Proton theory of acids

Lowry is best known to chemistry students through the tradition of eponymony, since the proton theory of acidity is known as the "Bronsted/Lowry theory" of proton donors. His most important experimental investigation likely was a long series of studies on optical rotatory dispersion.49 For our purposes, there is special interest in his discovery of mutarotation in camphor derivatives and his theory of dynamic tautomerism, which led him to an ionic theory of organic reaction mechanisms. 

From the above it is evident that the proton theory of acids has made possible considerable progress in acidic and basic catalysis and even permitted the quantitative prediction of the activity of new catalysts after a few experiments with other acids or bases. In some cases the prediction can be quite accurate (Kilpatrick and Kilpatrick, 42). 

Bronsted, Johannes Nicolaus (1879-1947) Danish physical chemist in 1923 he introduced the protonic theory of acid-base reactions, simultaneously with the English chemist Thomas Martin Lowry. 

The proton theory of acids and bases recognizes the existence of a large number and variety of bases, both molecular and ionic hydroxyl ion, amide ion, ethoxide ion, piperidine and other amines alcohols, ethers, acetate ion, hydrosulfide ion, cyanide ion, bisulfate ion, ketones, and many others. It also recognizes that acid-base phenomena do not depend upon the solvent. But its great weakness is that it ignores a large body of experimental data by restricting the use of the word acid to proton donors. For a time the only alternative to the proton theory seemed to be the theory of solvent systems. 

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... 

Hydrogen was recognized as the essential element in acids by H. Davy after his work on the hydrohalic acids, and theories of acids and bases have played an important role ever since. The electrolytic dissociation theory of S. A. Arrhenius and W. Ostwald in the 1880s, the introduction of the pH scale for hydrogen-ion concentrations by S. P. L. Sprensen in 1909, the theory of acid-base titrations and indicators, and J. N. Brdnsted s fruitful concept of acids and conjugate bases as proton donors and acceptors (1923) are other land marks (see p. 48). The di.scovery of ortho- and para-hydrogen in 1924, closely followed by the discovery of heavy hydrogen (deuterium) and... 

On the Brpnsted theory (p. 51), solutions with concentrations of H3O+ greater than that in pure water are acids (proton donors), and solutions rich in OH are bases (proton acceptors). The same classifications follow from the solvent-system theory of acids and bases... 

The Bronsted-Lowry theory of acids and bases referred to in Section 10.7 can be applied equally well to reactions occurring during acid-base titrations in non-aqueous solvents. This is because their approach considers an acid as any substance which will tend to donate a proton, and a base as a substance which will accept a proton. Substances which give poor end points due to being weak acids or bases in aqueous solution will frequently give far more satisfactory end points when titrations are carried out in non-aqueous media. An additional advantage is that many substances which are insoluble in water are sufficiently soluble in organic solvents to permit their titration in these non-aqueous media. 

The term proton in these definitions refers to the hydrogen ion, H+. An acid is a species containing an acidic hydrogen atom, which is a hydrogen atom that can be transferred as its nucleus, a proton, to another species acting as a base. The same definitions were proposed independently by the English chemist Thomas Lowry, and the theory based on them is called the Bronsted-Lowry theory of acids and bases. 

Bronsted-Lowry theory A theory of acids and bases involving proton transfer from one species to another. 

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 ... 

Bronsted theory a theory of acids and bases that defines acids as proton donors and bases as proton acceptors. 

Up to this point, we have dealt with the subject of acid-base chemistry in terms of proton transfer. If we seek to learn what it is that makes NH3 a base that can accept a proton, we find that it is because there is an unshared pair of electrons on the nitrogen atom where the proton can attach. Conversely, it is the fact that the hydrogen ion seeks a center of negative charge that makes it leave an acid such as HC1 and attach to the ammonia molecule. In other words, it is the presence of an unshared pair of electrons on the base that results in proton transfer. Sometimes known as the electronic theory of acids and bases, this shows that the essential characteristics of acids and bases do not always depend on the transfer of a proton. This approach to acid-base chemistry was first developed by G. N. Lewis in the 1920s. 

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 ... 

It now remains to explain why this tautomeric equilibrium shifts towards dominant O-protonation in concentrated and anhydrous acid. The reasons for the changeover can best be understood in terms of the developments in the theory of acidity functions, which have been outlined on page 293. Cation solvation must be taken into account in considering the tautomeric equilibrium (33). 

The Lewis theory of acids and bases defines an acid as an electron-parr acceptor, and a base as an electron-parr donor. Thus, a proton is only one of a large number of species that may function as a Lewis acid. Any molecule or ion may be an acid if it has an empty orbital to accept a parr of electrons (see Chapter 2 for orbital and Lewis theory). Any molecule or ion with a pair of electrons to donate can be a base. 

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 ... 

Johannes Nicolaus Bronsted and Thomas Martin Lowry simultaneously developed the proton donor/acceptor theory of acids and bases in the... 

Acid and Base Catalysis. Opportunities are now available for checking some of the theories of acids and bases which involve both the manner of readjustment within the molecule and the function of the catalyst. An acid dissociates according to the Bronsted theory to give a proton and a base. The rate of a given change will be faster for a proton than for a deuton but there will be little differ-... 

In his later studies Hantzsch assumed, according to the Bronsted theory of acids and bases, that nitric acid acts as a base towards sulphuric acid. According to Bronsted, an acid is a compound able to give off a proton while a base is a compound able to take up a proton. 

Bronsted acid-base theory — In 1923, Bron-sted and, independently of him, Lowry published essentially the same theory of acids and bases which can be applied not only to water as a solvent but also to all other - protic solvents, as well as to proton transfer reactions in gases. An acid is defined as a proton donor, i.e.,... 

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. 

This tendency of water molecules to break apart other molecules is part of the first definition of acids and bases, known as the Arrhenius theory of acids and hases and named after (surprise ) a chemist named Svante Arrhenius. According to Arrhenius, an acid is a substance that increases the concentration of H-i- ions in an aqueous (water) solution. So, hydrogen chloride (HCl) dissolved in water is an acid because the water breaks this molecule up into ions and CT ions. Actually, this statement is a bit of a lie because ions (which are simply protons—take away an electron from hydrogen, which is com-... 

NHj contains no hydroxide ions yet when you put it in water it increases the hydroxide ion concentration. To account for examples such as this we have what is known as the Bronsted-Lowry (Johannes Bronsted and Thomas Lowry) theory of acids and bases. In this definition an acid is any substance that donates protons in a reaction and a base is any substance that accepts protons in a reaction. This is basically (no pun intended) the same as the Arrhenius definition of acids because if a substance readily donates protons in a reaction it will also increase the hydronium ion concentration when placed in water. The definition of a base, though, is expanded to include any substance that readily accepts protons in a reaction, and that substance doesn t have to contain hydroxide ions. Let s continue with our above example. There, NH contributed to the Off concentration when dissolved in water. But NH3 is also a proton acceptor (which we have defined as a base) because it picks up a proton to become as in... 

Arrhenius theory of acids and bases. A theory of acids and bases in which an acid is defined as any substance that contributes protons or, more accurately, hydronium ions, in solution and a base is defined as any substance that contributes a hydroxide ion (OH ) in solution. 

The object of this book is to provide an introduction to electrochemistry in its present state of development. An attempt has been made to explain the fundamentals of the subject as it stands today, devoting little or no space to the consideration of theories and arguments that have been discarded or greatly modified. In this way it is hoped that the reader will acquire the modern point of view in electrochemistry without being burdened by much that is obsolete. In the opinion of the writer, there have been four developments in the past two decades that have had an important influence on electrochemistry. They are the activity concept, the interionic attraction theory, the proton-transfer theory of acids and bases, and the consideration of electrode reactions as rate processes. These ideas have been incorporated into the structure of the book, with consequent simplification and clarification in the treatment of many aspects of electrochemistry. 

The Bronsted theory of acids and bases defines an acid as a proton donor and a base as a proton acceptor, i.e. a pro tic acid such as hydrochloric acid is a source of protons. Although the idea of an acidic hydrogen in organic compounds may initially be understood in terms of a carboxyl hydroxyl group, a hydrogen atom may become weakly acidic in a number of other circumstances, e.g. when it is attached to a carbon atom that is adjacent to a carbonyl group. On the other hand, a base such as an amine, or a carboxylate anion, is capable of accepting a proton. 

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. 

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