Degrees of association of alkali tert-butoxides in tert-butanol in the absence and in the presence of crowns: An ebulliometric study

Abstract

Association degrees have been determined in 0.03-0.8 molal t-C4H9OLi-t-C4H9OH, t-C4H9ONa-t-C4H9OH and t-C4H9OK-t-C4H9OH solutions in the absence and also in the presence of three different crowns (18-crown-6, 15-crown-5 and 12-crown-4). It has been found that presence of any of the three crowns does not affect association degree in the alkoxide solutions. t-C4H9OLi is on average dimeric in 0.03 molal and tetrameric in 0.4-0.8 molal solution, whereas t-C4H9ONa and t-C4H9OK are in the full concentration range monomeric ion pairs, in the absence as well as in the presence of the crowns. Of the nine tert-butoxide-crown combinations studied, only two, t-C4H9OK-18-crown-6 and t-C4H9OK-15-crown-5, yielded complexes quantitatively, in the form of 1 : 1 complexed ion pairs. An incomplete complex formation (10-30%) has been observed with the combinations t-C4H9OK-12-crown-4, t-C4H9ONa-18-crown-6, t-C4H9ONa-15-crown-5, t-C4H9ONa-12-crown-4. No complex formation has been detected with t-C4H9OLi, in combination with any of the three crowns. Complex stability decreases in the order K(+) > Na(+) > Li(+), regardless of the crown cavity size. At variance with the prevailing opinion it indicates that steric fit between cation and crown is not the main factor which controls complex stability in the solution. A cooperative influence of anion on crown complex stability is considered in terms of the theory of hard and soft acids and bases.