He free radical chemistry of ROOH containing systems can proceed either by O or O homolysis. Here we only discuss the chemistry of the O bond; the interested reader is pointed to a review of the radiation and photochemistry of peroxides, which discusses a variety of O bond homolysis reactions.230 PCET reactions of organic peroxyl radicals have almost always been understood as HAT reactions, especially the chain propagating stepChem Rev. Author manuscript; available in PMC 2011 RR6 site December 8.Warren et al.Pagein autoxidation.17 This makes sense because of the strong ROO bonds, while PT-ET or ET-PT pathways are disfavored by the low basicity of ROO?and the moderate ROO?- potentials (Table 10). The most commonly employed organic hydroperoxide is tert-butyl hydroperoxide. The gas phase thermochemistry of organic peroxides has been widely discussed. Simmie et al.231 recently gave Hf?tBuOO? = -24.69 kcal mol-1, which, together with Hf?H? = 52.103 kcal mol-1 232 and Hf?tBuOOH) = -56.14 kcal mol-1 233, gives BDEg(tBuOOH) = 83.6 kcal mol-1.234 The pKas of several alkyl hydroperoxides and peracids have long been known,235 and pKa values for several peroxybenzoic acid have been reported.236 However, until recently, the reduction potentials of the corresponding peroxyl radicals have remained elusive. Das and co-workers indirectly measured the ROO?- couple for several peroxyl compounds in water (Table 10).237 Their value for E?tBuOO-/? is in good agreement with an earlier estimate made using kinetic and pKa data.238 In contrast, very little data exists on the redox potentials of percarboxylate anions. Peracids have gas phase BDFEs that are a little higher, and they are more acidic than the corresponding alkyl peroxides, which indicate that the RC(O)OO?- potentials are probably more oxidizing ( 1 V).239 Jonsson’s estimate of E?(CH3C(O)OO?-) = 1.14 V240 is in agreement with this estimate. Jonsson has also estimated thermochemical data for a variety of other peroxides but these need to be used with caution as they were extracted from electron RR6 custom synthesis transfer kinetic data240 and some of these values do not agree with those determined via more direct methods (e.g., Jonsson gives E?(Cl3COO?-) = 1.17 V while and Das reports E?Cl3COO?-) = 1.44 V237). 5.5 Simple Nitrogen Compounds: Dinitrogen to Ammonia, Amines, and Arylamines The previous sections all focused on reagents with reactive O bonds. With this section we shift to N bonds, and those below deal with S and C bonds. While the same principles apply, there are some important differences. N bonds are less acidic than comparable O bonds, and in general N-lone pairs are higher in energy so nitrogen compounds are more basic and more easily lose an electron to form the radical cation. Therefore, stepwise PCET reactions of amines typically involve aminium radical cations (R3N?), particularly for arylamines, while those of alcohols and phenols involve alkoxides and phenoxides. We start with the simple gas phase species from N2 to ammonia, then progress to alkyl and aryl amines, and finally to more complex aromatic heterocycles of biological interest. 5.5.1 Dinitrogen, Diazine, and Hydrazine–Dinitrogen (N2) is one of the most abundant compounds on earth, making it an almost unlimited feedstock for the production of reduced nitrogen species such as ammonia. The overall reduction of dinitrogen to ammonia by dihydrogen is thermodynamically favorable under standard conditions both in the gas phase and in aqueous s.He free radical chemistry of ROOH containing systems can proceed either by O or O homolysis. Here we only discuss the chemistry of the O bond; the interested reader is pointed to a review of the radiation and photochemistry of peroxides, which discusses a variety of O bond homolysis reactions.230 PCET reactions of organic peroxyl radicals have almost always been understood as HAT reactions, especially the chain propagating stepChem Rev. Author manuscript; available in PMC 2011 December 8.Warren et al.Pagein autoxidation.17 This makes sense because of the strong ROO bonds, while PT-ET or ET-PT pathways are disfavored by the low basicity of ROO?and the moderate ROO?- potentials (Table 10). The most commonly employed organic hydroperoxide is tert-butyl hydroperoxide. The gas phase thermochemistry of organic peroxides has been widely discussed. Simmie et al.231 recently gave Hf?tBuOO? = -24.69 kcal mol-1, which, together with Hf?H? = 52.103 kcal mol-1 232 and Hf?tBuOOH) = -56.14 kcal mol-1 233, gives BDEg(tBuOOH) = 83.6 kcal mol-1.234 The pKas of several alkyl hydroperoxides and peracids have long been known,235 and pKa values for several peroxybenzoic acid have been reported.236 However, until recently, the reduction potentials of the corresponding peroxyl radicals have remained elusive. Das and co-workers indirectly measured the ROO?- couple for several peroxyl compounds in water (Table 10).237 Their value for E?tBuOO-/? is in good agreement with an earlier estimate made using kinetic and pKa data.238 In contrast, very little data exists on the redox potentials of percarboxylate anions. Peracids have gas phase BDFEs that are a little higher, and they are more acidic than the corresponding alkyl peroxides, which indicate that the RC(O)OO?- potentials are probably more oxidizing ( 1 V).239 Jonsson’s estimate of E?(CH3C(O)OO?-) = 1.14 V240 is in agreement with this estimate. Jonsson has also estimated thermochemical data for a variety of other peroxides but these need to be used with caution as they were extracted from electron transfer kinetic data240 and some of these values do not agree with those determined via more direct methods (e.g., Jonsson gives E?(Cl3COO?-) = 1.17 V while and Das reports E?Cl3COO?-) = 1.44 V237). 5.5 Simple Nitrogen Compounds: Dinitrogen to Ammonia, Amines, and Arylamines The previous sections all focused on reagents with reactive O bonds. With this section we shift to N bonds, and those below deal with S and C bonds. While the same principles apply, there are some important differences. N bonds are less acidic than comparable O bonds, and in general N-lone pairs are higher in energy so nitrogen compounds are more basic and more easily lose an electron to form the radical cation. Therefore, stepwise PCET reactions of amines typically involve aminium radical cations (R3N?), particularly for arylamines, while those of alcohols and phenols involve alkoxides and phenoxides. We start with the simple gas phase species from N2 to ammonia, then progress to alkyl and aryl amines, and finally to more complex aromatic heterocycles of biological interest. 5.5.1 Dinitrogen, Diazine, and Hydrazine–Dinitrogen (N2) is one of the most abundant compounds on earth, making it an almost unlimited feedstock for the production of reduced nitrogen species such as ammonia. The overall reduction of dinitrogen to ammonia by dihydrogen is thermodynamically favorable under standard conditions both in the gas phase and in aqueous s.
