Carbon Isotopic Fractionation in the Schmidt Decarboxylation: Evidence for Two Pathways to Products

The evolution of CO₂ at 5 ± 2 °C from the H₂SO₄-catalyzed reaction of n-octanoic acid with HN₃ in CHC1₃ (the Schmidt decarboxylation) takes place in two kinetic phases with different carbon kinetic isotope effects. Experiments were carried out under high-efficiency stirring conditions at HN₃ concentrations ranging from 1.47 to 0.24 M. At an initial HN₃ concentration of 1.47 M, the pseudo-first-order rate constant for the first kinetic phase was 0.07 ± 0.03 h^-1, with ^12k/l3k = 1.0589 ± 0.0003. After a lag time of 48 min the rate of CO₂ evolution changed, with the pseudo-first-order rate constant (1.47 M HN₃) increasing to 0.45 ± 0.02 h^-1; ^12k/l3k ≈ 1.03. Results obtained at lower HN3 concentrations showed that the first, slower reaction phase displayed overall first-order kinetics, its rate being independent of HN₃ concentration. The first reaction phase could be suppressed by addition of KHSO₄ to the HM₂SO₄. Preequilibration of the HN₃/CHC1₃ with H₂SO₄ markedly affected the isotopic composition of the initial CO₂ product, apparently effecting the initial transient utilization of the second-phase reaction pathway through the action of the H2SO₄ on the HN₃. Although the kinetic phases could not be resolved at 22 °C, the isotopic composition of the evolved CO₂ indicated that both pathways to the product were being utilized.