Health & Environmental Research Online (HERO)


Print Feedback Export to File
8386218 
Journal Article 
Mechanism of Thermolysis of Endoperoxides of Aromatic Compounds. Activation Parameters, Magnetic Field, and Magnetic Isotope Effects 
Turro, NJ; Chow, MF; Rigaudy, J 
1981 
Yes 
Journal of the American Chemical Society
ISSN: 0002-7863
EISSN: 1520-5126 
103 
24 
7218-7224 
English 
A mechanistic investigation has been made of the thermolysis of several endoperoxides of anthracenes and naphthalenes which produce molecular oxygen and the parent aromatic species quantitatively. Qualitative thermochemical measurements in the solid state indicate that in all the cases studied, the reactions were endothermic. This situation appears to be valid in solution also. Clean first-order kinetics were observed for these thermolyses. Activation parameters were derived from the temperature dependence of the first-order rate constants. The primary yields of singlet molecular oxygen (1O2) from the several endoperoxides were determined, and a correlation was discovered between the A factors (∆S* values) for thermolysis and the yield of 1O2. It was found that high A factors (positive ∆S* values) correlated with relatively low yields of 1O2, and that low A factors (slightly negative or near zero ∆S* values) correlated with nearly quantitative yields of O2. These two results are interpreted in terms of a diradical mechanism which leads to low yield of 1O2 and a concerted mechanism which leads to quantitative yields of 1O2, respectively. This interpretation is consistent with the observation of a magnetic field effect on the yield of 1O2 from endoperoxides whose thermolyses proceed with positive ∆S* values and the absence of a magnetic field effect on the yield of 1Oi endoperoxides whose thermolyses proceed with near zero ∆S* values. Further support for the occurrence of a diradical mechanism is available from the demonstration of a special 170 isotope effect on the thermolysis of an endoperoxide which is postulated to undergo thermolysis principally via a diradical intermediate. The thermolysis of endoperoxides which decompose mainly by a diradical mechanism yields triplet molecular oxygen that is selectively enriched in 170. © 1981, American Chemical Society. All rights reserved.