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Solvent effects on acid-base behaviour Acidity constants of eight protonated substituted pyridines in (acetonitrile+water)
O-210 J Chem. Thermo&amics
1987, 19. 443-441
Solvent effects on acid-base behaviour Acidity constants of eight protonated substituted pyridines in (acetonitrile + water) ZENON
PAWLAK’
Department of Technology of Industrial Production and Science of Commodities, Universit~~ qf Gdansk, 81-824 Sopot, Poland (Received 8 August 1986; in final
form 13 October
1986)
Acidity constants K, at 298.15 K and a constant ionic strength of 0.01 mol. kg-’ of eight protonated substituted pyridines (BH+): 4-amino-, 2-amino-, 4-methyl-, 3-methyl-, 2-methyl-. 3-hydroxy-, 3-acetyl-, and pyridine in {.&H&H +(I -x)H,O}, where x = 0.2, 0.4. 0.6, 0.8. and 0.85. were determined by the electrometric-titration method. The pK, values reach a minimum at x = 0.45. The results are discussed in terms of preferential solvation of ions by the two types of solvent molecule.
1. Introduction It is known that pK, values of positively charged acids pass through a minimum when the composition of (solvent + water) is varied. This dependence has been confirmed for (a protic solvent + water) and (an aprotic solvent + water).“-‘) Braude and Stern’@ suggested that the water structure was broken down progressively on the addition of an organic component to a water-rich medium. Moreau and Douhe’ret”, *) have studied thermodynamic and physical properties of (xCH,CN + (1 - x)H,O} and proposed a structural description of these mixed solvents. There are three different regions: x = 0 to 0.2, x = 0.2 to 0.8, and x > 0.8. in these solvents, as demonstrated by sharp maxima of viscosity excess functions and dielectric properties. In the intermediate region, 0.2 < x < 0.8, large aggregates progressively break down into smaller entities with domination of dihydrates. Acetonitrile does not appear to be a strong breaker of the water structure. It is weaker, both as an acid and as a base. than water and has a dielectric constant of 34.8.“’ Symons(9) postulated that the auto-dissociation of bulk water proceeded ’ Present Address: Departments of Chemistry and Chemical Engineering. Brigham Young University, Provo. Utah 84602. U.S.A. 0071-9614/87/040443 f05
$02.00/O
;c’ 1987 Academic Press Inc. (London) Limited
444
Z.PAWLAK
according to + W%,,. (1) WZOhulk = W-L where 10 per cent of the molecules of water are free (OH)f,,, and free “lone pair” hexamethylv%ree. When a strongly basic aprotic solvent (dimethylsulphoxide, phosphotriamide) is added to water there is a fall in the absorption in the (OH)r,,, region, but for a weakly basic co-solvent such as acetonitrile or acetone, the absorption in the (OH)r,,, spectral region increases. In the strongly aqueous region, x < 0.2, we can write for a monobasic co-solvent: (HZO)bu,k + CH,CN
= CH,CN
. . . HOH + (LP)r,,,.
(2)
The lone-pair molecules of water seem to be of prime importance in its interaction with charged acids. Acidity constants K, of eight cation acids, BH+. were determined in (acetonitrile + water). Studied nitrogen bases are monosubstituted pyridines with pK, in water from 3.20 to 9.11 (see table 2).
2. Experimental Acetonitrile (analysis grade) of Prolabo was purified by distillation over A&O, and then from P,O,. The first and last fractions, each amounting to about 15 per cent of the total distillate, were discarded. The conductivity of the remaining solvent was 5 x lo-‘R-‘.cm-‘. Di-ionized water was distilled in an all-glass still before use. The mixed solvents were prepared by mass. Solutions of HBr were prepared by passing dry HBr into (acetonitrile + water) and standardized by titration with NaOH(aq). Solutions of the titrant were made from weighed amounts of base and 0.01 mol ’ kg- ’ HBr. Tris(hydroxymethyl)aminomethane (TRIS) was recrystallized from 75 mass per cent of methanol in water, 3-hydroxypyridine and 4-aminopyridine from benzene, and 2-aminopyridine from (cyclohexane+ toluene). The liquid bases: 2-, 3-, and 4-methylpyridine, and pyridine, were dried over KOH and then distilled. 3-Acetylpyridine was dried with molecular sieves and distilled under a partial vacuum. The titration cell, which had a volume of about 40 cm3, was immersed in a thermostat maintained at (298.15 kO.05) K. The e.m.f. was measured by a MeraElwro pH meter, Model N-517, with a precision of +0.5 mV.
3. Method and results The titration
method utilized the cell without liquid junction Glass electrode[HBr
+ BIAgBrJAg.
of the type: (1)
It has been described in detail. (3-5) The assembly was standardized by an initial measurement of e.m.f. E, in 0.01 mol. kg-’ HBr. Titrant solution containing the nitrogen base C (0.1 mol. kg- ‘) and HBr (0.01 mol. kg- ‘) were added in amounts usually sufficient to cover the range of buffer ratio 0.5 < {m(BH+)/m(B)} < 2 and
K, OF SUBSTITUTED
the e.m.f. way, the 0.01 mol. calculated
PYRIDINES
IN (ACETONITRILE+
WATER)
445
E, was measured after the addition
of each increment of titrant. In this ionic strength and the molality of Br-(aq) remained constant at kg-’ in the solution as the titration proceeded. Values of pKf were by pKz = F(E,-E,)/(RTln
10)+lg({m(BH+)/O.Olm(B)}.
(3)
There is evidence that silver-halide complexes such as AgBr; are formed in pure acetonitrile. However, in our studies the e.m.f.s of the AgBrlAg cells remained stable even at x = 0.85, indicating absence of AgBr; in (acetonitrile + water). Sample results for a titration of hydrobromic acid with 3-hydroxypyridine at constant ionic strength and Br-(aq) molality are given in table 1. Values of pKs for all eight protonated bases at 298.15 K are presented in table 2. Mean values of pKz and the corresponding standard deviations s are listed. The solvent effect of acetonitrile on the dissociation of the eight protonated basis is evident in figure 1. where the difference between pKz (the value of pK, in the fixed solvent) and pK, (the value of pK, in water) is plotted as a function of x. The pKf values of substituted pyridine acids measured at x = 0.45 (minimum of ApK,, figure 1) are lower than those measured in water by at least 0.5pK, to 0.9pK,. The pKf values of most pyridinium acids for x z 0.82 are close to those in TABLE
1. Results
H’ a -. g
for
2. pK,
w added
for BH+
B Y. 4-Aminopyridine 2-Aminopyridine 4-Methylpyridine 2-Methylpyridine 3-Methylpyridine Pyridine 3-Hydroxypyridine 3-Acetylopyridine
of
lO%i(BH
pKz -x)H,O}
+)
298.15 K of at x = 0.85
10%(B)
1.789 1.857 1.963 2.101 2.265 2.381
3-hydroxypyridinium
ion
m(BH + ) k ~ m(B)
PG
0.265 0.070 -0.110 -0.253 -0.340 -0.420
5.38 5.39 5.38 5.39 5.39 5.38
0.972 1.580 2.526 3.760 4.861 6.259
to 15.117 g of 0.01 mol. kg-’
in H,O
at
mol.kg-’
mol.kg-’
180 192 202 211 216 220
a Mass of titrant
1. 2. 3. 4. 5. 6. 7. 8.
determination {xCH,CN+(l
E,-E, mV
2.281 3.500 4.571 5.968 7.215 8.797
TABLE
the
in
HBr.
and in {xCH,CN+(l -x)H,O} standard deviation
at 298.15 K, where
s denotes
PK, 0
0.20
0.40
pK:+s 0.60
0.80
0.85
9.11”2 6.71”” 5.98”” 5.96r13’ 5.63’13’ 5.22”“’ 4.80”2’ 3.60”s’
8.7OkO.01 6.00~0.01 5.56+0.01 5.47 + 0.04 5.16+0.01 4.87kO.02 4.48 & 0.02 2.96 k 0.04
8.44*0.03 5.70 + 0.02 5.27 + 0.04 5.17+0.03 4.86iO.01 4.53 + 0.02 4.24kO.01 2.76kO.03
8.48kO.02 5.91 kO.01 5.34kO.02 5.27 +0.02 4.98 +O.Ol 4.61 +O.Ol 4.30*0.01 2.85&0.01
8.84&0.01 6.42 +O.Ol 5.80+0.02 5.76+0.01 5.41 kO.01 5.07*0.01 4.98kO.01 3.31*0.01
9.60+0.02 6.99kO.02 6.31+0.02 6.29_+0.01 5.93 kO.02 5.53+0.02 5.38+0.01 3.7OkO.02
the
446
Z. PAWLAK
I
I
I
I
FIGURE I. Effect of changes of solvent composition on the pK, of eight protonated substituted pyridines in {xCH,CH +(l -x)H,O) at 298.15 K. (I), 4-Aminopyridinium ion; (2). 2-aminopyridinium ion; (3), 4-methylpyridinium ion; (4), 2-methylpyridinium ion; (5). 3-methylpyridinium ion: (6). pyridinium ion: (7). 3-hydroxypyridinium ion: (8) 3-acetylopyridinium ion.
pure water. The form of (pK:-pK,) = f(x) closely resembles that observed for the + water)(4) and (N-methyl-2-pyrroliPKZ of acids in dimethylsulphoxide dinone + water)“’ and in binary mixed solvents. The observed changes of pKz and standard molar Gibbs free energy of transfer A,,Gk were calculated from: AtrGk = -RT
In lOApK,
= 5707(pKz(BH+)-pK,(BH+)}
J.mol-‘.
(4)
and are presented in table 3. The minimum in A,,Gz appears to lie not far from x = 0.4 to 0.5, where solvent-solvent interactions are presumed to be at a maximum. The ion-solvent interactions are much stronger than solvent-solvent interactions. In (acetonitrile + water) containing cationic acids (BH+), selective solvation by one component is preferred. The pKz is much lower in
K, OF SUBSTITUTED
PYRIDINES
IN (ACETONITRILE
447
+ WATER)
TABLE 3. Differences of standard molar Gibbs energy of transfer A,,GW at 298.15 K of protonated bases B for water to (acetonitrile+ water) B I, 2. 3. 4. 5. 6. 7. 8.
4-Aminopyridine 2-Aminopyridine 4-Methylpyridine 2-Methylpyridine 3-Methylpyridine Pyridine 3-Hydroxypyridine 3-Acetylpyridine
x:
0.2
0.4
-2.34 -4.06 -2.38 -2.80 -2.09 -2.01 - 1.84 -3.64
-3.81 -5.06 -4.06 -4.52 -4.39 -3.93 -3.18 -4.17
0.6 A,,G;/(kJ mol-‘) -3.60 -4.56 -3.64 -3.93 - 3.46 -3.41 -- 2.85 -4.27
0.8
0.85
- 1.55 - 1.63 -1.01 - 1.13 - 1.26 -0.84 -0.50 - I .63
2.80 1.59 1.88 I .88 1.72 1.76 3.31 0.59
(dimethylsulphoxide + water)‘4’ and in (N-methyl-2-pyrrolidinone + water)“’ than in (acetonitrile + water). The A,,Gi values are from 2 to 4 kJ mol- ’ more negative for x < 0.6. Proton stabilization is energetically favoured in (N-methyl-Zpyrrolidinone + water) and (dimethylsulphoxide + water) as compared with (acetonitrile + water). This observation confirms the hypothesis of Symons(‘* ‘O’ that more basic co-solvents such as dimethylsulphoxide and hexamethylphosphotriamide generate more (LP&,,,: see equation (1). The basicity of (LP)‘,,, groups in the mixture is much greater than that of monomeric water. This work was partly supported by a grant from CPBP-01.15. The collecting of some values by B. Przybyszewski and assistance in the preparation of this manuscript by Julie Woodfield is appreciated. REFERENCES
I. 2. 3. 4. 5. 6. 7. 8. 9. 10. I I.
DeLingy, C. L. Rec. Trav. Chim. 1960, 79, 731. Gutbezahl. B.; Grunwald, E. /. Am. Chem. SIX. 1953, 75. 559. Paabo, M.; Bates, R. G.; Robinson, R. A. .I. Phys. Chem. 1966, 70, 247. Pawlak, Z.; Bates, R. G. J. Solution Chem. 1975, 9, 817. Pawlak, Z.; Bates, R. G. J. SoIurion Chem. 1976, 5, 325. Braude, E. A.; Stern, E. S. J. Chem. Sot. 1978, 1976. Moreau. C.; Douhe’ret, G. J. Chem. Thermodynamics 1970, 8. 403. Moreau, C.; Douhe’ret, G. Thermochim. Acru 1975, 13, 385. Symons, M. C. R. Act. Chem. Res. 1981, 19, 179. Symons, M. C. R.; Fletcher, N. J.; Thompson, V. K. Chem. Phys. L,ett. 1979, 60, 323. Criss, C. M.; Salomon, M. Physical Chemistry of Organic So/vent Systems. Covington. A, K.; Dickinson, T.: editors. Plenum: New York. 1973. Chap. 2. 12. Christensen. J. J.; Smith, D. E.; Slade, M. D.; Izatt, R. M. Thermochim. Acfa 1972, 5, 35. 13. Mortimer, C. T.; Laidler, K. J. Trans. Faraday Sot. 1959, 55, 1751. 14. Andon, R. 1.; Cox, J. D.; Herington, E. F. G. Trans. Faraday Sot. 1954, 50, 918. 1.5. Cabani, S.: Conti, G. Gazz. Chim. Ital. 1965, 45, 533.
1987, 19. 443-441
Solvent effects on acid-base behaviour Acidity constants of eight protonated substituted pyridines in (acetonitrile + water) ZENON
PAWLAK’
Department of Technology of Industrial Production and Science of Commodities, Universit~~ qf Gdansk, 81-824 Sopot, Poland (Received 8 August 1986; in final
form 13 October
1986)
Acidity constants K, at 298.15 K and a constant ionic strength of 0.01 mol. kg-’ of eight protonated substituted pyridines (BH+): 4-amino-, 2-amino-, 4-methyl-, 3-methyl-, 2-methyl-. 3-hydroxy-, 3-acetyl-, and pyridine in {.&H&H +(I -x)H,O}, where x = 0.2, 0.4. 0.6, 0.8. and 0.85. were determined by the electrometric-titration method. The pK, values reach a minimum at x = 0.45. The results are discussed in terms of preferential solvation of ions by the two types of solvent molecule.
1. Introduction It is known that pK, values of positively charged acids pass through a minimum when the composition of (solvent + water) is varied. This dependence has been confirmed for (a protic solvent + water) and (an aprotic solvent + water).“-‘) Braude and Stern’@ suggested that the water structure was broken down progressively on the addition of an organic component to a water-rich medium. Moreau and Douhe’ret”, *) have studied thermodynamic and physical properties of (xCH,CN + (1 - x)H,O} and proposed a structural description of these mixed solvents. There are three different regions: x = 0 to 0.2, x = 0.2 to 0.8, and x > 0.8. in these solvents, as demonstrated by sharp maxima of viscosity excess functions and dielectric properties. In the intermediate region, 0.2 < x < 0.8, large aggregates progressively break down into smaller entities with domination of dihydrates. Acetonitrile does not appear to be a strong breaker of the water structure. It is weaker, both as an acid and as a base. than water and has a dielectric constant of 34.8.“’ Symons(9) postulated that the auto-dissociation of bulk water proceeded ’ Present Address: Departments of Chemistry and Chemical Engineering. Brigham Young University, Provo. Utah 84602. U.S.A. 0071-9614/87/040443 f05
$02.00/O
;c’ 1987 Academic Press Inc. (London) Limited
444
Z.PAWLAK
according to + W%,,. (1) WZOhulk = W-L where 10 per cent of the molecules of water are free (OH)f,,, and free “lone pair” hexamethylv%ree. When a strongly basic aprotic solvent (dimethylsulphoxide, phosphotriamide) is added to water there is a fall in the absorption in the (OH)r,,, region, but for a weakly basic co-solvent such as acetonitrile or acetone, the absorption in the (OH)r,,, spectral region increases. In the strongly aqueous region, x < 0.2, we can write for a monobasic co-solvent: (HZO)bu,k + CH,CN
= CH,CN
. . . HOH + (LP)r,,,.
(2)
The lone-pair molecules of water seem to be of prime importance in its interaction with charged acids. Acidity constants K, of eight cation acids, BH+. were determined in (acetonitrile + water). Studied nitrogen bases are monosubstituted pyridines with pK, in water from 3.20 to 9.11 (see table 2).
2. Experimental Acetonitrile (analysis grade) of Prolabo was purified by distillation over A&O, and then from P,O,. The first and last fractions, each amounting to about 15 per cent of the total distillate, were discarded. The conductivity of the remaining solvent was 5 x lo-‘R-‘.cm-‘. Di-ionized water was distilled in an all-glass still before use. The mixed solvents were prepared by mass. Solutions of HBr were prepared by passing dry HBr into (acetonitrile + water) and standardized by titration with NaOH(aq). Solutions of the titrant were made from weighed amounts of base and 0.01 mol ’ kg- ’ HBr. Tris(hydroxymethyl)aminomethane (TRIS) was recrystallized from 75 mass per cent of methanol in water, 3-hydroxypyridine and 4-aminopyridine from benzene, and 2-aminopyridine from (cyclohexane+ toluene). The liquid bases: 2-, 3-, and 4-methylpyridine, and pyridine, were dried over KOH and then distilled. 3-Acetylpyridine was dried with molecular sieves and distilled under a partial vacuum. The titration cell, which had a volume of about 40 cm3, was immersed in a thermostat maintained at (298.15 kO.05) K. The e.m.f. was measured by a MeraElwro pH meter, Model N-517, with a precision of +0.5 mV.
3. Method and results The titration
method utilized the cell without liquid junction Glass electrode[HBr
+ BIAgBrJAg.
of the type: (1)
It has been described in detail. (3-5) The assembly was standardized by an initial measurement of e.m.f. E, in 0.01 mol. kg-’ HBr. Titrant solution containing the nitrogen base C (0.1 mol. kg- ‘) and HBr (0.01 mol. kg- ‘) were added in amounts usually sufficient to cover the range of buffer ratio 0.5 < {m(BH+)/m(B)} < 2 and
K, OF SUBSTITUTED
the e.m.f. way, the 0.01 mol. calculated
PYRIDINES
IN (ACETONITRILE+
WATER)
445
E, was measured after the addition
of each increment of titrant. In this ionic strength and the molality of Br-(aq) remained constant at kg-’ in the solution as the titration proceeded. Values of pKf were by pKz = F(E,-E,)/(RTln
10)+lg({m(BH+)/O.Olm(B)}.
(3)
There is evidence that silver-halide complexes such as AgBr; are formed in pure acetonitrile. However, in our studies the e.m.f.s of the AgBrlAg cells remained stable even at x = 0.85, indicating absence of AgBr; in (acetonitrile + water). Sample results for a titration of hydrobromic acid with 3-hydroxypyridine at constant ionic strength and Br-(aq) molality are given in table 1. Values of pKs for all eight protonated bases at 298.15 K are presented in table 2. Mean values of pKz and the corresponding standard deviations s are listed. The solvent effect of acetonitrile on the dissociation of the eight protonated basis is evident in figure 1. where the difference between pKz (the value of pK, in the fixed solvent) and pK, (the value of pK, in water) is plotted as a function of x. The pKf values of substituted pyridine acids measured at x = 0.45 (minimum of ApK,, figure 1) are lower than those measured in water by at least 0.5pK, to 0.9pK,. The pKf values of most pyridinium acids for x z 0.82 are close to those in TABLE
1. Results
H’ a -. g
for
2. pK,
w added
for BH+
B Y. 4-Aminopyridine 2-Aminopyridine 4-Methylpyridine 2-Methylpyridine 3-Methylpyridine Pyridine 3-Hydroxypyridine 3-Acetylopyridine
of
lO%i(BH
pKz -x)H,O}
+)
298.15 K of at x = 0.85
10%(B)
1.789 1.857 1.963 2.101 2.265 2.381
3-hydroxypyridinium
ion
m(BH + ) k ~ m(B)
PG
0.265 0.070 -0.110 -0.253 -0.340 -0.420
5.38 5.39 5.38 5.39 5.39 5.38
0.972 1.580 2.526 3.760 4.861 6.259
to 15.117 g of 0.01 mol. kg-’
in H,O
at
mol.kg-’
mol.kg-’
180 192 202 211 216 220
a Mass of titrant
1. 2. 3. 4. 5. 6. 7. 8.
determination {xCH,CN+(l
E,-E, mV
2.281 3.500 4.571 5.968 7.215 8.797
TABLE
the
in
HBr.
and in {xCH,CN+(l -x)H,O} standard deviation
at 298.15 K, where
s denotes
PK, 0
0.20
0.40
pK:+s 0.60
0.80
0.85
9.11”2 6.71”” 5.98”” 5.96r13’ 5.63’13’ 5.22”“’ 4.80”2’ 3.60”s’
8.7OkO.01 6.00~0.01 5.56+0.01 5.47 + 0.04 5.16+0.01 4.87kO.02 4.48 & 0.02 2.96 k 0.04
8.44*0.03 5.70 + 0.02 5.27 + 0.04 5.17+0.03 4.86iO.01 4.53 + 0.02 4.24kO.01 2.76kO.03
8.48kO.02 5.91 kO.01 5.34kO.02 5.27 +0.02 4.98 +O.Ol 4.61 +O.Ol 4.30*0.01 2.85&0.01
8.84&0.01 6.42 +O.Ol 5.80+0.02 5.76+0.01 5.41 kO.01 5.07*0.01 4.98kO.01 3.31*0.01
9.60+0.02 6.99kO.02 6.31+0.02 6.29_+0.01 5.93 kO.02 5.53+0.02 5.38+0.01 3.7OkO.02
the
446
Z. PAWLAK
I
I
I
I
FIGURE I. Effect of changes of solvent composition on the pK, of eight protonated substituted pyridines in {xCH,CH +(l -x)H,O) at 298.15 K. (I), 4-Aminopyridinium ion; (2). 2-aminopyridinium ion; (3), 4-methylpyridinium ion; (4), 2-methylpyridinium ion; (5). 3-methylpyridinium ion: (6). pyridinium ion: (7). 3-hydroxypyridinium ion: (8) 3-acetylopyridinium ion.
pure water. The form of (pK:-pK,) = f(x) closely resembles that observed for the + water)(4) and (N-methyl-2-pyrroliPKZ of acids in dimethylsulphoxide dinone + water)“’ and in binary mixed solvents. The observed changes of pKz and standard molar Gibbs free energy of transfer A,,Gk were calculated from: AtrGk = -RT
In lOApK,
= 5707(pKz(BH+)-pK,(BH+)}
J.mol-‘.
(4)
and are presented in table 3. The minimum in A,,Gz appears to lie not far from x = 0.4 to 0.5, where solvent-solvent interactions are presumed to be at a maximum. The ion-solvent interactions are much stronger than solvent-solvent interactions. In (acetonitrile + water) containing cationic acids (BH+), selective solvation by one component is preferred. The pKz is much lower in
K, OF SUBSTITUTED
PYRIDINES
IN (ACETONITRILE
447
+ WATER)
TABLE 3. Differences of standard molar Gibbs energy of transfer A,,GW at 298.15 K of protonated bases B for water to (acetonitrile+ water) B I, 2. 3. 4. 5. 6. 7. 8.
4-Aminopyridine 2-Aminopyridine 4-Methylpyridine 2-Methylpyridine 3-Methylpyridine Pyridine 3-Hydroxypyridine 3-Acetylpyridine
x:
0.2
0.4
-2.34 -4.06 -2.38 -2.80 -2.09 -2.01 - 1.84 -3.64
-3.81 -5.06 -4.06 -4.52 -4.39 -3.93 -3.18 -4.17
0.6 A,,G;/(kJ mol-‘) -3.60 -4.56 -3.64 -3.93 - 3.46 -3.41 -- 2.85 -4.27
0.8
0.85
- 1.55 - 1.63 -1.01 - 1.13 - 1.26 -0.84 -0.50 - I .63
2.80 1.59 1.88 I .88 1.72 1.76 3.31 0.59
(dimethylsulphoxide + water)‘4’ and in (N-methyl-2-pyrrolidinone + water)“’ than in (acetonitrile + water). The A,,Gi values are from 2 to 4 kJ mol- ’ more negative for x < 0.6. Proton stabilization is energetically favoured in (N-methyl-Zpyrrolidinone + water) and (dimethylsulphoxide + water) as compared with (acetonitrile + water). This observation confirms the hypothesis of Symons(‘* ‘O’ that more basic co-solvents such as dimethylsulphoxide and hexamethylphosphotriamide generate more (LP&,,,: see equation (1). The basicity of (LP)‘,,, groups in the mixture is much greater than that of monomeric water. This work was partly supported by a grant from CPBP-01.15. The collecting of some values by B. Przybyszewski and assistance in the preparation of this manuscript by Julie Woodfield is appreciated. REFERENCES
I. 2. 3. 4. 5. 6. 7. 8. 9. 10. I I.
DeLingy, C. L. Rec. Trav. Chim. 1960, 79, 731. Gutbezahl. B.; Grunwald, E. /. Am. Chem. SIX. 1953, 75. 559. Paabo, M.; Bates, R. G.; Robinson, R. A. .I. Phys. Chem. 1966, 70, 247. Pawlak, Z.; Bates, R. G. J. Solution Chem. 1975, 9, 817. Pawlak, Z.; Bates, R. G. J. SoIurion Chem. 1976, 5, 325. Braude, E. A.; Stern, E. S. J. Chem. Sot. 1978, 1976. Moreau. C.; Douhe’ret, G. J. Chem. Thermodynamics 1970, 8. 403. Moreau, C.; Douhe’ret, G. Thermochim. Acru 1975, 13, 385. Symons, M. C. R. Act. Chem. Res. 1981, 19, 179. Symons, M. C. R.; Fletcher, N. J.; Thompson, V. K. Chem. Phys. L,ett. 1979, 60, 323. Criss, C. M.; Salomon, M. Physical Chemistry of Organic So/vent Systems. Covington. A, K.; Dickinson, T.: editors. Plenum: New York. 1973. Chap. 2. 12. Christensen. J. J.; Smith, D. E.; Slade, M. D.; Izatt, R. M. Thermochim. Acfa 1972, 5, 35. 13. Mortimer, C. T.; Laidler, K. J. Trans. Faraday Sot. 1959, 55, 1751. 14. Andon, R. 1.; Cox, J. D.; Herington, E. F. G. Trans. Faraday Sot. 1954, 50, 918. 1.5. Cabani, S.: Conti, G. Gazz. Chim. Ital. 1965, 45, 533.