Lewis theory of acids and bases

According to Lewis, acids are electron-pair acceptors (EPA) and bases electron-pair donors (EPD), connected through the following equilibrium [63, 65a]  

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

The HSAB behaviour is dependent on the medium in which EPD/EPA reactions are carried out. For example, the order of stability of complexes of metal ions with halide ions in the gas phase is F Cl Br I , which makes all metal ions appear hard in the gas phase. However, in aqueous solution, the stability order is reversed to F Cl Br 1 for those metal ions classified as soft [69], 

The application of the HSAB concept to solutions leads to the rule that hard solutes dissolve in hard solvents and soft solutes dissolve in soft solvents [66], This rule can be considered as a modem version of similia similibus solvuntur . For example, benzene is considered a very soft solvent since it contains only a basic function. Contrary to benzene, water is a very hard solvent, with respect to both its basic and acidic properties. It is the ideal solvent for hard bases and hard acids. The hardness of water is reduced by the introduction of alkyl substituents in proportion to the size of the alkyl group. In alcohols, therefore, softer solutes become soluble. Whereas oxalate salts are quite insoluble in methanol, the corresponding softer bisthiooxalate salts are quite soluble. 

Since hydrogen-bonding is a hard acid-hard base interaction, small basic anions prefer specific solvation by protic solvents. Hence, the reactivity of F , HO , or CH3O is reduced most on going from a dipolar non-HBD solvent such as dimethyl sulfoxide to a protic solvent like methanol. Dipolar non-HBD solvents are considered as fairly soft compared to water and alcohols [66], 

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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 theory of acids and bases a Lewis acid is a compound capable of accepting an electron pair, and a Lewis base is capable of donating an electron pair. 

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. 

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. 

Describe the following reaction in terms of the Lewis theory of acids and bases ... 

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. 

These reactions are Lewis acid-base reactions. The Lewis theory of acids and bases defines an acid as a substance capable of accepting a pair of electrons and a base as a substance that donates a pair of electrons. The terms acceptor and donor are sometimes used for acid and base, respectively. A Lewis acid-base reaction results in the formation of a coordinate bond, equation (9). 

Sometimes an aprotic solvent is used for acid-base reactions, that is, a solvent whose molecules do not donate a proton. Under these conditions the more general Lewis theory of acids and bases is applied. 

The Lewis Theory of Acids and Bases (Optional) COAL 4 Distingnish between a Lewis acid and a Lewis base. Given the strnctural formnla of a molecule or ion, state if it can be a Lewis acid, a Lewis base, or both, and explain why. 

Section 17.3 The Lewis Theory of Acids and Bases (Optional)... 

We now see more clearly the function of the Lewis theory of acids and bases. The elimination of the idea that acidity depends upon the presence of a particular element permits a much greater systematization of chemical reactions than has been possible in the past. 

One test of any new theory is Does it offer a usable explanation which succeeds in correlating a larger number of experimental facts We believe that according to this test the Lewis theory of acids and bases is successful. It does correlate a much larger body of experimental facts. On the other hand, a new theory in order to be convincing must not depart too far from previously accepted ideas. The wide interest in the Lewis theory indicates that it does not. 

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