Reactions of Alkyl Radicals with Substituted Toluenes and the Effect of Substituents on Dissociation Energies of Benzyl CH Bonds

Abstract

Reactions of isopropyl and of undecyl radicals with meta-and para-substituted toluenes are reported. The results demonstrate that the reactivities of toluenes are due to both benzyl-H abstraction and addition of the alkyl radicals to the aromatic ring. Relative reactivities yield curved Hammett plots, consistent with kinetic data reported by Dütsch and Fischer. Abstractions and ring additions occur with comparable rates, but opposite Hammett slopes. Addition is favored by electron-withdrawing and abstraction by electron-donating substituents. The effects of substituents on the dissociation energies of benzyl CÀH bonds are shown to be the major factor influencing reaction rates for benzyl-H abstraction by alkyl radicals.Introduction. -In 1982, Dütsch and Fischer published Nucleophilic Character of the tert-Butyl Radical. Absolute Rate Constants for the Reactions with Substituted Toluenes [1]. tert-Butyl radicals were produced by pulsed photolysis of di(tert-butyl) ketone at 488. The decay of tert-butyl radicals was measured by time-resolved electron paramagnetic resonance (EPR) spectroscopy in eleven meta-or para-substituted toluenes. A plot according to the Hammett equation (Eqn. 1) of the rate constants vs. Hammett substituent constants, s, was found to have a positive slope, 1. The s values reflect the electron-donating or -withdrawing abilities of the substituents, k X is the rate constant for reaction of tert-butyl radicals with the meta-or para-substituted toluene and k H is that with toluene. Because electron-withdrawing groups enhanced the rate of disappearance of tert-butyl radicals, following usual practice, these results were interpreted to mean that the transition state for benzyl-H abstraction by tertbutyl radicals is stabilized by a polar effect, with charge separation in contributing dipolar structures as shown in Eqn. 2. This conclusion was based on the assumption that the only mode of reaction between tert-butyl radicals and the toluenes is benzyl-H abstraction. The positive slope, 1 = 0.59, classified the abstracting tert-butyl radical as nucleophilic, capable of accommodating a partial positive charge, with a partial negative charge on the benzyl C-atom. Electron-withdrawing substituents, positive s, would stabilize the negative charge and speed the reaction. Electron donors, negative s, would do the reverse.

Effects of Molecule Stabilization Energies on Radical Reactions: G3 and G3(MP2) Model Chemistries Applied to Benzylic Systems

“…Negative slopes are obtained in Hammett plots of benzylic hydrogen abstraction rates by primary, secondary, and tertiary alkyl radicals, as found with all other H-abstracting radicals, the opposite of the positive slopes of halogen abstractions by • Sn( n -Bu) 3 , • Ge(Et) 3 , and • Si( n -Bu) 3 …”

Reaction mechanisms : Part (i) Radical and radical ion reactions

“…This examination begins with classical H-atom abstractions: Reactions of alkyl radicals with substituted toluenes were examined by Zavitsas et al in 2006. 2 It had been known that the rate constants for these reactions led to curved Hammett plots, with negative r-values for electron donating groups, and positive r-values for electron withdrawing groups. It was convincingly demonstrated that the negative r-values were the result of a hydrogen abstraction process (R d + XC 6 H 4 CH 3 -RH ), whose rate correlated to the strength of the C-H bond (as expected on the basis of the Evans-Polanyi relationship).…”

“…With electron withdrawing groups, rather than hydrogen abstraction, nucleophilic alkyl radicals undergo addition to the aromatic ring-a process facilitated by electron-withdrawing groups. 2 Galli et al provided evidence for the importance of stereoelectronic factors in Hatom abstractions reactions, finding that the rate constant for reaction of the nitroxyl radical BTNO d was consistently larger for cyclic vs. acyclic alkylarene substrates, e.g., Scheme 1. Alignment of the C-H bond with the adjacent aromatic system, enforced in the case of the cyclic systems, nicely explain the enhanced rates.…”

“…DFT/B3LYP calculations were used to explore the importance of electron transfer/deprotonation vs. SPLET pathways to understand the antioxidant properties of phenols. 10 Several elegant kinetic studies explored the role of pendant bases in promoting proton-coupled electron transfer (PCET) as illustrated in eqn (2), where the B: is carboxylate or amine functionality. [11][12][13][14] In addition, several theoretical papers in 2006 provided insight into the PCET process.…”

“…As noted earlier, the addition of alkyl radicals to substituted toluenes can be correlated to the Hammett equation, and a positive r-value was observed. 2 Alkyl radicals are considered ''nucleophilic'' and thus the addition to an aromatic ring is facilitated by electron withdrawing groups. Similarly, rate constants for the addition of aminoxyl radicals (R 2 NO d ) to arylketenes (XC 6 H 4 CHQCQO) were also subjected to a Hammett analysis, and a r-value of 1.6 was observed, also attributable to the nucleophilic character of the oxygen, 30 arising from the dipolar the resonance form depicted in eqn (4).…”