Complexation of Benzoic, 4-Methylbenzoic, and (R)- and (S)-2-Phenylpropanoic Acids and Their Conjugate Bases by 3^A-Amino-3^A-deoxy-(2^AS,3^AS)-β-cyclodextrin in Aqueous Solution

Abstract

A potentiometric titration study of the complexation of benzoic, 4-methylbenzoic, and (R)- and (S)-2-phenylpropanoic acids and their conjugate bases by 3^A-amino-3^A-deoxy-(2^AS,3^AS)-β- cyclodextrin, βCD3NH₂, in which the amino group may be protonated to produce a singly charged species, βCD3NH₃⁺, is reported. In aqueous solution at 298.2K and I = 0.10 mol dm-³ ( KCl ), the complexation constants for the complexes indicated have the values (in dm³ mol-¹) shown in parentheses: benzoic acid.βCD3NH₃⁺ (K_HA = 110±10); benzoate.βCD3NH₃⁺ (K_A = 19±2); 4-methylbenzoic acid.βCD3NH₃+ (K_HA = 210±10); 4-methylbenzoate.βCD3NH₃+ (K_A = 21±3); (R)- and (S)-2-phenylpropanoic acid.βCD3NH₃⁺ (K_RHA = 64±8, K_SHA = 57±5); (R)- and (S)-2-phenylpropanoate.βCD3NH₃⁺ (K_RA = 51±6, K_SA = 32±6); and (R)- and (S)-2-phenylpropanoate.βCD3NH₂ (K_RA′ = 13±7; K_SA′ is too small to quantify reliably). These complexation constants are substantially less than those for the host-guest complexes formed by the isomeric 6^A-amino-6^A-deoxy-β-cyclodextrin and also for those formed by β-cyclodextrin. The origins of these differences are discussed.