| Abstract
| - The first-order rate constants (kY) at several temperatures in CDCl3 were measured for thermaldecompositions of YC6H4CH2CO3C(CH3)3 with Y being p-OCH3, p-OPh, p-CH3, p-Ph, p-H, p-Cl, m-Cl,and p-NO2. The relative rates (kY/kH) exhibit excellent ρ+/σ+ Hammett correlations with ρ+< 0,indicating a polar TS. Activation parameters (ΔH⧧Y and ΔS⧧Y) and their differential terms (ΔΔH⧧Y-Hand ΔΔS⧧Y-H) were obtained from the Eyring plot. Differential activation terms (ΔΔH⧧Y-H andΔΔS⧧Y-H) disclose an isokinetic relation with p-CH3, p-Ph, p-H, p-Cl, and m-Cl (isokinetic temp,230 K). However, p-OCH3, and p-OPh show negative deviations, and a positive deviation occurswith p-NO2. Plot of ΔΔH⧧Y-H vs σ+ exhibits a good linear relation (r = 0.95) with a slope (α1 =−3.34). A better linear correlation (r = 0.97) and steeper slope (α2 = −5.22) were observed forTΔΔS⧧Y-H vs σ+. Negatively larger slope (α2 = −5.22) may point to entropy control of rates.Differential activation parameters (ΔΔH⧧Y-H and ΔΔS⧧Y-H) reflect variations of activation process.Differential activation entropies (ΔΔS⧧Y-H) are discussed in terms of contributions of translationaland rotational entropies. Similar deviation behaviors of p-OCH3, p-OPh, and p-NO2 were againobserved for the both plots. p-NO2 can strongly destabilize the cationic site of the polar TS butserves an eminent spin delocalizer for the homolytic TS.
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