Acids and Bases The Brpnsted-Lowry Definition

In addition to Arrhenius acids and bases, the Brpnsted-Lowry definition includes bases that have no hydroxide ions, yet can accept protons. Consider the following examples of acids donating protons to bases. NaOH is a base under either the Arrhenius or Brpnsted-Lowry definition. The other three are Brpnsted-Lowry bases but not Arrhenius bases, because they have no hydroxide ions. 

In Chapter 2, we will concentrate on two definitions of acids and bases the Brpnsted-Lowry definition, which describes acids as proton donors and bases as proton acceptors, and the Lewis definition, which describes acids as electron pair acceptors and bases as electron pair donors. 

Knowledge Required (1) The interpretation of acid equilibrium constants, K. (2) Meaning of terms associated with weak and strong acids and bases. (3) Brpnsted-Lowry acid-base definition. 

Section 1 13 According to the Brpnsted-Lowry definitions an acid is a proton donor and a base is a proton acceptor... 

This IS a very useful relationship You should practice writing equations according to the Brpnsted-Lowry definitions of acids and bases and familiarize yourself with Table 1 7 which gives the s of various Br0n sted acids... 

Before continuing on to the last definition of acids and bases, it will be helpful to consider the definitions for strong and weak acids within the context of the Br0nsted-Lowry model of acids and bases. The definitions are really an extension of the Arrhenius ideas. In the Arrhenius definitions, strong acids and bases were those that ionize completely. Most Brpnsted-Lo wry acids and bases do not completely ionize in solution, so the strengths are determined based on the degree of ionization in solution. For example, acetic acid, found in vinegar, is a weak acid that is only about 1 % ionized in solution. That means that when acetic acid, 11C, 11,(),. is placed in water, the reaction looks like... 

A more general definition of acid-base equilibrium in molecular solvents was proposed independently by Brpnsted and Lowry, who extended the term base . According to them, the acid is a donor of protons, and they defined a base only as an acceptor of H+. According to the Brpnsted-Lowry definition, the dissociation of an acid results in the formation of a proton and a conjugate base ... 

Of course, protons cannot exist as free particles in molecular solvents they should react with the solvent molecules, and such an interaction in aqueous solutions results in the formation of H5Oj (H+-2H20 or [H20--H--0H2]+) particles. Similarly, hydroxide ions in aqueous solutions exist as H302 (OH2- -OH-). The stronger the acid Ai compared to the acid A2, the more complete is the shift of reaction (1.1.6) to the right. The Brpnsted-Lowry definition thus extends the term acid-base interaction to reactions accompanied with proton transfer from one base to another. For example,... 

Usually, definitions of acids and bases are necessary for the classification of different kinds of chemical reactions, i.e. for dividing them into acid-base and other ones. For example, the Brpnsted-Lowry definition divides reactions into acid-base, which are characterized by redistribution of protons, and other ones. The wider Lewis definition makes the division of reactions into acid-base and redox ones, meaning that in the former case there is redistribution of electron density on account of electron pairs, and the latter case concerns reactions with the transfer of single electrons. Since chemistry concerns just the redistribution of electrons of external shells, we can classify all chemical reactions as acid-base ones by the Usanovitch definition. 

All nucleophiles are bases (1). In fact, within the definition of Lewis (2), nucleophilicity is basicity. Following Ingold (3), however, physical-organic chemists have normally used the Brpnsted-Lowry definition (4, 5) of bases as affinity for protons. Likewise, nucleophilicity referred to affinity for nuclei of other elements, most often carbon (3). Another classification reserves basicity, and its counterpart acidity, for equilibrium measurements, while nucleophilicity and its counterpart electrophilicity refer to rate measurements (6). The terms carbon basicity and hydrogen nucleophilicity have been employed (7-9). This classification does not seem to have gained much acceptance. 

Understand the Brpnsted-Lowry definitions of an acid and a base discuss how water can act as a base or as an acid and how an acid-base reaction is a proton-transfer process involving two conjugate acid-base pairs, with the stronger acid and base forming the weaker base and acid ( 18.3) (SPs 18.4, 18.5) (EPs 18.24-18.39)... 

The definitions of acid and base that we use now were provided by Brpnsted and Lowry in 1923. In the Br0nsted-Lowry definitions, an acid is a species that donates a proton, and a base is a species that accepts a proton. (Remember that positively charged hydrogen ions are also called protons.) In the following reaction, hydrogen chloride (HCl) meets the Brpnsted-Lowry definition of an acid because it donates a proton to water. Water meets the definition of a base because it accepts a proton from HCl. Water can accept a proton because it has two lone pairs. Either lone pair can form a covalent bond with a proton. In the reverse reaction, H3O is an acid because it donates a proton to CF, and CF is a base because it accepts a proton from H30. ... 

The Brpnsted-Lowry definition of acidity discussed in the previous four sections encompasses all compounds containing hydrogen. Of even more use, however, is the Lewis definition of acids and bases, which is not limited to compounds that gain or lose protons. A Lewis acid is a substance that accepts an electron pair, and a Lewis base is a substance that donates an... 

To discuss acid-base catalysis, it is helpful to recall the definitions of acids and bases. In the Brpnsted-Lowry definition, an acid is a proton donor and a base is a proton acceptor. The concept of general acid-base catalysis depends on donation and acceptance of protons by groups such as the imidazole, hydroxyl, carboxyl, sulfhydryl, amino, and phenolic side chains of amino acids all these functional groups can act as acids or bases. The donation and acceptance of protons gives rise to the bond breaking and re-formation that constitute the enzymatic reaction. 

The properties and reactions of acids and bases are central to our study of organic chemistry. We need to consider exactly what is meant by the terms add and base. Most people would agree that H2SO4 is an acid and NaOH is a base. Is BF3 an acid or a base Is ethylene (H2C=CH2) an acid or a base To answer these questions, we need to understand the three different definitions of acids and bases the Arrhenius definition, the Brpnsted-Lowry definition, and the Lewis definition. 

In the aqueous system, the Brpnsted-Lowry definitions of acids and bases are of general applicability. A Brpnsted acid a proton donor. Acidic oxides react with water to produce oxonium ions in solution, e.g. 

You have reviewed the Brpnsted-Lowry definition of acids and bases and the meanings of pH and p a- You have learned to identify the most acidic hydrogen atoms in a molecule based on a comparison of p a values. You will see in many cases that Brpnsted-Lowry acid-base reactions either initiate or complete an organic reaction, or prepare an organic molecule for further reaction. The Lewis definition of acids and bases may have been new to you. However, you will see over and over again that Lewis acid-base reactions which involve either the donation of an electron pair to form a new covalent bond or the departure of an electron pair to break a covalent bond are central steps in many organic reactions. The vast majority of organic reactions you will study are either Brpnsted-Lowry or Lewis acid-base reactions. 

A second, more widely apphcable definition of acids and bases, called the Brpnsted-Lowry definition, was introduced in 1923. This definition focuses on the transfer ofH ions in an acid-base reaction. Since an H ion is a proton—a hydrogen atom without its electron—this definition focuses on the idea of a proton donor and a proton acceptor ... 

Unlike the situation for Brpnsted acidity, manifest by complete or partial proton transfer from acid to base, Lewis acidity is a more general concept. It encompasses the Brpnsted-Lowry definition and thus there is no universal scale (in contrast to the Hammett acidity... 

At the microscopic level, the Arrhenius theory defines acids as substances which, when dissolved in water, yield the hydronium ion (H30+) or H+(aq). Bases are defined as substances which, when dissolved in water, yield the hydroxide ion (OH). Acids and bases may be strong (as in strong electrolytes), dissociating completely in water, or weak (as in weak electrolytes), partially dissociating in water. (We will see the more useful Brpnsted-Lowry definitions of acids and bases in Chapter 15.) Strong acids include ... 

Brpnsted-Lowry definition does not differ appreciably from the Arrhenius definition of hydrogen ions (acids) and hydroxide ions (bases) ... 

Section 1.17 The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Brpnsted-Lowry picture. A Lewis acid is an electron-pair acceptor. A Lewis base is an electron-pair donor. The Lewis approach incorporates the Brpnsted-Lowry approach as a subcategory in which the atom that accepts the electron pair in the Lewis acid is a proton. 

In 1923, Brpnsted and Lowry defined acids and bases on the basis of the transfer of protons. A Brpnsted-Lowry acid is any species that can donate a proton, and a Brpnsted-Lowry base is any species that can accept a proton. These definitions also include all the Arrhenius acids and bases because compounds that dissociate to give H30+ are proton donors, and compounds that dissociate to give OH are proton acceptors. (Hydroxide ion accepts a proton to form H20.)... 

Such considerations led to a more general definition of acids and bases, which was proposed independently by J. N. Brpnsted and T. M. Lowry in 1923. They defined acid as any substance (in either the molecular or the ionic state) which donates protons (H+), and a base as any substance (molecular or ionic) which accepts protons. Denoting the acid by A and the base by B, the acid-base equilibrium can be expressed as... 

In 1923, Johannes N. Brpnsted (1879-1947) and Thomas M. Lowry (1874-1936) independently defined acids and bases in a different way from the Arrhenins definitions. The resulting theory is sometimes called the Brpnsted-Lowry theory, bnt more often is referred to as just the Brpnsted theory. The Brpnsted theory extends the definitions of acid and base in a way that explains more than the Arrhenins definitions can explain. According to this theory, a Brpnsted acid is a proton donor, and a Brpnsted base is a proton acceptor. 

The Br0nsted-Lowry theory expands the definition of acids and bases to allow us to explain n ch more ol solution chemistry. For example, the Brpnsted-Lowry theory allows us to explain why a solution af ammonium nitrate tests acidic and a solution of potassium acetate tests basic. Most of the substances that we cofcider acids in the Arrhenius theory are also acids in the Brpnsted-Lowry theory, and the same is true of bases. Injboth theories, strong acids are those that react completely with water to form ions. Weak acids ionize only slightly. We can now explain this partial ionization as an equilibrium reaction of the weak acid, the ions, and the w ater. A similar statement can be made about weak bases ... 

Lewis defined a base as an electron-pair donor and an acid as an electron-pair acceptor. This definition further expands the list to include metal ions and other electron pair acceptors as acids and provides a handy framework for nonaqueous reactions. Most of the acid-base descriptions in this book will use the Lewis definition, which encompasses the Brpnsted-Lowry and solvent system definitions. In addition to all the reactions discussed previously, the Lewis definition includes reactions such as... 

Of the numerous definitions of acids and bases that have been employed over the years, the 1923 definitions of J. N. Brpnsted and T. M. Lowry have proven to be the most useful for discussions of ionic equilibria in aqueous systems. According to the Brpnsted-Lowry model, an acid is a substance capable of donating a proton to another substance, such as water ... 

The work of Brpnsted and Pedersen (23) on the catalytic decomposition of nitramide and the kinetic studies of Lowry and Faulkner (24) on the mutarotation of tetramethylglucose led to the formulation of a more general viewpoint on acids and bases which logically showed that the hydrogen ion and hydroxyl ion were not the unique carriers of acid and basic properties. An acid was defined as any substance capable of donating a proton, and a base any substance capable of accepting a proton. In accordance with this definition (Lowry, 25 Brpnsted, 26), the following substances are typical acids and bases ... 

The next theory of acids and bases is called the Brpnsted-Lowry Theory, proposed by Danish chemist Johannes Brpnsted and the English chemist Thomas Lowry, independently, in 1923. The definition of an acid, in this theory, sounds essentially the same as the Arrhenius acid. ABr0nsted-Lowry acid is defined as a substance that donates a proton to another species. Now, you might say, A proton is a hydrogen ion (H+), so what is the difference between the two definitions for acids Most notably, this definition doesn t require that the acid be in an aqueous solution. So a substance that donates protons, even when in a solid or vapor phase, is still acting as a Brpnsted-Lowry acid. 

By focusing on where the proton comes from and goes to, the Brpnsted-Lowry concept expands the definition of a base to encompass a host of species that the Arrhenius definition excludes a base is any species that accepts a proton to do so, the base must have a lone electron pair. (The lone electron pair also plays the central role in the Lewis acid-base definition, as you ll see later in this chapter.)... 

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