Single‐parameter versus dual‐parameter correlation for radical reactions. Addition of bromine atoms to α‐methylstyrenes

A rigorous procedure was developed for measuring the relative rates of addition of bromine atoms to eleven substituted α‐methylstyrenes (1‐Y, with Y = CF 3 , NO 2 , F, CN, Cl, Br, CO 2 Me, Me, COMe, OMe and SMe). The reaction was run in tetrahydrofuran in the presence of HBr, O 2 and dibutyl peroxyoxalate at 30°C. All products were derived from the YC 6 H 4 CMeCH 2 Br adduct radicals, which were immediately intercepted by O 2 . Correlation analysis of all the data confirmed the proposition that in the absence of measurable steric effects, the relative rates for radical additions can be correlated only by a dual‐parameter equation and not by a single‐parameter equation. Among various combinations of σ* and σ x , the (σ JJ * + σ mb ) combination yields the best correlation.