Decomposition rates of some trans-2-nitroso-1-phenylethane dimers in solution

Abstract

The rate of decomposition of some trans-α-alkyl-substituted 2-nitroso-1-phenylethane dimers, including one deuterium-substituted analogue, has been measured spectrophotometrically in the temperature range 15–60 °C in ethanol, ethanol–water, cyclohexane, carbon tetrachloride, and chloroform. From studies in ethanol the rate constant of monomer formation, k₁, the Arrhenius parameters, and the heat and entropy of activation were evaluated. The observed rate constant, k_obs, evaluated from a first-order plot of the decomposition of the secondary C-nitroso dimers decreased with increasing initial concentrations and with increasing size of the α-alkyl substitutent. C-Nitroso dimers with one α-hydrogen decomposed up to 1 200 times faster in ethanol than in non-polar solvents. The decomposition rate of one dimer without α-hydrogens was comparatively independent of the solvent used. However, the addition of water to the ethanol solutions had a stabilizing effect on both types of dimers. The isotope effect, k_H/k_D, was shown to be 2–5 on k₁, and 3–5 on k_obs for the dimer of 2-nitroso-1-phenylpropane.