Acids and Bases General Principles

FIGURE 4.5 Hydrogen bonding between molecules of ethanol and water. 

Because alkyl halides are insoluble in water, a mixture of an alkyl halide and water separates into two layers. When the alkyl halide is a fluoride or chloride, it is the upper layer and water is the lower. The situation is reversed when the alkyl halide is a bromide or an iodide. In these cases the alkyl halide is the lower layer. Polyhalogenation increases the density. The compounds CH2CI2, CHCI3, and CCLr, for example, are all more dense than water. 

All liquid alcohols have densities of approximately 0.8 g/mL and are, therefore, less dense than water. 

A soUd understanding of acid-base chemistry is a big help in understanding chemical reactivity. This and the next section review some principles and properties of acids and bases and examine how these principles apply to alcohols. 

According to the theory proposed by Svante Arrhenius, a Swedish chemist and winner of the 1903 Nobel Prize in chemistry, an acid ionizes in aqueous solution to liberate protons (H, hydrogen ions), whereas bases ionize to liberate hydroxide ions (HO ). A more general theory of acids and bases was devised independently by Johannes Brpnsted (Denmark) and Thomas M. Lowry (England) in 1923. In the Brpnsted-Lowry approach, an acid is a proton donor, and a base is a proton acceptor. 

Curved arrow notation is used to show the electron pair of the base abstracting a proton from the acid. The pair of electrons in the H—A bond becomes an unshared pair in the anion A. Curved arrows track electron movement, not atomic movement. 

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Accordingly, acids and bases were divided into three classes, labelled hard, soft and borderline. This classification is shown in Table 1.16. Its usefulness arises from the generalization that hard acids prefer to coordinate to hard bases and soft acids to soft bases, aU things being otherwise equal. This is the so-called Hard and Soft Acid and Base (HSAB) principle. A possible quantitative statement of the HSAB principle would be the four-parameter Equation 1.110 ... 

Another feature of the metal ions that are typically involved in cementitious bonding in AB cements is that most of them fall into the category of hard in Pearson s Hard and Soft Acids and Bases scheme (Pearson, 1963). The underlying principle of this classification is that bases may be divided into two categories, namely those that are polarizable or soft, and those that are non-polarizable or hard. Lewis acids too may be essentially divided into hard and soft, depending on polarizability. From these classifications emerges the useful generalization that hard acids prefer to associate with hayd bases and soft acids prefer to associate with soft bases (see Section 2.3.7). 

Many different types of reversible reactions exist in chemistry, and for each of these an equilibrium constant can be defined. The basic principles of this chapter apply to all equilibrium constants. The different types of equilibrium are generally denoted using an appropriate subscript. The equilibrium constant for general solution reactions is signified as or K, where the c indicates equilibrium concentrations are used in the law of mass action. When reactions involve gases, partial pressures are often used instead of concentrations, and the equilibrium constant is reported as (p indicates that the constant is based on partial pressures). and are used for equilibria associated with acids and bases, respectively. The equilibrium of water with the hydrogen and hydroxide ions is expressed as K. The equilibrium constant used with the solubility of ionic compounds is K p. Several of these different K expres-... 

Pyrones and thiopyrones react very readily with a variety of nucleophiles, but here the situation is complicated not only by the carbonyl group, but by the fact that these compounds show elements of both aromatic and aliphatic character. Pyran-2-one (9) can in principle react with nucleophiles at the 2-, 4- or 6-position and examples of each type of reaction are known. There is an approximate correlation with the hard and soft acid and base hypothesis, in that hard nucleophiles such as HO generally react at the 2-position while soft nucleophiles such as hydride ion generally react at the 6-position. Reactions at the 4-and 6-positions can, of course, be rationalized mechanistically simply as examples of conjugate addition. A similar situation obtains with pyran-4-ones (10), where reaction can in principle occur at the 2- or 4-position. Hard nucleophiles generally react at the 2-position,... 

In Chapter 16, we apply the fundamental general equilibrium expression to gaseous equilibrium reactions. In this chapter, we apply the same expression to the equilibria that involve weak acids and bases in aqueous solution, the principal difference being that all concentrations are expressed in moles/liter (rather than in atmospheres as for gases). All the general conclusions given in Chapter 16, and summarized in the principle of Le Chatelier, apply to equilibria in solutions as well as to those in gases. 

Hammett s success in treating the electronic effect of substituents on the equilibria rates of organic reactions led Taft to apply the same principles to steric, inductive, and resonance effects. The Hammett o constants appear to be made up primarily of two electronic vectors field-inductive effect and resonance effect. For substituents on saturated systems, such as aliphatic compounds, the resonance effect is rarely a factor, so the o form the benzoic acid systems is not applicable. Taft extended Hammett s idea to aliphatics by introducing a steric parameter ( .). He assumed that for the hydrolysis of esters, steric and resonance effects will be the same whether the hydrolysis is catalyzed by acid or base. Rate differences would be caused only by the field-inductive effects of R and R in esters of the general formula (XCOOR), where X is the substituent being evaluated and R is held constant. Field effects of substituents X could be determined by measuring the rates of acid and base catalysis of a series XCOOR. From these rate constants, a value a could be determined by Equation (5.9) ... 

Both metals and non-metals can be either (a) or (b) type of acids depending on their charge and size. Since the features which promote class (a) behavior are those which lead to low polarizability, and those which create type (b) behavior lead to high polarizability, it is convenient to call class (a) acids "hard" acids and class (b) acids "soft" acids. We then have the useful generalization that "hard acids prefer to associate with hard bases, and soft acids prefer soft bases" (Pearson, 1997). This is the Principle of Hard and Soft Acids and Bases, or the... 

The solubilities of salts in water (clearly of importance in aquatic, analytical, and geochemistry) can be fairly well predicted and explained using two principles later we shall return to the more familiar of these, the hard soft acid base (HSAB) principle (see Hard Soft Acids and Bases). To control this principle, we must first consider oifly salts of anions that are hard bases, that is, salts of 0x0 and fluoro anions, and oxides, hydroxides, and fluorides. The solubilities of salts of these anions can be fairly well predicted and explained on the basis of the acidity classification of the cation and the basicity classification of the anion, that is, on a principle of acid base strength. The numerous solubility rules taught in General Chemistry could be replaced with four solubility principles (Table 4), two of which are quite reliable and two of which are less reliable, for known reasons. 

Carbonyl functions often are protected as thioacetals, because these derivatives are stable to acids and bases. Dithio-acetals and -ketals generally are prepared by reaction of the carbonyl compound with a thiol in the presence of an acid catalyst. This transformation has been used in carbohydrate chemistry for a long time to lock aldoses in their acyclic forms (see equation 26). In recent years, this blocking principle for carbohydrates has been exploited in several chiral pool syntheses. Several other methods for... 

The principles of hard and soft acids and bases has also been applied to kinetic phenomena.20,21 In this connection, organic chemistry has provided most of the examples, because the reactions of organic chemistry are often slow enough for their rates to be easily measured. In organic chemistry, and in ionic organic chemistry in particular, we are generally interested in the reactions of electrophiles with nucleophiles. These reactions are a particular kind of the general acid-with-base type of reaction, and so the principle of hard and soft acids and bases applies equally to the reactions of electrophiles with nucleophiles. 

The approaches we have discussed so far to determine the precise location of the equivalence point use more than just one point, and are therefore in principle less prone to experimental error. The Schwartz and Gran plots rely on a linearization of the titration curve unfortunately, for samples that contain more than one monoprotic acid or base, linearization is no longer possible, nor is it (in general) for polyprotic acids and bases. And as for the alternative, we have seen that taking the derivative is easily overwhelmed by experimental noise. Is there no more robust yet general way to determine the equivalence volume with better precision ... 

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