
%0 Journal Article
%A Blackman, AG 
%A Buckingham, DA 
%A Clark, CR 
%T Reactions of Coordinated Ligands. The Bromination of 2,4-dimethyl-3H-imidazole and 3H-imidazole Coordinated (at N1) to Pentaamminecobalt(III)
%J Australian Journal of Chemistry
%D 1991/07/01
%V 44
%N 7
%P 981-994
%K 
%R https://doi.org/10.1071/CH9910981
%U https://www.publish.csiro.au/paper/CH9910981 
%X The pH dependence of the bromination of R-2,4-Me2ImH3+ [R=(NH3)5Co] by Br2 in acidic aqueous solution, giving R-2,4-Me2-5-BrImH3+, suggests that proton abstraction from a bromine-substituted Wheland intermediate is rate determining for pH < 3 (both OH-- catalysed and spontaneous paths are observed), while in the pH range 3-5, bromine addition is rate determining. For pH > 5 the reaction may be interpreted to occur through rate-determining Br2 addition to the conjugate base from of the reactant. Rate constants for bromine addition to R-2,4-Me2ImH3+ and R-2,4-Me2Im2+ are respectively 1.1&#215;104 and 3.4&#215;1010 dm3 mol-1 s-1 at 25.0&#176;C, I = 1.0 (NaClO4). Bromination of RimH3+, to give R-4-BrImH3+ initially, appears to occur largely via the conjugate base form of the reactant (k2 = 3.6&#215;109 dm3 mol-1 s-1), but for Ph < 2, addition of Br2 to RImH3+ contributes to the observed rate (k1 = 0.68 dm3 mol-1 s-1). Ea = 61±2 kJ mol-1 for the former process, and the primary kinetic isotope effect, kH/kD, increases from 1.3 in 0.05 M H+ to 2.4 in 5.42 M HClO4. These observations are discussed in terms of diffusion-controlled, or preassociation -type mechanisms. 
