839
Activity Coefficients of Substituted Benzoates B. D. Flockhart, J. Colloid. Sci., 16, 443 (1961). J. C. Eriksson and G. Gillbern, Acta Chem. Scand., 20, 2019 (1 966). T. Nash, J. Appl. Chem., 6, 539 (1956). T. C. Bruice, J. Katzhendler, and L. R. Fedor, J. Amer. Chem. SOC.,90, 1333 (19 6 8 ) . L. S. C. Wan, J. Pharm. Sci., 55, 1395 (1966). D. G. Herries. W. Bishoo. and F. M. Richards, J. Phvs. Chem., 68. I842 (1964). P. Molvneux and C T. Rhodes, Kolloid-2. Z. Polym., 250, 886 (1972): L. P. Hammett, J. Chem. Phys., 4, 613 (1936). J. Hermans. Jr., S.J. Leach, and H. A . Scheraga, J. Amer. Chem. SOC.,85, 1390 (1963). H. C. Saraswat and U. D, Tripathi. Bull. Chem. SOC.Jap., 38, 1555 (1965).
(23) R. Lumry and S.Rajender, Biopolymers, 9, 1125 (1970). (24) 0.Exner, Collect. Czech. Chem. Commun., 37, 1425 (1972), and references therein. (25) W. Good and M. H. Milloy, Chem. Ind. (London), 872 (1956). (26) C. A. Bunton in "Reaction Kinetics in Micelles," E. H. Cordes, Ed., Plenum Press, New York, N. Y . , 1973. (27) L. P. Fernandez and L. G. Hepler, J. Amer. Chem. SOC., 81, 1783 (1959). (28) P Mukerjee and K. Banerjee, J. Phys. Chem., 68, 3567 (1964). (29) A . L. Thakkar and N. A. Hall, J. Pharm. Sci., 57,1394 (1968). (30) R. C. Weast, Ed., "Handbook of Chemistry and Physics," The Chemical Rubber Publishing Co., Cleveland, Ohio, 1964-1970, p D-118. (31) N. Muller in "Reaction Kinetics in Micelles," E. H. Cordes, Ed., Plenum Press, New York, N. Y., 1973, and references therein. (32) G Nemethy and A. Ray, J . Phys. Chem., 77,64 (1973).
Transfer Activity Coefficients of Ortho-Su bstituted and Non-Ortho-Substituted Benzoates between Water, Methanol, and Polar Aprotic Solvents M. K. Chantooni, Jr., and I. M. Kolthoff* School of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received November 6, 7973)
Plots of dissociation constants (as pKd(HA)) of non-ortho-substituted benzoic acids in water, W (and methanol, M) us. pKd(HA) in acetonitrile (AN) are linear. However, values of pKd(HA) of ortho-substituted benzoic acids in M and much more so in W deviate from these plots. Plots of the electrostatic part of the transfer activity coefficient, pANyW,M(AeI-), of non-ortho-substituted benzoate ions us. Hammett substituent constant, u, are linear. Values of pANyW,M(Ael-) of ortho-substituted benzoates with the exception of 2,4,6-trimethylbenzoate deviate from the straight lines. From comparison of the deviation of ortho-substituted benzoic acids and benzoates from the two types of plots mentioned it has been concluded that o-nitrobenzoates exhibit abnormally large electrostatic free energies of solvation in the protic solvent W and less so in M with little or no contribution of steric effects (in all solvents), while the opposite is true for 2,4,64rimethylbenzoate. Both electrostatic .solvation and steric effects are encountered with o-chlorobenzoates. Comparable values of transfer activity coefficients of uncharged ortho- and similar non-ortho-substituted benzoic acids are obtained. From the practical viewpoint it is noteworthy that resolution of acid strength between W(M) and AN for ortho-substituted acids is considerably less than for non-ortho-substituted benzoic acids. Quite generally, resolution of acid strength between M and AN is less than between W and AN.
Introduction In a previous paper1 transfer activity coefficients, MySi,2 were determined for substituted benzoic acids (HA), their anions, and their methyl esters (MeA) between methanol (M) and S = acetonitrile (AN), N,N-dimethylformamide (DMF), or dimethyl sulfoxide (DMSO). The quantity pMySi is equal to AG0/2.303RT, where AGO is the standard free energy of transfer of specie i from M to S. Linear plots of pANyM(A-)and of pANyd(Ael-) (electrostatic contribution to transfer activity coefficient of A-) us. (pKd(HA))AN for non-ortho-substituted benzoates were obtained, with slopes of -0.54 and -0.43, respectively. Only two ortho-substituted benzoates were included in the plot of pANyM(A-)us. (pKd(HA))AN, which deviated considerably from the straight line. The present study is extended to several ortho-substituted acids in M and AN and we have also included water as a solvent for both ortho- and non-ortho-substituted benzoic acids. With regard to the non-ortho-substituted benzoates this has
yielded information on the effect of substituent on the difference in free energy of the solvation of these ions between water and AN, DMF, and DMSO. This provides further understanding in the nonaqueous solvents of the Hammett linear free energy p a relation, which is based on the ionization of benzoic acids in water. Furthermore, the well-known enhancement in acid strength of non-hydroxyortho-substituted benzoic acids in water and M is discussed in terms of the abnormally large electrostatic free energy of solvation of the corresponding anions in water and M. The following relations are used in the analysis of plots of pANyW,M(A-) or pANyW,M(AeI-)us. (pKd(HA))AN W,M
A AK pKd(HA)
=
+
pANyW3(H+) AN W,M
p y
pANyW3M(~~) = PANYW3Mn
+
(A-)
- P ~ ' V ~ ~ < ~ ((1) HA)
pANyWfl(~a)=
p AN yW M (MeA)
+ pANyW3M(Ha) (2)
The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
040
M. K. Chantooni and I.M. Kolthoff
TABLE I: Solubilities of Substituted Benzoic Acids, HA, and Their Methyl Esters, MeA, in Water, Acetonitrile, and Methanol W Substituent
SHA
IHAlsat.“
1. 4-Dimethylamino2. 3,4-Dimethyl3. 4-Methyl4. Unsubstituted 5. 4-Bromo6. 3-Bromo7. 3-Nitro8. 3,4-Dichloro9. 3,5-Dichloro10. 4-Nitro-
0.00040~ 0.00086g 0.002780 0 .028b 0.000280 0.00200 0.0204c 0.00032~ 0.00077g 0.00114~
0.00028 0.00070 0 .00246 0.028 0.00020 0.0015 0.0177 0.000132 0.00039 0.00087
11. 3-Nitro-4-chloro12. 3,5-Dinitro-
13. 14. 15. 16. 17. 18.
19. 20. 21. 22.
0.001720 0 . 006346 0.007340 0.0133Q 0 .0442b
i
4-Chloro3,5-Dinitro2-Chloro 2-Nitro2-Hydroxy2,4-Dinitro2,4-Dichloro2,6-Dichloro2,6-Dinitro2,6-Dihydroxy2,4,6-Trimethyl-
0.OGd 0 ,0490
0.00250 0.07460 0.07618 0.0620 0 ,004420
0.00099 0.00380 0.00394 0.013
0.044 0.012 0.0176 0.0010
0.0386 0.0251 0.013 0.0033
M
AN
SMBA~
0.000361 0.00130 0.00059 0.000994 0.00112~ 3 . 5 x 10-6; 0 . 00045k 0.00031 0.00056 0.000155
SEA
ShleAh
0 . 023h 0.091h 0.1630 0.85h
0.86
0. O w
0.80
0.167h 0.78h 0.028h 0 .035* 0 .041h
0 . 13sh
0.23h 1.22h
0.53h 0 .98h 0.570 1.02h 0 . 16gh 3.230 0.640 1.050 0.440
2.02 2.38 1.26 4.44i 0.55i 4.9k 1.44 1.15 1.70
SEA
0 . 086h O.6OA
3 .16d 0.12h 1.51h 3 .46d 0 .27h 1.25h 0 ‘20d
S&Ah
0.27
0.31 0.27 0.95 0.18 0 .76i 0.lli
0 .71h 1.23h
1.5v 2.53h 2.99d 2.57d 3 .23h P ,340
0.21 0.11
0.13
2.420 4.040 2.390
*
a Molecular solubility i n water after correcting for ionic dissociation. M. Randall and C. F. Failey, Chem. Rev., 4,291 (1927). N.Bjerrum and E. Larsson, 2. Phys. Chem., 127, 368 (1927). Reference 6. e A. Seidell, “Solubilities of Organic Compounds,” 3rd ed, Vol. 2, Van Nostrand, New York, N. Y. 1941. Reference 14. Q This work. Reference 1. Ethyl ester. 3 Isopropyl ester. n-Propyl ester.
*
been prepared by refluxing the alcohol with Aldrich 4nitrobenzoyl chloride. Since traces of unreacted acid in Substituting eq 2 and 3 into eq 1 and assuming pANyW,Mn the esters interfered with the spectrophotometric determi= pANyW.M(An-)eq 4 results nation of the solubility of the ester in W, the esters were recrystallized from water-ethanol mixtures at least four times. Techniques. Methods used for determination of solubility of benzoic acids and esters in AN and M have been deIn eq 2-4 pyn refers to that part of the transfer activity scribed.l The same alkalimetric titration procedure was coefficient of HA not involving the hydrogen bond of HA used for solubilities of substituted benzoic acids in water to the solvent, py(H,) to the difference in hydrogen bond as in the organic solvents. The solubilities of esters in accepting capacities between two solvents expressed as water were estimated by refluxing the filtered aqueous transfer activity coefficient. It is assumed that py(HA) saturated solution with 0.01 M sodium hydroxide for 3 hr py(Ha), which is denoted by pyn (eq 2) is equal to the nonelectric part of py(A-), denoted by py(An-) in eq 3. The. and spectrophotometrically determining the benzoate concentration in the uv. The following molar absorbance iny(AsPh4+) = y(BPh4-) assumption was used in the evaldices were determined in solutions of known acid concenuation of all single ion transfer activity coefficients. tration containing 0.01 M (excess) sodium hydroxide at the wavelength of maximum absorption: 4-bromo- 237 Experimental Section nm, amax = 1.4 x lo4; 3-nitro- 264, 7 . 0 , ~lo3; 3,4-dichloro- 234, 1.09 X lo4; 4-nitro- 273, 1.00 x lo4; 3-nitroSolvents, Acids, and Salts. Acetonitrile3 and methanol4 4-chloro- 270 (inflection point), 2.76 x 103; 3,5-dinitrowere purified and stored as described previously. Non236, 1.69 x lo4, and 4-chloro-3,5-dinitrobenzoate420 ortho-substituted benzoic acids,5 2-hydroxy-, 2,6-dihy(shoulder), 7.20 x lo3. The spectra are in agreement with droxy-, 2-chloro-, 2,6-dichloro-, 2-nitro-, and 2,4-dinitrobenzoic acids, and their tetraethylammonium salts were those entered in Sadtler‘s’ and Lang’ss compilations. Techniques employed in determination of the total solproducts used previously.5 2,6-Dinitro-, 2,4-dichloro-, and ubility of the silver salts by potentiometric titration with 2,4,6-trimethylbenzoic acids were Aldrich products, recrystallized from water. Tetraethylammonium salts of bromide, of paAg and paH measurements in saturated SOlutions of these salts in presence of the parent acid have these acids were prepared in the same way as for the benbeen described e l s e ~ h e r e . ~ zoic acids.5 Silver salts of ortho-substituted benzoic acids Instrumental. All ultraviolet spectra were recorded on a were prepared as described by Kolthoff, et aL6 Potassium Cary Model 15 recording spectrophotometer at sensitivity chloride was Merck Reagent Grade. 4, using 1- and 5-cm silica cells. The conductivity bridge Esters. Methyl esters of non-ortho-substituted benzoic and cell3 and potentiometric assembly for estimation of acids and ethyl- and isopropyl-4-nitrobenzoatesare prodpaHlO and paAg9 were described previously. ucts used previously,l while n-propyl-4-nitrobenzoate has The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
841
Activity Coefficients of Substituted Benzoates
Results Solubilities and of Substituted Benzoic Acids. Transfer activity coefficients of most of the acids
lytical acid concentrations: 0.0229 M HA, 6.62; 0.0476, 11.9; 0.0900, 22.2; 0.128, 33.0; 0.193, 54.3; 0.303, 96.0; and 0.408 M HA, 136.0, yielding log Kf(HA.Cl-) = 2.34 and log p(HA.Cl-) = +0.16. As previously,5 values of Xo(HA.Cl-) = 55, Xo(K+) = 86, and Xo(Cl-) = 92 were taken.
were obtained from solubility data. Dissociation constants of the acids in acetonitrile and methanol are so small that the ionic solubility is negligible. Whenever necessary, corDeterminution of p y ( A - ) from Solubilities and Activirection was made for the ionic solubility in aqueous medities of Silver Salts. Solubility products in water of silver um. The solubilities in water and AN of the 2-substituted salts of ortho-substituted benzoic acids were estimated benzoic acids, with the exception of 2,6-dichlorobenzoic from the total solubility by bromide titration, conductiviacid in AN, are so small that [HA],,,. can be taken equal ty of the saturated solution and paAg measured potentioto the activity of the acid in the saturated solution. This metrically in the saturated solution with the silver elecis no longer true for most of the 2-substituted benzoic trode. Values of [Ag+lSat. in Table 111 obtained by the acids in methanol. When the solubility in methanol exthree methods agree within experimental error, indicating ceeds 1.5 M , pANyM(HA)was found from the relation practically complete dissociation of AgA in water without p*NyM(HA) = p*NyW(HA) pWyM(HA).The values of complexation. In methanol, [Ag+],at. in saturated AgA SOpWyM(HA) were calculated from values of pWyM(A-) lutions containing HA from paAg and conductance meawhich were found from the solubility of the silver salts, surements agree (with the exception of unsubstituted benA&, in W, and M (vide infra). In the calculation of pANzoic acid), but are considerably smaller than the total solyW(HA) using eq 1 values of pKd(HA) in W and M were ubility (Table c)24 indicative of the presence of undissotaken from Table I1 and pWy’(H+) = 1.9, previously reciated AgA and absence of complexes of silver, Le., AgAz-. portedll on the basis of the tetraphenylborate assumpSilver benzoate in methanol, on the other hand, appears tion. Values of the solubilities of the acids in water, methto be complexed in the saturated solution both with and anol, and AN are tabulated in Table I and py(HA) values without added benzoic acid as [Ag+lSat.calculated from in Table 11. conductometric and potentiometric data was found to be Determination of p K d ( H A ) and K f ( H A 2 - ) . Values of considerably smaller than the total solubility (Table c) .24 pKd(HA) of many of the substituted benzoic acids in the The conductometric value of [Ag+],,t. in saturated silver three solvents are known. Those in water are taken from solution, 6.8 x M , agrees well with that of standard compilations,l2 while those in m e t h a n 0 1 ~ ~ - ~benzoate ~ 7.2 x M, reported by Kolthoff, et a1.6 and ANZ3 are reported elsewhere. In a few instances where Values of pKsp(AgA) in M were calculated in saturated discrepancy between reported values of pKd(HA) of 3AgA solutions containing HA from paH and paAg meaand 4-substituted benzoic acids in methanol exists in the surements, knowing pKd(HA), as done previously in literature, pKd(HA) values were checked by potentiometAN.19 Values of pKd(AgA) and pKsp(AgA) of silver orthoric determination of paH with the glass electrode using, substituted benzoates in water and methanol are tabulatapproximately equimolar solutions of the acid and its ed in Table ID, while Table cZ4lists calculated concentratetraethylammonium salt. Experimental results are in tions and activities of the various species. Table aZ4 and pKd(HA) values in Table 11. For some 2Values of pWrM(A-)of 2- and 2,6-substituted benzoates and 2,6-substituted benzoic acids values of pKd(HA) in in Table I1 were calculated from pKSP(AgA) values in W methanol and AN are not given in the literature. Conseand methanol in Table I11 using the relation MAWpKSp(Agquently they have been determined in the present study. pWy’(A-) and the value of A) = pWyM(Ag+) For 2,6-dinitro-, 2,4-dinitro-, and 2,6-dihydroxybenzoic pWyM(Ag+) = +1.3 previously reportedll on the basis of acids values of pKd(HA) in methanol were determined by the tetraphenylborate assumption. Satisfactory agreement the conductance method. Conductance data are presented of pWyM(A-)values of Z-nitrobenzoate of +I& derived in Table b.24 Values of pKd(HA) in AN and methanol and from values of pKsp(AgA) by Kolthoff, et a1.,6 and of +1.1 formation constants of homoconjugates, Kf(HA2-) and from data by K o n o ~ a l o vwho ~ ~employed the cell P(HA2-) (P(HA2-) = Kf((HA)2A-)/Kf(HA2-)) of orthosubstituted benzoic acids have been calculated from glass (H2)1 HA 1 AgA-Ag- Ag- AgA IHA 1 (HJ electrode potentiometric paH data (Table a) in mixtures Pt w M Pt of the acids and their tetraethylammonium salts. Plots of was found. paH US. log Ca/C, in Figure 1 were constructed from the Evaluation of p y n from Solubility of Methyl Esters. above data in AN where C, and C, denote the analytical Values of the nonhydrogen bonded contribution to the concentrations of acid and tetraethylammonium salt, retransfer activity coefficient of the 3- and 4-substituted spectively. A summary of pKd(HA) values of ortho-substituted benzoic acids in AN and methanol is in Table II. benzoic acids, Pyn, between a host of organic solvents, inThe following values of log Kf(HA2-) and log P(HA2-) cluding methanol, were derived from the solubilities of have been obtained in AN: 2-chloro- 4.0, 0.7; 2,4-dichloro- the methyl esters of these acids in these solvents.1 In the 3.95, 0.75; 2,6-dichloro- 3.8, 1.7; 2,6-dinitro- 3.7, 1.4; present investigation water was included as a solvent. 2,4,6-trimethyl- 3.46, 1.7, respectively. Previously23 log Solubilities of these esters in water are listed in Table I. Kf(HA2-) and log /3(HA2-) in AN have been reported as As the 2- and 2,6-substituted methyl benzoate esters are 3.98, 1.21, and 4.3, 0.89 for 2-nitro- and 2,4-dinitrobenzoic very soluble in, or miscible in all proportions with the oracids, respectively. ganic solvents, they were excluded from this study. The Heteroconjugation of 2,4,6-Trimethylbenzoic Acid with length of the alcohol chain to at least three carbon atoms Chloride in AN. Heteroconjugation constants of this acid did not appreciably affect PYester when organic solvents with chloride were found from the effect of the acid on the were involved.1 However, it is seen from the values of conductometrically determined ionic solubility of potassipANyWester in Table I1 for the 4-nitrobenzoate esters that um chloride in AN.5 The following values of [P[K+]z an increment of +0.6 in pANyWesterper methyl group of KSP(KCl)]/KSP(KC1)have been found at the various anathe alcohol part results. Therefore, a correction of -0.6
+
3 1 3
+
The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
M. K. Chantooni and I. M. Kolthoff 0
09
mm
m
9'9 m.
4 m 4
???
0 0
0
0 0 0
I
I I
I
I l l
m c? m
mo
o&b%ccr+4
mm
mm44mm*
0
??
?(9??(9c??
+ ++ +++++++ & a
z
09?(9???
m 4 * w w m I I I I I I
.n
m&&R
E
???Y?Y
>
oooorlo I I I I I I
v
'
~hlhlcomhlhlhlcvc??09".??(9c?(9?
??L?
(9
hlhlhl
hl
+ +++
.
t
.
(9?
% ???????
0 0
0 0 0 0 0 0 0
I I
I I I I I I I
h.
0 .l:
vv
t-a
(9F
'n
E t
%>;E
vv
?F".
v
4
??
++ +++ ++
++++++++++ rihlrirlrihlricvrirl
rid
cvcvrl v
b
3
The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
Y
hl
rlhl
843
Activity Coefficients of Substituted Benzoates
A
21
20
A
19
18
17
16
15
14
13 I
- I.o
+ 1.0
0
12,
t 2.0
1
I
I
6
7
1
2
I
log G a l
I
5
I
I
(pKdHA)M
4
3
I
I
I
(pKdHA)W
CS
Figure 1. Plots of paH vs. log Ca/Cs in mixtures of ortho-substituted benzoic acids and their tetraethylammonium salts in AN: (1) 2,4,6-trirnethylbenzoic acid, C, = 4.10 X M; (2) 2M; (3) 2,4-dichlorobenchlorobenzoic acid, C, = 3 . 4 9 X zoic acid, C, = 2.89 X l o W 3M ; (4) 2,6-dichlorobenzoic acid, c, = 5 . 4 5 X 1 0 - d M; (5) 2,6-dinitrobenzoic acid, C, = 5.84 X M. Dashed line has slope of -3.0.
unit was applied in the evaluation of PANTW, from pANyW(MeA), i.e., PANyWn = pANyW(MeA)-0.6, hence pANyW(H,) = pANyW(HA) (pANyW(MeA) -0.6). Values of pANyW(Ha) thus found are listed in the last column of Table 11.
-
Discussion Acid Strength of Non-Ortho-Substituted Benzoic Acids in A N and Water or Methanol. In Figure 2 are presented
Figure 2. Plots of ( p K d ( H A ) ) ~vs. ~ (pKd(HA))w (line A) or vs. ( P K ~ ( H A ) )(line ~ B) of substituted benzoic acids: (0) nonortho-substituted; (A)ortho-substituted excepting hydroxy; (A) ortho-substituted hydroxybenzoic acids. Numbers are those in Table I.
plots of (pKd(HA))AN us. pKd(HA) in W or M. These plots are linear for the non-ortho-substituted benzoic acids, the slopes pAN/pW and pANIpM,being 2.4 and 1.7, respectively, indicating less resolution of acid strength between M and AN than between W and AN. A plot of p'"yw(Ael-) US. (pKd(HA))AN-fornon-ortho-substituted bennnie o o i r l e in li'imrn v, '2 l i m n A hoc u o "'"y" clnna nf -n"."" G!? I ~ Pmrnpared to (pw - PAN)/PAN = -0.59 obtained by substitution of the Hammett pa relation into eq 4. This suggests that the effect of substituent on W A A N pKd(HA), is dependent almost entirely on pANyW(&,-) as was found for pANyM(Ae1-).IFrom a plot of AGO us. ASo of ionization of Y"l"
UVlU"
111 1 A b . 2 - V
1111V
'L,
l l U U
"A
U Y
""I.-
TABLE 111: Solubility and K"(AgA) of Silver Salts in M, W, and AN PK'YAgA) CAS
Acid
CHA, M
Solvent
Specific conductivity
(KBr or KI titration)
PaAg (Ag electrode)
Conduc- (pa(Ag+) paH
tivity
+
pa(A-)
Kd(AgA)
~
2-Chloro2,6-Dichloro2,4,6-Trimethyl2,4-Dinitro2,6-DinitroUnsubstituted
M
w M w M w M w
AN M
w
M M M M
0.00845
o 0.0106 o 0.00650 o 0.0113 o 0
0.0101
o
O
0.00294 0.0121 0.0102 (KBz)
2.01 1.71 1.83 1.31 1.90 1.46 1.85 2.29
x 10-4 x 10-3 x 10-4 x 10-3 x 10-4 x 10-3 x 10-4 x 10-3
1.23 x 10-4 2.57 x 10-3 5.70 x
6.85 x 10-3 2.08 x 10-2 3.79 x 10-3 1.58 x 6.98 x 10-3 1.87 x 3.40 x 10-3 2.95 X 0.314 2.16 x lo-* 3.54 x 10-2 9 . 5 >/: 10-4 8 . 7 x 10-4 8.8 X
4.77 x 10-4
2.68 1.78 2.74 1.84 2.76 1.84 2.77 1.71 1.87 2.91 1.62 3.30 3.28 3.32 4.29
7.64 6.31 7.92 5.80 5.43 8.70 8.12
5.34 3.48 5.43 3.71 5.40 3.59 5.43 3.22
5.41 3.56 5.45 3.68 5.64 3.67 5.37 3.42
1.0 X
5.77 3.12 6.27
5.75 3.24
N3
2 X 5 X
4 X
x
10-3
6.51 6.52
The Journal of Physical Chemistry, Vol. 78, No. 8,1974
844
M. K. Chantooni and I . M. Kolthoff M
A:5;l
22
- 10.0
- 8.0
z3I.A 14
19
- 6.0
A
17 2:
A
-4.0 I
-7.0
I
16
I
I
I
I
I
17
18
19
20
21
I
AN PYA;,
".
-10.0
A
A
1
I
9
-7.0
-6.0
-8.0 -9.0
- 5.0
20
A
-4.0
I
2 .o
I .5
-6.0 2.o
1.5
1.0
0.5
0.0
-0.5
d
Figure 3. Plots of p A N y W ( A e l - ) vs. ( p K d ( H A ) ) ~(line ~ A) or vs. Hammett u constants (line B) of substituted benzoates: ( 0 )
non-ortho-substituted and (A)ortho-substituted benzoates, and (0) acetate. Numbers are same as those in Table I .
non-ortho-substituted benzoic acids in water26327 it was concluded that the dissociation of these acids is essentially isoenthalpic, the AH" contribution being small in most cases. The slope of the AGO us. AS' plot was accounted for by the Bjerrum electrostatic theory.28 It is therefore likely that changes in pANyW(Ael-)with substituent can be identified with changes in ASo of ionization in water, since in aprotic media hydrogen bonding and ion-dipole interactions of anions with localized charge are much smaller than in ~ a t e r . ~Asg Hepler30 mentions it would be desirable to have AH" and ASo data of ionization of benzoic acids in nonaqueous solvents.
Acid Strength of Ortho-Substituted Benzoic Acids in Water and Methanol. Steric and Electric Effects. The en-
a
I
I
I
0.5
0.0
I
I .O
-0.5
Figure 4. Plots of pANyM(Ael-) vs. ( P K ~ ( H A ) (line ) ~ ~ A) or vs. Hammett u constants (line B) of substituted benzoates: ( 0 )
non-ortho-substituted and (A) ortho-substituted benzoates. Numbers are same as those in Table I .
hancement in acid strength in water of ortho-substituted benzoic acids and anilinium ions as compared to that of the meta- and para-substituted compounds is well known,31 and for many years has been interpreted on a structural basis by the concept of steric inhibition of resonance. More recently it has been apparent that solute-solvent intera~tions329~3 also can play an important role. An example cited by Hammett is the deviation of ortho-substituted benzoic acids from the linear plot of Kd(HA)/ Kd(HBz) of non-ortho-substituted acids in 1-butanol us. that in water.34 From Figure 2 it is clear that ortho-substituted benzoic acids (with the exception of 2- and 2,6dihydroxybenzoic acids) do not lie on the straight lines of the plots of ( P K ~ ( H A ) )us. ~ , (~P K ~ ( H A ) ) *the ~ , deviation indicating that as in 1-butanol these acids are relatively stronger in W and M than the corresponding nonortho-substituted acids. The deviation in M and 1-butanol
TABLE IV: Electrostatic Solvation and Steric Effect Abnormalities of Ortho-Substituted Benzoic Acids and Benzoates in Water and Methanol ~~
Substituent in benzoic acid
Deviationa in pANy W (As1-)
2-Chloro2,4-Dichloro2,6-Dichloro2-Nitro2,CDinitro2,B-Dinitro2,4,6-Trimethyl-
(-0.3) (-0.3)
-1
.o
-1.3 -1.2 -1.6 0 .o
Deviationd in ( P K ~ ( H AA) )N , w steric effect
(-0.5) (-0.4) -0.6 $0.2 +0.1 $0.4 -1 .o
Deviatione Deviationa in pANrX(A,i-)
(0.O) (-0.1) -0.3 -0.6 -0.5 -0.7 -0.2
in (PK~(HA))AN,M steric effect
(-0.2) (-0.2) -0.6 +O . 3 +0.2 +0.3 -1 .o
Angle of t W i s t b of -COOH group, deg
-10 -10
-45 -20 -20 -30 7OC
'From plots of pANyWfM(Ael-) us. C. Estimated from Fisher-Hirshfelder-Taylor atom models, using Van der Waal's covalent radii. Orientation of nitro group(s) taken so as to allow minimum rotation of carboxyl group. Spectroscopically determined angle of rotation of carboxyl group of 2,6-dimethylbenaoic us. (pKd(HA))w,see text. e Calculated acid, ref 35. Same angle of rotation assumed for 2,4,6-trimethylbenzoic acid. Calculated from plots of (~I@(HA))AN from plots of ( P K ~ ( H A ) ) AUS. N ( p ~ d ( ~ ~ )see )X text. , The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
Activity Coefficients of Substituted Benzoates
a45
4 in Table IV and the steric effect in columns 3 and 5 in Table IV. It is evident that the abnormally large free energies of solvation by water of ortho-substituted benzoates increase with increasing group dipole moment of the ortho substituent(s) which are -0, 1.70, and 4.21 D for methyl, chloro, and nitro groups, r e s p e c t i ~ e l y .It~ ~is possible that one or more water (or M) molecules are immobilized by ion-dipole, ion-quadrupole, and hydrogen bond interactions with substituents in the ortho position in addition to the carboxylate oxygen!. From Figures 3 and 4 and from Table IV it is seen that the abnormality in pANyM(bl-) for ortho substituents is smaller than that of pANyW(Ael-), which is plausible, considering that methanol possesses only one -OH group which can participate in hydrogen bonding to the orthosubstituted anion. The ring formation by hydrogen bondlog Kd(HA) = log Q - A E / R T = log Q ing of one molecule of water to both an oxygen of the car[AE, AEn AEst AEs(T>]/RT ( 5 ) boxylate ion and to the ortho substituent stabilizes the ion in water. comprised of the energy changes of the localized bonds, Considering the uncertainty of &0.15 unit in the u AEg, delocalized bonds, AE,, that due to steric hindrance, values of ortho substituents, additivity of these values in AEst (vide infra), and the solvation energy, AEs(T). In an 2,6-disubstituted benzoic acids and the applicability of eq attempt to eliminate the contribution of the steric factor 4, the values of the deviations in Table IV are approxiin lines A of Figures 3 and 4 and involve only inductive mate at best. From Figure 2 it is evident that the effects in the interpretation of transfer activity coeffipKd(HA) values of o-hydroxybenzoic acids hardly deviate cients of ortho-substituted benzoates, pANyW,M(&~-) from the straight line obtained with non-ortho-substituted values were plotted us. Hammett u substituent constants acids. This is accounted for by the strong intramolecular in Figure 3 and 4, lines B. The following values of orthohydrogen bond(s) in the o-hydroxybenzoates which inhibit substituent constants, uo, for nitro-,S6 c h l ~ r o - , ~and ' very strong solvations by W or M (see especially methyl37 groups were used: +1.20, +0.50, and -0.13. pANyW(A-) of 2,6-dihydroxybenzoate in Table 11). These values were found from nmr studies of substituted Steric Effects. Considering steric effects only it is gratiphenols in DMSO3? and hexamethylpho~phoramide~~ fying that the calculated deviations in pKd(HA) in W and which are strong hydrogen bond accepting solvents. In AN in the third column of Table IV from the plot of these studies steric effects and intramolecular hydrogen (pKd(HA))w us. (pKd(HA))*N are the same as those in bonding interactions are virtually absent. The magnitudes pKd(HA) in M and AN in the fifth column of Table IV of the deviations in pANyW,M(AeI-) of ortho-substituted from the plot of (pKd(HA))M us. (pKd(HA))*N. In generbenzoates from the linear plots of pANyW,M(Ael-)us. u in al, this deviation increases with increasing angle of twist Figures 3 and 4, lines B, are entered in Table IV. From of the carboxyl group out of the plane of the ring (Table this table it is apparent that the o-nitro- and, to a lesser IV), as is expected. To account for the negligible steric efextent, o-chlorobenzoates exhibit abnormally large elecfect in the o-nitrobenzoic acids it is necessary to postulate trostatic free energies of solvation in water as compared to that the nitro group(s) are not coplanar with the ring. It that of the non-ortho-substituted benzoates. This effect is appears that there definitely is a small steric effect in the greater for the 2,6-disubstituted than for the 2-substituted 2-chlorobenzoic acids which was considered as negligible benzoates. As the deviation in pANyW3M(AeI-) of o-nitroin an earlier publication,5 but has been considered by benzoates from the straight lines in Figures 3 and 4 (lines Taft.39 The value of -1.0 in Table IV for the steric effect B) roughly equals the deviation in ( P K ~ ( H A )of) ~the parin 2,4,6-trimethylbenzoic acid agress well with -0.8 unit ent acids from the linear plots in Figure 2, steric effects calculated by Taft39 for 2,6-dimethylbenzoic acid, taking are virtually absent (cf. eq 4 and 5). On the other hand, uo(CH3) = -0.1. The enhanced acidity of the carboxyl from the fact that the values of pANyW,M(Ael-) of 2,4,6group in 2,4,6-trimethylbenzoic acid deduced from the trimethylbenzoate lie on the straight lines in Figures 3 steric inhibition of resonance concept would be expected and 4, line B, it appears that the electrostatic free energy to result in a stronger hydrogen bond donor capacity of solvation of this ion is normal in W and M; i e . , the entoward chloride than would be predicted from electrical hancement in acid strength in water (and other solvents) effects alone. In fact, the value of 2.2 x 102 found for is entirely due to steric effects. o-Chlorobenzoates exhibit Kf(HA.C1-) in AN is twice that calculated from the both abnormally large electrostatic free energies of solvarelation40 log Kf(HA.C1-) = 2.23 + 0.470. using uo(CH3) tion in water and also steric effects. These effects are dif= -0.13. In all probability, the observed value of ficult to evaluate separately. The method used for their Kf(HA.Cl-) would be larger if steric interference with approximation is illustrated graphically for 2,6-dichlorohydrogen bond formation were absent. The large 1:1 hobenzoic acid in the plot of ( P K ~ ( H A ) ) us. *~ (PK~(HA))~ moconjugation constant of 2,4,6-trimethylbenzoic acid in in Figure 2. The magnitude of the steric effect, -0.6 unit AN parallels the large heteroconjugation constant with (the same for (pKd(HA))aN as for ( P K ~ ( H A ) )and ~ ) lachloride. beled St in Figure 2, has been chosen such that the deviaFrom the analytical point of view it is of interest to tion of pANyW(Ael-)(-1.0 unit from Table IV) labeled mention that resolution of acid strength between W (M) el, when added to St equals the total length of the horiand AN of ortho-substituted benzoic acids is considerably zontal line segment (between arrows). Calculated values less than that of non-ortho-substituted acids. of the abnormality in electrostatic free energy of solvation (deviation in p*NyW,M(Ael-))are listed in columns 2 and Acknowledgment. We thank the National Science
is considerably smaller than in water. In Table I1 it is shown that the deviation cannot be attributed to abnormal values of pANyM,W (HA) of ortho-substituted benzoic acids as compared to those of non-ortho-substituted acids. It is therefore postulated that the ortho-substituted benzoates are solvated to an abnormally large extent in M and more so in water, as discussed below. This is also apparent from the negative deviation of ortho-substituted benzoates from the linear plots of pANyW(A-)(not given), pANyW(Ael-) (Figure 3, line A), and pANyM(Ael-) (Figure 4, line A) us. ( P K ~ ( H A ) ) *The ~ . expression for the dissociation constant of an uncharged acid, HA, as proposed by Wilson, et a1.,35 in terms of the ratio of the partition functions, Q, and the difference in energy, AE, between that of the acid and its ions is given by eq 5 . In turn, AE is
+
+
+
The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
846
S. J. Thornton and Peter J. Dunlop
Foundation for Grant No. GP-20605 in support of this work. Supplementary Material Available. The data will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 x 148 mm, 24x reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for $3.00 for photocopy of $2.00 for microfiche, referring to code number JPC-74839. References and Notes (1) M. K. Chantooni, Jr., and I . M. Kolthoff, J. Phys. Chem., 77, 527 (1973). (2) i. M. Kolthoff and M. K. Chantooni, Jr., J. Amer. Chem. SOC., 95, 8539 (1973). (3) I. M. Kolthoff, S. Bruckenstein, and M. K. Chantooni, Jr., J. Amer. Chem. SOC., 83,3927 (1961). (4) I. M. Kolthoff and M. K. Chantooni. Jr., Anal. Chem., 44, 194 (1972). (5) M. K. Chantooni, Jr., and i. M. Kolthoff, J. Amer. Chem. SOC., 92, 7025 (1970). (6) I. M . Kolthoff. J. J. Lingane, and W. Larson, J. Amer. Chem. SOC., 60, 2512 (1938). (7) The Sadtler Standard Spectra, Sadtler Research Laboratories, Philadelphia, Pa. (8) L. Lang, Ed., "Absorption Spectra in the Ultraviolet and Visible Region," Academic Press, New York. N. Y., 1961. (9) M. K. Chantooni, Jr., and I. M. Kolthoff, J. Phys. Chem., 77, 1 11973) - -, (IO) I. M. Kolthoff and M. K. Chantooni, Jr., J. Amer. Chem SOC., 87, 4428 11965). (11) I. M . 'Kolthoff and M. K. Chantooni, Jr., J. Phys. Chem., 76, 2024 (1972). (12) G. KortOm, W. Vogel, and K. Andrussow, "Dissociation Constants of Organic Acids in Aqueous Solution," Butterworths, London, 1961. (13) J. Dippyand S. Hughes, Tetrahedron, 19, 1527 (1963). (14) J. Juillard, Ph.D. Thesis, University of Clermont-Ferrand, 1967; J. Juillard and N. Sirnonet, Bull. SOC.Chim. Fr., 1883 (1968). \
(15) J. Elliott and M. Kiipatrick, J. Phys. ?%em., 45, 454, 466 (1941). (16) I. M. Kolthoff and L. Guss, J. Amer. Chem. Soc., 61, 330 (1939). (17) I. M. Kolthoff, J. J. Lingane, and W. Larson, J. Amer. Chem. SOC., 60,2512 (1938). (18) W. Brightand H. Briscoe, J. Phys. Chem., 37, 787 (1933). (19) L. Minnick and M. Kilpatrick, J. Phys. Chem., 43, 259 (1939). (20) H. Goldschmidt and F. Aas, 2.Phys. Chem., 112, 423, 429 (1924). (21) M. Kilpatrick, J. Amer. Chem. SOC., 75, 584 (1953). (22) I . Tabagua, Tr. Sukhum. Gos. Pedagog. lnst., 15, 119 (1962). 92, (23) M. K. Chantooni, Jr., and I. M. Kolthoff, J. Amer. Chem. SOC., 7025 (1970). (24) See paragraph at end of paper regarding supplementary material. (25) 0. Konovalov, Russ. J. Phys. Chem., 39,364 (1965). (26) J. Christensen, R. Izatt, and L. Hanson, J. Amer. Chem. Soc., 89, 213 (1967). (27) P. D. Bolton, K. Fleming, and F. Hall, J. Amer. Chem. SOC.,94, 1033 (1972). (28) E. King, "Acid-Base Equilibria," Macmillan, New York, N. Y. 1965, p211. 163 (1962), (29) A. J. Parker, Quart. Rev., Chem. SOC., (30) J. W. Larson and C. Hepler in "Solute-Solvent Interactions." J. F. Coetzee and C. D. Ritchie, Ed., Marcel Dekker, New York, N. Y., 1969, p 37. (31) See, for example, L. P. Hamrnett, "Ph,ysical Organic Chemistry," 2nd ed, MacGraw-Hill, New York, N. Y., 1970, p 368; E. Gouid, "Mechanism and Structure in Organic Chemistry," Holt, Rinehart and Winston, New York, N. Y., 1959, p 256; H. Brown in "Determination of Organic Structures by Physical Methods," Vol. 1, E. Braude and F. Nachod, Ed., Academic Press, New York, N. Y., 1955, p 604. (32) L. P. Hamrnett, ref 31, p 371. (33) J. Steigman and I , Sussrnan, J. Amer. Chem. Soc., 89, 6406 (1967). (34) L. Wooten and L. P. Hamrnett, J. Amer. Chem. SOC., 57, 2289 (1935). (35) J. M. Wilson, N. Gore, J. Sawbridge, and F. Cardenas-Cruz, J. Chem. SOC.8,852 (1967). (36) M. W. Dietrich, J. Nash, and R. Keiler, Anal. Chem., 38, 1479 (1966). (37) M. Tribble and J. Traynham, J. Amer. Chem. SOC.,91, 379 (1969). (38) C. P. Srnyth, "Dielectric Behavior and Structure," McGraw-Hili, New York, N. Y., 1955, p 253. (39) R. W. Taft in "Steric Effects in Organic Chemistry," M. Newman, Ed., Wiley, New York, N. Y., 1956, p 581; R. Taft, J. Amer. Chem. SOC., 74, 3120 (1952). (40) I . M. Kolthoff and M. K. Chantooni, Jr., J. Amer. Chem. SOC., 93, 3843 (1971).
Isotope Effect in Diffusion of I4C-Substituted Benzenes in Benzene, n-Heptane,
n- Octane, and Cyclohexane at 25" S. J. Thornton and Peter J. Dunlop* Department of Physical lnorganic Chemistry, University of Adelaide, Adelaide, South Australia 5007 (Received October 26, 7973)
Tracer diffusion coefficients are presented for 14C-substituted benzenes of different molecular weights diffusing in benzene, n-heptane, n-octane, and cyclohexane at 25". The data which were obtained with one magnetically stirred diaphragm cell indicate that, in agreement with previous results, a small isotope effect is present in each system.
In a previous paper,l tracer diffusion coefficients, DT, were presented for 14C-substituted benzenes of varying molecular weight diffusing in unlabeled benzene. Those results indicated that, while the tracer diffusion coefficients were not inversely proportional to either the square root of the mass of the tracer species or to the square root of the reduced mass of the system, a very slight linear The Journal of Physical Chemistry, Vol. 78, No. 8, 1974
mass dependence was present. This information is contrary to both the theoretical prediction^^.^ and experimental findings4 for binary gaseous systems, to recent results for benzene tracers diffusing in benzene crystals, and to the results of Eppstein and Albright,6 who also studied benzene tracers diffusing in liquid benzene, but in agreement with the conclusions of PikaL7
