Thermolysis of tert-Butyl Phenylperacetates:  Delicate Control of the Rates through Contributions from Translational and Rotational Entropy

The first-order rate constants (k Y) at several temperatures in CDCl3 were measured for thermal decompositions 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 (k Y/k H) 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 - H and ΔΔS ⧧ Y - H) were obtained from the Eyring plot. Differential activation terms (ΔΔH ⧧ Y - H and ΔΔS ⧧ 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 occurs with 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 for TΔΔS ⧧ 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 translational and rotational entropies. Similar deviation behaviors of p-OCH3, p-OPh, and p-NO2 were again observed for the both plots. p-NO2 can strongly destabilize the cationic site of the polar TS but serves an eminent spin delocalizer for the homolytic TS.