Acidity Brpnsted-Lowry concept

An important feature of the Brpnsted-Lowry concept is the idea that the product formed when an acid gives up a proton is a potential proton acceptor and is called the conjugate base of the parent acid. For example, when the species acidj gives up a proton, the species basej is formed, as shown by the reaction... 

In the simple theory based on Lewis concepts exemplified above, the key aspects are an empty orbital on one atom and a filled orbital (with a pair of electrons present, the lone pair) on the other. Many of the ligand species providing the lone pair are considered bases in the classical Brpnsted-Lowry concept of acids and bases (which has as its focus the transfer... 

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.)... 

The final acid-base concept we consider was developed by Gilbert N. Lewis, whose contribution to understanding the importance of valence electron pairs in molecular bonding we discussed in Chapter 9. Whereas the Brpnsted-Lowry concept focuses on the proton in defining a species as an acid or a base, the Lewis concept highlights the role of the electron pair. The Lewis acid-base definition holds that... 

The first equation represents the Brpnsted-Lowry concept, with water reacting as both an acid and a base, forming a hydronium ion and a hydroxide ion. The second equation is a simplified version, indicating that water ionizes to give a hydrogen and a hydroxide ion. Actually, the proton (H" ) is hydrated and exists as a hydronium ion. 

In this reaction in aqueous solution, a proton, H, is transferred from the H30 ion to the NH3 molecule, giving H2O and NH4 (Figure 16.3). Here H30 is the proton donor, or acid, and NH3 is the proton acceptor, or base. Note that in the Brpnsted— Lowry concept, acids (and bases) can be ions as well as molecular substances. 

You have now seen several ways in which the Brpnsted-Lowry concept of acids and bases has greater scope than the Arrhenius concept. In the Br0nsted—Lowry concept... 

What is the difference between a strong acid and a weak acid, according to the Brpnsted-Lowry concept Give two examples of strong acids and two examples of weak acids. 

We can write the formation of a hydrochloric acid solution as a transfer of from hydrogen chloride to water. By accepting an H in the reaction, water is acting as a base according to the Brpnsted-Lowry concept. 

A particularly important concept in chemistry is that associated with proton loss and gain, i.e. acidity and basicity. Acids produce positively charged hydrogen ions H+ (protons) in aqueous solution the more acidic a compound is, the greater the concentration of protons it produces. In water, protons do not have an independent existence, but become strongly attached to a water molecule to give the stable hydronium ion H3O+. In the Brpnsted-Lowry definition ... 

E. Concepts of Acids and Bases — Brpnsted-Lowry approaches, Lewis theory, solvent system approaches... 

Acid-base reactions concepts of Arrhenius, Brpnsted-Lowry, and Lewis coordination complexes amphoterism... 

The Lewis concept deals with the behavior of electron pairs in chemical reactions. The same electron pairs we looked at when we discussed molecular geometry (see Chapter 6) can be involved in many reactions. Substances that can form a covalent bond by accepting an electron pair from another substance are known as Lewis acids. Substances that can form a covalent bond by donating an electron pair to another substance are known as Lewis bases. Be careful that you don t mix these up with the Brpnsted-Lowry acids and bases. It is easy to do since the words donate" and accept" are used, except they are associated with the opposite species (Brpnsted-Lowry acids donate protons, while Lewis acids accept electron pairs). 

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. 

In 1923 the Danish chemist Johannes N. Brpnsted (1879-1947) and, independently, the British chemist Thomas M. Lowry (1874-1936) pointed out that acid-base reactions can be seen as proton-transfer reactions and that acids and bases can be defined in terms of this proton (H ) transfer. According to the Br0nsted-Lowry concept, an acid is the species donating a proton in a proton-transfer reaction. A base is the species accepting the proton in a proton-transfer reaction. ... 

A further important concept related to electronegativity and polarity is that of acidity and basicity. We ll see, in fact, that much of the chemistry of organic molecules can he explained hy their acid-hase behavior. You may recall from a course in general chemistry that there are two frequently used definitions of acidity the Brensted-Lowry definition and the Lewis definition. We ll look at the Brpnsted-Lowry definition in this and the next three sections and then discuss the Lewis definition in Section 2.11. 

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... 

See pH. When dealing with chemical reactions in solvents other than water, it is sometimes convenient to define an acid as a substance that ionizes to give the positive ion of the solvent. The common definitions of acid have been extended as more detailed studies of chemical reactions have been made. The Lowry-Brpnsted definition of an acid as a substance that can give up a proton is more useful in connection with an understanding of bases (see base). Perhaps the most significant contribution to the theory of acids was the electron-pair concept introduced by G. N. Lewis around 1915. 

Thus, the Lewis concept radically broadens the idea of acid-base reactions. What to Arrhenius was the formation of H2O from H and OH became, to Brpnsted and Lowry, the transfer of a proton from a stronger acid to a stronger base to form a weaker base and weaker acid. To Lewis, the same process became the donation and acceptance of an electron pair to form a covalent bond in an adduct. 

The Acid-Base Concept According to Brpnsted and Lowry... 

Earlier in this chapter we considered Arrhenius s concept of acids and bases An acid is a substance that produces H+ ions when dissolved in water, and a base is a substance that produces OH ions. Although these ideas are fundamentally correct, it is convenient to have a more general definition of a base, which includes substances that do not contain OH ions. Such a definition was provided by Johannes N. Brpnsted (1879-1947) and Thomas M. Lowry (1874-1936), who defined acids and bases as follows ... 

Although the Arrhenius concept of acids and bases is useful, it is somewhat limited. For example, it tends to single out the OH ion as the source of base character, when other ions or molecules can play a similar role. In 1923, Johannes N. Brpnsted and Thomas M. Lowry independently noted that many reactions involve nothing more than the transfer of a proton (H ) between reactants, and they reaUzed that they could use this idea to expand the definitions of acids and bases to describe a large class of chemical reactions. In this view, acid—base reactions are proton-transfer reactions. 

According to the Lowry-Brdnsted theory, a Brdnsted acid is a proton donor, while a Brdnsted base is a proton acceptor. In Lewis concept, acid acts as electron-pair acceptor, while base is electron donor (such as molecules possessing electton lone pairs). Hence, a Lewis base is in practice equivalent to a Brpnsted base. However, the concepts of acidity are markedly different [27]. 

---