Donor-acceptor ability, classification

Figure 4.9 Classification of traus-4,4 -disubstituted stilbenes according to the donor-acceptor abilities of their substituents to stabilize the excited t state [12], (Reproduced with permission from [31].)...

Donors coordinating by F, O or N possess a general ability to coordinate both hard and soft acceptors, showing no special preference for soft ones. In the Pearson (2) classification such donors are termed hard. If their coordination are governed only, or at least mainly, by the electro-... 

Solvents can be classified as EPD or EPA according to their chemical constitution and reaction partners [65]. However, not all solvents come under this classification since e.g. aliphatic hydrocarbons possess neither EPD nor EPA properties. An EPD solvent preferably solvates electron-pair acceptor molecules or ions. The reverse is true for EPA solvents. In this respect, most solute/solvent interactions can be classified as generalized Lewis acid/base reactions. A dipolar solvent molecule will always have an electron-rich or basic site, and an electron-poor or acidic site. Gutmann introduced so-called donor numbers, DN, and acceptor numbers, AN, as quantitative measures of the donor and acceptor strengths [65] cf. Section 2.2.6 and Tables 2-3 and 2-4. Due to their coordinating ability, electron-pair donor and acceptor solvents are, in general, good ionizers cf. Section 2.6. 

The classic enzyme commission (EC) classification for GTs is on the basis of their donor and acceptor specihcity as well as the product formed. Currently, 295 entries are in this database (http //www.chem.qmul.ac.uk/iubmb/). The distinction between these enzymes is noted by their ability to catalyze the transfer of hexoses (EC 2.4.l.y, hexosyltransferases), pentoses (EC 2.4.2.y, pentosyltransferases), or other glycosyl groups (2.4.99.y, sialyltransferases). This classification is restricted to enzymes that are fully characterized, and it can be problematic for enzymes that act on several distinct acceptors but at different rates. It also does not take into account the origin of the enzyme or its three-dimensional stmcture. 

The ability of Lewis acids to bind to transition metals as o-acceptor ligands was recognized early on and the common classification of ligands as L (two-electron donor) and X (one-electron donor) was extended to Z (two-electron acceptor). In the latter case, the metal behaves as a Lewis base and engages in dative M Z interactions. Figure 7 provides a simplified orbital diagram for the corresponding two-center, two-electron interaction. ... 

HCl + H2O HsO + Clin view of the disadvantages, it can be questioned whether it is worth tr dng to extend the ideas of acid and base in the way that Lowry and Bronsted do. Chemistry might be better served if the classification of molecules and ions according to their ability to accept or donate protons - which is undoubtedly a very useful classification - were to be divorced Ifom the classification of substances into acids and bases, and be given a terminology of its own. In fact, such terminology lies readily to hand, the terms proton donor and proton acceptor being entirely suitable for this purpose. 

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