Conductometric study on higher ion aggregation of lithium and sodium nitrophenolates in aprotic solvents

Ion associations of lithium and sodium nitrophenolates (2-, 4-, 2,4-, 2,5-, and 2,4,6-derivatives) were examined by means of conductometry at 25 degrees C in several aprotic solvents (MeCN, DMF, acetone, and PhCN). The molar conductivities (Lambda) of lithium 2,4-dinitrophenolate in acetonitrile were explained by the formation of ''symmetrical'' triple ions (M(2)X(+) and MX(2)(-)) as well as ion pairs (MX); however, those of the sodium salt were explained by the formation of ion pairs alone. The salts of 2,5-dinitrophenol in acetonitrile gave larger formation constants than those of 2,4-dinitrophenol. The molar conductivities of lithium 2-nitrophenolate in acetonitrile were explained by the strong formation of quadrupoles (M(2)X(2)) in addition to triple ions and ion pairs, although the (direct) Shedlovsky analysis gave a ''pseudo'' value of K-a = 0 (no association between simple ions). Any species from lithium 2-nitrophenolate could not be ignored even at lower concentrations because the ratio of the concentration of a species (e.g., [X(-)]/C-s) to the salt concentration (C-s) was greater than ca. 1% for C-s = (0.16-2.0) x 10(-3) mol dm(-3). Lithium 4-nitrophenolate gave formation constants smaller than those of the 2-nitrophenolate because of no nitro-group at the ortho-position. Ion aggregations occurred to the higher extent for salts of ortho-substituted nitrophenols with weaker (Bronsted) acidities of phenols (2,4,6- < 2,4- < 2,5- < 2-). In the stronger solvating solvent, DMF, the high ion aggregation above ion pair formation was observed only for lithium 2-nitrophenolate but not for other salts. The logarithms of the formation constants for lithium 2,4-dinitrophenolate decreased linearly with increasing value of the (geometric) average between the donor and acceptor numbers [(DN x AN)(1/2)] of the solvents. The results of the present study provided the quantitative interpretation for the promotion of proton transfer from nitrophenols to amine or pyridine bases by the addition of LiClO4 or NaClO4 in acetonitrile.