Acid-base classification

In the context of AB cements, Al +, Mg , Ca and Zn are in class (a) while Cu is in the border region. Zn contains a completed 3d shell and forms stronger complexes with O than with S ligands, as do other class (a) cations. 

This concept of Chatt and his coworkers was developed further by Pearson (1963, 1966, 1968a,b) in his theory of hard and soft acids and bases. Hard acids correspond with class (a) acceptors and soft acids with class (b) acceptors. 

Hard acids prefer to react with hard bases and soft acids prefer to react 

Pearson (1966) defines a soft base as one in which the donor atom is of high polarizability and low electronegativity and is easily oxidized or associated with empty, low-lying orbitals . A hard base has opposite properties. The donor atom is of low polarizability and high electronegativity, is hard to reduce, and is associated with empty orbitals of high energy.  

The classification of Lewis acids and bases relevant to AB cements is shown below. 

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Table 1.7 Hard-Soft Acid-Base Classification of Metal Ions and Ligands...

The overall behavior pattern outlined can be further examined using the hard-soft acid-base classification approach, by further consideration of ligand-field effects, and by considering relativistic effects that influence the behavior of the heavier cations. 

The acid-base classification relies on the thermodynamic properties of the hydroxides in solution [2], and the periodic chemical properties of oxides have been extensively reviewed in dedicated textbooks [5]. Oxides are known for all the elements of the periodic table, except the lighter noble gases. As far as metallic oxides are concerned, they can be precisely stoichiometric (e g. CaO) or rather show stoichiomety variability over a narrow or wide range of composition, depending on the availability of different stable oxidation states for the metal (e.g. MnxOy). 

An even more general theory of acids and bases was given by the American chemist G. N. Lewis in 1923. In this theory, an acid is an electron acceptor and a base is an electron donor. This is a more general theory than the Br0nsted-Lowry theory, because it allows the acid-base classification to be applied to reactions in which neither H (aq) nor OH (aq) play a role, or even to reactions in which there is no solvent. For example, the following are acid-base reactions in the Lewis theory... 

A Figure 7.2 Basic drugs and classes, the largest subgroup in the acid-base classification scheme. The alkaloids are (or at least once were) derived from plant matter, while nonalkaloids are generally synthetic or semisynthetic. Tropane alkaloids Include cocaine, while tryptamines include mescaline and psilocyn. Caffeine and theophylline are xanthine alkaloids. The selective serotonin reuptake inhibitors (SSRIs) include the second-generation SSRI antidepressants. Prozac (fluoxetine, shown) and Paxil belong to this class. 

Cadmium salts are the best catalysts for the polymerization of thiiranes giving polymers of the highest stereoregularity, while they are unable to polymerize oxiranes. This behaviour can be explained by the hard-soft acid-base classification in which sulfur and cadmium are closer in their character than oxygen which belongs to "hard elements". 

Ambident reactivity of A-4-thiazoline-2-thione has been discussed (101) in terms of the Hard and Soft Acids and Bases classification (199) and the Klopman-Hudson approach (200). 

Classification according to Br nsted acid-base properties is useful. 

The Lux-Flood theory relates to oxide melts. Geologists have often used acid-base concepts for the empirical classification of igneous silicate rocks (Read, 1948). Silica is implicitly assumed to be responsible for acidity, and the silica content of a rock is used as a measure of its acid-base balance ... 

Casanola-Martin GM et al. (2006) New tyrosinase inhibitors selected by atomic linear indices-based classification models. Bioorg Med Chem Lett 16(2) 324-330 Tan CY, Wainman D, Weaver DF (2003) N-, alpha-, and beta-substituted 3-amino-propionic acids design, syntheses and antiseizure activities. Bioorg Med Chem 11(1) 113-121... 

From our previous treatment of the Arrhenius, Bransted and Lewis acid-base theories, the importance of the choice between the divergent solvent types clearly appeared if we now confine ourselves to solvents to which the proton theory in general is applicable, this leads to a classification of eight classes as already proposed by Bronsted35,36 (Table 4.3). 

Classification of biologically important metal ions and ligands according to the hard-soft acid-base concept and their general characteristics... 

Classification of reactions 190 Nucleophilic vs. general acid-base catalysis 191... 

As we have seen (Section 4, p. 191) the range of effective molarities associated with ring-closure reactions is very much greater than that characteristic of intramolecular general acid-base catalysis the main classification is therefore in terms of mechanism. By far the largest section (I, Tables A-D) gives EM s for intramolecular nucleophilic reactions. These can be concerted displacements (mostly at tetrahedral carbon), stepwise displacements (mostly addition-elimination reactions at trigonal carbon), or additions, and they have been classified in terms of the nucleophilic and electrophilic centres. 

The discussion and classification of reagents is masterful in identifying Ingold s new nomenclature and principles with more widely known oxidation-reduction and acid-base theory. The 1953 lectures at Cornell University, published as Structure and Mechanism in Organic Chemistry, follow this same strategy, showing how old classification schemes overlap with each other and how apparent inconsistencies disappear as old schemes are incorporated into the new one. Nineteenth-century Berzelian electrochemical dualism, revived by Lapworth and Robinson in the cationic/anionic schema, disappears into the electrophilic/nucleophilic language. 

Table 10.2 Classification of amino acids based on chemical structure...

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. 

In this section, enzymes in the EC 2.4. class are presented that catalyze valuable and interesting reactions in the field of polymer chemistry. The Enzyme Commission (EC) classification scheme organizes enzymes according to their biochemical function in living systems. Enzymes can, however, also catalyze the reverse reaction, which is very often used in biocatalytic synthesis. Therefore, newer classification systems were developed based on the three-dimensional structure and function of the enzyme, the property of the enzyme, the biotransformation the enzyme catalyzes etc. [88-93]. The Carbohydrate-Active enZYmes Database (CAZy), which is currently the best database/classification system for carbohydrate-active enzymes uses an amino-acid-sequence-based classification and would classify some of the enzymes presented in the following as hydrolases rather than transferases (e.g. branching enzyme, sucrases, and amylomaltase) [91]. Nevertheless, we present these enzymes here because they are transferases according to the EC classification. 

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