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 = CF3, NO2, F, CN, Cl, Br, CO2Me, Me, COMe, OMe and SMe). The reaction was run in tetrahydrofuran in the presence of HBr, O2 and dibutyl peroxyoxalate at 30°C. All products were derived from the YC6H4CMeCH2Br adduct radicals, which were immediately intercepted by O2. 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 σx, the (σJJ* + σmb) combination yields the best correlation.