Acid-base rationalization

2 Compound energetics rationalization schemes Acid-base rationalization 

Although periodic trends in enthalpies of formation are often striking, these trends can in general not be used to estimate accurate data for compounds where experimental data are not available. Other schemes are frequently used and these estimates are often based on atomic size and electronegativity-related arguments. As an example, the enthalpy of formation of a ternary oxide from the binary constituent oxides, i.e. the enthalpy of a reaction like 

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To maintain a stable chemical and physical environment for cells (this entails regulation of water, electrolytes, and acid/base rations)... 

In this initial section the reactivities of the major types of azole aromatic rings are briefly considered in comparison with those which would be expected on the basis of electronic theory, and the reactions of these heteroaromatic systems are compared among themselves and with similar reactions of aliphatic and benzenoid compounds. Later in this chapter all the reactions are reconsidered in more detail. It is postulated that the reactions of azoles can only be rationalized and understood with reference to the complex tautomeric and acid-base equilibria shown by these systems. Tautomeric equilibria are discussed in Chapter 4.01. Acid-base equilibria are considered in Section 4.02.1.3 of the present chapter. 

The base may control some of this chemistry by selectively converting 2-mercaptoethanol into a stronger nucleophile. Display an electrostatic potential map for 2-mercaptoethanol. Which proton, that attached to oxygen or sulfur, is more acidic, that is, more likely to be removed by strong base Rationalize your result. 

Isoperibol titration calorimetry was also extensively used by Drago s group [215] to determine enthalpies and equilibrium constants of a variety of reactions where acid-base adducts are formed. These results are the source of Drago s ECW model, which has been widely used to rationalize chemical reactivity [216-218]. 

As illustrated by the examples, the eatalytie properties of POMs may benefit from the assoeiation with the matrix, and in some eases both enhanced activity and selectivity have resulted from immobilization. That is often the ease of POMs embedding in silica matrix and may be attributed to a eonstrained environment effect . However, it is still very difficult to predict the catalytic performance after immobilization, and only experiment ean clarify this matter. Meanwhile, a rational modification of acid-base and/or hydrophobic-hydrophilic properties of a support is possible in some cases and may tune the catalytic properties. 

The alkane propane has pATa 50, yet the presence of the double bond in propene means the methyl protons in this alkene have pATa 43 this value is similar to that of ethylene (pATa 44), where increased acidity was rationalized through sp hybridization effects. 1,3-Pentadiene is yet more acidic, having pATa 33 for the methyl protons. In each case, increased acidity in the unsaturated compounds may be ascribed to delocalization of charge in the conjugate base. Note that we use the term allyl for the propenyl group. 

Early Qualitative Rationalizations of the Acid-Base Enthalpies. 89... 

Early Qualitative Rationalizations o Acid- Base Enthalpies... 

The extent to which an atom or molecule s charge distribution is affected by an external electric field E (which could be due to an approaching reactant) is governed, to first order, by its polarizability a. It was really a to which Pearson was referring in his hard and soft acid-base theory, which rationalizes a large number of chemical reactions. The terms hard and soft refer, respectively, to low and high polarizability. 

The effect of temperature on the acid base chemistry of the stationary phase can also play a role in separation. Free silanol groups on the stationary phase may exhibit changes in acid base chemistry with temperature [28]. Also, reverse phase columns with amine, amide, or acidic functional groups will be affected by the interaction of the temperature, the ionization state of the stationary phase, the mobile phase acidity, and the ionization state of the solute. Most non-linear van t Hoff plots can be rationalized in these terms, but it is difficult to predict a priori what the effects will be on a given system. Thus, it is important to characterize the system under study if a simple change in temperature produces unexpected effects. 

The rational design of chelating agents as antidotes requires a careful consideration of acid-base chemistry. Metal ions are Lewis acids, while the chelating agents or ligands are Lewis bases. The concepts of hardness and softness may be used to describe systematically the interaction between them. A hard metal cation is one that retains its... 

This latter reaction is very slow as written and is of more importance in the reverse, dehydration reaction.) The characterization of these metal and nonmetal oxides as acids and bases is of help in rationalizing the workings, for example, of a basic Bessemer converter in steetmatcing. The identification of these acidic and basic species will also prove useful in develop r a general definition of acid-base behavior. 

The 15N chemical shift tensors in purine are similar to those in other nucleic acid bases for similar types of nitrogens. Comparison of the calculated values in purine for the isolated molecule and for the molecule with its nearest neighbors shows well defined trends, which could be rationalized using the results in the model pyridine-methanol complex. 

Hardness and softness as chemical concepts were presaged in the literature as early as 1952, in a paper by Mulliken [138], but did not become widely used till they were popularized by Pearson in 1963 [139]. In the simplest terms, the hardness of a species, atom, ion or molecule, is a qualitative indication of how polarizable it is, i.e. how much its electron cloud is distorted in an electric field. The adjectives hard and soft were said to have been suggested by D.H. Busch [140], but they appear in Mulliken s paper [138], p. 819, where they characterize the response to spatial separation of the energy of acid-base complexes. The analogy with the conventional use of these words to denote resistance to deformation by mechanical force is clear, and independent extension, by more than one chemist, to the concept of electronic resistance, is no surprise. The hard/soft concept proved useful, particularly in rationalizing acid-base chemistry [141]. Thus a proton, which cannot be distorted in an electric field since it has no electron cloud (we ignore the possibility of nuclear distortion) is a very hard acid, and tends to react with hard bases. Examples of soft bases are those in which sulfur electron pairs provide the basicity, since sulfur is a big fluffy atom, and such bases tend to react with soft acids. Perhaps because it was originally qualitative, the hard-soft acid-base (HSAB) idea met with skepticism from at least one quarter Dewar (of semiempirical fame) dismissed it as a mystical distinction between different kinds of acids and bases [142]. For a brief review of Pearson s contributions to the concept, which has been extended beyond strict conventional acid-base reactions, see [143],... 

Woerpel and Nevarez demonstrated the synthetic potential of silaaziridines by selective insertion reactions (Scheme 7.52).123 Silver-catalyzed aldehyde insertion into the Si-N bond of 169b produced the N,0-cyclic acetal 180 as the cis isomer. In contrast to this process, insertion of tert-butyl isocyanide occurred into the weaker C-Si bond to afford imine 181. The authors rationalized the chemoselectivity for these two processes on the basis of Pearson s hard-soft acid-base theory 124-126 the more ionic Si-N bond reacted with harder benzaldehyde electrophile, whereas the more covalent Si-C bond reacted with the softer isocyanide. 

The appearance of SnF+ ions is also observed in the mass spectra of (p- or m-CF3C6FLt)4Sn and it becomes the major ion in the spectra of the perfluorinated (CgFs Sn species. This fluorine migration has been rationalized in terms of hard/soft acid base theory (HSAB) by considering Sn2+ as a hard acid that favors fluoride transfer55. 

Until recently, when Peri 155) reported on a model of the silica-alumina surface, there were no detailed models for the surfaces of mixed oxides available. Beside the presence of Br nsted and Lewis acid sites, Peri 156) had proposed the existence of a sites on the Si02—A1203 surface, which he described as acid-base pair sites rather than simple Lewis acid sites. Various molecules, such as acetylene, butene, and HC1, are adsorbed very selectively on these a sites, whereas NH3 and H20 are also held by many other sites 157). To rationalize the formation of these sites, Peri 155) developed a semiquantitative surface model for certain silica-aluminas, which were prepared by reaction of A1C13 with the surface silanol groups of silica and subsequent hydrolysis and dehydration. The model is entirely based on a surface model of silica, which suggests an external surface resembling a (100) face of the cristobalite structure 158). It should be mentioned in this connection that Peri s surface model of silica may... 

In our rationalization, we have relied heavily on the classical concept of aromaticity with particular emphasis on bond order and bond fixation. These concepts, together with acid-base and tautomeric equilibria and hydrogen bonding, are capable of explaining nearly all of the quantitative... 

Metal-inhibitor interactions can be viewed in terms of acid-base reactions. This type of rationalization has been done by Sastri in terms of the hard and soft acid-base principle (HSAB) at great length.18 The characteristics of hard acids, soft acids, hard bases and soft bases along with borderline acids are given below. 

Metal ions can be considered as Lewis acids, with the formation of complexes rationalized in terms of Lewis acid-base interactions. This interaction can be visualized in terms of the ability of a metal atom or ion M to accept a pair of electrons (and thus act as a Lewis acid) from a ligand X, which is an electron donating base with an accessible lone pair of electrons, i.e.,... 

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