RAPID KINETICS OF BROMINATION OF NITROPHENOLS IN AQUEOUS SOLUTION BY THE USE OF RPE J. B.BHORE , V.T. DANGAT* ,S.L.BONDE and V.T.BORKAR * Corresponding author : vijaydangat@gmail.com Nowrosjee Wadia College, Pune, affiliated to the University of Pune ,Maharashtra,India Abstract The kinetic study of the bromination of ortho, meta and para nitrophenols in aqueous solution at 7 pH by molecular bromine at 26.0 o C has been carried out. The rapidity of the reactions necessitated a special technique to follow the kinetics wherein the decaying concentration of bromine was followed by measuring the diffusion limited current at a platinum micro cathode rotated at 600 rpm versus an anode the SCE-saturated calomel electrode. The specific reaction rates determined from this study are found to be 7294, 6147 and 9286 M -1 s -1 respectively for ortho, meta and para- nitrophenol. These values have been invoked to justify the predicted relative reactivities of nitrophenols in aqueous solution in electrophilic aromatic substitution reactions. Key words: Bromination, nitrophenols, micro-electrode, structure-reactivity correlation INTRODUCTION Brominations of aromatic substrates aqueous solutions are known to be the most rapid amongst halogenations [1].We have studied the rapid kinetics of bromination of the ortho, meta and para nitrophenol regio-isomers in aqueous solution using molecular bromine at 7 pH. These kinetic data are invoked to justify the reactivities of nitrophenols in a quantitative manner. The rapidity of these reactions necessitate a special technique to follow the kinetics. These reactions being second order, their half-lives are extendable by diluting the solutions whereby convenient kinetic measurements are feasible [2]. The rotating platinum cathode, RPC was used as a sensor to monitor the decaying low concentrations of bromine in these reactions [3]. The saturated calomel electrode, SCE was the anode and the diffusion current generated by bromine at the cathode is a measure of the decaying halogen concentration during the course of the reactions. EXPERIMENTAL Chemicals : AR grade samples of the nitrophenol isomers, potassium nitrate and the required stock solutions of bromine in conductivity water were used. Bromine was standardized using iodometry. Electrodes: The Rotating Platinum Electrode fused to a glass tube was rotated at 600 rpm with aid of a synchronous ac motor. Saturated calomel electrode was used as the cathode. DIFFUSION CURRENT MEASUREMENT: A ] Calibration: The RPC and SCE were dipped in solutions of different concentrations of aqueous bromine solutions along with the phosphate buffer components for maintaining 7 pH and potassium nitrate which served as the supporting electrolyte. The diffusion currents recorded were proportional to the concentrations of bromine solutions. The plot of diffusion current against concentration of bromine obtained was linear. Kinetic Measurements: Equal volumes of identical concentrations of bromine and nitrophenol containing the supporting electrolyte and buffer components were mixed in the reaction vessel in which the RPE and SCE were dipped at constant temperature.A stop-clock was started simultaneously and at every 10 seconds, the diffusion current in terms of the galvanometer light spot ws recorded until the completion of the first half- life of the reaction. A plot of concentration of bromine Vs time was found to be linear .The slope of this plot is the specific reaction rate. j. b.bhore , v.t. dangat*,s.l.bonde and v.t.borkar, Int. J. Chem. Res., 2013v02i3 (01-06) ISSN: 2249-0329 IJCSR | MAY - JUNE 2013 Available online@www.ijcsr.co.in 1