ent. The strong positive rejections of ted phai~olsat high pW are significantly krettser tIt,i3.n RC) ~ ~ ~ e creported t ~ c ~ previously. ~ ~ s It ptmml. This is a consequence of the
had roughly the same size. It is per-
mn cmstants and (large) partition
C~DI.ffic%erXs. A n~i~rnber of ;,iizzling RCI results can be understood on
ifk I
the basis of these observations. The poor RO r:ejacti.on of urea, boric acid, m d aroma?k miiniines iq e;rpiicabie f-rem knowledge of their pki, values. Each o f these sohites exists, either partially or wholly, as an II ecies in , F O T this reason, cam? whit te their behavior, u k , water/na art;i&it?!-1 a t are larger than thoiie of i o n i z e d so%i!tes, and wo rejection that is poorer, In i: secondarqi alliphaaic amines &lave higher aromntia: amines, and exist ais chargec! 6ince they are charged., their good 3.9 have been predi c&d. 'l'he i w n e srgurria poor RO rejection of NW~6.4l-Iand good. A c k n m l e d g m m t s We are grateful to $4. M. T luninrrer for electron micrographs, to R. U1Xmasr tw liwftAl discussions, and to L.R.Mahoney for access to his unparaiieled supply o l suLstituted phenols
f Anilines with Water ullens, and P. Huyskens" Universitji of Lcuvain, Departmen! of Chemistry, 3030 Heverlee, Belgium
(Received August 9 , 7971)
Public8tlon cosls assisted by F. K . F. 0. Beigium and the University o f Louvain ( K .
U.6.)Belgium
?'!le exi,sapolated distribution coefficients PIa t infinite dilution of several methyl- and chloro-substituted aniliines between cyclohexane and water were determined at 15, 25, and 35" using an interferometric ~ ~the monomeric molecules from water to cyelohexmethod. From these values the transfer heats L L H of ane were deduced. Taking into account the effects of the various specific interactions o n the change of free energy that occurs when an aniline monomolecule passes from the former to the latter solvent, the foliowving equation IS deduced: log PI = log PIQ-t0.03($ - +O) - 0.28(pM, - pKaO) C.6062 - ~ N - H ) where a) is the molar volume of the aniline, n s - H the number of K-€3 links in the molecule and the superscript o refers LO the unsubstituted molecules A similar expression is derived for A&': A - 3 030i4 - 4') C 0.38(pKa - pKa0) - 0.82(2 - nN-H),where A f 1 1 O o refers to unsubstit&ed anilines. S'hc erq.xrimental values of PI fit by better than 20% with the calculated values and Ah! does not differ by more than 0.2 kcal from the experimental values. This demonstrates the intervention of N-H...O hy drogen bonds that stabilize the anilines with N-H groups in the water phase. The equi:ibrixn constant for t h e formation oT such bonds is of the order of magnitude of 0.1 M - l . Ox the other h ~ n dCI ..H-O hydrogen bonds do not play an important role in the stabilization of chloroanilines in water and the effects krfva'i'yi'lig the global dipole moments of the molecules seem to cancel out in the two solveits
+
The solubility of ,aniiines in water depends largely on the nature and position o f the substituents. For instance, a,(i-dimethylaniline is over five times as soluble in water a.s i"a,N-tJ.inrethgiI;ip-,elin~ at 25". The solubility of anilines in water is determined on one hand by the specific interactions which occur in the aqueous solvent and ,311 the other hand by the stabilization of the aniline molecules .in tneir own phase by the specific links that bring :about their association. The two effects can be separated by diluting the anilines in an inert solvent a.nd thus supprm3ing the specific interactions in the organic phase.
The limiting value of the partition coefficient at infinite dilution of the anilines between this inert solvent and water will be determined only by the specific interactions in the aqueous phase. In this work, we '.lave determined this limiting value of the distribution coefficient, of several anilines between cyclohexane and water at 15, 25, and 35" The enthalpy charge AH1 associated with the transfer of the monomolecule from water to cyclohexarie is deduced (1) (a) Holder cf a scholarship from the Belgian Ministry of National Education. ( b ) Ph. Mahillon assisted with tke measurements at 75 and 35'.
The Journai of Physicai Chemistry. Voi. 76, Ria. 26, 1972
~
A. Gomez, J, Mullens, and P. Huyskens
4611 2
ume increases. As a crude approximation we can write Aniline substituent
H 2-Methyl 3-Methyl 4-Methyl 2,6-.Dimethyl 2-Chioro
AS1" = AS1"' 1 . 1 3 3z 0 . 0 2 4 . 4 5 f 0.18 3.53 f 0.13 3 . 1 3 i 0.07 .!4,34 4 7.13 42.15 k 0 . 3 6
2,6-Dichloro N-Methyl
4.91 zk 0.11 3.94 f 0.05 IOS.6 4 6 . 2 0 15.50 =k 0.36
N,N - D i m e t h y l
147.6
3-ChlOPQ 4-Chioro
P
P1 a
3z 7.58
0.784 5.288 6.262 2.91 5 18.423 13.344 8.132 8.884 139.920 16.653 113.830
Y 1.161 0.986 1.435 2.238 --0.285 2.048 4.777 -1 1.840 17.21 0
-
0.931 -5.810
6 -0.300 -0.127 -0.215 0.000
0.000 -0.274
-1.100 0.000
0.000 -0.118 0.000
a 11' values given with standard deviation
from the variation of PI with the temperature. Equations relating Pi and hSrl with the molar volume of the aniline, its pK,, and the numbers of N-H links are deduced from theoretical considerations. The calculated values are compared with the experimental ones. I. Determination ef t h e Limiting Value Pl of t h e Partition Goeffzcient. The formalities F , and F , of the aniline in both phases are determined by an interferometric method.2 This niethod allows for accuracy of better than 0.001 M in these measurements. The PI values are obtained by the r?xi;rapolation of some 30 experimental values whose concentrations in the organic phase, F,, lie between 0.05 and 4 M depending on the solubility of the compound. The experimental values are thus represented by a polynomial. of the type
P= F,,/Fw= PI 3. /3Fo 4-yFo2 iSF,3
(1)
The coefficients nf tlne polynomials are given in Table 1. Table 11 gives the pK, values, the molar volume, and the Pa valuer; found using the polynomial form described above. At the same time we give the PI values reported by Cobumbic and Gsldbach3 and by Kemula, Buchowski, and Pawtowski,4 and the PI values calculated with the equation that will be derived later. The agreement bletween our data and those in the publications remains good until PIreaches very high values where, a t higt dilution in the organic phase, the concentration in the water phase is too low to be detected with su€ficient precision. We think that in these cases the valhes of CoBumbic and Goldbach are very high, because even a t higher concentrations in the organic phase we do not observe such high values for P. 9% Experirnenls a t Other Temperatures. Heat of Transfer. Similar experiments were also performed a t 15 and a t 35", From the values of PI a t different temperatures it is possible to derive the molar heat of transfer A H l o of the monomers from water to the organic phase. The values of AHl" can be compared to those obtained from the difference between the heats of solution of the anilines a t infinite dilution in cyclohexane and in water determined by direct ~alorirnretry.~~." From the comparison of AH,' and ~ G I the " moIsr entropy change AS,' when the monomolecules of aniline pass, from water to cyclohexane can be deduced. These results are given in Table ??I. Ht must be emphasized that t h e computed values of AS1 are heaviiy dependent on those of AH,.However, it appears from the data in Table .IT! that A S I increases when the molar volThe Jcurna! of Phvsicai Cnemistry. Vo!. 76. No. 26, 1972
+ 0.037(&- 4')
(2)
III. Deduction of a n Equation Relating P1 to Characteristics of t h e Substituted Anilines. The transfer of a monomolecule of aniline from the organic phase to water involves the following transformations. (1) Formation of a n Adequate Cavity in, Water and Suppression of a n Corresponding Cavity in t h e Organic Phase, This step involves the destruction of some hydrogen bonds between water molecules. The number of hydrogen bonds that must be destToyed is proportional to the volume of the cavity; consequently the corresponding free energy change will be proportional to the molar volume 4 of the aniline Ag,,, = a4 (31 An alternative method to evaluate g,,, could be derived from the scaled particle t h e ~ r which y ~ ~ expresses ~ this free energy charge as a polynomial of the third degree of the radius of the sphere which excludes the centers of the solvent molecules. However, approximations must be made owing to the lack of sphericity of the cavities considered here. (2) Formation of Hydrogen Bonds of' t h e T y p e O-fI...M between Water and t h e Aniline Molecules. Owing to the basicity of the anilines, these bonds may be expected to constitute the major specific interactions between the anilines and water. In a great variety of cases, ~ e e g e ~ s - ~ u y s showed ~ens~ that for a given proton donor, the logarithm of the complexation constants for such bonds varies in linear fzshion with the pK, of the proton acceptor. It may therefore be expected that the free energy variation corresponding to the formation of these O-H..*N bonds between the anilines and water will obey a relation of the form Ag0-H ... =
b
ppKa
(4)
where b and p are appropriate constants. (3) Formation o f N-I$ ...0 links between the Anilines in Water. These bonds are expected to be weaker than the previous ones. It may therefore be assumed that, in an initial approximation, the free energy change corresponding to the formation of one bond is the same for all the anilines. Under these circumstances the molar free energy change is proportional to the number o f Nthat the aniline molecule bears A ~ N - H . . =. ~- c ( w d
(5)
In principle the coefficient c also depends on the pK, of the aniline owing to the expected dependence of t,he acidiE. Meeussen and P. Huyskens, J. Chim. Phys., 63,845 (1966), C. Columbic and G. Goldbach, J. Amer Chem. Soc.. 73, 3966 (1951). (a) W. Kemula, H. Buchowski, and W. Pawtowski, Rocz. Chem., 42, 1951 (1968); (b) ibid., 43, 1555 (1969) A. Gomez, L. Lamberts, and P. Huyskens, Buil. SOC. Chim. F r . . 1734 (1972). J. Muliens and P. Huyskens, Annu. SOC.Scient. B:uxe/i., submitted for publication. H. Reiss, H. L. Fiesch, and L. Lebowitz, J. Chem Phys.. 3 6 , 369 (1959), R. A. Picrotti, J . Phys. Chem., 67, 1840 (1963); 69, 281 (1965). (a) A. M. Dierckx. P. Huvskens. and Th. Zeeaers-Huvskens. J. Ch>m. Phys., 62, 336 (1965); (b)' P. Lutgen, hl: P. Van' Damme, and Th. Zeegers-Huyskens, Buli. Soc. Chim. Beig.. 7 5 , 824 (1966); (c) D. Clotman, J. P. Muller, and Th. Zeegers-Huyskens, ibid., 79, 689 (1970); (d) G. Lichtfus, F. Lemaire, and Th. Zeegers-Huyskens. Spectrochim. Acta, in press.
1' :::i ,
4,: I. -i 3 3.53 3.73 14.34
12.15 4.91 3.94. 109.6 15.50 147.6
380 nD. D. ilsrrin, "::issociation Constanis c i Organic Bases in Aqi10ous Soiutions," Eutterworths, London, ?965.b H . A . Robinson, J. Res. N a t . Bur. Stand.. Sect. A: 68 121, 159 (1964). ' F . Aufauvre, I h 6 s e de Doctoral Clermont-Ferrand, France, 1969. dCalcul;?ted vaiue with the density iii the peltirig point (mp 43.5 , &'I5 = 11.9659: Handbook of Chemistry and Physics, 59th ed, The Chemical Rubber Publishing Co., Cleveland, Ohio, 1968-1969). e G . Debecker, These de !Doctorat, Louvain, Belgium. 1970. ?Calculated value with the measured density at melting point (mp -36.5", d = 1.34755). g R e f Srence 3. Reference ~ a 1 r?eierenca . 4.17.
t y of the W-13pratons on this factor. However, as a first approximat~.on,"ste variation of c in the case of the anilines studied hen: can lie disregarded, for the p values become /ow for weak complexation c ~ n s t a n t s . ~ (4) Formation of Specific Bonds of the T y p e U-N,.-T and Pcssibl,y of t h e T y p e O-..;.r betueen the ON Group or
the Lone Pairs of Electrons of the Water Molecules und the x Eiectrons (if itize Aromatic Ring of the Aniline. It may be asimmee-i as R first approximation that the c o r m riation 'of free eiiergy remains constant for all (6)
bigT...= --d
(5) f:hm?nges in the Nonspecific Interactions. These changes;, which must he o f minor importance, are pmportional to the su~faceof thc aniline molecules amd so, in a rough a-pproxima.kion,to their rnoia~volume Ag,,
!%gO-1- : . . . H
&!&H
( a 4. a')d, -- ppK,
Agns
...0
-.
'-
-
l o g ( P l / P l ~= ) O~~MNd _-40)-0.2ft(p.K, pK,') -b 0.60[2 -. (nx-H)j (IO) I__
_ll__lll
~
l
~
~
&\e,"
AH,".
AH,", (expt;). AS1", (caicd), Aniiine subsrituerrt
T,"c
kcai mol-'
p,
kcai
mol--'
cai "K PO!-'
kcal rnolF
_l_ll.ll_l-~
None
15 25
None None
2-Methyl
35 I5
0,90 1.13 1.35
3,57
3.48
2.86
3.07
3.83
2-ivliethyl 2-Methyl
25
4.45
35
.%-Methyl
16 25
35
5.30 2.60 3.13 3.74
:ti 25 35
13.90 '15~50 17.80
4-Methyl 4-Methyl N-MiJiefhyl N-Mebhyi N-Me:hyi
3.26
2.17
12.2 12.2 '12.2 12,6 12.6 12.6 -13.2 '13.2 13.2 12.8 12.7 3 2.8
3.02
3.17
2.25
By means ~f this equation, cve recalcuiased the v a ~ i o u sPI values. The results are grven in Table I The eatculated valuew do not differ by more than 20% from ow obserwatiQ1)S.
The transfer heat L
W 1 "
is related to PI h y the equation
Using eq 2 for evaluating AS^", AHl' car. be deduced from eX.pKe§SiOn 9 and ?;hi§gives AHl"
=z
~
~
l l _ _ _ 1 _ _ _
- c ( I c N _ ~ )- ( b + d ) (8)
B(pKaI;K,O) -t q 2 -- (.ap,.+H)] (9) IV,Comparisori L i j i t h the Experimental Values and Deductions. The ccreffiriients A., B, and C of eq 9 were comcxp'arimentat values of PI ?Ising a least; with tl1ese values eq 9 becomes -I
l from Water ~ t~
f
&, ... f
Comparing the limiting partition coefficient PI of an aniiirne with .t!iat ad' the msubstituted aniline P I 0 and owing to the presence of trijo w- !inks in unsubstituted aniline, we can c!xpect a relaicioor of tI?e form
log(Pll/P~*) A(w
and Entropy
~~~~~~~~~
~
(4)
= G'4
(Here also an aitemative way to compute this term could be deorved. h m t.he scaled particle theory.) Assernbiirtg these various contributions to the Bee energ y chzage, one call Wsile
R T l n PI =1 Agca\ "r
E i 11:
A.H-,"O.Q39(4 4') + 0.38(pK, - pK,O) -'
- 0~8262
nn-tr) (12)
-"
where AHI'* refers to the uasubstituted anilines. The values of L~H-,'calculated by means of this eqmation are shown in 'Table 111. The calculated valales and the experimental ones are again in fairly good a g r e e ~ e n t t, B u s justifying the approximations made in deti.ving eq 9. It can also be noted here that the v a h e of :MIogiven by caiorimeeric readings' for 2-chlQroXtI~~ine 2.60 kea: mol-1, while the calculated value from eq 1.2 i s 2.44 kcai mol-1. The Journal of Physisai Chemistry. Yo!. 76, No. 26. 7972
A. Gomez, d, Mullens, and P. Huyskens
40'14
n the other band, an expression for AH,' can also be found by differentiating eq 9 with respect to 1/T. From the Van't Hoff law .1H1"= -2.3R(d log Pl/dl/T)
(13)
it follows that Aff,' = AH1"O - 2.3R(d 4O)(dA/d 1/T) 2,3RA(d(4 - 4O)/d l/T) 2~3(pK,- pKAo)(dB/d, 1/T) 2.3R'B[d(pK, - pK,O)/d l/Tl 2.3R(2 - n,_,)(d C/d 1/T) (14) It can be assumed, as a first approximation, that the derivatives d ( 4 -- 4O)/(d l/T) and d(pK, - pK,O)/(d 1/T) are proportional :es;pectively to the differences (4 - +O) and (pK, - pMa0)).This leads to an expression of the form of eq 12. If, as a crude approximation, the two terms of eq 14 arising from the variation of ( 4 - 4 O ) and (pK, - pK,O) with the temperature are neglected, and if the values of A, B, and C at, 35 and 15" are used for estimating their derivatives with respect to 1 / T , one obtains the following expression AH10 = A H p
-'
0.0.4(4 - 40)-k
0.7(pKa - PIT,')
- 0.8(2 - 12N-H) (15)
whose coefficients remain of the same order of magnitude as those obtained using eq 2. For most of the anilines studied here that are not N substituted, the di.fferences between the PI and AH1 values are chiefly due to the changes in molar volume. This result is in agreement with the observations of Przybsrowska and 9oczewinski,1° who studied the chromatography parameters of methyl derivatives of aniline and concluded that their chromatographic behavior was mainly determined by the molecular volume of the solute while basicity plays only a minor part in the neutral systems. On the other hand, we can compare the coefficient A with the effect of increasing the molar volume of the alcohols upon their PI coefficient in the same solvents. From propanol to heptanol we determined in a previous w o r l P an increment of 0.65;; in log PI for each addition of a CH2 group. This addition corresponds roughly to an increase of 18 ml in the molar vohme, giving an increase of 0.038 per ml, of the same order of magnitude as the A constant found for the anilines. The last term of ieq 9 and the corresponding term of eq 12 are very important. When these terms are disregarded, the calculated va.luc?s of PI are four times too low for N methylaniline (and fifteen times too low for the Af,N-disubstituted anilines) and the transfer enthalpies are 0.8 kcal too high.
The Journal of Physical Cnemistry, Vol. 76, No. 26 1972
These terms were intro to take into account the formation of the N-H. ..O between the anilines and water. It is difficult to interpret as a result of steric hindrance of the H substituents, for such effect will also appear in the pK, values of the anilines. The importance of the last terms in eq 9 and 12 may therefore he considered as a strong indication of the formation of the N-H...Q bonds. The equilibrium constant KN-H c, of the complexatior. of water molecules by these bonds can be estimated. Taking into account the concentration of the water in the dilute aqueous solutions, this constant is related to the variation of free energy, and consequently to the C parameter by the expression
log(55.5KN I-i . o) = i:
(16)
This gives for KN-2 an order of magnitude of 0.1 M - l . The PI value of the C1 derivatives fits well with the proposed equation. This equation, ver, does not take into account the formation of . e l bonds between water and these derivatives. This shows that these bonds do not play an important role in the stabilization of the CI derivatives of aniline in water. Another factor that was not taken into account is the influence of the varying dipole moments of the compounds. That this influence can be disregarded may be due to the fact that it is cancelled out in the two phases.
Experimental Section The partition experiments were performed using a method described by Brown and Burrg.12 The Hilger RayPeigh M 75 interference refractometer with the cell M 160 €or liquids was used for the determination of the concentrations. Cell thicknesses were 1, 3, and 10 cm depending on the concentration range. About 60 standard solutions, covering the whole range of concentrations investigated, were used for the calibration of the interferometer. These solutions were saturated with the other solvent in order to take into account the mutual solubility of the phases. The reagents used were cyclohexane (Merck pro analysi) and distilled water. All the anilines were Fluka purissimum except 2,6-dimethylaniline which was purified by distillation in vacuo.
Acknowledgments. The authors wish to express their thanks to the Belgian Ministry of National Education and to the Katholieke Universiteit te Lewen, which gave a scholarship to A. 6. (IO) M. Przyborowska and E. Soczewinski, d. Chromatogr. Sci.. 42, 516 11969) \.___,.
(11) I. Hanssens, J. Mullens, Ch. Deneuter, and P. Huyskens, Bull. Soc. Chim. Fr.. 1342 (1968). (12) F. S. Brown and C. B. Burry, Chem. Soc., 123,2430 (1923).
