Evaluation of the Kinetic Data for Intramolecular 1,x-Hydrogen Shifts in Alkyl Radicals and Structure/Reactivity Predictions from the Carbocyclic Model for the Transition State

Abstract

Experimental and computational kinetic data for the intramolecular 1,x-hydrogen shift in alkyl radicals are compiled in Arrhenius format for x = 2-5. Significant experimental disparity remains, especially for x = 2 and 3. Experimental data for radicals with tert centers or bearing spectator substituents are lacking for all x, and none exist for x = 6. The common use of the strain energy of the unsubstituted (x+1)-carbocycle to coarsely model the activation energy for the 1,x-shift is extended to explore more subtle differences in progressively methyl-substituted systems by use of molecular mechanics estimates of differences in strain between radicals and carbocycles. For x = 5 and 6, a sterically driven increase in E is predicted for shifts in the tert --> tert class that apparently runs counter to the behavior of bimolecular hydrogen transfers. In contrast, a sterically driven decrease in E is predicted to result from spectator methyl groups for the prim --> prim reaction class for all x. There is no experimental basis to test these predictions; fragmentary computational evidence lends some support to the second but is ambiguous concerning the first. Possible deficiencies in the use of carbocycles as transition state models are discussed.

1,n‐Hydrogen‐Atom Transfer (HAT) Reactions in Which n≠5: An Updated Inventory

“…[8] Radical translocation implying 1,2-or 1,3-hydrogen-atom migration have highly strained distorted transition structures, and relevant examples are rather scarce. [9,12] They have little synthetic value; they are mainly relevant to biology and pyrolysis. A few examples of 1,4-HAT of synthetic value are known, but as discussed below, they display very special features.…”

Detailed mechanistic modeling of high-density polyethylene pyrolysis: Low molecular weight product evolution

“…c Value adapted from experimental value for polymerization [34]. d Values adapted from estimations of Poutsma [7,41] and Faravelli and coworkers [15]. e Values assigned based on the trends seen by Pfaendtner et al [33].…”

Mechanistic analysis and thermochemical kinetic simulation of the products from pyrolysis of poly(α-methylstyrene), especially the unrecognized role of phenyl shift

“…Similarly a 105-fold decrease in k 0 15 (p, p), if apportioned equally between E and A, would lead to 10 9.1 exp(À13,900/RT) s À1 , compared with the initially assigned value of 10 10.1 exp(À11,500/RT). In this case, there is even less basis for a firm assignment [46] but the change again seems large. Least palatable is the requirement that these changes be specifically coupled such that the k b (p, p)/ k 15 (p, p) ratio would be increased by !10 4 in going from PIB to PaMS.…”

“…These estimates will likely overestimate transfer to the extent that (1) transfer reactions are decelerated by steric effects as in PIB [2b], (2) a exceeds 0.5 for these highly endothermic reactions, and (3) the internal strain in PaMS stabilizes the reactant R tb compared with the products R s and R 0 p (see estimates above of isodesmic reactions with unstrained radicals). By the same protocol and with the same likelihood of overestimation, especially if 1,xshifts are sterically decelerated for tert radicals [46], we adjusted the values assigned for the nearly thermoneutral 1,x-shifts in PE [2a] to obtain the estimates shown in Table 2 for k 0 14 and k 0 16 . The rate constants for the reversals were estimated by adjusting E based on the same generic differences in radical stability.…”