Difluorine bonding

The (a-sulfanylalkyl)phosphonates and their difluorinated derivatives are also, by easy radical cleavage of the C-S bond, useful precursors of phospho-nomethyl or phosphonodifluoromethyl radicals, which can be added to double bonds and so, introduced in a variety of structures. Besides, the use of phospho-nodithioformates as radical trapping agents and their use as RAFT reagent for controlled polymerization open a new interesting field of appHcation for these dithioesters. 

Another fast reaction of radical initiation is the difluorine addition to the double bond of the unsaturated compounds [192-194] ... 

In fluorine thermochemistry, two key heat values frequently occur. They are the dissociation energy of difluorine, required for evaluation of fluorine bond energies and the heat of formation of hydrogen fluoride, a product in hydrolysis, hydrogenation, fluorine combustion, or neutralization reactions. These values have been difficult to measure and have changed considerably over the years. A recommended set of values has been reported in recent CODATA bulletins (60) which are collected in Table I together with older values and corrections to update them. 

Intuitively, one would expect a volume contraction on forming a strongly bonded compound from the elements. Indeed, Richards 190, 191) regarded heats of formation as heats of compression. The fractional volume contraction, AV = (molecular volume - 2 atomic vol-ume)/2(atomic volume), has been related to formation heats for NaCl or CsCl type structures 151). Even nonpolar compounds in the condensed state cohere in close-packed arrays. The packing density of difluorine, derived from the ratio of the van der Waals envelope to the molecular volume, is especially low, and a larger contraction would be expected for fluorides than for other halides. This approach has yet to be systematically examined. 

The thallium trinitrate-mediated ring contraction of frani-decal-2-ones has opened up a new route to the hydrindane system, and fluorinative ring contraction of cyclic alkenes to afford difluorocycloalkanes has been induced by iodotoluene difluoride and EtsN-HF. A possible mechanism is shown in Scheme 78. The double bond of the cyclohexene ring is attacked by iodotoluene difluoride activated by HF from the axial direction, followed by the addition of a fluoride ion from the trans direction. Reductive elimination of iodotoluene from the resulting adduct, ring contraction and the addition of the fluoride ion to the carbocation stabilized by fluorine then take place to give the ring-contracted difluorinated product. 

The standard state of fluorine is the difluorine molecule, F2, which has an electronic configuration identical with that of the peroxide ion. The two species are isoelectronic. The bond order is 1, and the bond dissociation energy of 155 kj mol-1 and bond length of 144 pm are very similar to the values for 022-. 

The selective oxidation of C—H bonds in alkanes under mild conditions continues to attract interest from researchers. A new procedure based upon mild generation of perfluoroalkyl radicals from their corresponding anhydrides with either H2O2, m-CPBA, AIBN, or PbEt4 has been described. Oxidation of ethane under the reported conditions furnishes propionic acid and other fluorinated products.79 While some previously reported methods have involved metal-mediated functionalization of alkanes using trifluoroacetic acid/anhydride as solvent, these latter results indicate that the solvent itself without metal catalysis can react as an oxidant. As a consequence, results of these metal-mediated reactions should be treated with caution. The absolute rate constants for H-abstraction from BU3 SnH by perfluorinated w-alkyl radicals have been measured and the trends were found to be qualitatively similar to that of their addition reactions to alkenes.80 a,a-Difluorinated radicals were found to have enhanced reactivities and this was explained as being due to their pyramidal nature while multifluorinated radicals were more reactive still, owing to their electrophilic nature.80... 

The dissociation of difluorine is a demanding test case used traditionally to benchmark new computational methods. In this regard, the complete failure of the Hartree-Fock method to account for the F2 bond has already been mentioned. Table 1 displays the calculated energies of F2 at a fixed distance of 1.43 A, relative to the separated atoms. Note that at infinite distance, the ionic structures disappear, so that one is left with a pair of singlet-coupled neutral atoms which just corresponds to the Hartree-Fock description of the separated atoms. 

As is the case for its neutral homologue, the difluorine radical anion is a difficult test case for the calculation of its bonding energy. At the Hartree-Fock level, the bonding energy is about 4 kcal/mol, depending on whether the ROHF or UHF method is used. The experimental bond energy is 30 kcal/mol. 

A versatile C4-building block, difluorinated Danishefsky s diene, was developed for the construction of fluorinated six-membered rings in the laboratory of K. Uneyama. The diene was prepared by the selective C-F bond cleavage of trifluoromethyl ketones. The reaction of this novel diene with benzaldehyde afforded the corresponding difluoro dihydropyrone in 92% ee in the presence of equimolar Ti(IV)-(R)-BINOL. ... 

Draw the Lewis structures of difluorine, F2, dioxygen, 02, and dinitrogen, N2. How many bonding pairs and how many nonbonding pairs of electrons are present in each structure ... 

The calculated bond dissociation energies are in good agreement with experiment except for difluorine. The theoretical result Dq = 46.0 kcaPmol is higher than the experimental value of Do = 36.9 kcaPmol. The trend however, that the BDE of F2 is smaller than the bond dissociation energies of CI2 and Br2 is preserved. Table 13.3 shows that the values of both and increase steadily from I2 to F2. We want to point out that... 

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