Lowry theory after the two scientists who proposed it, J. N. Brpnsted and T. M. Lowry. 

Any text on acids and bases would not be deemed complete if mention were not made of the extended definition of acids and bases that is embodied in the Lowry-Bronsted theory. The theory basically proposed a more general definition of acids and bases to overpower the limitations of the theory arising from the Arrhenius concept. 

The generalization was based on the introduction of the concept of donor-acceptor pairs into the theory of acids and bases this is a fundamental concept in the general interpretation of chemical reactivity. In the same way as a redox reaction depends on the exchange of electrons between the two species forming the redox system, reactions in an acid-base system also depend on the exchange of a chemically simple species—hydrogen cations, i.e. protons. Such a reaction is thus termed proto lytic. This approach leads to the following definitions ... 

According to the Arrhenius theory of acids and bases, the acidic species in water is the solvated proton (which we write as H30+). This shows that the acidic species is the cation characteristic of the solvent. In water, the basic species is the anion characteristic of the solvent, OH-. By extending the Arrhenius definitions of acid and base to liquid ammonia, it becomes apparent from Eq. (10.3) that the acidic species is NH4+ and the basic species is Nl I,. It is apparent that any substance that leads to an increase in the concentration of NH4+ is an acid in liquid ammonia. A substance that leads to an increase in concentration of NH2- is a base in liquid ammonia. For other solvents, autoionization (if it occurs) leads to different ions, but in each case presumed ionization leads to a cation and an anion. Generalization of the nature of the acidic and basic species leads to the idea that in a solvent, the cation characteristic of the solvent is the acidic species and the anion characteristic of the solvent is the basic species. This is known as the solvent concept. Neutralization can be considered as the reaction of the cation and anion from the solvent. For example, the cation and anion react to produce unionized solvent ... 

The limitations of the Arrhenius theory of acids and bases are overcome by a more general theory, called the Bronsted-Lowry theory. This theory was proposed independently, in 1923, by Johannes Br0nsted, a Danish chemist, and Thomas Lowry, an English chemist. It recognizes an acid-base reaction as a chemical equilibrium, having both a forward reaction and a reverse reaction that involve the transfer of a proton. The Bronsted-Lowry theory defines acids and bases as follows ... 

These ideas were rather limiting since they only applied to aqueous solutions. There were situations where acid-base reactions were taking place in solvents other than water, or even in no solvent at all. This problem was addressed in 1923 by the Danish chemist Johannes Bronsted (1879-1947) and the English chemist Thomas Lowry (1874-1936) when they independentiy proposed a more general definition of acids and bases, and the study of acids and bases took a great step forward. This theory became known as the Bronsted-Lowry theory of acids and bases. 

Qualitative and quantitative aspects of the Lewis theory of acids and bases, and practical applications of Lewis acids, are discussed in a series of monographs [1,4-6,30-46] and reviews [47-49], The following aspects are taken into account (a) electronic configuration of acceptors (A = M MX are generally metal and boron salts), (b) nature of anions (usually halides), (c) peculiarities of thin structure of donors (B are generally the compounds containing N, P, As, Sb O, S, Se, Te F, Cl, Br, I atoms) their electronic structure, spatial accessibility, and mutual position of donor centers. Moreover, the nature of X, order of binding of A and B in formation of adducts of type AB , nature of solvents, and evaluation of AH or AG of the processes (1.1)—(1.5) [31,48] should also be considered. 

The Lewis theory of acids and bases is more general than Bronsted-Lowry theory, but Bronsted-Lowry s definition is used more frequently. The terms "acid" and "base" most often refer to Bronsted acids and bases, and the term "Lewis acid" is usually reserved for chemicals like BF3 that are not Bronsted acids. 

In his oxonium theory of acids and bases (91), Werner anticipated by 16 years the now generally accepted views of Br0nsted (7, 8) and Lowry (40j 41) y which accords that acids are proton donors and bases are proton acceptors. Yet, despite the fact that Werner was the first to emphasize the critical role of the solvent in acid-base phenomena, his contributions to this field are almost universally ignored. 

The final acid-base theory that we shall consider was proposed by chemist Gilbert Lewis in the early 1920s. The Lewis Theory is the most general, including more substances under its definitions than the other theories of acids and bases. A Lewis acid is a substance that accepts a pair of electrons to form a covalent bond. A Lewis base is a substance that provides a pair of electrons to form a covalent bond. In order for a substance to act as a Lewis base, it must have a pair of unshared electrons in its valence shell. An example of this is seen when a hydrogen ion attaches to the unpaired electrons of oxygen in a water molecule, as shown here ... 

The theory of acids and bases, the Bronsted-Lowry theory, was dependent on the presence of proton (H ) to show acidic or basic properties, which may not always be the case hence in 1923, G. N. Lewis introduced a theory of acids and bases with a more general definition of acid-base reactions by examining what happens when an ion combines with an OH ion to form water. 

Lewis [13] proposed the electronic theory of acids and bases and gave a more general definition of acids and bases in its framework. Acids are determined as acceptors of an electron pair, and its donors are classified as bases. The principal scheme of the Lewis acid-base interaction is described by the following equation ... 

The theory of acids and bases postulated by Arrhenius, Br0nsted and Lewis is presently generalized in the Pearson theory of hard and soft acids and bases [26-31]. 

Perhaps you will not be surprised, then, you to learn that an even more general model of acids and bases was proposed by American chemist G. N. Lewis (1875-1946). Recall that Lewis developed the electron-pair theory of chemical bonding and introduced Lewis structures to keep track of the electrons in atoms and molecules. He applied his electron-pair theory of chemical bonding to acid-base reactions. Lewis proposed that an acid is an ion or molecule with a vacant atomic orbital that can accept (share) an electron pair. A base is an ion or molecule with a lone electron pair that it can donate (share). According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. Note that the Lewis model includes all the substances classified as Bronsted-Lowry acids and bases and many more. 

As another example, the Brpnsted theory of acids and bases replaced the older Arrhenius theory because of its more general applicability (e.g., it applied also to non-aqueous solutions and even to gases). Attempts have been made to familiarise secondary school students with principles of quantum mechanics and thermodynamics, albeit often in a simplified and sometimes distorted form. It was expected that this would save much time because formulas and physical and chemical properties of specific compounds could then simply be derived from theoretical principles instead of having to be memorised. It was decided that many aspects of descriptive chemistry could be left out when students were provided with a small handbook covering most of the relevant facts and formulas. 

The first complete theory of acids and bases was formulated by Arrhenius in 1897 (124) in water, acids dissociate to produce H, whereas bases produce OH . The more advanced and general Bronsted—Lowry theory was proposed in 1923 (125,126). According to this theory, acids are substances able... 

A more general concept of acids and bases was introduced by Gilbert N. Lewis. The Lewis theory deals with the way in which a substance with an unshared pair of electrons reacts in an acid-base type of reaction. According to this theory, a base is any substance that has an unshared pair of electrons (electron pair donor), and an acid is any substance that will attach itself to or accept a pair of electrons. 

The Arrhenius concept was the first successful theory of acids and bases. Then in 1923, Br0nsted and Lowry characterized acid-base reactions as proton-transfer reactions. According to the Br0nsted—Lowry concept, an acid is a proton donor and a base is a proton acceptor. The Lewis concept is even more general than the Br0nsted-Lowry concept. A Lewis acid is an electron-pair acceptor and a Lewis base is an electron-pair donor. Reactions of acidic and basic oxides and the formation of complex ions, as well as proton-transfer reactions, can be described in terms of the Lewis concept. 

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