Formation of Stable Carbocations or Zwitterions by a Specific Interaction with Alkali Metal or Alkaline-Earth Metal Ions in Acetonitrile

In an acetonitrile solution, the extents of ionization of trityl halides (Ph3CX: X = Cl, Br) in the presence of various concentrations of alkali metal (M+ = Li+, Na+) and alkaline-earth metal (M2+ = Mg2+, Ca2+, Sr2+, Ba2+) perchlorates were examined by spectrophotometry. The promoted formation of the trityl ion (Ph3C+) in the presence of metal ions was attributed to “chemical” interactions, such as coordination or covalent bonding between the halide ions and M+ or M2+ ions, and not merely the electrostatic interaction. The salt effects increased with methoxy-substituents on trityl chloride as non-substituted < 4-methoxy < 4,4′-dimethoxy < 4,4′,4″-trimethoxytrityl. At higher RX (or X−) concentrations, not only “coordination”, but also precipitation reactions between X− and M+ or M2+ (except for Li+ and Mg2+) seemed to promote the ionization of RX. In the co-presence of a small amount of CH3SO3H, trityl benzoate gave the Ph3C+ ion upon the addition of LiClO4 or Mg(ClO4)2. A red color with strong fluorescence due to the zwitterion (R+–COO−) of Rhodamine (Rhodamine B base) was observed upon the addition of M+ and M2+, which should have been caused by an interaction between the metal ions and the intramolecule carboxylate (from the γ-lactone) of Rhodamine in the solution. The γ-lactone ring of Crystal Violet lactone in acetonitrile was cleaved by the addition of Mg(ClO4)2 to give a strong violet color, whereas, LiClO4, NaClO4, or Ba(ClO4)2 had no (or very small) effects in the solution. It was found that the interactions between X− or RCOO− and M+ or M2+ in acetonitrile were strong enough to promote the ionization of partly ionized covalent bonds.