KINETICS AND MECHANISM OF OXIDATION OF MALIC ACID BY N-BROMONICOTINAMIDE (NBN) IN THE PRESENCE OF A MICELLAR SYSTEM

. The oxidation of malic acid by N-bromonicotinamide in the presence of micellar system is studied. First order kinetics with respect to NBN is observed. The kinetics results indicate that the first order kinetics in hydroxy acid at lower concentrations tends towards a zero order at its higher concentrations. Inverse fractional order in [H + ] and [nicotinamide] are noted throughout its tenfold variation. Variation of [Hg(OAc) 2 ] and ionic strength of the medium do not bring about any significant change in the rate of reaction. Rate of the reaction increases with a decrease in the percentage of acetic acid. Decrease in the rate constant was observed with the increase in [SDS]. The values of rate constants observed at four different temperatures were utilized to calculate the activation parameters. A suitable mechanism consistent with the experimental findings has been proposed.


INTRODUCTION
The kinetics of the oxidation of hydroxy acids has been studied with a number of oxidizing agents like potassium bromate, hexamethylenetetraminebromine, sodium Nchlorobenzenesulfonamide, N-bromoacetamide, ditelluratocuprate(III), 2,2-bipyridium chlorochromate, benzo-dipteridine etc. Although hydroxy acids have been utilized for a number of catalyzed reactions, nobody has examined the role of catalysts in NBN oxidation of hydroxy acids. Malic acid is a key intermediate in the major biochemical energy-producing cycle in cells, known as the Kreb's cycle, it takes place in the cells mitochondria in most living organisms. The body synthesizes malic acid during the process of converting carbohydrates to energy. Preliminary evidence suggests that individuals with the disease fibromyalgia (a disorder that involves fatigue and pain in the muscles) might have difficulty in creating or utilizing malic acid . Such a deficiency could interfere with normal muscle function. The presence of micelles can have marked effects on thermodynamic favorability and reaction kinetics as well as on many physical properties 1 . Organic reactions involving ionic, polar and neutral reactants in micellar solution are generally believed to occur in the stern layer of a micelle of an ionic surfactant. The catalysis and inhibition by ionic micelles is due to ionic micellar incorporation of both the reactants. Due to these facts a significant amount of systematic kinetic results have been reported on the effect of micelles on various organic reactions during past few decades.
A.K. Singh [1] studied the kinetics and mechanism of oxidation of some lactose by Nbromophthalimide. Chand Waqar [2] investigated the mechanism of Ru(III)-catalysed oxidation of glycollic and mandelic acids with N-bromosuccinimide in acidic media.
Ajaya Kumar Singh [4] followed the kinetic and mechanistic study on the oxidation of hydroxy acids by N-bromophthalimide in the presence of a micellar system. E.V. Sundaram [5] explained the oxidation of α-hydroxy acids with Quinolinium Dichromate .
Asim K Das [6] studied the micellar effect on the reaction of Chromium(VI) oxidation of some representative alpha-hydroxy acids in the presence and absence of 2,2'-bipyridyl in aqueous acid media.
A perusal of literature shows that the reactivity of N-bromonicotinamide (NBN) could be compared with other N-bromoimide such as N-bromosuccinimide (NBS) and N-bromosaccharin (NBSa). Since NBN is more stable than the latter, it is extremely stable in solid state when kept out of light and moisture. Its standard solution has excellent keeping qualities. There are several reports available in the literature on the oxidation of alpha-hydroxy acids by oxidants such as Nbromosuccinimide, N-bromoacetamide, potassium bromate, N-bromobenzenesuphonamide, and iodate [7]. However, the details of oxidation of malic acid by N-bromonicotinamide are yet unknown. This prompted the micellar effect on the kinetics of the oxidation of the malic acid by NBN in the acidic medium.

Materials
N-Bromonicotinamide (NBN) was prepared by the reported method [8]. The melting point of the sample was found to be 483 K. Solutions of NBN were prepared in 80% acetic acid and stored in a black-coated flask to prevent photochemical deterioration. The prepared solution was then standardized iodometrically against standard sodium thiosulphate using starch as indicator. Standard solutions of SDS (GR) and malic acid (Merck) were prepared using double distilled water. The standard solution of mercuric acetate (Merck) was acidified with 20% acetic acid. HClO 4 (A.R. grade) diluted with double distilled water was standardized via acid-base titration. All other standard solutions of NaClO 4 , KCl, KBr and nicotinamide were prepared using double distilled water. Double distilled water was distilled over KMnO 4 in an all glass (Pyrex) distillation set up. Distilled acetic acid was used throughout the experiment.

Kinetic Measurements
The solution of malic acid and oxidant were kept in black coated bottles separately. These solutions were kept in the thermostat to attain the thermostatic temperature. The appropriate quantity of oxidant was added to the substrate containing surfactant and other reagents and the reaction bottle was shaken well. The reaction was followed potentiometrically by setting up a cell made up of the reaction mixture into which the platinum electrode and reference electrode(SCE) were dipped. The e.m.f of the cell was measured periodically using a Equip-Tronics (EQ-DGD) potentiometer. The reactions were studied at constant temperature 35 • C. Different studies such as variation of malic acid, oxidant (NBN), perchloric acid, sodium perchlorate, nicotinamide, surfactant and temperature were carried out. The reaction was carried out under pseudo-first order condition ([malic acid] >>[NBN]). The pseudo-first order rate constants computed from the linear (r 2  0.9990) plots of log (E t E  ) against time. Duplicate kinetic runs showed that the rate constants were reproducible within 3%. The course of the reaction was studied for more than two half-lives.

PRODUCT ANALYSIS
The presence of 2-aldoethanoic acid as the main oxidation products was detected by the spot test [9] and the 2,4-dinitrophenylhydrazine method [10].

CMC DETERMINATION
Surfactants spontaneously aggregate above a certain concentration called critical micelle concentration (CMC) to form micelle, whose determination has considerable practical importance, normally to understand the self-organizing behavior of surfactants in exact ways. Micelles act as microreactors, which both speed or inhibit the rate of uni-and bimolecular reactions.  Table 1. The CMC value is lower than that given in the literature for aqueous solutions of SDS without added electrolyte, which was found to be approximately about 3.18×10 −3 mol dm −3 in reaction mixture for malic acid.  Successive addition of nicotinamide (as one of the oxidation products of NBN) to the reaction mixture showed a decreasing effect on the rate of oxidation of malic acid. Addition of NaClO 4 (to study the effect of ionic strength) in the reaction mixture showed an insignificant effect on the rate of oxidation. In order to find the effect of dielectric constant (polarity) of the medium on the rate, the oxidation of malic acid by NBN was studied in aqueous acetic acid mixtures of various compositions ( Table 2). The data clearly reveal that the rate of reaction increases with a decrease in the percentage of acetic acid, i.e., increasing dielectric constant or polarity of the medium leads to the inference that there is a charge development in the transition state involving a more polar activated complex than the reactants [12].  The effect of added salts on the rate of reaction was also explored because salts as additives, in micellar systems, acquire a special ability to induce structural changes which may, in turn, modify the substrate-surfactant interaction. In the present case, KCl has no effect whereas with the increasing concentration of KBr, rate of reaction increased.

Test for Free Radicals
To test for the presence of free radicals in the reaction, the reaction mixture containing acrylamide was kept for 24 h in an inert atmosphere. When the reaction mixture was diluted with methanol, the formation of a precipitate was not seen. This suggests that there is no possibility of formation of free radicals in the reaction.  Table 4). The inhibition effect is due to the fact that N-bromonicotinamide has N -Br bond which it binds to SDS micelles in Stern layer, while ionized malic acid, bearing negative charge is repelled by the head group of negatively charged SDS micelles.

Effect of temperature
Increase in temperature increases the rate of oxidation and plot of log k obs Vs reciprocal of temperature is linear. The oxidation of malic acid by NBN was studied at different temperatures in the presence and absence of SDS (308 to 323K) ( Table 5) and the activation parameters were evaluated (Table 6) . Activation parameters are believed to provide useful information regarding the environment in which chemical reactions take place.

Effect of Temperature on reaction rate in the presence of SDS
[SDS] = 0.004 mol dm -3

CONCLUSIONS
In the light of kinetic observations for the micellar effect on the kinetics of oxidation of malic acid by N-bromonicotinamide in the presence of perchloric acid, the following conclusions can be easily drawn: the reactive species of oxidant NBN is HOBr not NBN itself, the reaction rates are enhanced by increase in [malic acid] and temperature. Added nicotinamide retards the rate. 2-Aldoethanoic acid is the product of oxidation. Activation parameters were evaluated for both catalyzed and unanalyzed reactions. The critical micelle concentration value is much lower than that given in the literature for aqueous solutions of SDS without added electrolyte. The rate of oxidation slightly decreases with increasing concentration of SDS. The micellar effect can be correlated with the nature of the reducing substrates and the reactions conditions. These micellar effects are quite important to understand and to substantiate the proposed mechanistic pathways. This may widen the applicability of NBN as oxidant in organic synthesis.