The Lewis Definition of Acids and Bases

In the original Lewis definition (1923 [29], 1938 [30]), aeids are electron-pair acceptors and bases are electron-pair donors. The fundamental reaction between a Lewis acid A and a Lewis base B is the formation of a complex (or adduct or coordination compound or addition compound) A-B (reaction 1.17)  

In this reaction, the unshared electron pair of the base forms a coordinate covalent bond (or dative bond or dipolar bond) with an electron-deficient atom of the acid. The archetype of a Lewis acid/base reaction is 

BF3 is a Lewis acid because the boron atom has only six electrons in its valence shell and, having room for eight, can accept the lone pair of the nitrogen atom of ammonia. 

The proton is a Lewis acid because it can accept an electron pair into its empty 1 s atomic orbital. It follows that all Bronsted bases are Lewis bases. All Brdnsted acids are also Lewis acids because they are hydrogen-bond donors, that is, electron acceptors (see below). 

However, a much wider range of species can be classified as Lewis acids than can be classified in the Bronsted scheme. The translation of Lewis s definition into quantum-mechanical terms by Mulliken (1952) [31] further widened the definition, so as to include those reagents that donate or accept a fraction, possibly very small, of an electron. With this extension, the compounds in Tables 1.4 and 1.5 are considered as Lewis acids (electron 

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The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Br0nsted-Lowry pic ture A Lewis acid is an electron pair acceptor A Lewis base is an electron pair donor The Lewis approach incorporates the Br0nsted-Lowry approach as a subcategory m which the atom that accepts the electron pair m the Lewis acid is a proton... 

The Lewis definition of acids and bases is broader and more encompassing than the Bronsted-Lowry definition because it s not limited to substances that donate or accept just protons. A Lewis acid is a substance that accepts an electron pair, and a Lewis base is a substance that donates an electron pair. The donated electron pair is shared between the acid and the base in a covalent bond. 

In 1923. Lewis published a classic book (later reprinted by Dover Publications) titled Valence and the Structure of Atoms and Molecules. Here, in Lewis s characteristically lucid style, we find many of the basic principles of covalent bonding discussed in this chapter. Included are electron-dot structures, the octet rule, and the concept of electronegativity. Here too is the Lewis definition of acids and bases (Chapter 15). That same year, Lewis published with Merle Randall a text called Thermodynamics and the Free Energy of Chemical Substances. Today, a revised edition of that text is still used in graduate courses in chemistry. 

In both cases the nitrogen atom uses its pair of nonbonding electrons to make a new covalent bond. This similarity led G. N. Lewis to classify ammonia as a base in its reaction with B (CH3)3 as well as in its reaction with H3 O . Whereas the Br< )nsted definition focuses on proton transfer, the Lewis definition of acids and bases focuses on electron pairs. 

B The Lewis Definition of Acids and Bases 1. Lewis acid-base theory in 1923 proposed by G. N. Lewis (1875-1946 Ph. D. Harvard, 1899 professor, Massachusetts Institute of Technology, 1905-1912 professor, University of California, Berkeley, 1912-1946). 

A water molecule, HzO, becomes strongly held to a dissolved aluminum ion, Al3+. According to the Lewis definition of acids and bases, which is behaving... 

In this reaction, the proton behaves as an electron-pair acceptor, and the ammonia molecule behaves as an electron-pair donor. Consequently, the Lewis definition of acids and bases states that a Lewis acid is an electron-pair acceptor, and a Lewis base is an electron-pair donor. 

Which reaction below demonstrates the Lewis definition of acids and bases ... 

How would transition metal ions be classified using the Lewis definition of acids and bases What must a ligand have to bond to a metal What do we mean when we say that a bond is a coordinate covalent bond ... 

We are now in a position to reformulate the Lewis definition of acids and bases in terms of frontier orbitals A base has an electron pair in a HOMO of suitable symmetry to interact with the LUMO of the acid (although lone pair orbitals with the wrong geometry may need to be ignored). The better the energy match between the base s HOMO and the acid s LUMO, the stronger the interaction. 

Figure 15.11 shows x-ray diffraction (XRD) profiles of ceria powders produced by precipitation. The XRD data for the synthesized particles show characteristics of Ce02 with a typical fluorite stracture. Since the starting cerium salt was Ce(N03)3, it required the oxidation of Ce + to Ce + in the solution. In this system, there is a possible cause for this oxidation. According to the Lewis definition of acids and bases, Ce is a Lewis base and Ce + is a Lewis acid. Basic solution therefore favors Ce compared with Ce . The crystallite size was calculated from the Scherrer formula ... 

Use the Lewis definition of acids and bases to explain the example. given for the Br0nsted-Lowry, Lux-Flood, and. solvent system definitions (Eqs. 9.1-9.4, 9.5-9.8. 9.9-9.36). 

Although the Lewis definition of acids and bases has greater significance because of its generality, we normally speak of an acid and a base in terms of the Brpnsted definition. The term Lewis acid usnally is reserved for substances that can accept a pair of electrons bnt do not contain ionizable hydrogen atoms. 

Our emphasis throughout this chapter has been on water as the solvent and on the proton as the source of acidic properties. In such cases we find the Bronsted—Lowry definition of acids and bases to be the most useful. In fact, when we speak of a substance as being acidic or basic, we are usually thinking of aqueous solutions and using these terms in the Arrhenius or Bronsted—Lowry sense. The advantage of the Lewis definitions of acid and base is that they allow us to treat a wider variety of reactions, including those that do not involve proton transfer, as acid—base reactions. To avoid confusion, a substance such as BF3 is rarely called an acid unless it is clear from the context that we are using the term in the sense of the Lewis definition. Instead, substances that function as electron-pair acceptors are referred to explicitly as Lewis acids. ... 

The Lewis definitions of acids and bases provide for a more general view of acid-base reactions than either the Arrhenius or Br0nsted-Lowry picture. 

One method used for the preparation of [Pt(en)2]Cl2 or [Pt(en)3]Cl4 is the direct reaction between ethylenediamine and PtCU or PtCLt, respectively. The technique is to add slowly the solid platinum salts to the liquid ethylenediamine. This addition is accompanied by a vigorous evolution of heat, which is to be expected whenever a strong acid is added to a strong base. Recall (Section 2.1) that in terms of the Lewis definition of acids and bases, the formation of coordination compounds involves an acid-base reaction. In this particular case, the platinum ions are the acids and ethylenediamine is the base. Metal dimethylsulfoxide complexes have been prepared and characterized. One method used to prepare some of these complexes is a direct reaction (12) in the absence of any added solvent. 

You have reviewed the Bronsted-Lowry definition of acids and bases and the meanings of pH and pTQ. You have learned to identify the most acidic hydrogen atoms in a molecule based on a comparison of pIQ values. You will see in many cases that Brensted—Lowry acid-base reactions either initiate or complete an organic reaction, or prepare an organic molecule for further reaction. The Lewis definition of acids and bases may have been new to you. However, you will see over and over again that Lewis acid—base reactions which involve either the donation of an electron pair to form a new covalent bond or the departure of an electron pair to break a covalent bond are central steps in many organic reactions. The vast majority of organic reactions you will study are either Bronsted-Lowry or Lewis acid—base reactions. 

Finally, we learn the Lewis definitions of acid and base. A Lewis acid is an electron acceptor, and a Lewis base is an electron donor. The Lewis definitions are more general and inclusive than either Arrhenius or Bronsted-Lowry. 

The Lewis definition of acids and bases is broader than the Bronsted-Lowry definition. According to the Lewis definition, acidity and basicity are described in terms of electrons, rather than protons. A Lewis acid is defined as an electron acceptor, while a Lewis base is defined as an electron donor. As an illustration, consider the following Bronsted-Lowry acid-base reaction ... 

In Section 1.7 (p. 41), we introduced acids and bases. Now we know quite a bit more about structure and can return to the important subject of acids and bases in greater depth. In particular, we know about carbocations and carbanions, which play an important role in acid—base chemistry in organic chemistry The Lewis definition of acids and bases is far more inclusive than the Bronsted definition, which focuses solely on proton donation (Breasted acid) and acceptance (Breasted base). The archetypal Brensted acid-base reaction is the reaction between KOH and HCl to transfer a proton from HCl to HO. This reaction is a competition between the hydroxide and the chloride for a proton. In this case, the stronger base hydroxide wins easily (Rg. 2.57). 

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