A Comparison of Transition States During H-Atom Abstraction of Monophenols and Catechol by Methyl Radical

✍ Scribed by You-Min Sun; Cheng-Bu Liu

• Publisher: John Wiley and Sons
• Year: 2004
• Tongue: English
• Weight: 219 KB
• Volume: 2004
• Category: Article
• ISSN: 1434-193X
• DOI: 10.1002/ejoc.200300450

✦ Synopsis

Abstract

To compare the transition states of para‐substituted phenols with that of catechol, we employed the B3LYP/6−311+G(d,p) method to calculate the energy profiles, the geometries, the spin density distributions, and other physico‐chemical parameters. A comparison between catechol and para‐substituted phenols indicates that their transition state geometries for hydrogen atom abstraction are similar. Substituent effects on the transition states of o‐substituted phenols may differ from those of p‐substituted ones. The calculated value of E~a~ for catechol is much lower than that of phenol, which arises mainly from the electron‐donating property of the __ortho‐__hydroxyl group. Although, the intramolecular hydrogen bond (IHB) effect on E~a~ is small, the IHB effect on the transition state geometry is fairly strong. According to the regression equation of the relative O−H bond dissociation enthalpy (BDE) versus values of E~a~ for p‐substituted monophenols, the relative O−H BDE of catechol is −8.28 kcal mol^−1^, which is in agreement with the value obtained through a kinetic analysis (−7.2 kcal mol^−1^). The values of E~a~ are governed by resonance effects, which is similar to the observation found for the O−H BDEs. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)