Collect. Czech. Chem. Commun. 1983, 48, 1144-1157

Crowns in alkali alkoxide-conjugate acid solutions: A remarkably low propensity to complex formation

Václav Pechanec, Oldřich Kocián and Jiří Závada

Institute of Organic Chemistry and Biochemistry, Czechoslovak Academy of Sciences, 166 10 Prague 6


Complexing ability of the macrocyclic polyethers 18-crown-6, 15-crown-5 and 12-crown-4 has been investigated ebulliometrically in the homologous series of 0.6 molal ROM-ROH solutions (M = K, Na, Li; R = H, CH3, C2H5, n-C3H7, i-C3H7, t-C4H9). The values of complexation constants have been estimated in all the 54 crown-ROM combinations and the effects of cation, crown-cavity size and solvent have been examined. Two intriguing facts emerged from a comparison of the new results with the pertinent literature data. Firstly, it was found that the cation-crown cavity-complex stability variation in the ROM-ROH series disagrees with the well-known principle of steric match, the propensity of crowns to complexation decreasing in most instances in the order 15-crown-5 > 12-crown-4 > 18-crown-6 for the potassium and 12-crown-4 > 15-crown-5 > 18-crown-6 for the sodium alkoxide. An energetical preference of 2 : 1 over 1 : 1 crown-ROM complexes is presumably the responsible factor. Secondly, and even more importantly, it was found that complexation constants obtained in this study are strikingly small, the values for complexation of potassium and sodium alkoxides with 18-crown-6 being smaller by 3-7 power of ten than those which were previously obtained for the alkali ions, at lower concentrations. Evidence is provided that the difference does not originate from ion-pairing but from the operation of some effect which so far escaped notice.