THE LEWIS THEORY

The Lewis Theory of acids and bases does not feature the special role for the proton that it has in the Bronsted-Lowry theory. Here an acid is any electron-pair deficient species. A base from this viewpoint is a species capable of furnishing electron pairs. Thus, acid-base reactions are considered as coordination reactions. This theory is of great value in understanding metal coordination complex formation. 

The scientist G.N. Lewis put forward a more elaborate theory regarding acids and bases. According to him, a Lewis acid is a species that can accept a pair of electrons from another species. He defined Lewis base as any species that can donate a pair of electrons to another species. For a thorough understanding of this concept, let s look at a typical Lewis acid-base reaction. The reaction between ammonia (NH3) and boron trichloride (BCI3) is shown  

In this reaction, the electron acceptor is boron trichloride (Lewis acid) and the electron donor is ammonia (Lewis base). The new bond that is formed between the acid and the base is a coordinate covalent bond. 

Usually pure water is considered to be a nonelectrolyte. Nevertheless, it actually can conduct small amounts of electricity. This is because of the selfionization of water. Since the species that act as the acid and the base are one and the same, we can describe it as follows  

A proton is grabbed from one water molecule, and is accepted by another water molecule. The resulting products are hydronium and hydroxide ions. 

In 1923, Professor G. N. Lewis (1875-1946) presented the most comprehensive of the This is the same Lewis who made classic acid-hase theories. The Lewis definitions follow. many contributions to our 

An acid is any species that can accept a share in an electron pair. A base is any species that can donate or share one or more lone pairs of electrons. 

Lewis acids have low-lying empty valence orbitals that readily interact with lone pair(s) on other atoms (Lewis bases) to move towards a closed shell electronic configuration. These definitions do not specify that a lone pair of electrons must be transferred from one atom to another—only that an electron pair, residing originally on one atom (the Lewis base) must be shared with the Lewis acid. Neutralization is defined as coordinate covalent (or dative) bond formation. This results in a bond in which both electrons are furnished by one atom or ion (the Lewis base). 

The reaction of boron trichloride with ammonia is a typical Lewis acid-base reaction. The Lewis theory is sufficiendy general that it covers all acid-base reactions that the other theories include, plus many additional reactions such as metal complex formation (see Chapter 25). 

Unless otheiwise noted, all content on this page is Cengage Learning. 

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 

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A proton (H+) is an electron pair acceptor. It is therefore a Lewis acid because it can attach to ( accept") a lone pair of electrons on a Lewis base. In other words, a Bronsted acid is a supplier of one particular Lewis acid, a proton. The Lewis theory is more general than the Bronsted-Lowry theory. For instance, metal atoms and ions can act as Lewis acids, as in the formation of Ni(CO)4 from nickel atoms (the Lewis acid) and carbon monoxide (the Lewis base), but they are not Bronsted acids. Likewise, a Bronsted base is a special kind of Lewis base, one that can use a lone pair of electrons to form a coordinate covalent bond to a proton. For instance, an oxide ion is a Lewis base. It forms a coordinate covalent bond to a proton, a Lewis acid, by supplying both the electrons for the bond ... 

A note on good practice The entities that are regarded as acids and bases are different in each theory. In the Lewis theory, the proton is an acid in the Bronsted theory, the species that supplies the proton is the acid. In both the Lewis and Bronsted theories, the species that accepts a proton is a base in the Arrhenius theory, the species that supplies the proton acceptor is the base (Fig. 10.61. 

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. ... 

At about the same time that Brpnsted proposed his acid-base theoiy, Lewis put forth a broader theory. A base in the Lewis theory is the same as in the Brpnsted one. 

Although Lewis and Bronsted bases comprise the same species, the same is not true of their acids. Lewis acids include bare metal cations, while Bronsted-Lowry acids do not. Also, Bell (1973) and Day Selbin (1969) have pointed out that Bronsted or protonic acids fit awkwardly into the Lewis definition. Protonic acids cannot accept an electron pair as is required in the Lewis definition, and a typical Lewis protonic add appears to be an adduct between a base and the add (Luder, 1940 Kolthoff, 1944). Thus, a protonic acid can only be regarded as a Lewis add in the sense that its reaction with a base involves the transient formation of an unstable hydrogen bond adduct. For this reason, advocates of the Lewis theory have sometimes termed protonic adds secondary acids (Bell, 1973). This is an unfortunate term for the traditional adds. 

It is better than the Lewis theory for describing acid-base cements, for it avoids the awkwardness that the Lewis definition has with protonic acids. However, as Day Selbin (1969) have observed, the generality of the theory is such that it includes nearly all chemical reactions, so that acid-base reactions could simply be termed chemical reactions . 

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. 

C) The Bronsted-Lowry or proton theory interprets the acid-base reaction as a mere proton exchange between the acid (proton donor) and the base (proton acceptor) however, the Lewis theory or electron theory interprets the reaction as a donation and acceptance of a lone pair of electrons, where the... 

Comparison of Bronsted reaction 4.48 with Lewis reaction 4.49 shows that the Lewis theory is more generally applicable, but its interpretation is different in terms of the definition of acids and complexes. In fact, the Lewis theory is valid for all acid-base reactions (cf., eqns. 4.39 and 4.40). 

It is obvious that the Lewis theory and the HSAB concept are very important for the description of the mechanisms of chemical reactions however, for electrochemistry the Br0nsted theory is quite adequate. 

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. 

As we have seen, the Lewis theory of acid-base interactions based on electron pair donation and acceptance applies to many types of species. As a result, the electronic theory of acids and bases pervades the whole of chemistry. Because the formation of metal complexes represents one type of Lewis acid-base interaction, it was in that area that evidence of the principle that species of similar electronic character interact best was first noted. As early as the 1950s, Ahrland, Chatt, and Davies had classified metals as belonging to class A if they formed more stable complexes with the first element in the periodic group or to class B if they formed more stable complexes with the heavier elements in that group. This means that metals are classified as A or B based on the electronic character of the donor atom they prefer to bond to. The donor strength of the ligands is determined by the stability of the complexes they form with metals. This behavior is summarized in the following table. 

W. M. Latimer and W. H. Rodebush, J. Am. Chem. Soc. 42 (1920), 1419. This paper is often cited as the discovery of H-bonding (but see Jeffrey, note 5). Its title, Polarity and ionization from the standpoint of the Lewis theory of valence, reflects the strong influence of G. N. Lewis on all aspects of the early development of H-bond theory. 

The preceding historical survey is actually underlying a fundamental - but simple -aspect of the Lewis theory. Given an [(TVO L] complex ion (or complex HQ- 0), where Q represents the sum of metal-ccaijugafed charges, the bond is dissected... 

According to the Lewis theory, alkaline earth metal hydroxides are weaker bases than their oxides, the order of the strength of the basic sites being Ba(OH)2> SrO(OH)2 > Ca(OH)2 > Mg(OH)2. The hydroxides have been used recently as solid catalysts for organic transformations, such as the conjugate addition of methanol to a, S-unsaturated carbonyl compounds (12), cyanoethylation of alcohols (163,164), and transesterification reactions (166,167,171,172) which are described above. The extensive work of Sinisterra et al. (282) on the number and nature of sites and on the catalytic activity of the most basic alkali metal hydroxide, Ba(OH)2, is emphasized. It was found that commercial barium hydroxide octahydrate can be converted into... 

Problem 13.53 How does the Lewis theory of acids and bases explain the functions of (o) ZnCl, in the Lucas reagent (b) ether as a solvent in the Grignard reagent M... 

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. 

Lewis structures provide information about what atoms are bonded to each other, and the total electron parrs involved. According to the Lewis theory, an atom will give up, accept or share electrons in order to achieve a filled outer shell that contains eight electrons. The Lewis structure of a covalent molecule shows all the electrons in the valence shell of each atom the bonds between atoms are shown as shared pairs of electrons. Atoms are most... 

At about the same time that Bronsted proposed his acid-base theory, Lewis put forth a broader theory, A base in the Lewis theory is the same as in the Brpnsted one, namely, a compound with an available pair of electrons, either unshared or in a tt orbital. A Lewis acid, however, is any species with a vacant orbital.1115 In a Lewis acid-base reaction the unshared pair of the base forms a covalent bond with the vacant orbital of the acid, as represented by the general equation... 

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. 

Most chemists still tend to think about the structure and reactivity of atomic and molecular species in qualitative terms that are related to electron pairs and to unpaired electrons. Concepts utilizing these terms such as, for example, the Lewis theory of valence, have had and still have a considerable impact on many areas of chemistry. They are particularly useful when it is necessary to highlight the qualitative similarities between the structure and reactivity of molecules containing identical functional groups, or within a homologous series. Many organic chemistry textbooks continue to use full and half-arrows to indicate the supposed movement of electron pairs or single electrons in the description of reaction mechanisms. Such concepts are closely related to classical valence-bond (VB) theory which, however, is unable to compete with advanced molecular orbital (MO) approaches in the accurate calculation of the quantitative features of the potential surface associated with a chemical reaction. 

The Lewis theory does not differ from Bronsted theory with respect to substances classified as bases. A substance capable of donating an electron pair (i.e., any Lewis base) is capable of donating electron pair to a proton. Being proton acceptor it is also a Bronsted base. Thus both definitions label the following as... 

According to the Lewis theory of acids and bases (to be discussed in detail in Chapter 5), an acid is an electron pair acceptor and a base is an electron pair donor. Accordingly, the following reactions are acid-base reactions because they represent processes in which electron pair donation and acceptance occurs ... 

Electron donation-acceptance reactions, which are considered to be Lewis acid-base interactions, also include the formation of coordination compounds, complex formation through hydrogen bonding, charge transfer complex formation, and so on. It should be apparent that the Lewis theory of acids and bases encompasses a great deal of both inorganic and organic chemistry. 

The Lewis theory makes no prediction about molecular shapes, so it is permissible to arrange the dot pairs and other atoms around the central atom in an arbitrary way. Usually, a more or less symmetrical arrangement is written. 

The paramagnetism of oxygen is an anomaly in terms of the Lewis theory, although it is predicted by a more comprehensive theory that we will look at later. There are, however, a few other molecules that we would expect to be paramagnetic simply because they contain an odd number of valence electrons. The most well known example is nitric oxide, NO. Since oxygen has four and nitrogen has five outer electrons, the total number of valence electrons is nine, and magnetic measurements show that one of these is unpaired. 

Latimer WM, Rodebush WH (1920) Polarity and ionization from the standpoint of the Lewis theory of valence. J Am Chem Soc 42 1419-1433... 

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