- Email: [email protected]
Electrostatic model for substituent and solvent effects on organic acids
Fluid Phase Equilibria 136 Ž1997. 31–36
Electrostatic model for substituent and solvent effects on organic acids Xin-Gen Hu, Han-Xing Zong ) , Rui-Sen Lin Department of Chemistry, Zhejiang UniÕersity, Hangzhou 310027, China
Abstract On the bases of experimental results from gas phase and electrostatic theory a modified thermodynamic model for substituent and solvent effects on organic acids has been derived. It is found that the present model is quantitatively and simultaneously consistent with the experimental results obtained from gas phase, aqueous solution and mixed solvents such as ethanol–H 2 O. q 1997 Elsevier Science B.V. Keywords: Electrostatic model; Substituent effect; Solvent effect; Organic acids
1. Introduction Quantitative evaluation of substituent and solvent effects on the dissociation of organic acids has been playing an important role in physical organic chemistry. The well-known Hammett equation throws light on this problem empirically. Hepler’s ‘internal-environmental’ model w1,2x succeeds in predicting the general form of Hammett equation, with s A dDH int , r A 1 r T. dDG 0 s dD Hint Ž 1 q g . in which, g is a solute–solvent interaction parameter dependent on solvent, temperature, pressure and reaction type. In his modified electrostatic model w3x, g was defined as
g s B´ ir Ž 1 y ´ s . in which, B was considered to be a dimensionless and solvent independent parameter, ´ i and ´ s are dielectric constants of a hypothetical solvent and the real solvent, respectively. This earlier approach has been gradually leading to improved understanding of substituent and solvent effects. However, this model cannot be quantitatively and simultaneously consistent with the experimental results from gas phase, aqueous solution and other solutions of high dielectric constant w8x. In order to solve this problem, a modified electrostatic model, based upon Hepler’s idea, is derived in this paper. )
Corresponding author.
0378-3812r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 1 2 Ž 9 7 . 0 0 1 3 7 - 4
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
32
2. Construction of the model Proton transfer process such as HAX q AOys AXyq HAO
Ž1.
can be used to describe substituent effect, where HAO is a reference acid, HAX is a homologous substituted acid. Let’s consider the following thermodynamic cycle
d DG 0 s dDG int q DGt0 Ž AXy . y DGt0 Ž AOy . q DGt0 Ž HAO . y DGt0 Ž HAX .
Ž2.
Assuming that the difference in transfer free energies between neutral molecules Ž HAO and HAX. is negligible, and that the one between anions ŽAOy and AXy. is determined by the difference in solvation free energies, we have dDG 0 s dDGi nt q DGs0 Ž AXy . y DGs0 Ž AOy .
Ž3.
According to electrostatic theory for solvation, we obtain DGt0 Ž AXy . y DGt0 Ž AOy . s Ž qo2 y q x2 . Ž 1 y 1r´ s . r2 a
Ž4.
in which, qo and q x represent effective charges on the functional group of AOy and AXy, respectively, ´ s is the macroscopic dielectric constant of solvent. a is the solvation radius of the functional group. The relationship between qo and q x is assumed to be w3x q x s q o q Ž d qrd m . m x
Ž5.
in which, m x is the point dipole moment associated with the substituent x. So, we have DGt0 Ž AXy . y DGt0 Ž AOy . s qo m x Ž d qrd m .Ž 1 y 1r´ s . ra
Ž6. . 2 2x .
where we have omitted the relatively small terms involving Ž d qrd m m In gas phase, it was found that w4–6x dD Sint f 0 dDG int f dD Hint dD Hint f e m x cos frc 2
Ž7. Ž8. Ž9.
in which, e represents the magnitude of the electron charge; f represents the angle between the dipole line and the line drawn to connect the dipole with the center of the functional group. c is the distance between the substituent x and the proton to be dissociated from HAX.
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
33
Combination of Eqs. Ž 3., Ž6., Ž8. and Ž9. gives dDG 0 s e m x cos frc 2 y q0 m x Ž d qrd m .Ž 1 y 1r´ s . ra
Ž 10.
Define a dimensionless parameter B, B s Ž c 2 qorae cos f . Ž d qrd m .
Ž 11.
Therefore, Eq. Ž 10. can be rearranged as dDG 0 s e m x 1 y B Ž 1 y 1r´ s . cos frc 2 s e m x cos frc 2 y B Ž 1 y 1r´ s . e m x cos frc 2
Ž 12 .
The right side of Eq. Ž12., as shown above, has been grouped into two terms: solvent independent and solvent dependent terms. According to ‘internal-environmental’ model w1,2x, we know dDG 0 s dD Hint q dDGenv
Ž 13.
Comparison of Eqs. Ž12. and Ž13. gives dD Hint s e m x cos frc 2
Ž 14.
dDGenv s yB Ž 1 y 1r´ s . e m x cos frc 2 s yB Ž 1 y 1r´ s . dD Hint
Ž 15.
Eq. Ž15. shows clearly that dDGenv depends not only on dD Hint but also on parameters B and ´ s . Define another dimension less parameter g ,
g s yB Ž 1 y 1r´ s .
Ž 16.
Then, dDGenv s g dD Hint
Ž 17.
dDG 0 s dD Hint Ž 1 q g .
Ž 18.
Consider the parameter B, B s Ž c 2 q0rae cos f . Ž d qrd m . s qo Ž q x y qo . radD Hint s qo Ž d qrdD Hint . ra
Ž 19.
It is noticeable that B is related with gas phase proton affinity ŽydD Hint ., effective charges on the acid anions Ž qo and q x ., and solvation radius of the functional group Ž a. that is dependent on solvent. Therefore, B is considered to be a solvent dependent parameter.
3. Discussion 3.1. Gas phase (boundary condition) In the gas phase, ´s s 1. According to Eqs. Ž16. and Ž18. , we find
gs0 dDG int s dD Hint dD Sint s 0
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
34
The results are reasonable for reaction like the type of Eq. Ž1., except that HAX contains a substituent close to the functional group with proton w3x. So, we conclude that the present model is consistent with gas phase boundary condition. 3.2. Aqueous solution Many measurements of gas phase acidities have shown w4,5,8–10x the correlation dD HintrdDG 0 s ca. 4–10 for various organic acids or bases such as phenols, aliphatic carboxylic acids, benzoic acids, and pyridines. According to Eqs. Ž 16. and Ž 18. , we discover
g s ca. y0.90–y 0.75 B s ca. y0.91–y 0.76 For homologous acids, g and B are constants. Because the values of d q r dD Hint in gas phase and solution are also constant, we conclude that the values of a for solvation radiuses of the functional group in the same solution are unchangeable. In a special reaction series, if there were several substituted acids deviate from the linear relationship as shown by Eq. Ž 18. , there should be different values of B. For example, for a series of meta- or para-substituted phenols w7x, dD HintrdDG 0 s 6.6, so g s y0.848, B s 0.859. But some phenols with hydrogen-bond-accepting substituents drift off the linear relationship, i.e., they have different g and B values as shown in Table 1. We discover that the values of d BdD Hint are of the reasonable magnitudes for substituent HBA effects. For HAB meta-substituents, the values of d BdD Hint characterize the effects of the specific solvation of these Table 1 HAB effect of substituents on the acidities of phenols Substituents
dD Hint ŽkJ moly1 .
dDG 0 ŽkJPmoly1 .
g
B
d BdD Hint
paraCN CO 2 CH 3 SO 2 CH 3 CH 3 CO SOCH 3 NO 2 CHO
y16.6 y11.7 y17.6 y13.3 y11.4 y20.9 y15.8
y2.76 y2.05 y3.01 y2.66 y2.39 y3.89 y3.27
y0.834 y0.825 y0.829 y0.800 y0.790 y0.814 y0.793
0.844 0.835 0.840 0.810 0.801 0.824 0.803
0.249 0.281 0.334 0.652 0.661 0.732 0.885
metaCH 3 CO SO 2 CH 3 CO 2 CH 3 OCH 3 NH 2 NŽCH 3 . 2 SOCH 3
y6.5 y12.8 y5.1 y1.1 1.4 1.2 y7.8
y1.10 y2.16 y1.08 y0.48 y0.15 y0.20 y1.75
y0.831 y0.831 y0.788 y0.564 y1.107 y1.167 y0.776
0.842 0.842 0.798 0.571 1.121 1.182 0.786
0.111 0.218 0.311 0.317 0.367 0.388 0.569
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
35
Table 2 dDG 0 of substituted benzoic acids in ethanol–H 2 O mixtures Ž298.15 K. V%
dDG 0 ŽkJ moly1 . meta-
0 10 20 30 40 50 60 70 80
g
B
2 ´y1 s =10
y0.904 y0.887 y0.877 y0.871 y0.863 y0.861 y0.864 y0.858 y0.860
0.916 0.899 0.890 0.885 0.878 0.878 0.883 0.879 0.885
1.27 1.35 1.42 1.53 1.71 1.90 2.14 2.44 2.87
para-
NO 2
Cl
CH 3
NO 2
Cl
CH 3
y3.97 y4.76 y5.09 y5.34 y5.63 y5.92 y6.17 y6.31 y6.29
y2.03 y2.35 y2.36 y2.24 y2.11 y2.10 y2.07 y2.30 y2.33
0.48 0.09 0.22 0.42 0.76 0.87 1.08 0.94 1.08
y4.37 y5.05 y5.55 y5.91 y6.37 y6.50 y6.51 y6.65 y6.51
y1.17 y1.60 y1.75 y1.79 y1.84 y1.93 y1.98 y2.06 y2.06
1.05 0.72 0.79 1.00 1.22 1.32 1.60 1.62 1.99
substituents on normal fieldrinductive effects, while for HBA para-substituents, the values of d BdD Hint characterize the specific substituent solvation assisted resonance effects Ž SSSAR. w7x. 3.3. Mixed solÕent The values of ionization free energies Ž dDG 0 . of some meta- and para-substituted benzoic acids ŽNO 2 , Cl, CH 3 . in ethanol–H 2 O mixtures have been determined by us Ž Table 2. w11x. The results of gas phase acidities Ž GPA. for these acids have been reported by others w8x Ž Table 3. , from which we can calculate the values of dDG int . If dDG int are plotted vs. dDG 0 , fair correlations are observed throughout the range of compositions of the mixed solvents, as have been predicted by Eq. Ž18.: dD HintrdDG 0 s ca. 7.04–10.4 The values of g and B calculated are also shown in Table 2. The variation of them shows marked dependence on the alteration of solvent. The values of g Žnegative. increase gradually with increasing V % up to 40%, then remain nearly constant, while the values of B decrease progressively with increasing V % up to 40%, then somewhat increases. The micro-structure of ethanol–H 2 O mixtures is well known to be not successive w12x. The alteration of micro-structure of the mixed solvents leads to the variation in its solvation ability, which in turn leads to the change of a, the solvation radius of the functional group, and that of the parameter B. So, B is considered to be an important parameter which is related not only to the specific interaction between solute and solvent but also to the solvation structure parameter a. Table 3 dD Hint of substituted benzoic acids Substituents metapara-
GPA ŽkJ moly1 .
dD Hint ŽkJ moly1 .
H
NO 2
Cl
CH 3
96.56 96.56
y37.62 y46.40
y19.65 y18.60
2.93 4.39
36
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x
L.G. Hepler, J. Am. Chem. Soc. 85 Ž1963. 3089. J.W. Larson, L.G. Hepler, J. Org. Chem. 33 Ž1968. 3961. T. Matsui, L.G. Hepler, Can. J. Chem. 54 Ž1976. 1296. R.T. McIver Jr., J.H. Silvers, J. Am. Chem. Soc. 95 Ž1973. 8462. T.B. McMahon, P. Kebarle, J. Am. Chem. Soc. 99 Ž1977. 2222. C.S. Yorder, C.H. Yorder, J. Am. Chem. Soc. 102 Ž1980. 1245. M. Fujio, R.T. McIver Jr., R.W. Taft, J. Am. Chem. Soc. 103 Ž1981. 4017. R. Yamdagni, T.B. McMahon, P. Kebarle, J. Am. Chem. Soc. 96 Ž1974. 4035. K. Hiraoka, R. Yamdagni, P. Kebarle, J. Am. Chem. Soc. 95 Ž1973. 6833. M. Taagepera, W.G. Henderson, R.T.C. Brownlee, J.L. Beauchamp, D. Holtz, R.W. Taft, J. Am. Chem. Soc. 94 Ž1972. 1369. w11x Zhang Shen, Study on the ionization thermodynamics of substituted benzoic acids in ethanolrH 2 O mixtures, Ph.D. Dissertation, Zhejiang University Hangzhou, China, 1988, 33–38. w12x F. Franks, D.J.G. Ives, Q. Rev. 20 Ž1966. 1.
Electrostatic model for substituent and solvent effects on organic acids Xin-Gen Hu, Han-Xing Zong ) , Rui-Sen Lin Department of Chemistry, Zhejiang UniÕersity, Hangzhou 310027, China
Abstract On the bases of experimental results from gas phase and electrostatic theory a modified thermodynamic model for substituent and solvent effects on organic acids has been derived. It is found that the present model is quantitatively and simultaneously consistent with the experimental results obtained from gas phase, aqueous solution and mixed solvents such as ethanol–H 2 O. q 1997 Elsevier Science B.V. Keywords: Electrostatic model; Substituent effect; Solvent effect; Organic acids
1. Introduction Quantitative evaluation of substituent and solvent effects on the dissociation of organic acids has been playing an important role in physical organic chemistry. The well-known Hammett equation throws light on this problem empirically. Hepler’s ‘internal-environmental’ model w1,2x succeeds in predicting the general form of Hammett equation, with s A dDH int , r A 1 r T. dDG 0 s dD Hint Ž 1 q g . in which, g is a solute–solvent interaction parameter dependent on solvent, temperature, pressure and reaction type. In his modified electrostatic model w3x, g was defined as
g s B´ ir Ž 1 y ´ s . in which, B was considered to be a dimensionless and solvent independent parameter, ´ i and ´ s are dielectric constants of a hypothetical solvent and the real solvent, respectively. This earlier approach has been gradually leading to improved understanding of substituent and solvent effects. However, this model cannot be quantitatively and simultaneously consistent with the experimental results from gas phase, aqueous solution and other solutions of high dielectric constant w8x. In order to solve this problem, a modified electrostatic model, based upon Hepler’s idea, is derived in this paper. )
Corresponding author.
0378-3812r97r$17.00 q 1997 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 1 2 Ž 9 7 . 0 0 1 3 7 - 4
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
32
2. Construction of the model Proton transfer process such as HAX q AOys AXyq HAO
Ž1.
can be used to describe substituent effect, where HAO is a reference acid, HAX is a homologous substituted acid. Let’s consider the following thermodynamic cycle
d DG 0 s dDG int q DGt0 Ž AXy . y DGt0 Ž AOy . q DGt0 Ž HAO . y DGt0 Ž HAX .
Ž2.
Assuming that the difference in transfer free energies between neutral molecules Ž HAO and HAX. is negligible, and that the one between anions ŽAOy and AXy. is determined by the difference in solvation free energies, we have dDG 0 s dDGi nt q DGs0 Ž AXy . y DGs0 Ž AOy .
Ž3.
According to electrostatic theory for solvation, we obtain DGt0 Ž AXy . y DGt0 Ž AOy . s Ž qo2 y q x2 . Ž 1 y 1r´ s . r2 a
Ž4.
in which, qo and q x represent effective charges on the functional group of AOy and AXy, respectively, ´ s is the macroscopic dielectric constant of solvent. a is the solvation radius of the functional group. The relationship between qo and q x is assumed to be w3x q x s q o q Ž d qrd m . m x
Ž5.
in which, m x is the point dipole moment associated with the substituent x. So, we have DGt0 Ž AXy . y DGt0 Ž AOy . s qo m x Ž d qrd m .Ž 1 y 1r´ s . ra
Ž6. . 2 2x .
where we have omitted the relatively small terms involving Ž d qrd m m In gas phase, it was found that w4–6x dD Sint f 0 dDG int f dD Hint dD Hint f e m x cos frc 2
Ž7. Ž8. Ž9.
in which, e represents the magnitude of the electron charge; f represents the angle between the dipole line and the line drawn to connect the dipole with the center of the functional group. c is the distance between the substituent x and the proton to be dissociated from HAX.
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
33
Combination of Eqs. Ž 3., Ž6., Ž8. and Ž9. gives dDG 0 s e m x cos frc 2 y q0 m x Ž d qrd m .Ž 1 y 1r´ s . ra
Ž 10.
Define a dimensionless parameter B, B s Ž c 2 qorae cos f . Ž d qrd m .
Ž 11.
Therefore, Eq. Ž 10. can be rearranged as dDG 0 s e m x 1 y B Ž 1 y 1r´ s . cos frc 2 s e m x cos frc 2 y B Ž 1 y 1r´ s . e m x cos frc 2
Ž 12 .
The right side of Eq. Ž12., as shown above, has been grouped into two terms: solvent independent and solvent dependent terms. According to ‘internal-environmental’ model w1,2x, we know dDG 0 s dD Hint q dDGenv
Ž 13.
Comparison of Eqs. Ž12. and Ž13. gives dD Hint s e m x cos frc 2
Ž 14.
dDGenv s yB Ž 1 y 1r´ s . e m x cos frc 2 s yB Ž 1 y 1r´ s . dD Hint
Ž 15.
Eq. Ž15. shows clearly that dDGenv depends not only on dD Hint but also on parameters B and ´ s . Define another dimension less parameter g ,
g s yB Ž 1 y 1r´ s .
Ž 16.
Then, dDGenv s g dD Hint
Ž 17.
dDG 0 s dD Hint Ž 1 q g .
Ž 18.
Consider the parameter B, B s Ž c 2 q0rae cos f . Ž d qrd m . s qo Ž q x y qo . radD Hint s qo Ž d qrdD Hint . ra
Ž 19.
It is noticeable that B is related with gas phase proton affinity ŽydD Hint ., effective charges on the acid anions Ž qo and q x ., and solvation radius of the functional group Ž a. that is dependent on solvent. Therefore, B is considered to be a solvent dependent parameter.
3. Discussion 3.1. Gas phase (boundary condition) In the gas phase, ´s s 1. According to Eqs. Ž16. and Ž18. , we find
gs0 dDG int s dD Hint dD Sint s 0
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
34
The results are reasonable for reaction like the type of Eq. Ž1., except that HAX contains a substituent close to the functional group with proton w3x. So, we conclude that the present model is consistent with gas phase boundary condition. 3.2. Aqueous solution Many measurements of gas phase acidities have shown w4,5,8–10x the correlation dD HintrdDG 0 s ca. 4–10 for various organic acids or bases such as phenols, aliphatic carboxylic acids, benzoic acids, and pyridines. According to Eqs. Ž 16. and Ž 18. , we discover
g s ca. y0.90–y 0.75 B s ca. y0.91–y 0.76 For homologous acids, g and B are constants. Because the values of d q r dD Hint in gas phase and solution are also constant, we conclude that the values of a for solvation radiuses of the functional group in the same solution are unchangeable. In a special reaction series, if there were several substituted acids deviate from the linear relationship as shown by Eq. Ž 18. , there should be different values of B. For example, for a series of meta- or para-substituted phenols w7x, dD HintrdDG 0 s 6.6, so g s y0.848, B s 0.859. But some phenols with hydrogen-bond-accepting substituents drift off the linear relationship, i.e., they have different g and B values as shown in Table 1. We discover that the values of d BdD Hint are of the reasonable magnitudes for substituent HBA effects. For HAB meta-substituents, the values of d BdD Hint characterize the effects of the specific solvation of these Table 1 HAB effect of substituents on the acidities of phenols Substituents
dD Hint ŽkJ moly1 .
dDG 0 ŽkJPmoly1 .
g
B
d BdD Hint
paraCN CO 2 CH 3 SO 2 CH 3 CH 3 CO SOCH 3 NO 2 CHO
y16.6 y11.7 y17.6 y13.3 y11.4 y20.9 y15.8
y2.76 y2.05 y3.01 y2.66 y2.39 y3.89 y3.27
y0.834 y0.825 y0.829 y0.800 y0.790 y0.814 y0.793
0.844 0.835 0.840 0.810 0.801 0.824 0.803
0.249 0.281 0.334 0.652 0.661 0.732 0.885
metaCH 3 CO SO 2 CH 3 CO 2 CH 3 OCH 3 NH 2 NŽCH 3 . 2 SOCH 3
y6.5 y12.8 y5.1 y1.1 1.4 1.2 y7.8
y1.10 y2.16 y1.08 y0.48 y0.15 y0.20 y1.75
y0.831 y0.831 y0.788 y0.564 y1.107 y1.167 y0.776
0.842 0.842 0.798 0.571 1.121 1.182 0.786
0.111 0.218 0.311 0.317 0.367 0.388 0.569
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
35
Table 2 dDG 0 of substituted benzoic acids in ethanol–H 2 O mixtures Ž298.15 K. V%
dDG 0 ŽkJ moly1 . meta-
0 10 20 30 40 50 60 70 80
g
B
2 ´y1 s =10
y0.904 y0.887 y0.877 y0.871 y0.863 y0.861 y0.864 y0.858 y0.860
0.916 0.899 0.890 0.885 0.878 0.878 0.883 0.879 0.885
1.27 1.35 1.42 1.53 1.71 1.90 2.14 2.44 2.87
para-
NO 2
Cl
CH 3
NO 2
Cl
CH 3
y3.97 y4.76 y5.09 y5.34 y5.63 y5.92 y6.17 y6.31 y6.29
y2.03 y2.35 y2.36 y2.24 y2.11 y2.10 y2.07 y2.30 y2.33
0.48 0.09 0.22 0.42 0.76 0.87 1.08 0.94 1.08
y4.37 y5.05 y5.55 y5.91 y6.37 y6.50 y6.51 y6.65 y6.51
y1.17 y1.60 y1.75 y1.79 y1.84 y1.93 y1.98 y2.06 y2.06
1.05 0.72 0.79 1.00 1.22 1.32 1.60 1.62 1.99
substituents on normal fieldrinductive effects, while for HBA para-substituents, the values of d BdD Hint characterize the specific substituent solvation assisted resonance effects Ž SSSAR. w7x. 3.3. Mixed solÕent The values of ionization free energies Ž dDG 0 . of some meta- and para-substituted benzoic acids ŽNO 2 , Cl, CH 3 . in ethanol–H 2 O mixtures have been determined by us Ž Table 2. w11x. The results of gas phase acidities Ž GPA. for these acids have been reported by others w8x Ž Table 3. , from which we can calculate the values of dDG int . If dDG int are plotted vs. dDG 0 , fair correlations are observed throughout the range of compositions of the mixed solvents, as have been predicted by Eq. Ž18.: dD HintrdDG 0 s ca. 7.04–10.4 The values of g and B calculated are also shown in Table 2. The variation of them shows marked dependence on the alteration of solvent. The values of g Žnegative. increase gradually with increasing V % up to 40%, then remain nearly constant, while the values of B decrease progressively with increasing V % up to 40%, then somewhat increases. The micro-structure of ethanol–H 2 O mixtures is well known to be not successive w12x. The alteration of micro-structure of the mixed solvents leads to the variation in its solvation ability, which in turn leads to the change of a, the solvation radius of the functional group, and that of the parameter B. So, B is considered to be an important parameter which is related not only to the specific interaction between solute and solvent but also to the solvation structure parameter a. Table 3 dD Hint of substituted benzoic acids Substituents metapara-
GPA ŽkJ moly1 .
dD Hint ŽkJ moly1 .
H
NO 2
Cl
CH 3
96.56 96.56
y37.62 y46.40
y19.65 y18.60
2.93 4.39
36
X.-G. Hu et al.r Fluid Phase Equilibria 136 (1997) 31–36
References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x
L.G. Hepler, J. Am. Chem. Soc. 85 Ž1963. 3089. J.W. Larson, L.G. Hepler, J. Org. Chem. 33 Ž1968. 3961. T. Matsui, L.G. Hepler, Can. J. Chem. 54 Ž1976. 1296. R.T. McIver Jr., J.H. Silvers, J. Am. Chem. Soc. 95 Ž1973. 8462. T.B. McMahon, P. Kebarle, J. Am. Chem. Soc. 99 Ž1977. 2222. C.S. Yorder, C.H. Yorder, J. Am. Chem. Soc. 102 Ž1980. 1245. M. Fujio, R.T. McIver Jr., R.W. Taft, J. Am. Chem. Soc. 103 Ž1981. 4017. R. Yamdagni, T.B. McMahon, P. Kebarle, J. Am. Chem. Soc. 96 Ž1974. 4035. K. Hiraoka, R. Yamdagni, P. Kebarle, J. Am. Chem. Soc. 95 Ž1973. 6833. M. Taagepera, W.G. Henderson, R.T.C. Brownlee, J.L. Beauchamp, D. Holtz, R.W. Taft, J. Am. Chem. Soc. 94 Ž1972. 1369. w11x Zhang Shen, Study on the ionization thermodynamics of substituted benzoic acids in ethanolrH 2 O mixtures, Ph.D. Dissertation, Zhejiang University Hangzhou, China, 1988, 33–38. w12x F. Franks, D.J.G. Ives, Q. Rev. 20 Ž1966. 1.