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 orde r to obtain key data for the reactivity of the succinimidyl radical, S .. The products of the reaction after gamma-radiolysis are dependent o n 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 hydroly zed ring-opened product, 3-bromopropionamide, increases at the expense of (CH3)2C(OH)CH2Br when the RH concentration is lowered. The chain-p ropagating 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 corres ponding rate constants for the Br. mediated chain under the same condi tions were 2.5 x 10(9) and (5 +/- 3) x 10(3) M-1 s-1. Under pulsed con ditions with initial radical concentrations around 10(-4) M, the chain length is almost-equal-to 10. The ratio between k(ii) and the rate co nstant for ring opening of S., k(ii) (S. --> .CH2CH2CONCO), was determ ined to be 77 M-1, significantly higher than the corresponding value w ith neopentane as substrate and with dichloromethane as solvent (0.10 M-1). The k(ii) value places S. as having significantly lower reactivi ty than OH. in hydrogen atom abstraction reactions, whereas a theoreti cal estimate based on electron affinities puts it in between Cl. and O H., 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. (i v). 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.