J. Phys. Chem. 1996, 100, 891-896
891
Conductometric Study on Higher Ion Aggregation of Lithium and Sodium Nitrophenolates in Aprotic Solvents Masashi Hojo,* Hiroshi Hasegawa, and Noritaka Hiura Department of Chemistry, Faculty of Science, Kochi UniVersity, Kochi 780, Japan ReceiVed: June 6, 1995; In Final Form: August 21, 1995X
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 °C in several aprotic solvents (MeCN, DMF, acetone, and PhCN). The molar conductivities (Λ) of lithium 2,4-dinitrophenolate in acetonitrile were explained by the formation of “symmetrical” triple ions (M2X+ and MX2-) 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 (M2X2) in addition to triple ions and ion pairs, although the (direct) Shedlovsky analysis gave a "pseudo" value of Ka ) 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-]/Cs) to the salt concentration (Cs) was greater than ca. 1% for Cs ) (0.16-2.0) × 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 (Brønsted) 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 × 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.
Introduction The complexation ability of alkaline metal ions has been believed to be very weak in solutions. The complex reaction of alkaline metal ions in aqueous solution may not be observed easily, except with very strong complex forming ligands, such as EDTA.1 That is, the Coulombic interaction between ions is the only main force governing the ion-ion interaction in dilute aqueous solution at ordinary temperature. However, Oelkers and Helgeson2 suggested the formation of higher ion aggregates (multiple ion association) from concentrated NaCl in supercritical aqueous solutions. The theory of conductometry has been developed to account for the conductivities of higher concentrations of electrolytes. With the most sophisticated theories,3-5 it is said that the equations can describe conductivities up to 0.03 mol dm-3 for uni-univalent strong electrolytes in aqueous solutions. However, abnormal behaviors in conductivities have often been reported for simple salts, such as lithium picrate6 and chloride7 in nonaqueous solvents, especially non-hydrogen-bonding solvents, such as nitrobenzene or acetone, with rather higher permittivities. Such abnormal behavior seems to be not exclusively comprehended in spite of great efforts by many chemists. We have demonstrated8-17 by means of spectrophotometry, polarography, and conductometry that the “minor” complexation ability of lithium and sodium ions can be definitely observed if very low solvation media are employed. The presence of “reverse-coordinated” species, (M+)2X-, and “coordinated” species, M+(X-)2, in nonaqueous solution has been discovered; where X- ) RCO2-,8-10 Cl-,11 SCN-,12 picrate ion,13 RSO3-, and (PhO)2PO2-,14 and M+ ) Li+ and Na+ for RCO2-, and Li+ for other anions. Even in a protic solvent, EtOH, higher X
Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-0891$12.00/0
ion aggregates were observed for lithium and sodium tropolonates (C7H5O2-M+).15 The temperature dependency of the formation constants of aggregates from R3NH+X- (X- ) Cl-, Br-, and I-) in benzonitrile strongly suggested that the ion associations were governed mainly by hydrogen-bonding forces and not merely by Coulombic interactions.17 The specific salt effects on acid-base reactions in nonaqueous solvents have been quantitatively explained by the “complex” formation16,18,19 and not merely the ion exchange or the exchange between the contact ion pair (CIP)20 and the solvent-separated ion pair (SSIP).20 Thus, the cause was elucidated of a large variation in the indicator acidity (the Hammett acidity function) with the addition of various “indifferent” salts in acetonitrile.19 In a previous paper,21 we have reported that the proton transfer from nitrophenols to amine or pyridine bases in acetonitrile is promoted by the addition of M+ClO4- (M+ ) Li+ and Na+) and M2+(ClO4-)2 (M2+ ) Mg2+, Ca2+, Sr2+, and Ba2+). In the presence of appropriate bases, the extent of the proton transfer increased with the decreasing (Brønsted) acidity of the nitrophenols: 2,4,6- , 2,4- < 2,5- < 2-derivative. The present work provides the quantitative evaluation for the interaction between M+ and the phenolate ions in aprotic solvents. Ion associations of lithium and sodium nitrophenolates (2-, 4-, 2,4-, 2,5-, and 2,4,6-derivatives) in aprotic solvents (MeCN, DMF, acetone, and PhCN) were examined by means of conductometry. The solvent effects on ion associations are discussed from the viewpoint of solvation ability toward both anions and cations and not of the permittivity of solvent alone. Observed molar conductivities were analyzed by our method,10,14 considering higher ion aggregation above ion pairs. Theoretical Our method of analyzing conductivity data is outlined as follows. For a uni-univalent electrolyte (MX), the formation © 1996 American Chemical Society
892 J. Phys. Chem., Vol. 100, No. 2, 1996
Hojo et al.
of an ion pair (M+ + X- a M+X-, K1), triple ions (2M+ + X- a (M+)2X-, K2; M+ + 2X- a M+(X-)2, K3), and a quadrupole (2MX a M2X2K41) may be accounted for. Note that K2 and K3 are the overall formation constants of triple ions from simple ions. The equilibrium concentration of X- ([X] ) [M]) at a given salt concentration (Cs) can be found by solving a fourth-order equation, if K2 ) K3.
2K41K12[X]4 + 3K2[X]3 + K1[X]2 + [X] - Cs ) 0 (1) The concentrations of the ion pair, triple ions, and quadrupole are [MX] ) K1[X]2, [M2X] ) K2[X]3 ) [MX2] ) K3[X]3, and [M2X2] ) K41K12[X]4, respectively. In order to correct the effects of ion atmosphere, Onsager’s limiting law is applied for the limiting molar conductivities of the simple ions (Λ0, [M+X-]) and triple ions (ΛT, [(M+)2X- M+(X-)2]) at low ionic strength. The whole molar conductivity (Λ/S cm2 mol-1) consists of the Λ0′ and ΛT′ values corrected by Onsager’s limiting law from Λ0 and ΛT, respectively.
Λ)
[MX2] [X] Λ0′ + ΛT′ Cs Cs
(2)
Activity coefficients of ions are estimated by the Debye-Hu¨ckel equation with a ) 4 Å, and those of uncharged species are taken to be unity. In the higher permittivity media (r > 20), we can safely state that tetraalkylammonium salts (R4N+X-), perchlorate salts (M+ClO4-), and R4N+ClO4- can behave as “strong” electrolytes, or they do not form higher ion aggregates above ion pairs at low salt concentrations. Thus, reliable Λ0 values of weak electrolytes, MX, can be easily calculated by Kohlrausch’s additivity law. The value of 0.693 has been adopted for the ratio of ΛT to Λ0. Now, up to three variables, K1, K2 ) K3 (or Ka1′, Ka2′ ) Ka3′), and K41, can be defined by the trial and error method, fitting every calculated Λ value to the corresponding observed Λ value of wide concentration ranges. It has been clarified that, for weak electrolytes in solution, the following classification10 can be made of the relation between the limiting molar conductivity (Λ0(direct)), obtained directly by the Shedlovsky analysis, and Λ0(calcd), calculated from Kohlrausch’s additivity law for strong electrolytes: (a) Λ0(direct) ≈ Λ0(calcd). No triple-ion formation or only weak triple-ion formation occurs in addition to ion-pair formation. (b) Λ0(direct) < Λ0(calcd). Strong triple-ion formation with no quadrupole formation or relatively weak quadrupole formation. (c) Λ0(direct) > Λ0(calcd). Weak triple-ion formation with strong quadrupole formation. Experimental Section Commercially obtained sodium 2- and 4-nitrophenolates (both TCI, GR grade) were dried in vacuo at 160 °C. Lithium 2and 4-nitrophenolates were prepared by the neutralization between a LiOH aqueous solution and the equivalent amount of the nitrophenols in methanol. The solutions were evaporated to dryness, adding MeOH, acetone, and diethyl ether, successively, under reduced pressure at
