Kinetics and mechanism of the oxidation of neutralized a-hy- droxy acids by tris(pyridine-2-carboxylato)manganese(III) Kalyan K. Sen Gupta*, Nandini Bhattacharjee and Biswajit Pal Department of Chemistry, Jadavpur University, Calcutta 700 032, India Saktiprosad Ghosh Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700 032, India Summary The kinetics of oxidation of the neutralized a-hydroxy acids: lactic, a-hydroxyisobutyric, mandelic, benzilic and atrolactic acids by tris(pyridine-2-carboxylato)manga- nese(III) have been studied. The reactions were carried out in a Na(pic)-picH [Na(pic)  sodium salt of pyr- idine-2-carboxylic acid and picH  pyridine-2-car- boxylic acid] buer medium in the 4.89±6.10 pH range. The oxidation rate was found to be independent of pH, and rate follows the order: benzilate > mandelate > atrolactate > lactate > a-hydroxy isobutyrate. The oxidation products are MeCHO, Me 2 CO, PhCHO, Ph 2 CO and PhCOMe for the respective reactions. A mechanism is proposed involving intermediate formation of hepta-coordinated Mn III complexes in a fast step. The complexes then decompose to give free radicals and Mn II in the rate determining step. The free radicals subse- quently react with another molecule of the Mn III species to give the respective carbonyl compounds in a fast step. Introduction Kinetic studies of the oxidation of neutralized a-hy- droxyacids by hexachloroiridate(IV) (1) to yield alde- hydes and ketones have been studied. Kinetic investigations have also been carried out on the oxida- tions of a-hydroxy acids in acid media by mangane- se(III) pyrophosphate (2) , bromate (3) , vanadium(V) (4) , cerium(IV) (5±9) and chromic acid (10,11) . However, there is no literature data to date concerning the oxidation of a-hydroxy acids by tris(pyridine-2-carboxylato) manganese(III) (12) . An attempt has therefore been made to study the kinetics and relative reactivities of neu- tralized a-hydroxy acids towards this oxidant. Since the complex is unstable in acidic and alkaline media, the reactions have been carried out in a Na(pic)-picH buer (pH range  4.22±6.45) which stabilizes the oxidant in aqueous medium. Experimental Reagents Lactic acid (Qualigens, India), a-hydroxyisobutyric acid (Fluka AG) mandelic acid (Lancaster), benzilic acid (E. Merck), atrolactic acid (Aldrich) and pyridine-2- carboxylic acid (Lancaster) were used without further puri®cation and their sodium salts were prepared by the addition of the requisite amount of NaOH. All solutions were made using doubly distilled H 2 O. The oxidation studies were carried out in Na(pic)-picH buer which was prepared by mixing pyridine-2-carboxylic acid (pK a  5.52) (13) with its sodium salt in calculated amounts. The oxidant, tris(pyridine-2-carboxylato)manganese (III), was prepared (14) by a slightly modi®ed procedure. MnCl 2 á 6H 2 O (2.5 g) was dissolved in H 2 O (25 cm 3 ). The solution was warmed and H 2 O 2 (25 cm 3 ) was ad- ded, followed by NaOH (1 g) dissolved in H 2 O (25 cm 3 ). The mixture was stirred vigorously for at least 30 min and the precipitated MnO 2 was removed by ®ltration through a sintered glass crucible. The precipitate was washed thoroughly with H 2 O and then warmed with an aqueous solution, containing an excess of pyridine-2- carboxylic acid, with vigorous stirring. The mixture was then re®ltered through a sintered glass crucible. The scarlet red ®ltrate deposited reddish brown crystals of tris(pyridine-2-carboxylato)manganese(III) on cooling (Found C, 49.95; H, 3.3; N, 9.4. [Mn III (C 5 H 4 NCO 2 ) 3 ] á H 2 O calcd.: C, 49.2; H, 3.2; N, 9.6%). Thermogravi- metric analysis of the solid product was carried out by heating a sample (10 mg) at a rate of 10° per min. The result indicates that loss of water of crystallization takes place in one step. From the weight loss calculation based on [Mn III (C 5 H 4 NCO 2 ) 3 ] á H 2 O we con®rm that the water of crystallization is lost in one step. The structure of the manganese(III) complex has been reported (12) to be a tetragonally distorted octahedron with the axial positions occupied by N-atoms and with the axial bonds elongated. The room temperature (ca. 25 °C) magnetic moment of the solid was found to be 4.92 BM which is little dierent from the reported value (15) (4.90 BM). The cyclic voltammogram of the manganese(III) com- plex exhibited an irreversible reductive response at 0.8 V and the corresponding oxidative response was observed at )0.47 V (DE p  330 mV, E 1/2  )0.63 V). The manganese(III) solution was estimated (16) as fol- lows. To a solution (25 cm 3 ) containing [Mn III (C 5 H 4 NCO 2 ) 3 ] á H 2 O a known excess of a standard Mohr salt solution was added. The excess of Mohr salt was determined by titration with standard K 2 Cr 2 O 7 solution using H 3 PO 4 and barium diphenylamine sul- phonate indicator. All inorganic materials used were of highest purities available. Instrumentation Absorbances were measured on a Systronics (India) spectrophotometer. C, H and N analysis were per- formed by the Microanalytical Laboratory using a Perkin-Elmer 240 C, H, N Analyser. Thermogravimetric studies were performed on a Shimadzu Corporation (Japan) TG 50 in normal atomospheric environment. Ó 1999 Kluwer Academic Publishers. Printed in the Netherlands. * Author to whom all correspondence should be directed. 268 Gupta et al. Transition Met. Chem., 24, 268±273 (1999)