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

TLDR: Findings indicate that quantum mechanical tunneling plays an important role in these reactions of PINO(*) with substituted toluenes, benzaldehydes, and benzyl alcohols, and the second-order rate constants were correlated by means of a Hammett analysis.

Abstract: 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 × 103 L mol-1 cm-1 at λmax 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 °C. 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 σ+ (ρ = −1.3 and −0.41), and the reaction with benzaldehydes correlates better with σ (ρ = −0.91). The kinetic isotope effect was also studied and significantly large values of kH/kD were obtained:  25.0 (p-xylene), 27.1 (toluene), 27.5 (benzaldehyde), and 16.9 (benzyl alcohol) at 25 °C. From the Arrhenius plot for the reactions with p-xylene and p-xylene-d10, the difference of the activation energies, EaD − EaH, was 12...