Transition Metal Chemistry, 19 (1994) 169 172 Kinetic studies succinimide Ahmed A. Abdel-Khalek Chemistry Department, Faculty of Science, Cairo University, Beni-Suef Branch, Beni-Suef Egypt Alaa E.-D. M. Abdel-Hady* and Mohamed F. EI-Shahat w Chemistry Department, Faculty of Science, Ain Shams University, Abbassia, Cairo, Egypt N-Bromosuccinimide oxidation of Cr m 169 of the oxidation of chromium(III) by N-bromo- Summary A kinetic study of the oxidation of chromium(III) by N-bromosuccinimide (NBS) in aqueous solutions and H20-MeOH solvent mixtures were performed. The kinetics in aqueous solutions obeyed the rate law: d[CrVl]/dt = {k4KhK2[NBS] [CrI"]T}/ [H+]{I + Kh/[H+]+(KI + KhKz/[H+][NBS]} where Kh, K1 and K2 are the hydrolysis constant of [Crm(HzO)6] 3 +, and pre-equilibrium formation constants for the protonated and deprotonated precursor complexes, respectively. An innersphere mechanism is proposed. An argument based on isokinetic correlations among acti- vation parameters for the oxidation of a series ofeobalt(II) and chromium(III) complexes including [Cr(HzO)6] 3+ is presented in support of a common mechanism for these reactions. Introduction Few kinetic studies on the oxidation of inorganic substrates by NBS have been reported (1-4). Recently, we studied the oxidation of [Con(EDTA)] (2-5) (EDTA = ethylenediamin- tetraacetate) and [CoII(HEDTA)]-~6) [HEDTA = N-(2- hydroxyethyl)-ethylene-diamine-N,N',N'-triacetate] by N- bromosuccinimide in aqueous solutions, and it was found that these reactions proceed via the initial formation of cobalt(III) products, consistent with an innersphere mechanism (5'6). Also, the oxidation of [Crm(EDTA)] - by NBS was investigated (7). It was assumed that electron transfer proceeds through an innersphere mechanism with the hydroxo ligand of the chromium(III) complex bridging the two reactants. NBS molecules may replace a coordinated H20 molecule prior to electron transfer. It was observed that in the oxidation of the closely related series of cobalt(II) and chromium(III) complexes of EDTA and HEDTA by NBS, only (s-7) a relatively small variation in the rate occurred under identical con- ditions. This may be due to parallel changes in the enthalpy, AH*, and the entropy, AS*, of activation. A linear relationship between AH* and AS*, leading to small variations in the free energy of activation, AG*, may be considered as strong support for a common mechanism for the series (8). In this study the kinetics of oxidation of [Cr(H20)6] 3+ by NBS were investigated in order to observe the effect of the absence of a chelating ligand on the mode of oxidation. * Abstracted from the Ph.D. Thesis (Ain Shams University) of A. E.-D. M. Abdel-Hady. Author to whom all correspondence should be directed. In order to test whether the solvent influences the rate and/or the mechanism of this reaction, we investigated its kinetics in various mixtures of H20 with MeOH. Experimental Materials All chemicals were of reagent grade (BDH) or AnalaR. A stock solution of Cr(NO3)3-6H20 was prepared by accurate weighing. Freshly prepared solutions of NBS were used. Buffer solutions were made up from NaEB407 and KHePO4 solutions of known concentrations. The ionic strength in the different buffered solutions was adjusted with NaNO3. Double distilled H20 was used in all kinetic experiments. Procedure A Shimadzu-UV-1500-02 spectrophotometer was used to follow the rate of the reaction. The chromium(III) and NBS solutions in the required buffers were thermo- stated for ca. 20 min, then mixed thoroughly and quickly transferred to an absorption cell. The formation of the chromium(VI) was followed in the 365-372nm range where the absorption of chromium(VI) is maximal at the pH of the reaction, which was measured using a Schott- Gerate CG728 D628 Hofheim a.Ts pH meter. Pseudo-first order conditions were maintained in all kinetic runs by using at least a 10-fold excess of [NBS] over [Crnl]. Results and discussion Kinetics in aqueous solutions The stoichiometry of the chromium(III) NBS reaction was characterised as 2.9 _+ 0.1 moles of NBS per mole of chromium(Ill), at least during the time of reaction, so that the reaction is stoichiometrically described by Equation 1: Cr m + 3NBS , Cr w + 3Br- + other products (1) [Cr m] represents total concentration of all chromium(III) species in solution. The reaction rate was measured at the start of the slow reaction at fixed NBS concentration, ionic strength, pH and temperature. Plots of ln(A~- At) versus time were linear up to >~80% of the reaction, where A~o and A t are the absorbances of Cr w at infinity and time t, respectively. The pseudo-first order rate constant, kous, was obtained from the slopes of the first-order plots. The constancy of kobs, over the chromium(III) concentration range (1.15-9.00) x 10-4moldm -3, shown in Table 1, is in 0340-4285 9 I994 Chapman & Hall