CARBON ISOTOPIC FRACTIONATION IN THE SCHMIDT DECARBOXYLATION - EVIDENCE FOR 2 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. © 1979, American Chemical Society. All rights reserved.