Decomposition of Acetylsalicylic Acid and Its Derivatives in Solid State

Abstract

The decomposition of acetylsalicylic acid (I), acetyl-5-nitrosalicylic acid (II) and acetyl-5-chlorosalicylic acid (III) in solid state by water vapor were studied for an approach to elucidate the decomposition mechanisms of organic crystals by water vapor. All three materials showed the sigmoid-type decomposition curves and the order of the intensity of acceleration at early stage was II>III>I. From the decomposition study of pre-decomposed I and II, it was revealed that the accumulation of formed salicylic acids and/or acetic were not the cause of acceleration. Microscopic observation of advancement of reaction about II showed that the reaction starts partially on the crystals and then spread over all the crystals. It was concluded that the acceleration is attributed to the formation and growth of reaction nucleus as its wider sense. The decomposition curves were anaylzed by the empirical equation (dX/dt=(1-X)^m×k^l+1×t^l), where X, t, and k are the decomposition fraction, decomposition time, and the apparent decomposition rate constant, respectively, and where m and l are parameters. The apparent activation energies of decomposition of three materials were 30 kcal/mol, and the k of II linearly increases as the relative humidity rises.