Acid-base concepts summary

In Section IV-VI we systematically discuss the acidity, basicity and, where appropriate, the hydrogen-bonding of nitrones, nitriles and thiocarbonyls. Since this review very much relies on physical measurements and the discussion of acid-base reactions, Section HI provides a brief introduction to proton affinity and a short summary of the acid-base concept and the quantitative measure of basicity. 

The amphoteric nature of the nitrosyl ligand has been extensively discussed by several authors in terms of molecular orbital models and in particular the utility of the extended Hiickel approach for understanding the bonding has been demonstrated by Hoffmann, et al. This latter work has been extended to sulfur dioxide complexes by Ryan and Eller The interested reader is referred to the recent review by Mingos for a summary Although detailed molecular orbital treatment will no doubt be necessary for a complete understanding of SO2 bonding, we find that the simple acid/base concept serves to correlate the presently known structural information. This point of view will be emphasized in the present article and extended to include >7 -802 as a r acid 1... 

In summary, the Brpnsted theory of acids and bases has a much broader scope than previous acid-base concepts. According to the Bronsted theory, acids and bases can be ions as well as molecular substrates acid-base reactions are not restricted to aqueous solutions various chemical entities can act either as acids... 

The concept of hard and soft acids and bases can be used to interpret many trends in chemical reactivity. These trends are summarized in the hard-soft acid-base principle (HSAB principle), an empirical summary of results collected from many chemical reactions studied through decades of research. 

In summary, acid-base chemistry is conceptually rather simple, but the multiplicity of factors involved makes its treatment somewhat involved. Until more unifying concepts are developed, as they undoubtedly will be. it will be necessary to apply to each problem that is encountered the ideas, rules, and (when available) the parameters applicable to it. 

Summary of the Relationship between Diastereoselectivity and the Transition Structure. In this section we considered simple diastereoselection in aldol reactions of ketone enolates. Numerous observations on the reactions of enolates of ketones and related compounds are consistent with the general concept of a chairlike TS.35 These reactions show a consistent E - anti Z - syn relationship. Noncyclic TSs have more variable diastereoselectivity. The prediction or interpretation of the specific ratio of syn and anti product from any given reaction requires assessment of several variables (1) What is the stereochemical composition of the enolate (2) Does the Lewis acid promote tight coordination with both the carbonyl and enolate oxygen atoms and thereby favor a cyclic TS (3) Does the TS have a chairlike conformation (4) Are there additional Lewis base coordination sites in either reactant that can lead to reaction through a chelated TS Another factor comes into play if either the aldehyde or the enolate, or both, are chiral. In that case, facial selectivity becomes an issue and this is considered in Section 2.1.5. 

In summary, the concepts of electrophilies and nucleophiles are very similar to those of Lewis acids and bases. A more thorough discussion of what makes good electrophiles and nucleophiles is left to Chapter 8. Until then, it is instructive to simply realize that trends of preferential reactivity fall into classes defined as hard and soft species, where nucleophiles and electrophiles within these individual classes prefer to react. The reactivity of the soft species is primarily due to better overlap of the orbitals, while for the hard species the electrostatic attraction dominates. 

Earlier definitions of acids and bases as species producing respectively hydrogen ions and hydroxyl ions are only valid in aqueous solution. The concept of Br nsted and Lowry, while extending the classical definition, does not exclude the treatment of Arrhenius for aqueous media. The extended view regards acids as proton donors and bases as proton acceptors regardless of whether substances concerned are ionic or neutral. A terse summary of this definition may be given as follows... 

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