Bond Dissociation Energy Halogenation

The last example represents a fairly rare elimination of hydrogen fluoride in preference to hydrogen chloride, a reaction that deserves a more detailed discussion A comparison of bond dissociation energies of carbon-halogen bonds shows that the carbon-fluorine bond is much stronger than the carbon-chlorine, carbon-bromine, and carbon-iodme bonds 108-116, 83 5, 70, and 56 kcal/mol, respec-... 

FIGURE 2.18 The bond dissociation energies of the hydrogen halide molecules in kilojoules per mole of molecules. Note how the bonds weaken as the halogen atom becomes larger. 

The physical properties of the interhalogens are intermediate between those of their parent halogens. Trends in the chemistry of the interhalogen fluorides can be related to the decrease in bond dissociation energy as the central halogen atom becomes heavier. The fluorides of the heavier halogens are all very reactive bromine trifluoride gas is so reactive that even asbestos burns in it. 

The reactivity shown in Scheme 3 results from the low bond dissociation energy (BDE) of the P-H bond [11] k=l.2 10 M s for the H-transfer from R02P(0)H to a primary C-centered radical) and the fast halogen-atom transfer from a C-halogen bond to a phosphonyl radical [9,12] (fc=4 10 M s for f-Bu-Br and k=83 10 M s for Cl3C-Br). Piettre et al. [13] pointed out that these chain reactions were even more efficient when dialkylthiophosphites and the corresponding dialkylphosphinothioyl radicals were involved. 

The effect of TOS on the product distribution during the pyrolysis of R22 over CU-AIF3 catalyst is shown in Fig. 3. The amoimt of halogen ion trapped in NaOH solution was determined by IC. The concentration of Cl formed during the pyrolysis of R22 was higher than the concentration of F at all TOS. This result is a consequence of the facile cleavage of the C-Cl bond in comparison to the C-F bond. Bond dissociation energy for the C-element of R22 is followed by the order C-C1[Pg.235]

The validity of equation (12) has been checked for several families of alkyl halides for which D and E /x- are known (Ref. 32, see particularly figure 6 therein). It was thus found that for v = 0.1 V s the constant is equal to 0.3 eV at 20°C (expressing D in eV and the potentials in V). Equation (12) was then applied to the approximate determination of unknown BDEs in several series of compounds undergoing dissociative electron transfer, namely, TV-halosultams,32 sulfonium cations,33 vicinal dihalides,34 1,3-dihaloadamantes, 1,4-dihalo-bicyclo[2.2.2]octanes, and l,3-dihalobicyclo[l.l.l]pentanes.35 In the latter case, the mutual influence of the two halogens could be rationalized thanks to the conversion of the peak potential data to bond dissociation energies. 

The Arrhenius parameters and the thermochemical sum of the phenyl-carbon and phenyl-halogen bond dissociation energies are shown in Table 8. The extent of the diphenyl mercury decomposition was determined from the weight of mercury produced. It is the present author s opinion that in calculating the Arrhenius parameters for this compound Carter et al.81 gave too great a statistical... 

Similar decomposition is observed in p-bromoacetophenone, o-bromo-, p-bromo, and p,p -dibromobenzophenone, and p-iodobenzophenone44 but not in the fluoro- and chloro-substituted compounds. This order of reactivity follows the bond dissociation energies for aromatic halides which are about 90 kcal/mole for chlorobenzene, 70 kcal/mole for bromobenzene, and 60 kcal/ mole for iodobenzene. The lowest-lying triplet of p-bromoacetophenone is 71.2 kcal45 while that of the substituted benzophenones is slightly lower since benzophenone itself has a lower triplet energy than acetophenone. p,p Dibromobenzophenone was the least reactive of the compounds that photoeliminated halogen atoms. 

Fig. 17.12 Bond dissociation energies and bond lengths of the hydrogen halides, methyl halides, and halogen molecules. Note that this figure, which is taken directly from Politzcr s work, portrays in a different way relationships that are closely rebled to Fig. 9.7. (From Politzer. P. J. Am. Chetn. See. 1969. 91.6235. Reproduced with permission.)...

Table 1. Generalized Carbon-Halogen sp3-Bond Dissociation Energies...

Table 9.4 Bond Dissociation Energies of Halogen Compounds (kcal mole-1)0...

By comparison, the bond dissociation energies of the halogens range from 151 kj/mol for I2 to 243 kj/mol for Cl2. Because of the strong H-H bond, H2 is thermally stable. Even at 2000 K, only 1 of every 2500 H2 molecules is dissociated into H atoms at 1 atm pressure. 

TABLE 18. Derived silicon-halogen bond dissociation energies (kJmol l)a... 

Many other bond dissociation energies have been derived from theoretically calculated enthalpies of formation of radicals in recent years. These include Si—H, Si—C, Si—Si, Si—halogen, Si—O and Si—N. These are too many examples to consider in the context of this (mainly) experimentally orientated account. Where comparisons exist, as indicated,... 

Of this group only benzyl chloride is not an aryl halide its halogen is not attached to the aromatic ring but to an. v/r -hybridized carbon. Benzyl chloride has the weakest carbon-halogen bond, its measured carbon-chlorine bond dissociation energy being only 293 kJ/mol (70 kcal/mol). Homolytic cleavage of this bond produces a resonance-stabilized benzyl radical. 

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