Electron pair acceptors and donors

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

We can also examine isomeric pairs of H-bonded complexes in which the roles of electron-pair donor and acceptor are reversed, and the mutual dipole orientations are therefore fundamentally altered. As an example, Fig. 5.4 shows the isomeric pair of complexes between ammonia and hydrogen fluoride,... 

Lewis (1923) put forward another definition of acids and bases solely dependent on giving or taking of an electron pair. According to Lewis— an add is an electron pair acceptor, whereas a base is an electron pair donor . Therefore, it is obvious that whenever any neutralization occurs the formation of an altogether new coordinate covalent bond between the electron pair donor and acceptor atoms take place. 

Although many other acid-base definitions have been proposed and have been useful in particular types of reactions, only a few have been widely adopted for general use. Among them are those attributed to Arrhenius (based on hydrogen and hydroxide ion formation), Br0nsted-Lowry (hydrogen ion donors and acceptors), and Lewis (electron pair donors and acceptors) [6,67-70]. 

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 designation of electron-pair donors and acceptors as Lewis bases and Lewis acids is firmly and fittingly ingrained in the language of chemistry. G. N. Lewis laid the foundation for this important theory approximately 80 years ago and Lewis acids have since become increasingly important because of their central role in synthetic organic chemistry. This is clearly illustrated by an ever-increasing number of publications and books. 

Electron pair donors and acceptors Consider the reaction between a hydrogen ion (H+) and a fluoride ion (F ) to form a hydrogen fluoride (HF) molecule. The role of the electron pair is illustrated through the following Lewis structures. 

Quantitative Calculations The stoichiometry of complexation reactions is given by the conservation of electron pairs between the ligand, which is an electron-pair donor, and the metal, which is an electron-pair acceptor (see Section 2C) thus... 

Lewis defined a base as an electron pair donor and an acid as an electron pair acceptor. Lewis electron pair donor was the same as Bronsted-Lowry s proton acceptor, and therefore, was an equivalent way of defining a base. Lewis acids were defined as a substance with an empty valence shell that could accommodate a pair of electrons. This definition broadened the Bronsted-Lowry definition of an acid. The three definitions of acids and bases are summarized in Table 13.3. 

Basicity (including the abilities as proton acceptor, hydrogen-bond acceptor, electron pair donor, and electron donor)a)... 

The molecules or ions that surround the central metal ion in a coordination compound are called ligands, and the atoms that are attached directly to the metal are called ligand donor atoms. In cisplatin, for example, the ligands are NH3 and Cl-, and the ligand donor atoms are N and Cl. The formation of a coordination compound is a Lewis acid-base interaction (Section 15.16) in which the ligands act as Lewis bases (electron-pair donors) and the central metal ion behaves as a Lewis acid (an electron-pair acceptor). 

Lewis bases are electron pair donors and the electrons are given up to Lewis acids (electron acceptors). Unshared electron pairs are the more common type of electron pairs to be donated to Lewis acids, although on occasion shared pairs can be donated to the Lewis acid. In die case of Lewis acid-base reactions, the product is termed a Lewis acid-base complex. 

Since they have both a vacant low-energy orbital and a lone pair, silylenes might behave either as electron pair donors or acceptors. There is scant evidence for silylenes reacting as Lewis bases, but complexes of silylenes acting as Lewis acids are now well-established these complexes can also be described as silaylides, R2>Si —B+262. Trinquier has calculated that even SiLL should form a weak complex with 112S i , in which a silane hydrogen binds to the p-orbital of the silylene263. 

A vast generalization beyond the Bronsted—Lowry acids and bases concepts is the concept of a Lewis29 base (an electron pair donor) and a Lewis acid (an electron pair acceptor). This concept has been used extensively in all branches of chemistry. In physical organic chemistry, quantities of the type pA = —logio[A] have used extensively to study reactivities—for example, in the Hammett equation. 

The preceding example demonstrates the general view that the procedure most likely to alter the crystallization thermod)mamics of true race-mate systems will entail the formation of dissociable diastereomer species [50-54]. In most instances, these diastereomers are simple salts formed between proton donors and proton acceptors, or electron-pair donors and electron-pair acceptors. For example, the first resolving agents introduced for acidic enantiomers were alkaloid compounds, and hydroxyl acids were used for the resolution of basic enantiomers. This t) e of resolution procedure has been known since the time of Pasteur, and extensive tables of resolving agents and procedures are available [48,55,66]. 

In 1923 G. N. Lewis" proposed a definition of acid-base behavior in terms of electron-pair donation and acceptance. The Lewis definition is perhaps the most widely used of all because of its simplicity and wide applicability, espeaally in the field of organic reactions. Lewis defined a base as cn electron-pair donor and an acid as an electron-pair acceptor. In addition to all of llie reactions discussed above, the Lewis definition includes reactions in which no ions are formed and no hydrogen ions or other ions are transferred 2... 

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... 

The continuum model has been applied to an experimental study of the solvent effect on the 6-chloro-2-hydroxypyridine/6-chloro-2-pyridone equilibrium in a variety of essentially non-hydrogen-bonding solvents (Beak et al., 1980). In this study, a plot of log A nh/oh) versus (e - 1)/ (2e + 1), the solvent dielectric term, yielded a linear least-squares fit with a slope of 2.5 0.2, an intercept of -1.71, and a correlation coefficient of 0.9944. This result was used to estimate the gas phase free-energy difference of 9.2 kJ mole-1, which compares favorably with the observed value of 8.8 kJ mole-1 for this system. The authors also reported that alcohol solvents are correlated fairly well in this study but that other solvents seem to be divided into two classes, those that are electron-pair donors and those that are electron-pair acceptors in a hydrogen bond. The hydrogen bonding effect is assumed to be independent from the reaction field effect and is included in the continuum model by means of the Kamlet and Taft (1976) empirical parameters. The interested reader is referred to the original paper for a detailed discussion of the method and its application. 

A number of years ago G. N. Lewis extended our understanding of acid-base behavior to include reactions other than proton transfers. According to Lewis, an acid is an electron-pair acceptor and a base is an electron-pair donor. Thus, carbocations are electron-pair acceptors and are Lewis acids. Halide anions are electron-pair donors and are Lewis bases. It is generally true that electrophiles are Lewis acids, and nucleophiles are Lewis bases. 

While nonbonded electron pairs in molecules do not enter into covalent bonding in the usual sense, they may exhibit a secondary kind of valency by being transferred into vacant molecular orbitals in suitable acceptor molecules. This results in the transformation of a coordination complex in which the bond formed between the electron-pair donor and the acceptor is said to be a coordinate covalent or dative bond. Brpnsted basicity is the simplest example of coordinate covalent bond formation. A Brpnsted base donates a pair of nonbonded electrons to a vacant Is orbital of a hydrogen ion to form the conjugate acid. The o-bond formed between the base and the hydrogen ion results in the loss of identity of the nonbonded pair previously localized on the base. The formation of coordination complexes has significance in the interpretation of spectra of compounds having nonbonded electron pairs. 

A Lewis acid is an electron pair acceptor while a Brpnsted-Lowry acid is a proton donor. The proton of a Brpnsted-Lowry acid fits the definition of a Lewis acid because it accepts an electron pair when it bonds with a base. All Lewis acids are not Br0nsted-Lowry acids. A Lewis base is an electron pair donor and a Brpnsted-Lowry base is a proton acceptor. All Brpnsted-Lowry bases can be Lewis bases, and vice versa. 18.85(a) No, Zn(H20)6 ""(fl< ) + 6NH3(flf/) Zn(NH3)6"++ 6H20(/) NH3 is a weak Brpnsted-Lowry base, but a strong Lewis base. 

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