Kinetic study of the phthalimide N-oxyl radical in acetic acid.: Hydrogen abstraction from substituted toluenes, benzaldehydes, and benzyl alcohols

被引:125
作者
Koshino, N [1 ]
Saha, B [1 ]
Espenson, JH [1 ]
机构
[1] Iowa State Univ Sci & Technol, Dept Chem, Ames, IA 50011 USA
关键词
D O I
10.1021/jo0348017
中图分类号
O62 [有机化学];
学科分类号
070303 ; 081704 ;
摘要
The phthalimide N-oxyl (PINO) radical was generated by the oxidation of N-hydroxyphthalimide (NHPI) with Pb(OAc)(4) in acetic acid. The molar absorptivity of PINO. is 1.36 x 10(3) L mol(-1) cm(-1) at lambda(max) 382 nm. The PINO radical decomposes slowly with a second-order rate constant of 0.6 +/- 0.1 L mol(-1) s(-1) at 25 degreesC. The reactions of PINO. with substituted toluenes, benzaldehydes, and benzyl alcohols were investigated under an argon atmosphere. The second-order rate constants were correlated by means of a Hammett analysis. The reactions with toluenes and benzyl alcohols have better correlations with sigma(+) (rho = -1.3 and -0.41), and the reaction with benzaldehydes correlates better with sigma (rho = -0.91). The kinetic isotope effect was also studied and significantly large values of k(H)/k(D) were obtained: 25.0 (p-xylene), 27.1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25 degreesC. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d(10), the difference of the activation energies, E-a(D) - E-a(H) was 12.6 +/- 0.8 kJ mol(-1) and the ratio of preexponential factors, A(H)/A(D), was 0.17 +/- 0.05. These findings indicate that quantum mechanical tunneling plays an important role in these reactions.
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页码:9364 / 9370
页数:7
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