Pseudobase formation from 9-substituted 10-methylacridinium cations in aqueous solution

Rates of cation–pseudobase equilibration have been measured for a series of C-9 substituted (CH3, CH3CH2, C6H5CH2, (CH3)2CH, C6H5, 4-(CH3)2NC6H4) 10-methylacridinium cations in aqueous solution at 25 °C over the pH range of approximately 9–13. Separation of the rate constants for formation (kOH) and decomposition (k2) of each of these pseudobases allowed the calculation of the [Formula: see text] value for each cation. The presence of a C-9 isopropyl substituent enhances the stability of the pseudobase relative to the cation, whereas all other C-9 substituted cations have [Formula: see text] values greater than the 10-methylacridinium cation. There is no simple quantitative relationship between [Formula: see text] and the size of the C-9 substituent. Rate constants (kOH) for hydroxide ion attack on these C-9 substituted cations are in the order: H > primary alkyl > secondary alkyl > aryl, while rate constants (k2) for pseudobase decomposition are less predictable but generally follow the order: H > primary alkyl > aryl > secondary alkyl. These phenomena are shown to be consistent with a competition between destabilization of the cation by peri interactions between the C-9 substituent and H(1) and H(8) and reduced pseudobase solvation for large C-9 substituents. Resonance interactions of 9-aryl substituents with the acridine moiety of these cations are shown to be quite small. 9-Aryl-10-methylacridinium cations are approximately 30-fold more reactive towards hydroxide ion attack than are their triphenylmethyl carbocation analogues.