Fluoride donors and acceptors

A major part of the chemistry of xenon fluorides and oxide fluorides is associated with their reactions with fluoride donors and acceptors to produce a variety of cations and anions. The known species are summarized in Scheme 1. In addition, there are a number of more unusual entities, notably the dixenon cation [Xe2]+ and [Xe03X] (X = F, Cl, or Br). The preparations, structures, and chemistry of these species was comprehensively reviewed in 1984 (12), so only developments since 1979 are covered in this present account. 

The compounds act as both fluoride donors and acceptors. Thus, with tetramethylammo-nium fluoride in acetonitrile under rigorously anhydrous conditions, XeF4 forms the unique pentagonal-planar [XeFsJ" anion ... 

Reactions 17.23 and 17.24 showed the fluoride donor and acceptor abilities of CIF. All the higher interhalogens undergo similar reactions, although CIF5 does not form stable complexes at 298 K with alkali metal fluorides. However, it does react with CsF or [Me4N]F at low... 

The xenon fluorides were synthesized by the direct reaction of the element shortly after Bartlett s announcement. The tetrafluoride is the stablest and best characterized, although its hydrolysis yields the dangerously explosive xenon trioxide. Xenon difluoride is an excellent oxidizing agent and fluoride ion donor. Xenon hexafluoride is both a fluoride donor and acceptor. 

As in the case of C1F, the trifluoride can behave as both a fluoride ion donor and acceptor, as illustrated by the reactions carried out in liquid C1F3 ... 

We can also examine isomeric pairs of H-bonded complexes in which the roles of electron-pair donor and acceptor are reversed, and the mutual dipole orientations are therefore fundamentally altered. As an example, Fig. 5.4 shows the isomeric pair of complexes between ammonia and hydrogen fluoride,... 

BF3-OEt2 Unnatural P-sialoside derivatives are produced preferentially by using O-acetylated glycosyl fluoride donor and BF3-OEt2 as the promoter. When P-sialyl fluoride derivative 28 was reacted with acceptor 22 in the presence of BF3-OEt2 in dichloromethane, the P-sialoside 28 was obtained in a mixture of anomers (a/p 17/83) (Scheme 2.11) [44],... 

In their review of the classification of donors and acceptors in inorganic reactions, Williams and Hale (7) pointed out that for reactions in water, class (a) character was exhibited most strongly by lithium and least by caesium, which was indeterminate between classes (a) and (b). Here class (a) character means that the fluoride is more stable in water than the iodide. In general Group IA metals prefer hard ligands, F, O, N their interaction with sulphur and carbon is considered in para. IV. 

Types of Solvent.—In order that a particular solvent may permit a substance dissolved in it to behave as an acid, the solvent itself ifiust be a base, or proton acceptor. A solvent of this kind is said to be proto-philic in character instances of protophilic solvents are water and alcohols, acetone, ether, liquid ammonia, amines and, to some extent, formic and acetic acids. On the other hand, solvents which permit the manifestation of basic properties by a dissolved substance must be proton donors, or acidic such solvents arc protogenic in nature. Water and alcohols arc examples of such solvents, but the most marked protogenic solvents are those of a strongly acidic character, e.g., pure acetic, formic and sulfuric acids, and liquid hydrogen chloride and fluoride. Certain solvents, water and alcohols, in particular, are amphiprotic, for they can act both as proton donors and acceptors these solvents permit substances to show both acidic and basic properties, whereas a purely protophilic solvent, e.g., ether, or a completely protogenic one, e.g., hydrogen fluoride, would permit the manifestation of either acidic or basic functions only. In addition to the types of solvent already considered, there is another class which can neither supply nor take up protons these are called aprotic solvents, and their neutral character makes them especially useful when it is desired to study the interaction of an acidic and a basic substance without interference by the solvent. 

The membranes used for analytical pervaporation are hydrophobic membranes of the types usually employed in ultrafiltration and gas-diffusion processes. In practice, PTFE is the most frequently used membrane material, followed by hydrophobic polyvinylidene-fluoride (PVDF). Ultrafiltration membranes are very thin, which, in combination with the large surface area of both the donor and acceptor chamber, leads to their easy bending. This results in changes in the ffux of the permeating component through an altered membrane area and hence in changes in the efficiency of the process. As a result, membranes must be replaced fairly often. Because of their thickness, gas-diffusion membranes are not so easily bent, so the same membrane can be used over long periods. The pore size of the... 

Table 2 Glycosynthase reactions of E383A /3-glucosidase using a-giucosyi fluoride donor and different acceptors...

Electron pair donors and acceptors Consider the reaction between a hydrogen ion (H+) and a fluoride ion (F ) to form a hydrogen fluoride (HF) molecule. The role of the electron pair is illustrated through the following Lewis structures. 

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