Fluorination hydrogen bonds

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

It has recently been shown that the same principle can be applied to deep eutectic solvents by using small quaternary ammonium cations such as ethylammonium and fluorinated hydrogen bond donors such as trifluoroacetamide. However, there is only a limited benefit that can be achieved using this approach as the physical parameters cannot be varied totally independently of one another. For example there will be an optimum ion size too small and the lattice energy will increase the surface tension, too large and the ionic mobility will be impeded. 

The hydrogen difluoride ion, FHF-, exists in crystals (such as KHF., which contains the ions K+ and FHF-) and in aqueous solution. It has an unusual structure, in that the hydrogen atom (proton) is midway between the two fluorine atoms (fluoride ions). The two fluorine-hydrogen bonds in FHF- have the length 1.13 A. 

Other complex structures which show similar packing arrangements based on charge-transfer interactions include the 1 1 complexes of mesitylene-hexafluorobenzene, thymine-p-benzoquinone, and the 1 2 complex formed by phlorogludnol and p-benzoquinone. The mesitylene complex consists of alternately stacked molecules of mesitylene and hexa-fluorobenzene [separation 3.56 A]. There are also nearly linear C-H F [3.24 A] intermolecular contacts, and the possibility of methyl-fluorine hydrogen-bonding cannot be discounted. The structures of both the thymine and phlorogludnol complexes with p-benzoquinone may be described In... 

Fluorine. Hydrogen bonded cluster formation by F and HF has been studied using and F NMR spectroscopy. ... 

Secondary bonds Hydrogen bonds Hydrogen bonds involving fluorine Hydrogen bonds excluding fluorine van der Waals bonds Up to 40 10-25... 

Egli M (2012) The steric hypothesis for DNA replication and fluorine hydrogen bonding revisited in hght of strucmral data. Acc Chem Res 45 1237... 

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. 

Thermochemistry. Thermodynamic considerations ate of utmost importance in fluorinations. Table 1 is based on JANAF data (25) for CH, which indicate an average carbon-hydrogen bond strength of 410.0 kj/mol (98 kcal/mol) based on the atomization energy of CH. ... 

The physical properties of hydrofluorocarbons reflect their polar character, and possibly the importance of intermolecular hydrogen bonding (3). Hydrofluorocarbons often bod higher than either their PFC or hydrocarbon counterparts. For example, l-C H F bods at 91.5°C compared with 58°C for n-Q and 69°C for Within the series of fluorinated methanes, the boiling point reaches a maximum for CH2F2, which contains an equal... 

In dimers composed of equal molecules the dimer components can replace each other through tunneling. This effect has been discovered by Dyke et al. [1972] as interconversion splitting of rotational levels of (HF)2 in molecular beam electric resonance spectra. This dimer has been studied in many papers by microwave and far infrared tunable difference-frequency laser spectroscopy (see review papers by Truhlar [1990] and by Quack and Suhm [1991]). The dimer consists of two inequivalent HE molecules, the H atom of one of them participating in the hydrogen bond between the fluorine atoms (fig. 60). PES is a function of six variables indicated in this figure. 

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... 

Systems usually fluonnated by electropositive fluorine reagents include acti-vated alkenes (enol ethers, enol acetates, silyl enol ethers, and enamines), activated aromatic systems, certain slightly activated carbon-hydrogen bonds, and selected organometallics. 

The types of reactions covered in this segment are those in which the overall transformation is the conversion of a carbon-hydrogen bond to a carbon-fluorine bond through the use of electropositive fluorine reagents [7, 2, 3, 4, 5, 6]. 

Fluorine is well known for its ability to fluonnate seleeti vely terttary carbon-h drogen bonds that are slightly activated by polar substituents Hypofluorite reagents also react with carbon-hydrogen bonds, both secondary and tertiary but tertiary bonds react better [6]... 

Fluoroxytiifluorouiethane ettectively fluorinates tertiary carbon-hydrogen bonds in materials with biologieal applications [2 3] (equations 23 and 24)... 

Xenon difluoride fluorinates adamantane in low yield [45] (equation 22) When the carbon-hydrogen bond is activated by an a-sulfur atom, fliiorination occurs readily The reactions involve intermediates that contain sulfur-fluorine bonds. At-Fluoropyridinium reagents behave similarly [99, 100, 101, 102] (equations 55-57)... 

Benzylic carbon-hydrogen bonds in compounds such as methylpentafluoro-benzene, fluoromethylpentafluorobenzene, and difluoromethylpentafluoroben-zene are not capable of metalation by butyllithium Instead nucleophilic substitution of the para fluorines occurs m each example [55] (equation 13)... 

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... 

Spectroscopic evidence indicates that protonation of 2-fluoro-and 2-chloro-quinoline is not appreciable in O.OlJf aqueous hydrochloric acid. Protonation becomes evident in more strongly acidic solution in the case of the chloro compound without any accompanying decomposition, but the fluoro compound hydrolyzes to carbostyril under the latter conditions. The hydrolysis is acid-catalyzed, but it is doubtful whether protonation on the heterocyclic nitrogen is responsible, owing to its low basicity (presumably below that for the chloro compound). An alternative explanation in this case would be hydrogen bond formation with fluorine, Ar—F. .. H-O+H2. 

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