Fluorine with molecular hydrogen

FIGURE 2 The reaction of atomic fluorine with molecular hydrogen produces vibrationally excited hydrogen fluoride. The reaction coordinate represents the breaking of the molecular hydrogen bond as the fluorine atom approaches and the subsequent formation of the HF bond as the hydrogen atom departs the collision center. 

I. F + D2 -> DF + D. The reaction of atomic fluorine with molecular hydrogen containing molecules has been demonstrated to... 

The determination of absolute rate coefficients of transfer reactions of bromine atoms is much more favourable than for the corresponding reactions of fluorine or chlorine atoms. This arises because the dissociation constant of molecular bromine is high at normal experimental temperatures and the chain lengths in bromination are relatively short. The rate constant of the reaction of bromine atoms with molecular hydrogen was the first quantitative kinetic study of a radical reaction [96]. Fettis and Knox [52] evaluated the data for the Br—Hj reaction and their results are given in Table 7. Trotman-Dickenson [1] has pointed out that the subsequent data of Timmons and Weston [80] for the reaction with Hj, HD and HT are not fully compatible with the conclusions of Pettis and Knox [52]. 

Eveiy bond has a characteristic length and strength. Example compares molecular hydrogen with molecular fluorine. 

The type of reactivity shown by the formation of Chichibabin s hydrocarbon extends to the activation of substituents other than hydrogen as well. For example, molecular silver will not usually remove fluorine from organic compounds, not even from triphenylmethyl fluoride. Yet the radical parafluorophenyldiphenylmethyl reacts with molecular silver to give silver fluoride.88... 

From specular reflection experiments with molecular beams of hydrogen or helium (4) it may be concluded that cleavage surfaces of LiF or NaCl are very smooth surfaces indeed, the inequalities being caused only by the temperature movement and amounting to the order of magnitude of 10 8 cm. Diffraction spectra obtained with helium beams (5), indicate moreover (6) that the distance between two fluorine ions in the outer layer of LiF is exactly the same as in the crystal. 

Although controlled substitution with molecular fluorine is difficult to attain, noteworthy is the recent success in perfluorinating both saturated and unsaturated heterocycles by electrolysis in anhydrous hydrogen fluoride. Cf. T. C. Simmons and F. W. Hoffmann, J. Am. Chem. Soc. 79, 3429 (1957), for the preparation of undecafluoropiperidine (from piperidine). The latter compound can be converted into pentafluoropyridine by passing it over an iron contact at 600° [R. E. Banks, A. E. Ginsberg, and R. N. Haszeldine, J. Chem. Soc. p. 1740 (1961)]. 

Molecular Orbital Description. The molecular orbital description of the H bond has received surprisingly little attention. Pimentel considered HF2 qualitatively with molecular orbitals composed of linear combinations of atomic orbitals (1634). (See also 1529.) Using only fluorine p orbitals directed along the bond pA and Pb) and the hydrogen atom Is orbital (j), three molecular orbitals result. These are shown in the second column of Fig. 8-4. Since the H bond involves four electrons,... 

By-products. The presence in the products of small quantities of compounds which would arise from combination of free radical intermediates can provide evidence for a free radical process. For example, the explosive reaction of methane with fluorine gives mainly hydrogen fluoride and a mixture of mono-, di-, tri- and tetrafluoromethanes, but small quantities of fluorinated ethanes, including C2F6, are also produced. These two-carbon products cannot be readily explained on the basis of possible molecular reactions (see reaction 6.16), but would arise naturally as combination products of the fluorinated methyl radicals produced in a radical chain reaction sequence (reaction 6.17). 

---