Carbon Isotope Effects in the Decarboxylation of Oxaloacetic Acid

Wood, Alexander (1961) Carbon Isotope Effects in the Decarboxylation of Oxaloacetic Acid. PhD thesis, University of Glasgow.

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Kinetic isotope effects in the decarboxylation of oxaloacetic acid were studied at 25° in aqueous solution for the acid alone, and then in turn in the presence of the cations of the rare earth metals: yttrium, dysprosium and gadolinium, which act as catalysts through complex formation. Both acid and complexes decompose by known mechanisms with first order kinetics and at convenient rates. The three isotopes investigated were C12 , C13 and C14 , and the reactions were, R.C12OOH--k12→RH + C12O2; R.C13OOH---k13→RH + C13O2; R.C14OOH---k14→RH + C14O2. The purpose of the work was to show the presence of an Isotope effect (i. e. , to show that the reaction rates de- creased as the mass of the Isotopic carbon increased), to investigate the effect of the catalytic metal ions and obtain values of the Isotope effect for the various decarboxylating species, and to ascertain any possible effects due to the paramagnetic ions of dysprosium and gadolinium, in view of the known paramagnetism of C13 caused by its nuclear spin. Experimental results were also to be compared with theoret- ically calculated isotope effects. Ordinary carbon contains about 1% of C13, so ordinary oxaloacetic acid can be used to determine the first isotope effect, i.e. , the ratio k12/k13. This was done by analysing the purified effluent carbon dioxide from the reaction, at various fixed stages of completeness, by means of a mass spectrometer. The reaction involving the heavy isotope Is a little slower than the light one, so the carbon dioxide appears to have an abnormally low C13 content and from this k12/k13 can be calculated. The experiments were repeated using oxaloacetic acid enriched in oxaloacetic acid-1-C13 to about 4%, in order to facilitate the mass-spectrometric measurements. This enriched material was prepared from enriched barium carbonate via a Grignard reaction, giving enriched sodium acetate. The salt was converted into ethyl acetate and thence diethyloxaloacetate obtained by means of a Claisen condensation with diethyl oxalate. The enriched acid was obtained in a satisfactory state of purity by acid hydrolysis of the diethyl ester. Isotope effects similar to those already observed for ordinary oxaloacetic acid were found on repeating the experiments with the enriched material. The k12/k14 k ratios were obtained using oxaloacetic acid enriched in oxaloacetic acid-l-C14. The preparation was the same as before, but started from labelled sodium acetate. Since C14 is beta-radioactive, the samples of effluent carbon dioxide were analysed by counting measured volumes in a Geiger counter, using a simple standard technique, being derived in a manner similar to that used for k12/k13. The results of the work showed the presence of definite isotope effects. Under the reaction conditions the most important species in the catalysed reactions are the complexes shown between the metal ions and the dianion A2- of oxaloacetic acid. (n) indicates the paramagnetic ions. The theoretical values for k12/k13 and k12/k14 are 1.044 and 1. 083 respectively at 25. The similarity between the results in each row for the complexes is ascribed to the similar nature of the metal ions in the complexes and gives no indication of any paramagnetic effect. The difference between the kinetic isotope effect for the uncatalysed reaction and. the corresponding effect for the complexes is attributed to the inductive effect of the metal ions. It was observed that the theoretical approach gave a fairly satisfactory value for k12/k13, but a less satisfactory result for k12/k14.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Organic chemistry
Date of Award: 1961
Depositing User: Enlighten Team
Unique ID: glathesis:1961-79387
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 05 Mar 2020 10:28
Last Modified: 05 Mar 2020 10:28

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