Medium Activity Coefficients of Substituted Benzoic Acid
527
Medium Activiit
oefficients in Methanol and Some Aprotic Solvents of nzoic Acids and Their Anions as Related to Their Hydrogen
Chantooni, dr., and I . M. Kolthoff* Schooi of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received October 5 , 79721
Hydrogen bond accepting capacities of solvents (S) relative to that of acetonitrile (AN), expressed as pANy’(H,). were obtained by assuming that pANyS(H,) = pANys(HA)- p*NyS(MeA) = pANys(HA) p*PJyA(n), HA denoting substituted benzoic acids, MeA, their methyl esters, and pANyS(n) the nonhydrogen bonded part of pANys(HA). The same value of pANyS(Ha)was found from log ANASKf(HA:!-)or *NASKf(HACl-) using eq 7a. The following average values of pANyS(H,)were obtained: -1.9, -1.5, -1.4, -1-0.1, H . 4 , and C3.0 for S = dimethyl sulfoxide (DMSO), methanol (M), N,N-dimethylformamide (DMF), acetone (Ac), methyl isobutyl ketone (MIBK), and nitrobenzene (NB), respectively. The electrostatic part of p*NyDMl’J’Mso(A-), pANyDMF*DMSO(A-)el,calculated from eq 3, has been found to be small and independent of the basic strength of A-, being C0.8 and -0.3, respectively. On the other hand, both pMyAN,DMF,DMSO(A-) and pMyAN,DMF,DMSO(A- are large and increase with increasing basic strength of A- , the organic contribution to the former being small and of the order of -0.7 f 0.2. Acetic acid and its methyl ester are miscible in all proportions with the organic solvents used. Indirectly it was found that pA’qy“,DMF,DMSO(H,) values of this acid are the same as of the substituted benzoic acids, but that p**q-y“’DMFJJMso(n) are 0.5, 1.5, and 1.7 units more positive than those of the substituted benzoic acids.
Introduction In a previous study,l dissociation constants of substituted benzoic acids ( H A ) have been determined in the three polar aprotic solvcnts, acetonitrile (AN), N,N-dimethylformamide (DMF), rind dimethyl sulfoxide (DMSO). It was found that DMF,DMsoAANpKd(HA) is a constant, indicating no difference i n resolution of acid strength between the three solvents, or expressed in terms of medium activity coefficients pAP~yl)MF,DMSO(HA)- pANyDMF,DMSO(A-) = constant in
pair water and methanol. In eq 3, P A ” ~ S ( A - ) ~ ,denotes the sum of the hydrogen bond donating effect of the solvent and ion-dipole and ion-quadrupole interactions. The Born effect is small, as M, AN, DMF, and DMSO are close to isodielectric. Also in eq 3 pANyS(A,) refers to the neutral component of the medium transfer coefficient of A-. Introducing eq 2 and 3 into eq 1 and assuming that *NyS(n) = ANyS(A,), eq 4 results S
+
AAVL ~ K ~ ( H=A pANYW+) )
pANYS(A-),l - pANYS(H,) (4) In an independent way, pANyDMF,DMsO(Ha) has been estimated from In order to gain some insight into the relative hydrogen bond accepting properties of the three aprotic solvents and methanol (M), expressed as pANyDMF,DMSo,M(Ha) we have determined the solubility of various substituted benzoic acids and their methyl esters (MeA) in these solvents. Parker, et a1.,2 considered py(HA) to be the sum of a nonhydrogen bond accepting component, denoted by us as py(n), and of py(H,). We assume that py(n) = py(M1.A) and write
pA”YS(MeA)
+ pANYS(H,) (2)
the ester having similar size and structure as HA. In order to arrive t i t an estimate of the relative hydrogen bond donating capacities of M and the aprotic solvents toward A we assume
.
as was done by Alfenaar and de L i g n ~for ~ , the ~ solvent
from values of A N A S log Kf(HA2-) and ANAs log Kf(HAC1-), denoted by A N A s log Kf(HAA’- ). We consider that pANys(HAA’-) consists of the neutral nonhydrogen bonded component of HA, the nonelectrostatic (neutral) part of A’- plus the electrostatic part of the conjugate, viz. P*”Y~(HAA’-)= p * v y n )
+
pA”YS(A’),,
pAN7”(HAA’-),~ (6)
I. M. Kolthoff and M. K. Chantooni, Jr., I. Arner. Chern. SOC.. 93, 3843 (1971). E. Clare, D. Cook, E. KO, Y . Mac, and A . J. Parker, J. Amer. Chern. Soc., 88, 1911 (1966). M. Aifenaar and C. L. de Ligny. Red. Trav. Chim. Pays-Bas. 86, 929 (1967). D. Ea%,C. de Ligny, and M .Aifenaar, R e d . Trav. Chim. Pays-Bas. 91,452 (1972).
The Journal of Physical ChernisVy, Vo!. 77, No. 4, 7973
M. K. Chantooni, Jr., and I. M. Kolthoff
528
1o4
1,000
100
/ 4 / P
102 0.010
0.1
10
CHR
Figure 1. Plots of [12[Na+]2- K”]/K”” vs. C(HR) on logarithmic scales for sudium salts in AN: 0 , p-bromobenzoate: 0 3,4-dichlorobeatzoate;A , 3,4-dimethylbenzoate.
t
1
0.01
,
e
1
3
.
a
l
l
&
0.1 C~~
In eq 6 it is assumed that hydrogen bonding of HAA’-- to the solvent is negligible. Substituting eq 2, 3, and 6 into eq 5 , the following results
+
ANAs log Kf(HAA’-)
= -pANYS(H,) s p Y (HAA’-),I pAP;YS(A’-),I (7)
-
AY
In deriving eq 7 it is not necessary to assume *NyS(n) = A N Y S ( A ’ ) ~ . When the values of the electrostatic medium transfer coefficients pANys(A’-)el and pANys(HAA’-),l are small, as is encountered between close to isodielectric aprotic solvents, they may be taken equal in eq 7. Hence
Equation 7a states that the difference in the logarithm- of the formation constant of a 1:I conjugate in a given pair of solvents is a measure of the relative hydrogen bond accepting propertie., of the solvents. A similar treatment for the 2:1 conjugate (
ANAs log Kf((I-IA),A’-) p
AX
S
Y ((WA)2A’-),i
-
+
-2p*’?”(H,) p4V((A’-),l
-2pAUYS(H,) (8)
where Kf((HA)zA’-1 denotes the overall formation constant of ( H A ) & - . ExperimentaF Section
Solvents, Act&, and Salts. Acetonitrile,5 N,N-dimethylformaniide,~ dimethyl s ~ l f o x i d e ,methan01,~ ~ and acetone6 were purified as described elsewhere, while methyl isobutyl ketone was a product purified in this laboratory.? Subtditut ed benzoic acidsa and acetic acids were products used pyeviously, while p-dimethylaminobenzoic The Journal of Physical Chemistry, Yo/. 77, No. 4, 1973
Figure 2. Plots of [f2[K+]* - K s P ] / K s p vs. C ( H A ) on logarithmic scales for potassium chloride with substituted benzoic ’acids, and acetic and dichloroacetic acids in A N , Ac, and DMF: 1, benzoic: 2, p-bromobenzoic; 3, 3,4-dichlorobenzoic;4, p-nitrobenzoic: 5; 3,5-dinitrobenzoic: 6, acetic; 7, monochloroacetic; and 8, dichloroacetic acids: unprimed, in DMF; singly primed, in Ac, and doubly primed, in AN. Left-hand scale constructed for curves 2-6, l’, and 7”; right-hand scale for curves 4’-6‘, 8’, e”, and 8”. and 4-chloro-3,5-dinitrobenzoicacids were Aldrich products recrystallized from ethanol-water mixtures. Monoand dichloroacetic acids were Eastman IKodak White Label products distilled at 20 mm pressure. Sodium and potassium chlorides were Merck Reagent Grade, while the sodium salts of the substituted benzoic acids were prepared in the same way as thep-bromoben~oate.~ Esters. Methyl p-bromo-, p-iodo-, and p-nitrobenzoates were Eastman Kodak White Label products. All the other methyl esters except p-dimethylaminobenzoate were prepared by reaction of the acid with Eastman White Label phosphorus pentachloride followed by treatment with methanol.10 The p-dimethylaminobenzoate ester was prepared by esterification of 1 M solution of the acid in boiling methanol in presence of 0.05 M sulfuric acid (in excess). Ethyl and isopropyl p-nitrobenzoates were prepared by reacting Aldrich p-nitrobenzoyl chloride with the appropriate alcohol at the boiling point.lO All esters were recrystallized from ethanol-water mixtures and dried at 50” (5) I. M. Kolthoff and M. K. Chantooni, Jr., J . Phys. Chem., 76, 2024 (1972), and references therein. (6) T. Jasinski and Z.Pawlak, Rocz. Chem., 41, 1943 (1967). (7) J. Juillard and I. M. Kolthoff, J. Phys. Chem., 75, 2496 (1971). (8) . . M. K. Chantooni, Jr., and I. M. Kolthoff. J Arner. Chem. Soc.. 92, 7025 (1970). (9) I. M. Kolthoff, M. K. Chantooni, Jr., and S. Bhowmik, J . Arner. Chem. SOC..90, 2 3 (1968). (IO) R . L. Shriner and R. C. Fuson, “‘The Systematic identification of Organic Compounds,” Wiley, New York. N. Y . , 1948, p 165.
Medium AcZivity Coefficientsof Substituted Benzoic Acid
529
TABLE I: Specific Conductivity of Saturated Solutions, Lsat., and pKsP of Sodium Salts DMF
DMSO l__l___
pAN~
Salt
A
p-Dimethylaminobenzoate
sx Lsat., 104 ~
10.08
PK~P
6.54'
xLsat., 104
PKSP
4.70 5.36a.b
3,4-Dimethylbenzoate Benzoate p-Bromobenzoate 3,4-Dichlorobenzoate
8.1 e 8.7b 8.44e 8. 14e
1.83e 5.2e 4.78e 3.8a3e 1.28e 5.4e 1.82e 5.3a3@ 8.45e 3 . 1 4 e 3 . 1 F 4 . 8 " ~ ~9 . 3 5 " ~ 3.22are ~ 9.3e 3.2e
p-Nitrobenzoate
8.24d 3.88e
4.6"3e 4.1c
rn-Nitrobenzoate 7.gd 5.78e 4,3e 3-Nitro-4-chlorobenzoate 7.44d 10.6e 3,74a3e Acetate 9.16 6.Tb
*
a Salt solvated in solid phase. Reference 5. ref 19. ReferenceB. eThis work.
pKSP
4,8b of potassium salt:
sium chloride in presence of HA was described elsewhere.8 Solubilities of benzoic acids in the various solvents were determined by titrating alkalimetrically in ethanol-water Figure 3. Plots of p A N y M ( A - ) , and pANyM(A-),] vs, mixtures aliquots of the filtered saturated solutions, phen( P K ~ ( H A ) ) A(A~ and B, respectively) for substituted benzoates olphthalein serving as indicator. and acetate: Filled in circles, meta- and para-substituted benMethods involving distribution of H A between S (AN, zoates, open circles, ortho-substituted benzoates: half-filled circles, acetate. Numbers are those in Table I l l . Least-squares DMF, DMSO, etc.) and a hydrocarbon for the estimation slopes of A and B are -0.54 and -0.43; the intercepts are of pANys((HA) were found to be subject to complications 4-4.95and 9 2 . 2 3 , respec:tively. due to complex formation between HA and DMSO or DMF in the hydrocarbon layer saturated with S, Such a at atmospheric pressure. Their melting points were 1.02, solution became considerably undersaturated in the pres80, 113, 78, 46.5, 96, 55.5, 110, 82, 110, and 105.5", the es- ence of HA and a long studyl3 for each acid would be necters listed in order of appearance in Table 111. These melt- essary to calculate the solubility of S in hexane containing a known concentration of HA. ing points agree within 1-2" with the literature values.11 Solubilities of esters in M and AN were found by taking Crystal Soluatm of Salts and Acids Crystal solvates of sodium 3,4-dichloro- 3,4-dimethyl-, p-dimethylamino-, the filtered saturated solutions to dryness under a heat lamp and weighing the residue to constant weight, taking p-bromo-, p-nitro- Iind 3-nitro-4-chlorobenzoates with care not to volatilize the esters. Solubilities of the esters DMF and DMSO (Table I, footnote tu) were prepared by in DMF, DMSO, and MIBK were found by introducing filtering a slurry of the salt in DMF or DMSO and washing with n-hexane or (ethyl ether, as described p r e v i ~ u s l y . ~ aliquots of the filtered saturated solutions into 10 volumes of water, in which the esters are sparingiy soluble, removAssay, by dissolving the salt in 0.3 ml dry acetic acid, ing the water together with DMF or DMSO by means of a adding 5 ml AN, and titrating the benzoate with standard 0.5 M perchloric acid in acetic acid using a-naphtholben- filter stick, washing the ester with water, drying at 50", and weighing. The MIBK was removed as an azeotrope zein as indicatorlz corresponds to NaA.O.25DMF,DM SO with water by carefully heating after flooding the saturatfor 3,4-dichloro- and p-bromobenzoate and to NaA.l.00 & ed MIBK solution with water. O&DMF,DMSCs for the other solvates. Blanks were run with the corresponding amounts of DMF and DMSO in Results AN, Crystalline solvates of benzoic acids with DMF or Ionic Mobilities. In the calculation of Ksp(NaA) and DMSO were prepared in the same way as those of the Kf(HACl-)(Kf((HA)&l-)) from conductance data the salts. Their melting: points were not sharp. Assay by alkafollowing values of ionic mobilities at infinite dilution limetric titration in water-ethanol mixtures using phenolhave been used previously: N a t , 70 in AN,14 30 in phthalein as indicator corresponded to monosolvatt. of 90.6 in Ac;18 DMF,I5 13.1 in DMSO;16 K + , 85 in monoultra-substitual ecl and disolvate of dinitro-substituted (11) J. Pollock and R . Stevens, Ed., "Dictionary of Organic Combenzoic acids. Crystalline solvates with DMF or DMSO pounds," Vol. 1-5, Oxford University Press, London, 1965. were not formed with benzoic acids having other substitu(12) I . M. Kolthoff. M . K, Chantooni, Jr., and S . Bhowmik, Anal. Chem., ents. I t was also found that all the solid benzoic acids and 39, 1627 (1967). (13) R . van Duyne, S. Taylor, S. Christian, and h. Affsprung, J. Phys. their sodium salts studied remained unsolvated in M and Chem., 71, 3427 (1967). AN (14) S. Minc and L. Werblan, Rocz. Chem., 40, 1537 (1966). (15) J. E. Prue and P. J. Sherrington, Trans. Faraday SOC., 57, 1795 Techniques 'The conductometric determination of ionic (1961). solubilities of lwdium benzoates in the various solvents (16) P. J, Sears, G. Lester, and L. Dawson, J. Phys. Chern., 60, 1433 was the same as for wdium acetate,s while that of potas(1956). I
The Journal of Physical Chemistry, Vol. 77, No. 4, 1973
M. K. Chantooni,Jr., and I . M. Kolthoff
530
TABLE II: Homo- and Heleroconjugation Constants with Chloride of Substituted enzoic, Acetic, and Chloroacetic Acids in Aprotic Solventsa AN Acid
Benzoic p-Bromobenzoic 3,4-Dichlorobenzoic p-Nitrobenzaic 3,5-Dinitrobe~zoicJ Acetic Monochloroacetic Dichloroacetick
Log Kf (HACI-)
Log K f (HA2-)*,T
3.60'
2.2' [-0.1]' 2.3@
2.4
2.5e
3.85' 4.0'
3.6g
DMSO
DMF
Log K f (HA2-)*
2.5'[0.33]" 2.9' [0.65]' 2.5~~3~
2.6 2.6
2.73d[0.71d 3.4d [O.4Id
Log K f (HACi-) b,d
Log K f (HA*-)'*g
Ac Log K f (HAP-) b , h
- Log K f (HACI-)
NM
~
_
_
Log K f (HA>-) 6 , h
1.8
0.9 1 .o 1.2 1.5 1.3
4.1 1.5
U5 3.5
n A c = acetone, RIM = nitromethane. * N o data available of Kf((HA)2A-) of Kf((HA)zCI-); values expected to be very small. Reference 8. dThis work. e From Hammett plot of log Kf(HACI-) vs. 6,ref 8. f I. M. Kolthoff, M. K. Chantooni, Jr., and H. Smagowski, Anal. Chem., 42, 1622 (1970). g Reference 9. 'T. Jasinski and 2. Pawlak, Rocz. Chem., 43, 605 (1969), and references therein. & Reference 18. 'Log Kf(HA2-) =: 4.3, ref 7. Log Kf(HAz-) = 6.3, log Kr(HACI-).= 6.7 in NE, ref 18.
e l - , 105.2 in Ac;18 A- (acetate or substituted benzoate) in AN assumed the same as 3,5-dinitrobenzoate, 100.5 Values of Ao(A-1 in DMF and DMSO equal to 43 and 17.6, respectively, were calculated from that in AN from the Walden product. The Walden product for anions holds for our three aprotic protophobic solvents. Previously, the value of Xo(H,4CI-) = 55 in AN8 was used, while those in Ac and DN[F equal to 63 and 24, respectively, were calculated from the Walden products. Solubilities of Sodium Salts. Specific conductivities of saturated solutions and values of pKsP of sodium salts of substituted benzoic acids in DMF and DMSO are listed in Table I. Since these salts are not sufficiently soluble in AN to allow the direct determination of ionic solubility from the conductivity of their saturated solutions, values of pK*P(NaA; in Table I were found conductometrically from the ionic solubility in presence of p-bromophenol as described previously.8 Plots of p [ N a 1J2 - Ksp(NaA)]/ K$P(NaA)1)s. C(EIA) are presented in Figure 1. Conjugatlor; of HA with Chloride The formation constants of HACl- and (HA)ZCl- in AN, DMF and Ac were estimated BS previously in AN8 conductometrically from the ionic sokbillity of potassium chloride in presence of substituted benzoic acids, acehic or dichloroacetic acid. Plots of p [ K + ] z - Ksp(KCl)]/Ksp(KCl) us. C(HA) are presented in Figure 2. Values of pKSP(KC1) were reported previously as 8.1),5 5.FjY5 and 9.2'8 in AN, DMF, and Ac, respectively. In the present investigation the specific conductivity of a saturated solution of potassium chloride in acetone was found to be 2.3 x 70-6 ohm-1 cm-I, yielding pPW(KC1) = 9.8, this value being used in our calculations. Previously18 Pawlak reported pKSP(KC1) = 9.2. Because of the low solubility of this salt in acetone and the possible effect of small amounts of water, values of pKSp( KG1) aw approximate. Evidence of formation of (HA)2C1- us found in the present study in acetone, as in but not in the stronger hydrogen bond acceptor DMF. Values of log Kf(HACl-) and log P(HACl-) (in brackets) are presented in Table I1 where P(HAC1-) = [(HA)&- l / [ ~ A ~ [ ~ ~ A(f((HA)ZCl-) ~l-J = f(HACl-)). Since the effect of HA 0x1 the total solubility of potassium chloride in DMSO i s very small,18 considerable uncertaint y 1s involved in the estimation of Kf(HAC1-) in this solThe Journal of Physical Chemistry, Vol. 77, No. 4, 1973
vent; consequently these values have been omitted from Table 11. Solubility of Brnzoic Acids and Their Esters The molar solubilities of substituted benzoic acids in N1, AN, DMF, and DMSO are listed in Table 111, the ionic solubilities being negligible. Solubilities of the methyl esters of the substituted benzoic acids in the four solvents are also entered in Table 111. Not included in Table I11 are the solubilities of 3,5-dinitrobenzoic acid and its methyl ester in methyl isobutyl ketone, 0.37 and 0.53 M . respectively. 'Po account for the effect of the alcohol residue on the medium activity coefficient of the ester, solubilities of methyl-, ethyl-, and isopropyl-p-nitrobenzoates in M, AN, and DMSO were determined and the data reported in Table 111. Hardly any effect of the alcohol residue containing three carbon atoms or less on pANyS(ester) is encountered among the aprotic solvents and M, p*NyM(MeA), pANyM(EtA), pANyM(i-PrA)being 0.84, 0.77, and 0.69, respectively. Between AN and DMSO the corresponding values are 0.05,0.06, and 0.24, respectively. Dissociation and Homoconjugation Constants of Acids. Values of the dissociation constants of substituted benzoic acids and acetic acid in M,19 AN,I DMF,1 and DMSO1 were used in the calculation of pANys(HA) from eq 1and
Since the values of pKd(HA) of ~ - d ~ ~ e t h y i a m i n o ~ e n z o i ~ acid in M, AN, DMF, and DMSO are not available in the literature, they were determined in the present study. In M the following paH values were found in mixtures of the acid and its potassium salt a t constant ratio C,(M)/ C,(M): C, = 1.86 X 10-3, Cs = 2.50 X 10.32; 4.4 X 10-3, 5.98 x 10-3, 10.25; 8.25 x 10-3, 1.11 x 10-2, 1 0 . 2 ~ ; and 1.09 x 1.47 x 10.18. Extrapolation to zero concentration as recommended by Juillard3g yields pKd(HA) = 10.40. The paH of equimolar mixtures p dimethylaminobenzoic acid and its t e ~ r a b u ~ , ~ l a n ~ ~ o n ~ u m M , 3.4.40 in salt was 22.89 in AN when C = 3.64 X DMF when C = 2.80 X M , and 13.02 in DMSO when (17) J. Forcier and J. Olver, Nectrochim. Acta, 7,257 (1962). (18) Z. Pawlak. Rocz. Chem., 46, 249 (1972). (19) J. Juillard,J. Chim. Phys. Physicochim. B i d , 691 ('1970).
Medium Activity Coefficients of Substituted Benzoic Acid
531
TABLE Ili: Solubilities of Substituted Benzoic Acids (HA) and Their Methyl Esters (MeA) in Various Solvents
1 :3,4-Dimethyl2. p-Dimethylamino-
0.595' 6.086'
3. Unsiibstitijlted 4. p-Bromo5. p-lodo-
3.1P
6 , m-Brorno7. m-Nitro8. 3,4-Dichloro9. :3,5-Dichioco-. 10. ~i-Nitro-
1'1, 3-Nitro-4-chioro-
12. 3,5-Dinitrw13. 4-ChlOro3.5-dinitro14. o-Chloro15. o-Nitro16. 2,4-Dinitro17. 2,4-Diehliaro-
0.12' 0.068' 1.51c 3.46a Q.27C 1.25' 0.20a
0.70~~ 1 .23c
1.%ic 2.53' 2.9ga 3.23' 1.34c
0.273 0.31 0.076 0.265 0.95
0.18 0.757d 0.11e 0.21 0.10~ 0.12,
a i. M. Kolthoff, 3. J. Lirigane, and W. Larson. J. Amer. Chem. Ethyl ester. e Isopropyl ester.
0.091 0.023 0.85 0.018 0.0098 0.167 0.;78 0.028 0.085 0.041
0.80 0.156
1.19 1.10 5.35 0.93 0.72
2.02 2.38
1.906 2.86
2.29 3.75
2.5,b Z14
1.60 2.66
1.26 4.4p
0.2g6
1.73
o.80b
1.1, 3.88 0.312e
1.426
2.16 1.86
2.47b
1.15
1"576
1.33
0.86
3.07 1.95
1.96
0.605
2.45 2.14
0.93 0.41
2.22 1.69
0. 0.133 0.23
1.44 1.15
1.22 0.!53
1.70
1.306
o.:m
4.74 2.58'
1.02 0. '1 ei8
2.93
Sac..60,2512 (1938). bAcid solvated in solid phase. CThis work; all esters this work.
and AHf(Ha)(DMSO) - AHf(Ha)(2-butanone) = -1.98 kcal/mol as compared to pM1BKyL7MF,DMsoO(Ha) free energy values of -1.3 and -2.0 kcal/mol, respectively, found in this study. It may be concluded, therefore, that AASf(H,) between DMF, DMSO, and MIBK is equal or close to zero, within experimental error, assuming that pMxBKDiscussion ?%butanone (Ha) = 0. From ir studies of Av(0H) and Kf in carbon tetrachlorHydrogen Bond Arcepting Properties of Solvents. The ideZ0and I9F nmr studies of A and Kf in cyciohexanezl of agreement of ~AN,~DMF,DMSOO,~~~~(H.) values found for the hydrogen bonded complex between p-fluorophenol and benzoic acids from solubility data in Table 111 using eq 2 various bases, the same order of hydrogen bond accepting with those from Kf(HAz-) and Kf(HAC1k) (Table 11, eq capacity DMSO > DMF > 2-butanone, benzonitrile was 7a) is gratifying, ceneidering the different assumptions on found, as given above. The latter two bases have compawhich these equations are based. pANyDMF*DMso,Ac*NM(Ha) rable hydrogen bond accepting capacities as AN. values derived from Kf(WAC1-), Kf(HA2-) values For acetic acid, pANyM,DMF,DMSO(n) has been evaluated in Table TI for acetic and chloroacetic acids are the same in the present study, using eq 2, from known values of as for the benzoic acids. It is of interest to note that pANyM%DMF.DMSO(HAc) equal to -0.4, -0.2, and -1.2 pANyDMFJIMso(Haj equal to -1.2 and - 1.8, respectively, (Table IV) and the average values of pANyM*"MF,DMso(H,) derived from Kf( H W - ) values previously reported1 using given above. The value of p*NyM(HOAc) has been calcueq 7a (HIE = p.bromopheno1) agree well with those fTom lated from eq 1 using previously reported values of carboxylic acids.. Average values ot pANy5(Ha) equal to pANyM(H+) = -6.2,5 pANyM(Ac-) = -7.0,5 and -1.9, -13, -1.4, 4-0.1, +0.4, and +3.0 were found, S 12.8. Resulting values of ANAMpKd(HA~) = being DMSO, M, DMF, Ac, MIBK, and NB, respectively, pANyMvDMFTDMSO(n) are 0.5, 1.5, and 1.7 unit, sespectiveiy, in order of decreasing hydrogen bond accepting capacity more positive for acetic acid than for the average of the toward HA. benzoic acids. The values of the stepwise formation constants of Medium Transfer Coefficient of A - . Values of (HA)&l- 1x1 Table IL are approximately the same in AN pAKyS(A-) and pANyS(A-),, reported in this paper are and Ac and much smaller in DMF as predicted from eq 7a based on the tetraphenylborate assumption. Among the and 8 using the average values of pANyAc,DMF(H,). aprotic solvents AN, DMF, and MSO, values of It i s of interest to compare our data of Py(Ha) with values pANyDMF,DMSo(A-),l in Table IV are found essentially inof the enthalpy of' formation AHf(Ha) of the hydrogen dependent of the basic strength of the substituted benzobond between phenol and various solvent bases by Arnett, et al20 They a plied the pure base method, taking (Ha)= AMf(phenol) - AHf(anisole), the latter ac(20) E. M. Arnett, L. Joris, E. Mitchell, S. Murty, T. Gorrie. and P. v. R. Schleyer, J. Amer. Chern. Soc., 92,2365 (1970). counting for the nonhydrogen bonded contribution. They (21) L. Joris, J. Mitsky, and R. W. Taft, J. Amer, Chem. Soc., 94, 3438 found A ~ e ( I ~ a -~ AHf(Ha) ( ~ ~ ~(2-butanone) ~ ) = -1.67 (1972).
C = 4.0 x Air, yielding pKd(HA) = 23.0, 14.5, and 13.1 in AN, DMF, and DMSO, respectively. Previously reported values of homoconjugation c'onstants of substituted benzoic and acetic acids in the various solvents are tabulated in Table 11.
The Joornai of Physical Chemistry, Voi. 77, No. 4, 1973
M. K. Chantooni, J r . , and I . M. Kolthoff
532
TABLE 1V: Medium Activity Coefficients of Substituted Benzoic Acids, Methyl Esters, and Benzoates between AN, SAANpKSp(NaA) Benzoic acid
“AANpKd(HA)
Obsd
Calcd
DMF, and DMSOa
pANyS(HA) Obsd
Calcd
pANyS(MeA)
pANyS(A-)
l_ll^___l_-___lll____
pANyS(A-),~ pANyS(H,) -._________-
s = DMF 3,4-Dirneth,ylp-DimethylaminoUnsubstituted p-Bromop-lodo-
3,4-Dichlorop-Nitro3-Nitro-4-chlororn-Nitro3,5-Dinitro-
-8.2 -8.5 -8.4 -8.2 -8.2 -8.1 -8.1 -8.5 -8.5 -8. -8.
Acetic
-2.9
-3.5
- 3.3 -3.lC
-3.3
-3.3c -3.6c -3.7c -3.6
-3.5
-1.1 -1.7 -1.3 -1.7 -1.9 -2.0
-0.4
-0.9b -1.0b -0.4b -1.7 -0.7~~ -0.2
-2.4
-0.3 -0.6 -0.2 -0.14 -0.2 -0.1 -0.2
1.1 0.5
0.9
-1.3
0.7 0.7
1 .o
0.5
0.7
-1.4 -l.3 -1.8
0.4
0.5
-1.6
Av +0.8
Av -1.5
1.6 Av -0.3
p-Dimethylaminop-Bromop-lQdO-
3,$-Dichlorop-Nitro3-Nitro-4-chloio3 , s -Dinitro-
rn-NitroAcetic
-9.8 -9.9 -9.8 -9.8 -9.8 -9.7 -10.0 -9.5 -10.1 -9.7
-4.3c -5.3
-4.7
-5.3 -4.9
S = DMSO -1.5 -1.9 -2.1 -2.3 -1.9 - 1.3O 1.3b
-
-o.ab
f0.2
+0.1 -0.5 -0.5
-0.2
-0.1
-0.1
-1.6 -2.1 -1.9 -1.9
-0.5
+0.1
-0.1 +0.1
-0.5b
-4.3
-0.3 0.0 -0.4 0.0 +0.1
k0.2
-1.2
+0.5
Av -0.1
Av -0.3
kO.2
Av -1.9
f0.2
--
a PAN Y DMF(Hi) --10.6,5 pANyDMSo(Hf) = -11.4,5 pANyDMF(Na+) = -4.0,5 pANyDMSo(Na+) = -4.8.5 Acid solvated in solid phase; py(HA) calculated as described in Discussion. Salt solvated in solid phase: sAANpKSP(NaA)calculated as described in Discussion.
tuted acetate and benzoate, respectively, as compared to PM~DMF(A-),~values of 8.4 and 7.3 in the present paper. The difference of about 2 units in MyDMF(A-)H and pMyDMF(A-)el is probably due to nonhydrogen bonding ion-solvent interactions (ion-dipole, etc.), but may be accounted for in part or entirely by the validity of the y(MeA) = y(n) assumption, the tetraphenylborate assumption, and Parker’s assumption that k M / k D M F = y(A-)H. Crystalline Solvates of HA and NaA. In systems in which either the acid or its sodium salt foirms solid solor vates in DMF and DMSO either pAWyDMF@fsO(HA) pANyDM5,DMSo(A-) has been estimated from eq 2 using the data in Table IV. No crystalline solvates of HA or NaA are found in AN. From py(HA) (or py(A-)) thus found the solubility of HA (or KsP(NaA)) is calculated; hence p*NyDMFJJMso(HA) or p*NyDMF,DMsO(A- 1 of the unsolvated species become known. The free energy of solvation (in kcal/mol) is equal to 1.36 (pANyDMF.DMSo(HA)unsolv- pANyDMF~DMso(HA),olv) for 1.A and 1.36 (~ANYDMF,DMSO(A-)~~~~~~ -- P A N ~ D M ~ , D M S Q ( A - ) ~ for ~~~) A M A ~ p =~ApSYM(A-),I d ~ ~ ~ -~ ~ NaA. For the three sodium salts which form solvates calA ~ ~ T Y H , ) A ~ ~ T ~ ( A - ) , culated , values of sA*NpK5p are presented in Table IV. In an excellent and thought provoking paper Parker, et Considering the uncertainties in values of py(HA) and aC.,z attributed the large changes in AApKd(HA) with the py(A-) of 10.1 to iO.2, we may conclude that AFRolvof basic strength of A- going from M to DMF to changes in these sodium salts in DMF and DMSO is equal to -0.3 f the hydrogen bond accepting property of A- expressed by 0.2 kcal/mol. ~ rates Parker them as y(.4-)H. la fact, from S N reaction From the data in Table IV an average value of reported p”yDMF(A-)H equal to 6.2 and 5.5 for unsubstip*NyDMF(HA) of the unsolvated acid, excluding 3,4-dirnethate. Furthermore, they are numerically small as compared to those of the aprotic solvents and methanol. Average values of p*N-:DMF,DMsO(A-),i are found equal to +Q.8 i 0.2 and -Q.3 i 0.2. On the other hand, values of pANyM(A-) as well as those of p*”yM(A - ),I are very negative (indicating strong solvation in M) and become increasingly so with increasing basic strength of the substituted benzoate ion, as ilus. lustrated by the plots of pANyM(A-) and P*N~M(A-)~] (pKd(HA))AN in Figure 3. The slopes of these plots are -0.54 and -0.43, respectively, as compared to (p(M) p(AN))/p(AN) = -0.44 predicted from combining the Hammett p a yelation of pKd(HA) with eq 1 or 4, respectively, arid taking py(WA) or p?(Ha), respectively, as constant. Values of p ( M ) = -1.36l9 and p(AN) = -2.411 have been previously reported. It is therefore evident that the effect of substituent on ANAMpK*(HA)is accounted Emost entirely for by changes in P * N ~ M ( A - - )It~ ~ follows . from eq 4 that for two acids
-
The Journal of Physical Chemistry, Vol. 77, No. 4, 7973
Solubilitiesof Alkali Metad Chlorides ylbenzoic acid is - li.7. Assuming this value for pANyDMF= - 1.6 (H,A) of unsolvated nz-nitrobenzoic acid, AF,,,, kcal/mol for this acid i n DMF. From the observation that a considerable quantity of heat is evolved accompanying solvate formation of HA and NaA and that values of AFsolv are small, it appears that large negative entropy changes are involved. On the other hand, dissolulion of the esters in all solvents studied
535
(saturated solutions) is considerably less exothermic than the corresponding acids which indicates that the former do not form crystalline solvates.
Acknowledgment. We thank the National Science Foundation for a grant (No. GP-20605) in support of this work.
ali Metal Chlorides in Some Amine and Ether Solvents trong and T. R. Tuttle, Jr.* Department of Chemistry, Brandeis University, Waltham, Massachusetts 02154 (Received June 16, 1972)
Solubilities of alkali metal chlorides except lithium were determined in ammonia, EDA, methyl- and ethylamines, DME, and THF at three different temperatures. From these data, standard thermodynamic functions of solution and solvation with respect to ion formation were calculated, The standard free energies of solvation increased linearly as a function of D - I . However, the requirements of the Born equation were not satisfied because the slopes and intercepts of these plots were not equal. Values of the entropies of solvation are roughly accounted for on the basis of the freezing out of a fixed number of solvent molecules. The superiority of ammonia as an agent for solvating ions is attributed to its relatively Zuw molecular entropy compared to other solvents. A criterion for salt-like behavior is introduced and used to suggest that saturated alkali metal solutions are salt like and are probably dominated by the M+oM- ion pairs. On this basis solubilities of metals in ammonia are too large to be accounted for withiout introducing additional species.
Introdamtion Solubilities of alkali metal halides have been measured in liquid ammonia,' but not in alkylated amines, although these solvents exhibit somewhat similar properties. In order to compare the behavior of ionic solutes in these solvent systems solubility measurements of slightly soluble alkali chlorides, with the exception of LiCl, have been made. From thic; information standard free energies of solution and solvation are calculated, and comparisons made of trends with respect to changes in solvent dielectric constant and trends with respect to cation size in a given solvent system. The measurements were carried out in ammoinia, methylamine, ethylamine, and ethylenediamine (EDA), as well as in two ethers, tetrahydrofuran (THF) and 1,2-dirnethoxyethane (DME). The ethers were included due to their similarity to amines with respect to formation of alkali metal solutions and were also of interest with respect to specific sol~renteffects as they have dielectric constants which are very close to that of ethylamine. A major stimulus for these solubility measurements was our interest in metal isolutions in these polar solvents. The question of whether the metal solutions behave like solutions of salts i s of considerable importance and interest in assessing the nature ofthe metal solutions.
Experimental Section Materials. Solutes were of the highest purity commercially available. NaCl and KC1 were Fisher Reagent Grade, CsCl was 99.97% from K & K Laboratories, and RbCl was Mann analyzed, optical grade from Mann Research Laboratories. All salts were dried over P& in vacuo at 100"for 24 hr and stored over drierite. Ammonia from Matheson was dried in uucuo over Na-K alloy and used without further purification. Ethylamine and methylamine from Matheson were treated to remove ammonia impurities.2 Gas chromatographic analysis3 showed the final ammonia content of each to be less than 0.01 mol %. These solvents were dried in wacuo over Na-K alloy prior to use. EDA from Eastman Organic Chemicals was predried over CaHz for several months and fractionally distilled from CaHz onto sodium pieces in a dry nitrogen atmosphere. The fraction boiling at 115.9-116.0' was collected and vacuum distilled onto Na-K alloy for final drying. The characteristic deep blue color of alkali metal solution in EDA was taken as an indication of dryness. (1) G. Heymer and A , Schneider. Z. Anory. Allg. Chem., 302, 306 (1959). See also ref 1 1. (2) Ian Hurley, Thesis, Brandeis University, Waltham, Mass., 1970, (3) I . Hurley, T. R. Tuttle, Jr., and S. Golden, J. Chem, Phys., 48,2818 (1968).
The Journal of Physical Chemistry. Vol. 77, No. 4, 1973
