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3 Polish Journal of Chemistry

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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.

Carbon-13 Isotope Fractionation in the Decarboxylation of Phenypropiolic Acid in Orthophosphoric Acid and in Diphosphoric Acid

Zieliński M., Ogrinc N., Zielińska A., Koblyuk N., Papiernik-Zielińska H.

Polish Journal of Chemistry

2000

Vol. 74, nr 5

707-715

C-13 isotope effects in the decarboxylation of phenylpropiolic acid (PPA) in water solution of formic acid (FA)

Zieliński M., Zielińska A., Ogrinc N., Kobal I., Paul H., Bernasconi S., Papiernik-Zielińska H.

Polish Journal of Chemistry

1999

vol. 73 nr 5

813-820

Kinetics and the carbon-13 kinetic isotope effects in the decarboxylation of phenylpropiolic acid in HCOOH/H2O, 1:1/V:V, solution have been examined between 80-140.13oC. 13C KIE in the decarbonylation of formic acid, proceeding with measurable rate in this medium between 130-150oC, has determined also and compared with corresponding values found in the pure HCOOH/H2O solution. Kinetics and C-13 KIE in the decarboxylation of phenylpropiolic acid in pure water have been investigated subsequently between 100-143oC in all glass reaction vessels sealed under vacuum. The enthalpy of activation of decarboxylation of PPA in pure water, 30.20 kcal/mol, and the entropy of activation, DS* = -3.7 e.u., are by 6.7 kcal/mol and 12.5 e.u., respectively, higher than the DH* and DS* values found in the decarboxylation of PPA in pure formic acid. The C-13 KIE equal to 1.004-1.005 between 70-100oC in the pure HCOOH medium increased to C-13 KIE of 1.020 in the case of decarboxylation of PPA at 133.7oC in the initially pure water.