Bond dissociation enthalpy peroxides

If we make the assumption that the reverse of reaction 15.5 is diffusion-controlled and assume that the activation enthalpy for the acyl radicals recombination is 8 kJ mol-1, the enthalpy of reaction 15.5 will be equal to (121 - 8) = 113 kJ mol-1. This conclusion helps us derive other useful data. Assuming that the thermal correction to 298.15 K is small and that the solvation enthalpies of the peroxide and the acyl radicals approximately cancel, we can accept that the enthalpy of reaction 15.5 in the gas phase is equal to 113 kJ mol-1 with an estimated uncertainty of, say, 15 kJ mol-1. Therefore, as the standard enthalpy of formation of gaseous PhC(0)00(0)CPh is available (-271.7 5.2 kJ mol-1 [59]), we can derive the standard enthalpy of formation of the acyl radical Af//°[PhC(0)0, g] -79 8 kJ mol-1. This value can finally be used, together with the standard enthalpy of formation of benzoic acid in the gas phase (-294.0 2.2 kJ mol-1 [59]), to obtain the O-H bond dissociation enthalpy in PhC(0)0H DH° [PhC(0)0-H] = 433 8 kJ mol-1. 

A variety of bond dissociation enthalpies at 298 K have been calculated at the G2MP2, CBS-Q and G2 levels of theory and a summary of typical 0-0 BDEs are presented in Table 1. It becomes immediately obvious that the estimate of the 0-0 BDE of a generic peroxide should be iucreased by 10 kcalmoD to ca 44 kcalmoE . 

Peroxides are often added to free-radical reactions as initiators because the oxygen-oxygen bond cleaves homolytically rather easily. For example, the bond-dissociation enthalpy of the O —O bond in hydrogen peroxide (H —O —O —H) is only 213 kJ/mol (51 kcal/mol). Give a mechanism for the hydrogen peroxide-initiated reaction of cyclopentane with chlorine. The BDE for HO — Cl is 210 kJ/mol (50 kcal/mol). 

Vertical exceptions within a group also occur. The E.A. for F is less than that for Cl, for example. The smaller-than-expected E.A. for fluorine can be rationalized because of fluorine s extremely small radius. The addition of an electron to its valence shell would therefore increase the magnitude of the electron-electron repulsions. Consequently, the E.A. for fluorine is somewhat less than that for chlorine. Additionally, the bond dissociation enthalpy for Fj (155 kj/mol) is considerably less than that expected based on the other members of its group. For comparison, Clj, Br2, and I2 have bond dissociation enthalpies of 242, 193, and 151 kj/mol. Other anomalies occur for N and O, whose electron affinities are also less than the group trend would have predicted. By analogy with the F-F bond strength, the N-N and 0-0 bonds are likewise weaker than those for P-P or S-S. In fact, both the hydrazine (N-N) and peroxide (0-0) classes of compounds are particularly reactive. Hydrazine, N2H4, was once used as a rocket fuel, and many peroxides are potentially explosive. 

Figure 4 Selectivity in epoxidation for a range of substrates plotted against the dissociation enthalpy of the weakest C-H bond in the olefin (m) TS-1 peroxide system (u) silver-oxygen system. 1. 1-octene, 2. 1-butane, 3. 2-butane, 4. gropene, 5. 4-unyltoluene, 6. 1-3 butadiene, 7. styrene, 8. 4-vinylpyridine, 9. ethylene.2...

The enthalpy of dissociation decreases as the mass number increases along the series. However, there exists a discontinuity (the value for F2 is similar to that of I2), and thus there is an increase when going from F2 to CI2. This trend is also observed in other groups of the periodic table. In groups 15 and 16, the single bonds of the first elements in those groups, N-N in hydrazine and 0-0 in peroxides, are weaker than those of the following heavier elements, P-P and S S, respectively. Some of these properties are snmmarized in Table 2. 

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