The Lewis Concept

Lewis defined in a very general way an acid as an electron pair-acceptor and a base as a electron-pair donor . The criterion for the occurrance of an acid-base reaction is the formation of a coordinate covalent link26.27 0 g 

In this way both the acidic and the basic function is independent of an ion-transfer mechanism and independent from a solvent. In fact it covers all reactions considered in coordination chemistry. The terms Lewis acid and Lewis base are applied mainly in the field of organic chemistry. 

In order to avoid confusion with acids in the Bronsted sense, coordination chemists may prefer the terms acceptor and donor in the place of Lewis acid and Lewis base respectively and we shall follow this convention in the course of this presentation. 

Kolthoff28 suggested the term Protoacid for Lewis acids except proton donors, and Bjerrum prefers the term Antibase.  

2 Luder, W. F., and S. Zufpanti The Electronic Theory of Acids and Bases, Wiley, New York 1946. 

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The Lewis concept permits inclusion of acids and oxidants in a single... 

Usanovich (1934) modified the Lewis concept of acid and base by removing the restriction of either donation or acceptance of the electron pair in a more generalized fashion. According to him ... 

It should be kept in mind that the terms acidity and basicity of the solvent have to be intended not only according to the Lewis concept (electrophilic vs. nucleophilic properties), but also according to the Bronsted concept (proton donor vs. proton acceptors), or to the hydrogen bonding capacity (hydrogen bond donor vs. hydrogen bond acceptor). 

A Lewis acid (electrophile) shares an electron pair furnished by a Lewis base (nucleophile) to form a covalent (coordinate) bond. The Lewis concept is especially useful in explaining the acidity of an aprotic acid (no available proton), such as BFj. 

An example of the different points of view and different tastes in the matter of acid-base definitions was provided to one of the authors in graduate school while attending lectures on acid-base chemistry from two professors. One Felt that the solvent system was very useful, but that the Lewis concept went too far because it included coordination chemistry. The second used Lewis concepts in all of his work, but felt uncomfortable with the Usanovich definition because it included redox chemistry To the latter s credit, however, he realized that the separation was an artificial one, and he suggested the pyridine oxide example given above. 

The Lewis definition thus encompasses all reactions entailing hydrogen ion. oxide ion. or solvent interactions, as well as the formation of acid-base adducts such as R,NBF, and all coordination compounds. Usage of the Lewis concept is extensive in both inorganic and organic chemistry, and so no further examples will be given here, but many will be encountered throughout the remainder of the book.11... 

NUCLEOPHILE. An ion or molecule that donates a pair of electrons to an atomic nucleus to form a covalent bond. The nucTeus that accepts the electrons is called an electrophile. This occurs, for example, in the formation of acids and bases according to the Lewis concept, as well as in covalent bonding in organic compounds. 

Even more general is the Lewis concept of acids and bases a Lewis base has a lone pair available for formation of a coordinate bond, and a Lewis acid has a vacant acceptor orbital handy. This concept is applicable to reactions in the gas phase or in inert solvents (as discussed in the previous section) as well as to complex formation in solution and the acid/ base phenomena studied by Arrhenius, Br0nsted and Lowry. 

Lewis has defined acids and bases in a general way as electron acceptors and donors, respectively (21). Accordingly, a compound or element capable of accepting electrons (electron seeking) is termed a Lewis acid. Conversely a compound or element capable of giving (or sharing electrons) is a Lewis base. In other words, those elements which are deficient in electrons --that is, have unfilled electron shells -- will seek out those elements carrying extra electrons (lone pairs). This is the basis for the Lewis concept of "acid -base interaction. Thus ... 

Despite the quantitative victory of molecular orbital (MO) theory, much of our qualitative understanding of electronic structure is still couched in terms of local bonds and lone pairs, that are key conceptual elements of the valence bond (VB) picture. VB theory is essentially the quantum chemical formulation of the Lewis concept of the chemical bond [1,2]. Thus, a chemical bond involves spin-pairing of electrons which occupy valence atomic orbitals or hybrids of adjacent atoms that are bonded in the Lewis structure. In this manner, each term of a VB wave function corresponds to a specific chemical structure, and the isomorphism of the theoretical elements with the chemical elements creates an intimate relationship between the abstract theory and the nature of the... 

The Lewis concept of the acid is included in the AP curriculum but is not emphasized as much as the other two descriptions. As a result, we will just look at the main ideas. 

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 7) 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 Bronsted-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 (Bronsted-Lowry acids donate protons, while Lewis acids accept electron pairs). 

A similar trend in the change of the formal or E /2 potentials was observed for solvated cations, for instance for the Eu /Eu couple in different solvents [87, 88]. The analysis of Gibbs energies, entropies and enthalpies of single ion transfer led Gritzner [89] to search for general trends in the interaction of different ions with solvent molecules. The interactions of cations with different solvents were considered, in terms of the Lewis concept, i.e., as a reaction of the acid (cation) with the base (solvent). 

Adds and electrophiles are electron-deficient species. According to the Lewis concept, all electrophiles (e.g., cations, carbenes, metal ions) are acids by definition. However, from long usage the term acid is frequently used to refer to a proton donor, whereas the term Lewis acid usually refers to charged electrophiles in general. 

Audrieth and Moeller (61) have applied the Lewis concept to poly acids, and Hill and Audrieth (62) have shown that fluoride acts as a strong anionic base and catalyzes the high-temperature depolymerization of the polymetaphosphate and polyphosphate in the fused state. 

The Lewis concept of acids and bases (G. N. Lewis, 1923) interprets the combination of acids with bases in terms of the formation of a coordinate covalent bond. A Lewis acid can accept and share a lone pair of electPDns donated by a Lewis base. Because protons readily attach themselves to lone electron pairs, Lewis bases are also Biyinsted bases. Lewis acids, however, include a large number of substances in addition to proton donors for examjjle, metal ions, acidic oxides, or atoms. 

We shall find the Lewis concept of acidity and basicity fundamental to our understanding of organic chemistry. To make it clear that we are talking about this kind of acid or base, we shall often use the expression Lewis acid (or base) or sometimes acid (or base) in the Lewis sense. 

These definitions describe the Lowry-Bronsted concept of acidity and basicity. The Lewis concept of acids and b s is more general, and may be useful for reactions in which protons are not involved. A Lewis acid is any substance that can accept electrons, and a Lewis base is any substance that can donate electrons. Small, high-charge metal ions such as Al + and Fe " are strong Lewis acids because they tend to complex with functional groups on molecules (Lewis bases) by accepting electrons from them ... 

What is valence bond theory How does it differ from the Lewis concept of chemical bonding ... 

The significance of the Lewis concept is that it is much more general than other definitions. Lewis acid-base reactions include many reactions that do not involve Brpnsted acids. Consider, for example, the reaction between boron trifluoride (BF3) and ammonia to form an adduct compound (Figure 15.11) ... 

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... 

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 wave mechanics contains in it the Lewis concepts of atomic structure, of the rule of eight, of the rule of two, and of the valence property of the unpaired electron. How these atoms can be combined to form the molecules of Chapter I with their localized bonds, geometrical arrangements, and free or restricted rotations, etc., will be taken up in the following chapter. 

Organic chemists find that the information obtained from MO theory, where valence electrons occupy bonding and antibonding molecular orbitals, does not always yield the needed information about the bonds in a molecule. The valence-shell electron-pair repulsion (VSEPR) model combines the Lewis concept of shared electron pairs and lone-pair electrons with the concept of atomic orbitals and adds a third principle the minimization of electron repulsion. In this model, atoms share electrons by overlapping... 

The importance of the Lewis concept of acids and bases is that it can help explain reactions between molecules that are not strictly acids or bases, so it broadens the applicability to include nearly every reaction you can think of. 

SECTION 16.11 The Lewis concept of acids and bases emphasizes the shared electron pair rather than the proton. A Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor. The Lewis concept is more general than the Bronsted Lowry concept because it can apply to cases in which the add is some substance other than H. ... 

The Lewis concept has been called the most widely used model in contemporary chemistry. Frenking, G. Shaik, S. /. Comput. Chem. 2007, 28,1. 

LEWIS ACIDS AND BASES (SECTION 16.11) The Lewis concept of... 

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