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Bond dissociation enthalpies (BDE) of hydroxylamines containing alkyl, aryl, vinyl, and carbonyl substituents at the nitrogen atom have been determined by using the EPR radical equilibration technique in order to study the effect of the substituents on the O-H bond strength of these compounds. It has been found that substitution of an alkyl group directly bonded to the nitrogen atom with vinyl or aryl groups has a small effect, while substitution with acyl groups induces a large increase of the O-H BDE value. Thus, dialkyl hydroxylamines have O-H bond strengths of only ca. 70 kcal/mol, while acylhydroxylamines and N-hydroxyphthalimide (NHPI), containing two acyl substituents at nitrogen, are characterized by BDE values of ca. 80 and 88 kcal/mol, respectively. Since the phthalimide N-oxyl radical (PINO) has been recently proposed as an efficient oxidation catalyst of hydrocarbons or other substrates, the large BDE value found for the parent hydroxylamine (NHPI) justifies this proposal. Kinetic studies, carried out in order to better understand the mechanism of the NHPI-catalyzed aerobic oxidation of cumene, are consistent with a simple kinetic model where the rate-determining step is the hydrogen atom abstraction from the hydroxylamine by cumylperoxyl radicals.

The results of a detailed thermodynamic and kinetic investigation on the homolytic reactivity of phenothiazine, phenoxazine, and phenoselenazine, of several substituted phenothiazines, and of related tricyclic aromatic amines are reported. All these compounds give, by hydrogen atom abstraction from the N−H group, persistent aminyl radicals. Equilibration of each of these radicals with the parent amine and a reference compound having an easily abstractable hydrogen allowed us to determine, by using EPR spectroscopy, the N−H Bond Dissociation Energies (BDE) of the amines. These are characterized by low BDE values (in some cases lower than the O−H bond strength of α-tocopherol, i.e 78.3 kcal/mol) and therefore are very good hydrogen atom transfer reagents. To check the efficiency of tricyclic amines as antioxidants and as polymerization inhibitors, absolute rate constants were determined for their reaction with alkyl, alkoxyl, and peroxyl radicals by using competitive techniques in the first two cases and by autoxidation studies under controlled conditions in the last one. All amines have been found to be highly reactive toward these radicals which makes them very good autoxidation and polymerization inhibitors.

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The autoxidation of organic materials is a detrimental radical chain process often leading to their rapid deterioration unless they are protected by preventive and/or chain breaking antioxidants. The properties of the more important family of the latter ones, that one of phenols, are illustrated in this tutorial review. A short outline of diarylamine antioxidants is also given. We describe simple experimental methods employed for the determination of the two parameters more useful for estimating the inhibiting power of antioxidants, that is the kinetic rate constant for their reaction with the chain carrying peroxyl radicals, k(inh), to give persistent phenoxyl or aminyl radicals and the bond dissociation enthalpy BDE(X-H) (X = O, N) of the bond cleaved in the inhibition process. The dependence of these parameters on the number and nature of the substituents is discussed and, in the case of phenols, a simple rule allowing to predict with reasonable accuracy the BDE and k(inh) values, from their structure. The effect of solvent polarity on the antioxidant power is also described. Finally, the information on the mechanism of reaction between phenols and peroxyl radicals provided by both experiments and theoretical calculations are examined. Because of difficulties associated with the analysis of non-homogenous systems all the reported results refer to homogenous solution in which experimental data can be analysed by means of more reliable and complete treatments.