Carbon-13 kinetic isotope effects in the decarbonylation of lactic acid of natural isotopic composition in phosphoric acid medium |
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Authors: | M Zieliński G Czarnota H Papiernik-Zielińska G Kasprzyk L Gumółka W Städter |
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Institution: | (1) Isotope Laboratory (Laboratory of Nuclear Chemistry), Faculty of Chemistry, Jagiellonian University, Kraków, (Poland);(2) Present address: PETER FRITZ at UFZ-Leipzig, (Germany) |
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Abstract: | The13C kinetic isotope effect fractionation in the decarbonylation of lactic acid (LA) of natural isotopic composition by concentrated phosphpric acids (PA) and by 85% H3PO4 has been studied in the temperature interval of 60–150°C. The values of the13C(1) isotope effects in the decarbonylation of lactic acid in 100% H3PO4, in pyrophosphoric acid and in more concentrated phosphoric acids are intermediate between the values calculated assuming that the C(1)–OH bond is broken in the rate-controllin gstep of dehydration and those calculated for rupture of the carbon-carbon bond in the transition state. In the temperature interval of 90–130°C the experimental13C fractionation factors determined in concentrated PA approach quite closely the13C fractionation corresponding to C(2)–C(1) bond scission. the13C(1) kinetic isotope effects in the decarbonylation of LA in 85% orthophosphoric acid in the temperature range of 110–150°C coincide with the13C isotope effects calculated assuming that the frequency corresponding to the C(1)–OH vibration is lost in the transition state of decarbonylation. A change of the mechanism of decarbonylation of LA in going from concentrated PA medium to 85% H3PO4 has been suggested. A possible secondary18O and a primary18O kinetic isotope effect in decarbonylation of lactic acid in phosphoric acids media have been discussed, too. |
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