Acid/base character

Dehydrogenation processes for acetone, methyl isobutyl ketone [108-10-1], and higher ketones (qv) utilizing, in one process, a copper-based catalyst have been disclosed (18,19). Dehydrogenation reaction is used to study the acid—base character of catalytic sites on a series of oxides (20,21). 

The amino acids, basic building blocks of proteins, all share this dual acid-base character. See Chapter 13 for a description of the amino acids and their biological chemistry. Organic bases also have a long and varied history as painkillers and narcotics, as our Chemishy and Life Box on the next page describes. 

In this book we will focus on physicochemical profiling in support of improved prediction methods for absorption, the A in ADME. Metabolism and other components of ADME will be beyond the scope of this book. Furthermore, we will focus on properties related to passive absorption, and not directly consider active transport mechanisms. The most important physicochemical parameters associated with passive absorption are acid-base character (which determines the charge state of a molecule in a solution of a particular pH), lipophilicity (which determines distribution of a molecule between the aqueous and the lipid environments), solubility (which limits the concentration that a dosage form of a molecule can present to the solution and the rate at which the molecule dissolves from... 

The oxyanion containing Zn2+, Zn022-, is known as a zincate, and it equivalent to Zn(OII)42, which is named as the tetrahydroxozincate(II) anion (see Chapter 16). From these reactions, it is clear that ZnO can react as either an acidic or basic oxide and it is therefore known as an amphoteric oxide. In essence, there are some oxides that are clearly acidic, some that are dearly basic, and some that are in between. There is, in fact, a continuum of acid-base character for the oxides of elements that is shown in Figure 9.2. 

Acid-base reactions are called neutralization reactions because the reaction of an acid with a base generally produces a salt with little or no acid-base character and, in many cases, water. 

Two important zeolite properties are (1) the intra-pore electrostatic field, and (2) its acid-base character. As discussed below post-synthetic modifications of many zeolites to fine-tune these properties are possible and provide a unique opportunity to influence reaction outcome. 

There is a fourth category, consisting of salts that contain the cation of a weak base with the anion of a weak acid. Prediction of the acid—base character of these salts is less obvious, because both ions undergo hydrolysis. The two equilibria not only alter the pH of the solution, but also interfere with each other. Predictions require a comparison of the K values for... 

Amino acids and proteins have groups that can release and bind protons (H ) in other words, they have acid-base character. The groups with acid-base character can be generally represented in one of the two ways shown below. 

Amino acids are classified as acidic or basic according to their R groups because in proteins, these are the only groups that can dissociate. The a-amino and a-carboxyl groups are in peptide bonds and lose their acid-base character. This system of classification can be confusing since the words add and base are used in a way slightly different than discussed in the section above. 

Proton acid-base reactions are not particularly sensitive to stoic compression, and hence provide a good measure of inductive effects. For acid-base character, three sets of reference reactions can be used. The easiest of these to perform experimentally requires an analogy be drawn between the relative pK values of a series of protonated annelated pyridines and the pK values of the analogous isoelectronic benzene. The second is a direct measure of the kinetic acidity of the a- and P-sites on a soies of annelated benzenes. The third is a related direct assessment of kinetic acidity by protodetritiation. 

This chapter begins with a general discussion about acid-base character of solid surfaces. It continues with the information gained from microcalorimetry... 

Why do changes in pH interfere with the function of proteins For your answer, consider the acid-base character of many amino acid side chains. 

The acid-base properties, and hence ionic character, of peptides and proteins also can be used to achieve separations. Ion-exchange chromatography, similar to that described for amino acids (Section 25-4C), is an important separation method. Another method based on acid-base character and molecular size depends on differential rates of migration of the ionized forms of a protein in an electric field (electrophoresis). Proteins, like amino acids, have isoelectric points, which are the pH values at which the molecules have no net charge. At all other pH values there will be some degree of net ionic charge. Because different proteins have different ionic properties, they frequently can be separated by electrophoresis in buffered solutions. Another method, which is used for the separation and purification of enzymes, is affinity chromatography, which was described briefly in Section 9-2B. 

Wooding, N. S., and W. C. E. Higginson Anionic polymerisation. Part II. The acid-base character of anionic polymerisation. J. Chem. Soc. (London), p. 774. 

As the two binary compounds approach each other in acid/base character, the enthalpy of formation gets less negative. For ternary compounds formed from very similar binary oxides (for which the data are not shown), such as (AI2O3 + SiC>2), (CuO + Fe2C>3), and (Fe2C>3 + TiC>2), the AtH values are, in fact, endothermic. 

The acid-base character of solids was studied very early by Tanabe s group31,32 and was first described in a landmark volume.33... 

This discussion should not be seen as explaining the acid/base character of oxides, i.e. their solubilities in water at various pH values. We are emphasising the close relationship between the acid/base behaviour of oxides and the nature of aqueous species. The dissolution of an oxide (other than a neutral oxide) in water, or in acids/alkalies, is an acid-base process, a chemical reaction rather than a mere separation of ions. The relative acid/base strengths of oxides are further discussed in Section 9.2. 

The acid-base character of a chemical and the pH of the aqueous phase determine the distribution of ionized-nonionized species in solution. Starting from the equilibrium dissociation of a weak acid, HA,... 

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