Rate-limiting formation of diazonium ions in the aqueous decomposition of primary alkanediazoates

Article abstract of DOI:10.1021/ja00094a016

Rate constants, k₀, for the buffer-independent decomposition of four primary (E)-alkanediazoates and one primary (Z)-alkanediazoate in aqueous media at 25 °C, ionic strength 1 M (NaClO₄) are reported. Between pH 4 and 12 the plot of log k₀ against pH is biphasic with a hydrogen ion-dependent reaction at the high-pH end changing to a pH-independent region at lower pH. The change in absorbance at 235 nm of the relatively stable (E)-trifluoroethanediazoate as a function of pH gives a good fit to a simple titration curve for a monofunctional acid with pK_a = 7.00 ± 0.04 that is in good agreement with the kinetic pK_a determined from the pH-rate profile between pH 4 and 12. The plot of log k₁, the pH-independent rate constant, against σ* for the primary (E)-alkanediazoates, and (E)-methane- and (E)-2-butanediazoates previously reported, yields a common line with a slope ρ* = -4.4. Of the 1-butanol formed from the decomposition of (E)-1-butanediazoate in D₂O at pH_obs = 10.50,6% contains deuterium, while, of the 1-butanol formed in the presence of 1 M NaOD in D₂O, 25% contains deuterium. A total of 96% of the iodotrifluoroethane formed upon decomposition of (E)-trifluoroethanediazoate in a D₂O solution that is 1 M in NaI contains at least a single deuterium atom. These data combined with small negative values of ΔS^# normal solvent deuterium isotope effects, and the decreases in k₁ of between 500- and 1000-fold on change of solvent to ethanol are consistent with a mechanism that involves the rate-limiting unassisted heterolytic bond fission of the diazoic acid to yield the diazonium ion. The decomposition of [¹⁶O]-(E)-1-butanediazoate in water containing 47 ± 2% ¹⁸O yields 1-butanol of which 49 ± 2% contains ¹⁸O. This observation, in combination with the observed pH-dependent deuterium incorporation into 1 -butanol during decomposition of (E)-1-butanediazoate, above, indicates that the 1-butanediazonium ion is a diffusionally equilibrated intermediate. The rate constant for the pH-independent decomposition of the (Z)-trifluoroethanediazoate is 2600 times greater than that for the corresponding (E) isomer. The decomposition of the (Z)-trifluoroethanediazoate, but not its (E) isomer, is catalyzed by buffer acids, and the catalysis by carboxylic acids is characterized by a Bronsted plot with a slope a = 0.41. Catalysis by methoxylammonium ion is comparatively weak, the rate constant falling a log unit below the aforementioned Bronsted line. The solvent deuterium isotope effect for formic acid catalysis is k_HA/k_DA = 3.1 ± 0.2. It is concluded that the mechanism for general acid catalysis of the decomposition of the (Z)-diazoic acid entails proton transfer to oxygen that is concerted with N-O bond heterolysis to yield the diazonium ion.