KINETICS OF RADICAL-INITIATED CHAIN BROMINATION OF 2-METHYL-2-PROPANOL BY N-BROMOSUCCINIMIDE IN WATER

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 gamma-radiolytic methods in order to obtain key data for the reactivity of the succinimidyl radical, S.. The products of the reaction after gamma-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) x 10(8) (determined in three ways) and k(ii) = (1 +/- 0.5) x 10(7) 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 x 10(9) and (5 +/- 3) x 10(3) M-1 s-1. Under pulsed conditions with initial radical concentrations around 10(-4) M, the chain length is almost-equal-to 10. The ratio between k(ii) and the rate constant for ring opening of S., k(ii) (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 PI 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) x 10(3) M-1 and SBr2. is probably unreactive, the propagation rate of a bromine atom chain is expected to decrease with increasing SBr concentration.