Hydrogen fluoride, proton-donor

With very electrophilic olefins, an alternative hydrogen fluoride addition process is often preferred This process, involving reaction of the olefin with fluoride ion in the presence of a proton donor, is applicable to certain perhalogen ated alkenes [/] and substrates with other electron attracting groups attached to the double bond [i5, 36] (equations 4 and 5)... 

It might be pertinent to consider the basis of the extremely facile isomerization in anhydrous HF or pyridinium poly(hydrogen fluoride). HF is an extremely strong proton donor, but it is also a potent fluorinating agent. It is highly probable that the postulated cationic intermediates in these isomerizations are fluori-nated and serve as reactive intermediates in the same way as the fructofuranosyl... 

Protic character. The protic character of the solvent is an important consideration because electrochemical intermediates (particularly radical anions) frequently react rapidly with protons. The classification of solvents into protic or aptotic solvents is somewhat arbitrary. A simple classification1 is that protic solvents (such as hydrogen fluoride, water, methanol, formamide, and ammonia) are strong hydrogen-bond donors, exchange protons rapidly, and in-... 

For this reason, electrophilic additions such as addition of hydrogen halides are very difficult, as can be seen in Explanations 13, 14, and 15. But because of the low overall electron density, the double bonds in perfluoroalkenes are easily attacked by nucleophiles such as fluoride ion. If perfluoropropene is treated with dry potassium fluoride in a proton-donor solvent such as formamide, the fluoride anion joins the less negative carbon of the double bond and forms a negatively charged intermediate that readily reacts with the proton of formamide to give 2 H-perfluo-ropropane [5/]. Such a nucleophilic addition takes place with other fluorides such as cesium fluoride, tetraethylammonium fluoride, and sil-... 

Proton Sponge-HF (Figure 3.24) is a particularly useful system which is an effective fluoride-ion donor for appropriate fluorinations [38,155] it is likely that the proton in this system is much less involved in H-bonding to fluoride than is the proton in hydrogen fluoride. 

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. 

Mixtures of hydrogen fluoride and boron trifluoride are very powerful proton donors the substances react as shown. 

This is not, however, the only way in which these quantities can be determined, and Levi et al. (1963) have used a more orthodox approach in which the electrical conductivity of ice was studied as a function of the quantity of proton donor and proton acceptor impurity present for ice crystals cross-doped with ammonia and hydrogen fluoride. Their results, which differ slightly from those of Eigen et al., are also shown in table 7.1. 

This explanation of acids and bases depends on the choice of solvent. If a solvent is chosen that is a strong donator of protons, then it is considered the acid in the reaction, even if it s not technically called an acid. Or, if two acids are mixed together, the weaker donor of protons is instead classified as the base. Take, for example, the case of hydrogen fluoride mixed in perchloric acid ... 

Table 17.2 presents binding energies, AE s, for a series of complexes of TT-electron systems with molecules acting as proton donors, hydrogen fluoride, HF, and acetylene, [32]. Certainly for complexes with HF the interactions are stronger than the corresponding interactions for complexes with C2H2 since hydrogen fluoride is the stronger Lewis acid than acetylene. Except of two triplet cases the singlet systems are presented in Table 17.2. In two cases there are the meaningful...

In this process it may be more realistic to assume that fluoride ion transfer is responsible rather than proton transfer. Hydrogen fluoride is indeed a solvent in which an acid is either a proton donor according to Bronsted or a fluoride-ion acceptor, and a base either a proton acceptor or a fluoride ion donor... 

In principle two different definitions can be given for acids and bases analogous to those used in the liquid hydrogen fluoride solvent system. An acid may be described as either a proton donor or a halide ion acceptor and a base as a proton acceptor or a halide ion donor . ... 

Studies on the interaction between hydrogen fluoride and boron trifluoride have been documented. According to Sharp °, reaction to produce HBF4 is unlikely unless an electron donor is present to solvate the proton. McCaulay et investigated the... 

The stereospecificity of the formation of 2fl-nonafluoropent-2-ene when the method employed undoubtedly involves a nucleophilic mechanism [equation (3)], coupled with the resistance shown by perfluoropenta-1,2-diene towards attack by hydrogen halides except for hydrogen fluoride, expected to be the most powerful proton donor, has led to the suggestion" that the... 

Liquids and polymer surfaces can include three types of hydrogen-bonding molecules (a) proton acceptors (electron donors or bases) such as esters, ketones, ethers or aromatics including polymers such as PMMA, polystyrene, copoly(ethylene vinyl acetate), polycarbonate (b) proton donors (electron acceptors or acids) such as partially halogenated molecules, including polymers such as PVC, chlorinated polyethylenes or polypropylenes, poly(vinylidene fluoride) and copoly(ethylene acrylic acid) and (c) both proton acceptors and proton donors such as amides, amines and alcohols, including polyamides, polyimides and poly(vinyl alcohol). 

---