Concepts Lewis

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 

Lewis acid-base adducts involving metal ions are called coordination compounds (bonds formed with both electrons from one atom are called coordinate bonds) their chemistry will be discussed in Chapters 9 through 14. 

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Cations at the surface possess Lewis acidity, i.e. they behave as electron acceptors. The oxygen ions behave as proton acceptors and are thus Bronsted bases. This has consequences for adsorption, as we will see. According to Bronsted s concept of basicity, species capable of accepting a proton are called a base, while a Bronsted acid is a proton donor. In Lewis concept, every species that can accept an electron is an acid, while electron donors, such as molecules possessing electron lone pairs, are bases. Hence a Lewis base is in practice equivalent to a Bronsted base. However, the concepts of acidity are markedly different. 

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. 

The generalized Lewis concept of acids and bases also includes common proton-transfer reactions.1 Thus water acts as a base because one of the... 

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 ... 

Lewis Concept According to this concept an acid is a substance (molecule or ion) that can accept an electron pair to form a covalent bond and base is a substance that can supply an electron pair to form a covalent bond. Thus an add is electron pair acceptor and a base is an electron pair donor. An add need not contain hydrogen. 

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

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