Complexation of amino acid methylesters and amino alcohols by 18-crown-6 and benzo-18-crown-6 in methanol

The complexation of protonated amino acid methylesters; and amino alcohols by the ligands 18-crown-6 (18C6) and benzo-18-crown-6 (B18C6) has been stud ied in methanol using calorimetric titrations. No influence of the anions u pon the stability constants and thermodynamic data for the reaction of prot onated amino acid methylesters with both ligands has been noticed, which in dicates the complete dissociation of the salts employed. A similar effect h as been observed for the complexation of protonated and unprotonated amino alcohols with 18C6 and B18C6. The values obtained of the reaction enthalpie s for the complexation of protonated amino acid methylesters with 18C6 are larger than those corresponding to the complexation with B18C6. The results demonstrate that the complex formation of unprotonated amino alcohols is f avored by entropic contributions, while the complexation of protonated amin o alcohols is favored by enthalpic contributions with both ligands. The inf luence of various substituents on the complexation behavior of amino acid a nd amino alcohol has also been investigated.