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1 2003

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Solvent Deuterium Isotope Effect in the Acid Catalyzed Decarboxylation of Phenylpropiolic Acid in 85 % D3PO4 in D2O

Zieliński M., Zielińska A., Papiernik-Zielińska H.

Polish Journal of Chemistry

2003

Vol. 77 / nr 2

211-219

A comparative study of the solvent deuterium isotope effect in the decarboxylation of phenylpropiolic acid (PPA) in 85% perdeuteriated orthophosphoric acid, D3PO4/D2O, has been carried out and the ratio of the decarboxylation rate constants, k(H2O)/k(D2O) equal 1.88š0.07, interpreted as caused by different dissociation constants, Kh/Kd, of normal and perdeuteriated phosphoric acids in H2O and in D2O respectively, and by the deuterium discriminations in the subsequent steps of hydrogen transfer from the solvent H3O+/D3O+ to triple bond of PPAand finally by proton transfer from _-complex to nearest C_-carbon and sp2(C_-H) covalent bond formation. The detachment of carbon dioxide from PPA(decarboxylation step) is the fast process taking place or directly after the rate determining T.S. formation or stepwise by hydration of the vinyl cation and formation of the benzoylacetic acid intermediate compound, which then decarboxylates in the kinetically insignificant step. The discussion is supplemented by calculating the k(H2O)/k(D2O) ratios for decarboxylation scheme involving protonation of the triple bond in the rate determining step and for the reaction scheme involving fast isotope equilibrium between protonated water and protonated triple bond, followed by the rate determining transfer of proton from _-complex to the usual sp2 (C-H) covalent bond. Agood agreement between experimental solvent D2O I.E. and values calculated for reaction scheme involving proton transfer in the r.d.s. (what follows also from the 13CK.I.E. determinations in decarboxylation of PPAinH3PO4/H2OandD3PO4/D2Osolvents) was found.