Spektroskopische Untersuchungen über die Protonierung von Anthracenderivaten durch verdünnte Lösungen von Schwefelsäure in Nitromethan*

The reversible protonation at 25° of seven different anthracene derivatives (An, -4 M) by anhydrous sulfuric acid (1 M) is investigated spectrophotometrically, using anhydrous oxygen-free nitromethane as a solvent. Primary proton complexes, [AnH+] [HSO4-] (I), displaying long wave-length light absorption, are formed rapidly; they are unstable and change into at least two different secondary proton complexes (II and III) within a few minutes. I and III are markedly different either from the well-known proton complexes formed by dissolution of aromatic hydrocarbons in concentrated strong acids, or from the cation radicals into which these proton complexes are transformed by oxidation. From the observed dependence on sulfuric acid concentration of the long wave-length absorbancy of I, extrapolated to t = 0, the equilibrium constants for the exchange equilibria between the proton complexes of nitromethane and of the anthracence derivatives, respectively, are estimated. Different kinetic models, proposed for describing the change of I into II and III, are tested by comparison of the calculated and observed time-dependences of the absorbancy at four different wave-lengths of solutions of anthracene ( -4 M) and sulfuric acid (1 M) in nitromethane. It can be concluded therefrom that II is identical with the covalent proton complex of anthracene formed with concentrated acids, whilst I is a novel species with the proton probably not fixed at a well-defined carbon atom of the ring skeleton. Species III, originating from a bimolecular reaction step, has probably the composition [AnH+An] [HSOt-].