Stability of PhCH2CH2ĊHX (X = H, CF3, Me, CO2Me) Radicals

Using EPR spectroscopy, β-fragmentation of the radicals PhCH₂CH₂ĊHX (R¹, X = H; R², X = CF₃; R³, X = Me; R⁴ X = CO₂Me) accompanied by C—C bond cleavage has been found and studied, and the activation energy of this reaction has been calculated, thus permitting us to determine the rate constants for fragmentation of R¹ (k_3a = 3.9( ± 0.2) × 10⁴ s⁻¹, R² (k_3b = 6.5( ± 0.4) × 10³ s⁻¹ and to evaluate the value of this rate constants for radical R³ and R⁴ (k₃ = 10² s⁻¹) at 22 °C.