Hydrogen atom abstraction from 0-H bonds

Hydrocarbon radical cations, structure and reactivity of, 38, 87 Hydrocarbons, small-ring, gas-phase pyrolysis of, 4, 147 Hydrogen atom abstraction from 0—H bonds, 9, 127 Hydrogen bonding and chemical reactivity, 26, 255... 

Some of the particular kinetic problems of hydrogen atom abstractions from 0—H bonds have been discussed in the context of questions of general interest. The aim has been to make unsolved problems explicit rather than to give final solutions. It seems desirable to realize explicitly that many of our theories and treatments are entirely empirical (Guggenheim, 1938) strongly resembling the entirely empirical origin of the rate constant (Harcourt and Esson, 1866, 1867). ... 

Essentially no incorporation of deuterium label was observed when the same reaction was carried out in toluene-dg. Since the formation of more than one equivalent of CO(n) hydroxide requires the transfer of hydrogen atoms, we hypothesized that the mechanism of this transformation involved the formation of a dinuclear p-peroxo complex, followed by rapid cleavage of the 0-0 bond and hydrogen atom abstraction from the Tp-ligand. The inferred activation of the primary C-H bonds of the rm-butyl groups of the ligand requires a highly reactive species thus the "activation" of O2 seems to have been accomplished. 

Figure Vni2. Rates of hydrogen atom abstraction from cyclohexane by various radicals at 2.5-28 C vs the formed 0-H bond strength.

No trapped hydrogen atoms have been observed except in CH4 at 5K, where G(H ) = G(CH3 ) = 3.3 (100 eV) and G(e ) = 0. The hydrogen atoms react either while hot or by tunnelling abstractions from C—H bonds by thermalized atoms. 

Rate constants for hydrogen atom abstraction from dihydroanthracene vs. the strength of the 0-H bond formed by oxygen radicais and manganese complexes. Adapted with permission from Mayer, J. M. Acc. Chem. Res. 1998, 31,441. 

Fig. 3. Arrhenius parameters of hydrogen abstraction reactions. The curve drawn suggests the trend of the compensation phenomenon. Key o abstraction by alkyl radicals from C—H bonds (Kondratiev, 1970) A abstraction by F or Cl atoms and HO radicals from C—H bonds (Kondratiev, 1970) O abstraction by CCls radical from methane (Hautecloque, 1970) further data are given in Table 7, as follows No. 1 Nos. 2-9 No. 10 BNo. 11 0 Nos. 12,13 x No. 14 No. 15 No. 16 V Nos. 17,20,21 + Nos. 18, 19, 22, 23.

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