MECHANISM OF OXIDATIONS BY CHLORINE

Abstract

The present investigation on the •Mechanism of Oxidations by Chlorine" has been undertaken under the guidance 9tt<i supervision of Dr. S.K.Mukherjee, M.5c.,Ph.D. He pointed out that although chlorine is frequently used for the disinfection of drinking water »nt^ waste waters, the detailed mode of its action as oxidizing agent has not been investigated. The Sanitary Chemists mS'vmced »w© views to account for the action of chlorine upon bacteria. According to the first view, chlorine on hydrolysis yields HOC1 and HC1, and H0C1 oxidizes some of the essential constituents of the bacterial cell. The second view Is that the destruction of living organisms is brought about by direct chemical combination of chlorine with the protoplasm of the microorganisms. More data thus seems to be necessary in order to arrive at definite conclusions. Dr. Mukherjee suggested that a systematic kinetic study of the reactions of chlorine with simple organic molecules would throw light on the mechanism and with this view a general research scheme was prepared. Chlorine is a powerful oxidizing agent. Its reversible electrode potential for the two electron reduction to Cl" is 1.3583 Volts. It is known to hydrolyse In water to yield HOC1.H and CI . Beside these, CI and Cl3 ions have also been detected under different conditions. From a general chemical point of view, it Is of prime Importance (a) to Investigate the causes that lead to the formation of different (11) types of products by action of chlorine, (b) to find out if the oxidation involves the transfer of an oxygen atom from the oxidant molecule or it Is a simple case of dehydrogenatlon of the organic compound, (c) to identify which of the species of chlorine in water, Cl2, HOCl.Ci*, Clg, or CI", Is the principal effective ©xidant, (d) to know if the reaction Is of electron-transfer type. If so, whether the reaction takes place by one electron or two electron step, A complete elucidation of the above research project requires many sided Investigations. To start with at the first instance, the kinetic study of the reactions of chlorine with monohydrlc alcohols and formaldehyde was alloted to the author by Or.Mukhtrjee with a view to elucidate the mechanisms of these reactions* A critical examination of the attempts made by various Investigators to postulate the reaction mechanisms with chlorine shows that with aromatic compounds, generally the chlorination of the organic molecule, and with aliphatic compounds, complicated reactions. Involving both oxidation and chlorination, take place. For example, (A) Rodd pointed out that chlorine attacks aqueous methyl alcohol quite easily. The dichloro di-methyl ether, (CI CH2)20, is first produced as an lntermedlate,whlch Is (A) E.H.Rodd," Chemistry of Carbon Compounds,* Elsever Publ, Co., Amsterdam (1961), Vol.1, Part A, p.298, 305. (ill) cortverted by water to formaldehyde and hydrochloric acid. Similarly, ethyl alcohol gets oxidized to acetaldehyde.whlch forms acetal with excess of alcohol. Further chlorination leads to the formation of mono and dlchloroacetal, finally to chloral alcoholate. These complications further aggravate the kinetic Investigations. The following Is the brief summary of the present investigation reported in this thesis i The kinetics of reactions of 1. Methyl Alcohol CHgOH 2. Fthyl Alcohol G^OH 3. IsoPropyl Alcohol CRj *>y CHOH 4. Sec.Butyl Alcohol CH^ and 5. Formaldehyde HCHO with chlorine In aqueous solution has been studied in detail. MLtil all the above compounds the following investigations have been made t (1) Th<* orders of the reactions with respect to each reactant at various sets of concentrations in absence and in presence of perchloric acid have been determined. This we have done by considering initial rates. (2) The influence of ionic strength on the reaction rates has been studied. ^> CHOH (iv) (3) The catalytic effects of about two dozen metallic salts on the reaction rates have been investigated. (4) The rates of the reactions have been measured at various temperatures in the range of 25° to 55°C and the various activation parameters have been calculated, (5) The products of the reactions have been Identified and the alkyl hypochlorites have been isolated. (6) The influence of sodium chloride on the reaction rates has been investigated in detail. The reactions were found to be very complex. Besides oxidation, chlorination of the methyl group also follows through consecutive reactions. Vie established that molecular chlorine is the principal effective oxidant and hypochlorous acid plays a minor role in all the cases. Consistent with the observed facts the mechanism of reactions of monohydric alcohols and formaldehyde with chlorine in aqueous solution have been proposed. The initial rates of the reactions of seven aliphatic alcohols (Methyl, rthyl, n-Propyl, Iso-Propyl, n-Butyl , IsoButyl and Sec.Butyl Alcohols) with chlorine in aqueous solution have been measured at 25° and 35°C with a view to find out the effect of structure of organic molecule on the reaction rate. The relative thermodynamic activation parameters, enthalpy, free energy, and entropy of activation, have been calculated and en attempt has been made to interpret the data in a proper way. The orders of (v) the reactions of chlorine in aqueous solution with n-Propyl» n-Butyl and IsoButyl Alcohols with respect to each reactant In absence and In presence of perchloric acid have also been determined.