Alkyl chain length and metallic cation effects on the acid-base properties of C-alkylresorcin[4]arenes in aqueous media

Abstract

The polyprotic behavior of C-alkylresorcin[4]arenes was investigated in aqueous media in a large interval of pH values by potentiometry and spectrophotometry. The pK a1 ; pK a2 ; pK a3 and pK a4 were determined. The pK ai values determined by both methods correlate very well. The influence of the alkylic chain length on the bridge carbon of the resorcin[4]arene were determined for C-methylresorcin[4]arene (C1RA); C-butylresorcin[4]arene (C4RA) and C-heptylresorcin[4]arene (C7RA). This effect was observed in the lower pK a1 determined for C7RA (2.3) respect to the similar values determined for the others resorcin[4]arenes (3.2) and an increase in the values of the pK a3 as the length of the alkyl chain is longer (8.3; 9.9; 10.5) without influence on pK a2 (6.9) and pK a4 (10.4). Also, the influence of the metallic cation of the base (Li + , Na + or K + ) was determined on the acid-base constants of C4RA. The values for the first and four dissociation constants are practically the same without influence of the counter ion of the base (pK a1 = 3.1 and pK a4 = 10.5); but the influence of Li + is observed as a higher pK a2 (7.7) and a lower pK a3 (9.0) than with Na + and K + (pK a2 = 7.0 and pK a3 = 9.8). These effects were interpreted on the basis of the ditopic characteristic of the resorcin[4]arenes, the interaction/complexation of the solvate of each cation with the acid-base specie of each macrocycle and their stabilization.