Hydrogen fluoride, bonding

Hg. 5.15 Hydrogen bonding between molecules of hydrogen fluoride (hydrogen bonds are usually represented by dotted lines). 

With the exception of ammonia, water and hydrogen fluoride, hydrogen bonds are not present in the hydrides of elements in groups 15, 16 and 17. The increase in the boiling points for the hydrides of each periodic group is therefore due to the increase in London (dispersion) forces as the molecular size increases. 

The structure of ammonium fluoride hydrogen bonds are shown in purple. Note the similarity to the structure of ice (Figure 5.12b). 

The ability to form hydrogen bonds explains the formation of complex ions such as HFJ and HjFj when a fluoride salt, for example potassium fluoride, is dissolved in aqueous hydrofluoric acid ... 

Halogen exchange with KF is not successful ia acetic acid (10). Hydrogen bonding of the acid hydrogen with the fluoride ion was postulated to cause acetate substitution for the haUde however, the products of dissolved KF ia acetic acid are potassium acetate and potassium bifluoride (11). Thus KF acts as a base rather than as a fluorinating agent ia acetic acid. 

Hydrogen bonding aeeounts for the abnormally high boiling points of, e.g., water, hydrogen fluoride, ammonia, and many organie eompounds (see later) sueh as aleohols. 

During electrochemical fluorination retention of important functional groups or atoms in molecules is essential. Acyl fluorides and chlorides, but not carboxylic acids and anhydrides (which decarboxylate), survive perfluorination to the perfluorinated acid fluorides, albeit with some cyclization in longer chain (>C4) species [73]. Electrochemical fluorination of acetyl fluoride produces perfluoro-acetyl fluoride in 36-45% yields [85]. Electrochemical fluorination of octanoyl chloride results in perfluorinated cyclic ethers as well as perfluorinated octanoyl fluonde. Cyclization decreases as initial substrate concentration increases and has been linked to hydrogen-bonded onium polycations [73]. Cyclization is a common phenomenon involving longer (>C4) and branched chains. a-Alkyl-substituted carboxylic acid chlorides, fluorides, and methyl esters produce both the perfluorinated cyclic five- and six-membered ring ethers as well as the perfluorinated acid... 

In media such as water and alcohols, fluoride ion is strongly solvated by hydrogen bonding and is neither very basic nor very nucleophilic. On the other hand, the poorly solvated, or naked, fluoride ions that aie present when potassium fluoride dissolves in benzene in the presence of a crown ether aie better able to express their anionic reactivity. Thus, alkyl halides react with potassium fluoride in benzene containing 18-crown-6, thereby providing a method for the preparation of otherwise difficultly accessible alkyl fluorides. 

The crystal structure of many compounds is dominated by the effect of H bonds, and numerous examples will emerge in ensuing chapters. Ice (p. 624) is perhaps the classic example, but the layer lattice structure of B(OH)3 (p. 203) and the striking difference between the a- and 6-forms of oxalic and other dicarboxylic acids is notable (Fig. 3.9). The more subtle distortions that lead to ferroelectric phenomena in KH2PO4 and other crystals have already been noted (p. 57). Hydrogen bonds between fluorine atoms result in the formation of infinite zigzag chains in crystalline hydrogen fluoride... 

An extreme example of the fluorine-hydrogen bond is found in the hydrogen difluoride ion, HFS. This ion exists in acidic solutions of fluorides,... 

Hydrogen abstraction reactions potential surfaces for, 25-26,26,41 resonance structures for, 24 Hydrogen atom, 2 Hydrogen bonds, 169,184 Hydrogen fluoride, 19-20, 20,22-23 Hydrogen molecules, 15-18 energy of, 11,16,17 Hamiltonian for, 4,15-16 induced dipoles, 75,125 lithium ion effect on, 12... 

Due to its strong hydrogen bonds, in the vapor state hydrogen fluoride is found as short chains and rings. Draw the Lewis structure of an (HF)3 chain and indicate the approximate bond angles. 

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