Salt effects and correlations with the Hammett acidity function in the decompostion of trioxane by aqueous hydrochloric acid

The acidity function, H_o, of Hammett and Deyrup for a neutral basic indicator has been shown to correlate with the specific reaction rate constant, k, for a number of acid-catalyzed reactions with an A-1 mechanism. Correlations of this type have been reported for many of the strong mineral acids and certain weak acids over a wide temperature range and in aqueous, nonaqueous, or mixed solvent systems. The acid-catalyzed depolymerization of trioxane is one specific reaction which has shown relationships between rate constants and H_o in aqueous media and has been studied in the present investigation for the purpose of providing a suitable background for a study of acidity in non-aqueous media. The approach was to study the rate of depolymerization in solutions of hydrochloric acid at several temperatures and in mixtures of hydrochloric acid and various chloride salts at one particular temperature.

The reaction mixtures of these solutions were prepared by mixing and diluting stock solutions of hydrochloric and trioxane (with weighed amounts of salt for salt mixtures) with water. After removal of the dissolved air, the dilatometric technique was used to determine the rate of reaction by the Guggenheim method. A set of readings, r_t, taken at times, t, and a set of readings, r_t+G, taken after allowing a fixed amount of time, G, to elapse after the first set, are related by the equation ln (r_t+G - r_{t</sub = - kt + constant and the slope of a plot of ln (rt+G - rt) against t gave the specific reaction rate constant, k, when multiplied by 2.303.}

The results obtained in this investigation ruay be summarized by the following statements:

1. The specific reaction rate constants (sec.⁻¹) for the decomposition of trioxane in aqueous solutions of hydrochloric acid over the temperature range of 25-60° C. can be summarized by the equation k = 3.6 x 10¹³ (h_o)^1.17_e - 28,600/RT where h_o is the value of the acidity function for hydrochloric acid.

2. The addition of a chloride salt to a reaction mixture results in an increase in the rate of reaction by an amount which is roughly in inverse proportion to the size of the cation of the salt. More exactly, the specific reaction rate constants (min⁻¹) for the decomposition of trioxane in aqueous solutions of hydrochloric acid and lithium chloride at 40° C. can be summarized by the equation log k = -1.16 H_o + 0.326 C_S 4.603 where H_o refers to the measured value of the acidity function for hydrochloric acid and C_S refers to the LiCl concentration.