Chemical reactions Lewis acid-base

Usually, definitions of acids and bases are necessary for the classification of different kinds of chemical reactions, i.e. for dividing them into acid-base and other ones. For example, the Brpnsted-Lowry definition divides reactions into acid-base, which are characterized by redistribution of protons, and other ones. The wider Lewis definition makes the division of reactions into acid-base and redox ones, meaning that in the former case there is redistribution of electron density on account of electron pairs, and the latter case concerns reactions with the transfer of single electrons. Since chemistry concerns just the redistribution of electrons of external shells, we can classify all chemical reactions as acid-base ones by the Usanovitch definition. 

The definition of a species as a Lewis acid or a Lewis base is based on whether a species has a tendency to donate its electrons to another species or whether it tends to accept electrons from another species in a chemical reaction. Lewis acids are electron acceptors, while Lewis bases are electron donors. As you can probably guess, Lewis acids tend to react with Lewis bases, since they each have what the other one is looking for. 

When we mix two solutions the result is often simply a new solution that contains both solutes. However, in some cases the solutes can react with each other. For instance, when we mix a colorless aqueous solution of silver nitrate with a clear yellow aqueous solution of potassium chromate, a red solid forms, indicating that a chemical reaction has occurred (Fig. 1.1). This section and the next two introduce three of the main types of chemical reactions precipitation reactions, acid-base reactions, and redox reactions, all of which are discussed in more depth in later chapters. (The fourth type of reaction discussed in this text, Lewis acid-base reactions, is introduced in Section 10.2.) Because many chemical reactions take place in solution, particularly in water, in this section we begin by considering the nature of aqueous solutions. 

Write the balanced chemical equation for (a) the thermal decomposition of potassium chlorate without a catalyst (b) the reaction of bromine with water (c) the reaction between sodium chloride and concentrated sulfuric acid, (d) Identify each reaction as a Bronsted acid—base, Lewis acid—base, or redox reaction. 

Lewis acid-base chemistry provides one of the most useful tools ever devised for systematizing an enormous number of chemical reactions. Because the behavior of a substance as an acid or a base has nothing to do with proton transfer, many other types of reactions can be considered as acid-base reactions. For example,... 

The principle of hard and soft Lewis acids and bases, proposed by Pearson (1963), is useful to describe these reactions. A Lewis acid is any chemical species that employs an empty electronic orbital available for reaction, while a Lewis base is any chemical species that employs a doubly occupied electronic orbital in a reaction. Lewis acids and bases can be neutral molecules, simple or complex ions, or neutral or charged macromolecules. The proton and all metal cations of interest in subsurface aqueous solutions are Lewis acids. Lewis bases include H, O, oxyanions, and organic N, S, and P electron donors. A list of selected hard and soft Lewis acids and bases found in soil solutions is presented in Table 6.1. 

In this article, I shall begin by showing the tremendous scope of Lewis acid-base considerations. Although it is not fully reeilized, it is very difficult to find chemical reactions in which these effects are not operative. This will be followed by a discussion of the kind of data that should be obtained and analyzed in order to learn about the strength of bonding. Since data selection is important, a good deal of space is devoted to complications that can arise from improper design of experiments and improper analysis of experimental results. 

Acyl cations are now well-established chemical species. They can be prepared in quantity in solution, and several have been isolated as their crystalline salts. Recent papers have described their formation from carboxylic acids and esters under strongly acidic conditions81214, but they are most conveniently available from the reactions of acyl halides with L.ewis acids21 well-defined Lewis acid-base complexes are formed, which decompose in a second stage to the acyl cations21-23, viz. 

This and the next two sections introduce three of the main types of chemical reactions precipitation reactions, acid-base reactions, and redox reactions, all of which are discussed in more depth in later chapters. The fourth type of reactions discussed in this text, Lewis acid-base reactions, are introduced in Chapter 2. 

Boron and aluminum atoms need five electrons to complete their octets. However, they may be unable to acquire that number of electrons from the atoms to which they are bonded. As a result, the compounds these elements form have special chemical characteristics. Moreover, they introduce an important class of reactions that are called Lewis acid-base reactions. ... 

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. 

Sections 2.2.6 and 2.6 dealt with Lewis acid/base equilibria in which principally the solvent itself is involved in the chemical reaction, either as a Lewis acid (EPA solvents) or as a Lewis base (EPD solvents). This Section includes some examples of solvent-dependent Lewis acid/base equilibria in which the solvent is not directly involved as the reaction partner, but as the surrounding and interacting medium. 

The Usanovich definition of acids and bases has not been widely used, probably because of (I) the relative inaccessibility of the original to non-Russian-reading chemists and (2) the awkwardness and circularity of Usanovich s original definition. The Usanovich definition includes all reactions of Lewis acids and bases and extends the latter concept by removing the restriction that the donation or acceptance of electrons be as shared pairs. The complete definition is as follows An add is any chemical species which reacts with bases, gives up cations, or accepts anions or electrons, and, conversely, a base is any chemical species which reacts with acids, gives up anions or electrons, or combines with cations. Although perhaps unnecessarily complicated, this definition simply includes all Lewis acid-base reactions plus redox reactions, which may consist of complete transfer of one or more electrons. Usanovich also stressed unsaturation involved in certain acid-base reactions ... 

Many organic and biological reactions are acid-base reactions that do not lit within the Arrhenius or Bronsted-Lowry theories. Experienced chemists find the Lewis theory to be very usefol because so many other chemical reactions are covered by it. The less experienced sometimes find the theory less useful, but as their knowledge expands so does its utility. 

Lewis acid-base reactions in which a metal cation combines with a Lewis base result in the formation of complex ions. Thus, we can define a complex ion as an ion containing a central metal cation bonded to one or more molecules or ions. Complex ions are crucial to many chemical and biological processes. Here we will consider the effect of complex ion formation on solubihty. In Chapter 22 we will discuss the chemistry of complex ions in more detail. 

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