Spectrophotoinetric studies of the spontaneous and magnesium-ion-catalyzed decarboxylation of oxalacetic acid in Hz0 and D?O have been carried out over a range of pH, pD, and magnesium ion concentrations.The rate of decarboxylation of oxalacetic acid depends on the proposed equilibrium system between the acid anion and magnesium chelate under a variety of conditions.The absorbancy indexes and the apparent equilibrium constant of the keto and en01 forms of the magnesium chelate were estimated from a combination of the kinetic and spectral data. IKTRODUCTIONOxalacetic acid has been shown t o decompose to COz and pyruvic acid spontaneously (1, 2, 3), and catalytically by the action of polyvalent cations (1, 4, 5) and enzymes (6). The catalysis by metal ions has served as a model for the enzymatic reaction (7,8). The kinetics of the spontaneous decarboxylation of oxalacetic acid in aqueous solution have been studied over a range of temperature and a t varying degrees of dissociation (9,lO). Steinberger and Westheimer (8) have proposed that the keto form of the acid decarboxylates by using the a,a-dimethyl substituted oxalacetic acid, in which only the keto form can exist. In this paper the decomposition was followed spectrophotometrically as a function of pH and pD. The results indicate that the keto form of the monoanion of oxalacetic acid is the predominant species leading t o decarboxylation.The decarboxylation of oxalacetic acid is also catalyzed by a variety of multivalent cations (11)(12)(13)(14). Magnesium ion was found to have a low catalytic activity compared t o other divalent metals, but still it involves a change of spectrum indicating the formation of a chelate. The results of the present study show a direct correlation between the rate of decarboxylation of oxalacetic acid and the nlagnesium ion concentration as a function of pH and pD. The range of optimal activity (pH 5 to 6) for the magnesium-ion-catalyzed reaction coincides with the range found for the enzj-rnatic decarboxylation (11,15,16). E X P E R I M E N T A LThe following materials were commercial preparations: oxalacetic acid (OAA) (Sigma Chemical Co.), MgC12.6H20 reagent grade, Tris(hydroxymethyl)ami~~omethane (Tris) was primary standard grade.The D2O (99.78 atom % excess D) was purchased from the Atomic Energy Commission of Canada Limited, Ottawa.Measurements of pH and p D of each reaction mixture were made a t the end of the experiment with a Beckman Model G pH meter standardized with pH 7 buffer. When DzO was used as a solvent the measurements were made with the same glass and calomel electrodes and the p D was calculated by adding 0.4 units to the observed meter reading (17). The buffer system used was a Tris-acetate mixture. T o adjust the pH, 1 M basic Tris and 1 Macetic acid were mixed in the proper proportions. For the DzO systems all the reagents were made u p in DzO. Absorption spectra were measured with a Bausch and Lomb Spectronic 505 spectrophotometer. Rates of reaction were followed either in the Beckman DU spectropho...
The utility of energy transfer processes for the stabilization of polymers will depend to a large extent on the lifetime of the excited states involved. In the present instance involving aliphatic carbonyl groups, the excited state lifetimes, t, are so short that excessive concentrations of the stabilizer molecules would be required to obtain an economically desirable effect. However, other polymers, particularly those containing phenyl groups, might be expected to have longer lifetimes and hence would be easier to stabilize by this mechanism. In view of this, the study of energy transfer processes in other polymers may well lead to important advances in the development of stabilizer systems.
The initial stage of the oxidation process can be accounted for by diffusion control of the oxidation rate of polymer to hydroperoxide. The kinetics of the initial oxidation reaction follows quantitatively a type of Bolland mechanism. Chain scission consists of a random process due to hydroperoxide decomposition. Energies of activation have been determined for all the reactions involved in the oxidative process. The kinetics and diffusion equations for the initial stage of the oxidation are well obeyed by the experimental results. Later stages of the reaction can only be described in a qualitative or at best semiquantitative way.
synopsisPoly-p-xylylene prepared by pyrolysis of di-p-xylylene has been degraded under vacuum and in the presence of oxygen as a function of temperature and oxygen pressure. The vacuum pyrolysis is mainly due to "abnormal" structures. Volatiles are initially produced quite slowly, but t>he reaction accelerates subsequently. Arrhenius equations were derived for various ranges of volatile formation. A mechanism has been formulated consisting of random chain scission followed by depropagation (dimers to pentamers); simultaneously another zip reaction produces hydrogen. The thermal, oxidative degradation has been studied above and below the softening point of the polymer as a function of oxygen pressure. A first-order reaction of volatile formation due to "abnormal" chain scission is followed by normal chain scission, which is also first order. The postulated mechanism leads initially to hydroperoxide formation. Arrhenius equations for volatile formation are different below and above the softening point. ' Oxygen consumption also follows a first-order reaction with an energy of activation of 31.5 kcal/mole.
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