Kinetics of radical-initiated chain bromination of 2-methyl-2-propanol by N-bromosuccinimide in water

Article abstract of DOI:10.1021/ja00062a014

The kinetics and product distribution of the radical chain bromination of 2-methyl-2-propanol (RH) by N-bromosuccinimide (SBr) in water have been studied by pulse-radiolytic and γ-radiolytic methods in order to obtain key data for the reactivity of the succinimidyl radical, S^?. The products of the reaction after γ-radiolysis are dependent on the concentration of RH, being essentially made up of equal amounts of (CH₃)₂C(OH)CH₂Br and succinimide at [RH] > 0.5 M, while the hydrolyzed ring-opened product, 3-bromopropionamide, increases at the expense of (CH₃)₂C(OH)CH₂Br when the RH concentration is lowered. The chain-propagating steps R^? + SBr → RBr + S^? (i) and S^? + RH → SH + R^? (ii) of the S^? mediated chain reaction had rate constants k_i = (1.8 ± 0.3) × 10⁸ (determined in three ways) and k_ii = (1 ± 0.5) × 10⁷ M^-1 s^-1 (determined in four ways), respectively, whereas the corresponding rate constants for the Br^? mediated chain under the same conditions were 2.5 × 10⁹ and (5 ± 3) × 10³ M^-1 s^-1. Under pulsed conditions with initial radical concentrations around 10^-4 M, the chain length is ≈10. The ratio between k_ii and the rate constant for ring opening of S^?, k_iii (S^? → ^?CH₂CH₂CONCO), was determined to be 77 M^-1, significantly higher than the corresponding value with neopentane as substrate and with dichloromethane as solvent (0.10 M^-1). The k_ii value places S^? as having significantly lower reactivity than OH^? in hydrogen atom abstraction reactions, whereas a theoretical estimate based on electron affinities puts it in between Cl^? and OH^?, i.e. at higher reactivity than OH^?. This provides kinetic evidence for the II nature of ground state S^?. It was also established that Br^? complexes rather strongly to SBr according to SBr + Br^? → SBr₂^? (iv). As k_iv = (4-7) × 10³ M^-1 and SBr₂^? is probably unreactive, the propagation rate of a bromine atom chain is expected to decrease with increasing SBr concentration.