1734
N.E.VANDERBORGH, N. R.ARMSTRONG, AND W. D. SPALL
creasing temperature, but not varying linearly with l / T , in the system of C U ( T F A ) ~ . ~ - > I ~AsP ~eq. 1 implies, the hyperfine coupling constant, A , can be obtained from the Iinear plot of contact shift us. I / T . For the C U ( H F A ) ~ . ~ ( ~ - Mand ~ P ~CU(HFA)~. ) 2(4-RIePyO) complexes, the calculated hyperfine coupling constants for y-CHa are, respectively, +3.0 X 105 Hz and +7.7 X 105 Hz. Kluiber and Horrocks5 have reported a similar abnormal temperature effect for Cu(TFA)t .4-MePyO complex. They have proposed that a five-coordinated complex of low symmetry or a tetrahedral four-coordinated complex (with the opening of one TFA ring) accounts for the observed temperature variation a t elevated temperature (above 30") for this complex. However, this model of Kluiber and Horrocks is not consistent with our results on the Cu(HFA)z complexes. C U ( H F A )forms ~ tetragonally distorted octahedral bis
adducts with 4-methylpyridine and 4-methylpyridine K-oxide in chloroform solution. Since CU(HFA)~is less stable than C U ( T F A ) ~ ,one ~ ' would expect ring opening to be easier for CU(HFA)~ than CU(TFA)~ at elevated temperatures. The fact that CU(HFA)~. 2(4-MePy) and C U ( H F A ) ~ . ~ ( ~ - X I ~show P ~ O ) appreciable spin delocalization and a normal temperature effect is not consistent, then, with a tetrahedral species being present for Cu(HFA)* complexes. Thus, since this tetrahedrally coordinated complex does not occur for the Cu(HFA)z complexes, we think it even less likely to occur for the CU(TFA)~ complexes. Acknowledgment. The authors thank Dr. A. Allendoerfer of the State University of New York at Buffalo for his assistance in obtaining the esr g values. (27) L.G.Van Uitert, W. C. Fernelius, and B. E. Douglas, J.Amer. Chem. Soc., 75,457 (1953).
A Cryoscopic Study of the Association of Phenolic Compounds in Benzene by Nicholas E. Vanderborgh, NeaI R. Armstrong, and W. DaIe Spa11 Department of Chemistry, University of New Mexico, Albuquerque, New Mexico 87106
(Received June I d , 1969)
The cryoscopic behavior of phenol, positional isomers of chlorophenol and cresol, and 2,5-, 2,6-, 3,4-, and 3,5dimethyl phenol were studied in benzene, and equilibrium constants describing this behavior in terms of association were determined for the concentration range 0-0.8 rn. The results indicate that substitution of a ring hydrogen of phenol by either chlorine or methyl decreases the amount of association of the parent phenol, chloro isomers having less association than methyl isomers. The degree of association is qualitatively related to the effects of the substituents on the T electron cloud of the phenyl ring. The measurement of colligative properties of liquid solutions, those properties which depend upon the number and not the type of dissolved species, has long been recognized as an important method for the study of molecular association. Of the several colligative property techniques, one of the most accurate and experimentally simple is the depression of the freezing point, cryoscopy. Early workers studying this technique discovered that many organic solutes, when used to depress the freezing points of aprotic solvents, showed smaller freezing point depressions than would be predicted on the basis of their formula weights. The most commonly used explanations for this observed nonideal behavior are the formation of solid solutions, changes in activity coefficientswith concentration, and molecular association. The Journal of Physical Chemistry
Solid solution formation greatly complicates the interpretation of cryoscopic data. The existence of solid solutions may be detected either by chemical analysis of the frozen solid or by the method of Van Bijlert. Compounds suspected of forming solid solutions or mixed crystals in benzene solutions include acetic a ~ i d z -and ~ pheno1.2J However, in these previous studies, the existence of solid solutions was not verified by chemical analysis. The usual method for correcting cryoscopic data for the effects of solid solu(1) A. Van Bijlert, 2.Phgs. Chem. (Leipzig), 8, 343 (1891). (2) C.R. Bury and H. 0. Jenkins, J . Chem. SOC.,688 (1934). (3) A. G.Milligan, J . Phys. Chem., 33, 1363 (1929). (4) R.Marc and W.'Wenk, Z. Phys. Chem. (Leipzig), 68, 104 (1910). (5) F. Garelli, ibid., 21, 122 (1890). (6) J. A, Davison, J. Amer. Chem. Soc., 67, 222 (1945).
1735
ASSOCIATION OF PHENOLIC COMPOUNDS IN BENZENE Table I: Reagents Anal. of purityd”
Reagent
Re1 wt 5% major
HzO wt %
B
a
N
b
99.6 99.9+ 99.9+ 99.7 99.3 99.3 99.4 99.6 99.6 99.8 99.1 99.1 99.3 99.2
0.025 Not detectable Not detectable 0.055 0.030 0.030 0.030 0,029 0.033 0,019 0.041 0.048 0,043 0.047
Symbol
Benzene Naphthalene Biphenyl Phenol o-Cresol m-Cresol p-Cresol o-Chlorophenol m-Chlorophenol p-Chlorophenol 2,5-Dimethylphenol 2,6-Dimethylphenol 3,4-Dimethylphenol 3,5-Dimethylphenol
>
Method of purifioation
BP
C
P
a
oc
MC PC OCP MCP PCP 25DMP 26DMP 34DMP 35DMP
a
a
b a a
c and b b b b b
’
Fractional crystallization Fractional distillation from Linde 4A molecular sieves; system flushed with dried and purified argon. from benzene, vacuum dried. Sublimation. d Glpc: 2-m Silicone Oil DE-710 on firebrick; T = 190’ for phenols, T = 80” for benzene. e Error in major component analysis is rrt0.5 wt %. (1
tion formation is the use of an expression of the form7v8
AT
=
KtCo(1
- k)
where AT is the observed temperature lowering, Kt the freezing depression constant, Cothe “observed concentration,” and k is the distribution coefficient for the solute between the solid and liquid phases. Using an expression this form, Bury and Jenkins2 report a value of 0.47 for k while Davidson6 reports a value of 0.41 for solutions of phenol in benzene. Even though the existence of solid solutions is suspected, it is not reasonable to interpret cryoscopic data solely on this basis for systems which might show other causes for nonideal behavior. Phenolic solutions in benzene show infrared spectra characteristic of hydrogen bonded systems and the nonideal cryoscopic behavior should be, in part, attributed to association. Deviation from ideal solution behavior for these systems may also be due to activity effects. Several theories for the determination of activity coefficients of nonelectrolytes in nonaqueous media have been put forth; the one derived from the normal theory of liquidss is perhaps the most used. These theories are generally unsatisfactory in explaining the data since quite large activity corrections are necessary. For example, phenol has been reported to have an activity coefficient of less than 0.3 at 0.5 m benzene solution.1° Another cause for nonideal behavior may be loosely classified as molecular association. The solute may associate with itself, the solvent, or with impurities present. I n this investigation, equilibrium constants for the self-association of phenol (P), orthocresol (OC), metacresol (MC), paracresol (PC), orthochlorophenol (OCP), metachlorophenol (MCP), parachlorophenol (PCP), 2,5-dimethylphenol (25DMP), 2,6-dimethyl-
phenol (26DMP) , 3,4-dimethylphenol (34DMP), and 3,5-dimethylphenol (35DMP) are reported in the solvent benzene. Infrared spectra of these compounds in benzene solution are accepted as showing hydrogen bonding;lo it is assumed that self-association of the phenols is the cause of this spectral behavior. The formation of solid solutions of phenol in benzene has also been investigated. I n this investigation we shall assume that the cryoscopic data can be explained solely on the basis of molecular association after the effects of possible solid solution formation have been explored.
Experimental Section Reagents. The chemicals used for this study as well as the method of purification and analyzed purity are shown in Table I. Apparatus. The apparatus used for these investigations was designed to measure the freezing points of solutions at millimolal and higher concentrations using a procedure to maximize accuracy and to minimize time spent in data collection. The test solution was maintained under an atmosphere of dried argon at all times. Stirring was accomplished by a Teflon disk stirrer driven at 600 rpm by a synchronous motor. The apparatus was sealed except when solution concentrations were changed. (See Figure 1.) A Veco 51A1 100,000-ohm thermistor was used as (7) S. Glasstone, “Textbook of Physical Chemistry,” 2nd ed, Van Nostrand-Reinhold Co., Inc., Princeton, N. J., 1849,p 649. (8) See ref 7, p 660. (9) G. N. Lewis and M. Randal, revised by Pitzer and Brewer, “Thermodynamics,” 2nd ed, McGraw-Hill Publications, New York, N. Y., 1961, chapter 20. (10) G. C. Pirnentel and A . L. McClellan, “The Hydrogen Bond,” W. H. Freeman and Co., San Francisco, Calif., 1960. Volume 74, Number 8 April 16, 1OYO
1736
N. E. VANDERBORGH, N. R. ARMSTRONG, AND W. D. SPALL
60 RPM SYNCHRONOUS
MOTOR.
14:1
I
i I,,/
GEAR
RATIO
I/
Figure 1. Cryoscopic cell assembly, constructed from Pyrex, described in text.
pure solvent was placed in the cell, a blanket of argon introduced, and the solvent benzene frozen by raising a constant temperature bath (0’) into position. The cooling curve was recorded on a recorder. After melting the benzene frozen during the initial determination, a known volume of a solution of known composition of the solute in benzene was introduced and the freezing curve for this solution determined. Repeated additions allowed the measurement of the cryoscopic solution behavior throughout the desired concentration range. Solid Solution Studies. Benzene solutions of phenol, o-chlorophenol, and p-cresol were prepared by weight, maintained under dry argon, and while vigorously stirred partially frozen by emersion of the flask containing the solution into a bath maintained at 2’. The partially frozen solution was then transferred under an argon layer into a refrigerated Hirsch funnel and the liquid fraction separated. The remaining solid was washed with 5.0 ml of 6” benzene; these washings were added to the liquid fraction and the total volume noted. The collection vessel was then changed, the solid allowed to melt, and collected. Analyses of the resulting solutions were made on a Varian Auto-Prep gas chromatograph. The analytical results were compared to results obtained with standard solutions of the same compounds. From these measurements, the value for the distribution coefficient for each solute was determined. Concentration values for the liquid fraction were corrected for dilution by the 5.0-ml wash fraction of benzene.
Experimental Error
-1 R2
Figure 2. Temperature measuring circuit; Ei, = 1.35 V Mallory mercury battery; R1 = Ra = R3 = 500,000-ohm cermet trimpot, Rf = 100,000 ohm Veco 5 l A l thermistor; 0-1, Fairchild ADO-44, operational amplifier.
the temperature sensing device with the measuring circuit shown in Figure 2 . The apparatus shown is capable of an output sensitivity of approximately 150 mV/deg C and can detect 0.001” changes in temperature under the conditions used in this study. Use of the full sensitivity of this circuit would allow investigation of 2.5 X lo-* m solutions. Operating conditions were such that approximately a 2’ change in the temperature corresponded to maximum response. Discussion of the measuring circuit may be found elsewhere.”
Experimental Procedure The apparatus was carefully cleaned and dried in a vacuum oven (1lOO) before use. A weighed charge of The Journal of Physical Chernietry
An error analysis showed the average error in the analytical solution concentrations was A 0.5% and 1.0% in the cryoscopically observed concentration. Maximum error occurred in the lowest analytical concentrations studied, below 100 mm, where the error in CA was from 10.8 to 1.0% and the corresponding error in CO was from k2.0 to 3.0%. The measuring circuit was calibrated by comparison with thermometers traceable to the National Bureau of Standards.
Results Determination of the Cryoscopic Constant of Benzene. Cryoscopic techniques require a known relationship between the observed freezing point lowering and the concentration of a species known to behave ideally in the solvent under study. If such ideal behavior has not been reported, a “reference compound’’ may be selected and all concentrations can be related to this standard. I n this study, naphthalene and biphenyl were chosen as the nonassociated standards. Biphenyl is known to be nearly ideal in benzene solu(11) “Handbook of Operational Amplifiers,” BurrBrown Research Corp., Tucson, Ariz., 1960,p 30.
1737
ASSOCIATION OF PHENOLIC COMPOUNDS IN BENZENE tions;12 positive deviations from Raoult’s law for the liquid-vapor equilibrium are less than 2% at relatively low concentrations. The freezing points of a series of solutions of naphthalene and biphenyl were determined; the value of K fdetermined with either compound was 5.112”C/m, in good agreement with 5.070 reported by Barton and Kraus,13 5.122 by Bury and Jenkins,2 5.10 by Peterson and Rodebush,145.12 from latent heat of fusion by Huffman, Parks and Daniels,15 and 5.11 by Auwers,16 but differing with the value of 5.492 reported by White and Kilpatrick.’7 When using the apparatus shown, the output of the temperature measuring circuit was linear with concentration of the standard species to within 0.0091 Q unit with an intercept less than experimentally observable. of 6 X The apparent linearity of data is attributed to a fortuitous cancellation of the nonlinear response of the temperature-measuring device with the change of Kt with temperature. Interpretation of Experimental Data The experimental data determined must be examined in terms of association since reliable theories of nonelectrolyte activity are nonexistent. As has been pointed out succinctly by Rossotti and Rossotti, l8 association data of this sort result in essentially two equations with n unknowns, a problem which may in theory be solved; however, experimental error usually makes the set of equations ill-conditioned. Attempts to increase the number of equations available for data analysis results in fixing some arbitrary relationship between successive equilibrium constants for the associative reaction. Notable contributions on this line have been made by L a ~ s e t t r e , l ~Dunken,22t2a -~~ and BejerrumlZ4modified by Fr0naeus.~5 A second approach is
.
0.5.
0 c
E
e 0.4-
. . 4
c
8 0.3‘0
2
2 0.2. 0
V
0
0.1
0.2
0.4 0.5 0.6 Analytical concentration, m.
0.3
0.7
0.8
Figure 3. Cryoscopic behavior of the chlorophenols. Observed concentration ( m )us. analytical concentration; line calculated from equilibrium constants shown in Table 11; 0, OCP; 0 , MCP; A, PCP.
0.6
0.5
1
6
._ c 0.4 a C C
v
0.3.
>
L
: 8 0.2. 0.1 I V
0
0.1
0.2
0.3
0.4 0.5 0.6 Analytical concentration, m.
0.7
0.8
Figure 4. Cryoscopic behavior of cresolb and phenols. Observed concentration ( m ) us. analytical concentration; line calculated from equilibrium constants shown in Table 11; 0, OC; 0 MC; A, PC.
based on the minimization of undetermined thermodynamic functions and on statistical curve fitting.2e,27 The relationship of CO, the “observed concentration” determined from the apparent freezing point depression, to CA, the “analytical concentration,” for the compounds used in this investigation is shown in Figures 3-6. Figures 3 and 6 include the data of Davison and those of Bury and Jenkins for purposes of comparison. Table I1 lists equilibrium constants determined by curve fitting, those determined by Lassettre’s method, as well as those previously reported. (12) Laszlo and Szabados, Kozp. Fiz. Kut. Intez., Kozlem, 15, 283 (1967). (13) B. C. Barton and C. A. Kraus, J . Amer. Chem. Hoc., 56, 2017 (1934). (14) J. M. Peterson and W. H. Rodebush, J. Phys. Chem., 32, 709 (1928). (15) H. M. Huffman, G. 8. Parks, A. C. Daniels, J . Amer. Chem. SOC., 52, 1547 (1930). (16) K. Auwers, Z . Phys. Chem., 42, 513 (1902). (17) N. E. White and M. Kilpatrick, J. Phys. Chem., 59, 1044 (1955). (18) (a) F. J. C. Rossotti and H. Rossotti, “The Determination of Stability Constants,” McGraw-Hill Publications, New York, N. Y., 1961; (b) F. J. C. Rossotti and H. Rossotti, J . Phys. Chem., 65, 926 (1961). (19) E. N. Lassettre, Chem. Rev., 20, 259 (1937). (20) E. N. Laasettre, J. Amer. Chern. Soc., 59, 1383 (1937). (21) E. N. Lassettre and R. G. Dickinson, ibid., 61, 54 (1939). (22) H. Dunken, 2.Phys. Chem., 45B, 201 (1940). (23) K. L. Wolf, H. Dunken, and K. Merkel, 2.Phys. Chem. (Leip zip), 46B,287 (1940). (24) J. Bjerrum, Kem Maanedsbl. Nord. Handelsblad Kem. Ind., 24, 21 (1943). (25) S. Fronaeus, “Komplexsystem hos Koppar,” Dissertation, University of Lund, 1948. (26) F. J. Zlesnik and S. Gordon, Ind.Eng. Chem., 60,27 (1968). (27) A. Pullman and H. Berthod, Theor. Chim. Acta, 10, 461 (1968). Volume 74, Number 8 April 16, 1970
1738
N. E. VANDERBORGH, N. R. ARMSTRONG, AND W. D. SPALL
Table I1 : Summary of Cryoscopically Determined Equilibrium Constants for the Association of Phenolic Compounds in Bensene
V
Compd
% assocd“
Kiz
PB 34DMP 35DMP 25DMP 26DMP
84.75 21.81 19.35 31.95 14.51 16.27
125 0.30 0.20 0.47 0.17 0.58
450 2.1 2.0
23.18 18.96
0.50 0.30
oc
MC
PC OCP
9.09 31 -51 35.49
MCP PCP a
V
Y B- - - - - -
Equil oonst for amocn This investigationourve fitting-BY method of L a s s e t t r e Kin Kiz Kis
=
% associated, defined in text.
0.10 0.46 0.55
b b b
0.556 0.653 0.556 0,309
0 190 0.640 0.464 0,143
0.18
0.14
1.80 1.60
0.578 0.675
0.955 0.595
--Previously KII
repdRef
61
32
1.1 0.28
3 19
...
*..
b b b
0.006 0.168 0.242
1.1
3
0.42
19
0.004 0.04 0.09
* Kla < 0.01.
The equilibrium constants shown in Table I1 were determined by a curve-fitting technique. A series of CA vs. CO curves were generated using incremented values of monomer concentration and overall equilibrium constants. The experimental data were then displayed graphically and the resulting curves overlayed on the various calculated curves to achieve a “best fit.” Criteria for “best fWJ were that the deviations of the experimental points from the calculated curves were a minimum, the same order of magnitude as the experimental error, and that the least number of equilibrium constants necessary t o describe the data were used. This leads to the simplest model to adequately explain the experimental data. To obtain a reasonable value of the initial dimerization constant, the data obtained for low analytical concentrations were used since the
0.6
I
7
contribution of higher order polymers t o CO, the observed concentration, should be small; the initial part of the CO-CA curve, then, was used to evaluate the dimerization association equilibrium constant. Once this was done, this constant was used to evaluate the trimerixation constant. I n no cases were association constants higher than the trimerization necessary. This approach is generally similar to that often used for the determination of successive formation constants of complex ions.ls Also shown in Table I1 is the function V, the per cent of association, evaluated a t an analytical concentration of 0.5 m. V = concentration of all associated species X 100 divided by the concentration of all species; V = (CO - Cmonomer) lOO)/(CO). The monomer concentration was obtained from solution of the equation relating
1
!
0.6
I i
e 0.5
0.5
.-i
c
._
-
c
e
2 0.4
c
0.4
8
’
s
0
” 0.3
Bt
v
$
0.3
6 0.2
2 0.2
0.1
0.1
0.0
0.1
0.2
0.4 0.5 0.6 Analytical concentration, m.
0.3
0.7
08 .
Figure 5. Cryoscopic behavior of dimethylphenols. Observed concentration ( m )vs. analytical concentration; line calculated from equilibrium constants shown in Table 11. 0, 2,6-; 0, 2,5-; 0, 3,4-; A, 3,6-. The Journal of Physical Chemistry
0.0
0.1
0.2
0.4 0.5 0.6 Analytical Concentration, m.
0.3
0.7
0.8
Figure 6. Cryoscopic behavior of benzene solutions of phenol. Observed concentration ( m ) vs. analytical concentration; 0, this investigation; 0, Bury and Jenkins; A, corrected for solid solution formation.
1739
ASSOCIATION OF PHENOLIC COMPOUNDS IN BENZENE CO to the monomer concentration through the equilibrium constants. If a large concentration range is studied and the resulting data are used to determine equilibrium constants, corrections for the temperature dependence of these constants may be necessary since the measurements are not isothermal. In theory, this effect may be used to predict thermodynamic parameters9 derived from the van’t Hoff equation. For phenolic systems in benzene, these deviations of the equilibrium constants with temperature are limited by values of AH for the associative reaction. lo Calculations based on an average value of AH for OH. hydrogen bonds indicate that for observed concentrations of 0.5 m, only 5% change in the equilibrium constants results from these effects. Since the highest observed concentrations found in this investigation were of this magnitude] the temperature dependence effects should introduce error of no greater than 5%. Table I1 also includes the results obtained by analysis of our datausing the method described by Lassettre. 19-21 This approach assumes three possible functional forms for the relationship between CO and CA; two of these describe a hyperbole and one a quadratic. When the proper relationship has been selected] the maximum number of equilibrium constants is determined by the degree of experimental error. Using the equilibrium constants obtained from this method, we attempted to reconstruct our experimental data. It was found that the reconstruction required a greater number of equilibrium constants than the experimental error indicated were significant. and Rossotti and Rossotti’8 have pointed out that the precision of measurements as well as computational errors introduced by smoothing of experimental error seldom justifies more than three independent parameters. The errors introduced by forcing the data to conform to a predetermined function may actually increase the overall error. I n a similar data analysis, Sil16n28 introduced a graphical curvefitting ppocedure which has been expanded in an attempt to reduce the error. We feel that the curve for fitting technique which we have described allows a
0
~
~
~~
Table 111: Solid Solution Analysis of Benzene Solutions
Solute
o-Chlorophenol p-Cresol Phenol
Originalu
-Concn Liquid phase”’
Solid phase*
kd
0.287
2.99
0.57
0.189
0,263 0.258 0.267
2.99 2.88 2.64 6.00 7.59
0.54
0.179 0.200 0.349 0.340 0.386
0.481 0.797
0.58 0.92
2.04 2.93
am. Wt %. Corrected for dilution. tribution coefficient] solid: liquid.
Apparent dis-
more latitude in suggesting B model than does an explicit solution. It has been reported previously2J**that phenol forms solid solutions with benzene. It was therefore rrecessary to investigate the extent of this solid solution formation. The results of these studies are shown in Table 111. I r data indicate that OCP is slightly associated,lO in agreement with our cryoscopic studies. It is therefore reasonable to assume that the partition coefficient determined for OCP is one which results from a residual absorption and occlusion with the solid phase and not from solid solution formation. Due to the observed partition coefficient for PC, a similar conclusion results even though the cryoscopic behavior of PC is indicative of considerably increased association, OCP and PC mere chosen as reference compounds due to their structural similarity to phenol. Both show partition coefficients of 0.19 f 0.08; this value is taken as representative of absorption and occlusion effects. No attempt to utilize the Van Biljert method was made since it was felt that addition of a third component to the cryoscopic solution could alter the solvent properties, or, more importantly, alter the extent of solid solution formation. The average value of the partition coefficient of OCP and PC was subtracted from the observed value for phenol, 0.36, to yield a partition coefficient of 0.17 for phenol corrected for surface absorption and occlusion inherent in this procedure. Using this corrected value, the cryoscopic data were then corrected for the effects of solid solutions. These data are shown in Figure 6 and equilibrium constants are included in Table 11.
Discussion Phenolic compounds are thought to self-associate in solution because of the favorable difference in energy between the associated and monomeric forms. This energy difference is termed the delocalization energy; a significant fraction of this difference is due to the hydrogen bond. Theories of hydrogen bonding piedict that several factors contribute to the total energy of the bond. The differences in magnitude of these factors in different systems have made the complete description of the hydrogen bond difficult. Hydrogen bonding has been extensively studied using various spectral techniques. Ultraviolet methods indicate varying degrees of n cloud interactions while infrared methods indicate varying amounts of hydroxyl interactions in the phenolic compounds under study.1° Recent theoretical s t ~ d i e s ~show ~ , ~T~interactions -~~ may arise in several ways and may contribute to the stability of the asso(28) L. G. Sill&, Acta Chem. Scand., 10, 186 (1956). (29) 9. Brator, Advan. Quantum Chem., 3 , 207 (1967). (30) K. Morokumo, H. Kato, T. Yonerawa, and K. Fukui, Bull. Chem. SOC.Jap., 38, 1263 (1965). (31) A . Ocvirk, H. A. Zuman, and D. Hadji, Theor. Chem. Acta (Bed), 10, 187 (1968).
Volume Y4, Number 8 April 16, 19YO
1740
N. E. VANDERBORGH, N. R. ARMSTRONG, AND W. D. SPALL
Table IV: Selected CA and CO Values: Cryoscopic Data; CA, Analytical Concentration, m; CO, Cryoscopically Observed Concentration, m
Phenol CA
co
0.0261 0.0382 0.0498 0,0608 0,0714 0.0912 0.1262 0.1562 0.1927 0.2494 0.3091 0.3545 0.4077 0.4508 0,5326 0.6091 0.6317 0.6809 0.7672 0.7661
0.0216 0.0305 0.0368 0.0432 0,0495 0.0584 0.0800 0,0953 0.1143 0,1329 0.1567 0.1828 0,2089 0,2374 0.2706 0.3062 0.3098 0.3264 0.3610 0.3691
CO' oor for solid soln
-0-ChlorophenolcA4 co
0.0260 0.0616 0.0367 0.1073 0.0443 0.1207 0.0520 0.1578 0 0596 0.1697 0.0704 0.1773 0.0964 0.2062 0.1148 0.2317 0.1377 0.2528 0.1601 0,2715 0.1888 0.2839 0.2202 0.2977 0.2517 0.3340 0.2860 0.4614 0.3260 0,4937 0.3689 0.5538 0.3733 0.6037 0.3933 0.6681 0.4349 0.7022 0,4447 I
0.0693 0.1021 0.1151 0.1495 0.1650 0.1685 0.1994 0,2231 0.2434 0.2587 0.2730 0.2908 0.3216 0,4439 0,4842 0.5459 0.5815 0.6527 0.6836
-p-ChlorophenolCA co
--o-CresolCA
co
0.0480 0.0958 0.1485 0.1835 0.2247 0.2642 0.3386 0.3567 0.4123 0.4400 0.5273 0.6431 0.7100 0.7443
0.0635 0.0735 0.1239 0 1437 0.2109 0.2776 0.2883 0.3381 0.3910 0.4567 0.4853 0.5627 0.6330 0.6591 0.7031 0 7512
0.0570 0.0636 0.1128 0.1306 0.1899 0.2469 0.2587 0.2955 0.3442 0.3988 0.4178 0.4795 0.5329 0.5554 0.5839 0.6231
0,0463 0.0950 0.1389 0.1721 0.2160 0.2492 0.3086 0.3204 0,3537 0.3869 0,4356 0.5222 0,5887 0.6005
I
I
-mChloroph CA
enol-
0.0552 0.0932 0.1082 0.1390 0.1591 0.1822 0.2081 0.2218 0.2552 0.3030 0.3662 0,4107 0.4535 0.4948 0.5412 0.5730 0.6111 0.6615 0.6870 0.7779
0.0510 0.0748 0.0973 0.1199 0.1424 0.1602 0.1840 0.1958 0.2219 0,2635 0,3264 0.3572 0.3928 0.4273 0.4652 0.4902 0.5175 0.5720 0.5934 0.6551
-m-CresolCA
0.0315 0,0711 0,1391 0,1796 0.2041 0.2463 0.2800 0.3104 0,3428 0.3836 0.4310 0.4932 0.5427 0.6012 0.6708 0.7014 0.7646 0.8120
co
co 0.0368 0.0653 0.1222 0.1531 0.1721 0.2018 0.2279 0,2492 0.2730 0.3026 0,3323 0.3679 0.4023 0,4320 0.4724 0.4866 0,5246 0.5519
ciated phenolic species much more than was originally believed. Initial theories t o explain the hydrogen bond used a purely electrostatic approach; these have been shown to be less than completely adequate.1° However, an electrostatic parameter might be quite successful to predict trends of associative behavior within a group of chemically similar compounds, especially if one particular compound can be selected as an arbitrary reference. A logical choice for such a reference compound among the compounds reported in this work might be the unsubstituted basis compound, phenol. Phenol is the most associated of all the compounds investigated, exhibiting a cryoscopic behavior quite similar to that pf The Journal of Physical Chemistry
---p-Creaol-CA
0.0587 0.0993 0.1157 0.1682 0.2193 0.2503 0.3152 0,4033 0.4467 0.5230 0.6050 0.6763 0.7234
co 0.0570 0.0961 0.1104 0.1519 0.1899 0.2101 0.2635 0.3228 0.3442 0.3964 0.4379 0.4866 0.5020
--2,6-XylenolCA co
0.0724 0.1124 0.1407 0.2053 0.2137 0.3056 0.3244 0.3891 0.4317 0.5013 0.5289 0.6306 0.6588 0,7413 0,7725
,----3,4-Xylenol---CA
co
0.0754 0.1142 0.1470 0.2150 0.3100 0.3415 0 3946 0.4566 0.5079 0.5617 0.6077 0.6582 0.6963 0.7888 0.7998 0.9410
0.0697 0.0963 0.1284 0.1842 0,2540 0.2749 0.3098 0,3489 0.3768 0,4061 0.4312 0.4605 0.4745 0.5191 0.5219 0,5861
I
0.0683 0.1088 0.1353 0.1981 0.2079 0.2958 0.3210 0.3642 0.4228 0.4843 0.5164 0.6015 0.6378 0.6880 0.7466
--2,5-Xylenolco CA
0.0941 0.1058 0.1518 0.1710 0.2011 0.2439 0.2566 0.3135 0.3179 0.3780
------3,6-Xylen01----~ CA
0.0438 0.0581 0.0881 0.1128 0.1644 0.2283 0.2593 0.3171 0.3445 0.3937 0.4215 0.4808 0.5238 0.5711 0.6405 0.7616
0.0907 0.1004 0.1395 0.1646 0.1842 0.2233 0.2400 0.2889 0.2930 0.3377
co 0.0418 0.0558 0,0809 0.0990 0.1437 0.1967 0,2233 0.2651 0.2791 0.3182 0.3307 0.3726 0,3935 0.4187 0.4605 0.5164
the carboxylic acids. Properties of phenol such as the dielectric constant, dipole moment, aqueous pK,, and absorption spectra are not dissimilar to that of the substituted phenols. This study indicates that addition of a substituent to phenol lowers the degree of association, analogous to the cryoscopic behavior of benzoic acid and its substituted analogs; benzoic acid is also the most highly associated. lo For these reasons, phenol cannot be taken as a reference point. For the compounds investigated in this study, the methyl derivatives are more highly associated than the corresponding chloro compounds. From the extent of association of the cresols, as well as that of the chloro substituted compounds, one is led to the conclusion that
ASSOCIATION OF PHENOLIC COMPOUNDS IN BENZENE charge density of the carbon atoms ortho to the hydroxyl group is qualitatively related to the extent of association; increasing this charge density leads to the increasing association while decreasing the charge density leads to the opposite effect, on a relative basis. The order of decreasing association for an ortho-para directing substituent would then be meta, para, ortho. For a substituent that directs meta, the order would be para, ortho, meta. The low extent of association of the ortho isomers has been attributed to steric effect^;^'-^^ recent work, however, indicates the effect may be primarily a resonance or electrical one.as In either case, the degree of association for ortho-substituted phenols is less than would be predicted on the basis of association exhibited by meta and para isomers. Greater complexity is observed with the dimethyl phenols. If the above assumption is correct and the effects are additive, a dimeta (3,4-) substitution should show the highest extent of association followed in order of decreasing association by meta-para (3,4-) , ortho-meta (2,5-, then 2,3-), orthopara (2,4-) and the least associated should be the ortho-ortho compound (2,6-). Furthermore, one would expect the 3,5-isomer to be slightly more associated than the meta compound, the 3,4isomer slightly more than the para compound, and the 2,3- and 2,5-isomers to both be more associated than the ortho compound. The 2,6-isomer should show a
1741 small extent of association. On this basis, one would expect phenol to exhibit an association intermediate to the chloro and methyl substituted phenols. Clearly, phenol is more highly associated than either. T h e effects of solid solution formation upon observed cryoscopic data are large and thus a relatively small error in the determination of the distribution coefficient, k , can markedly influence the data. However, a distribution coefficient considerably greater than that reported here would be necessary to yield the expected behavior. The general trends predicted above are shown by the data in Table IV with the sole exception of the reversal of the expected behavior of the meta and para isomers of chlorophenol. Calculations are now in progress to evaluate the net charge densities of the carbon atoms ortho to the hydroxyl group; these might help to place the above observations on a more quantitative basis and, hopefully, help explain the anomaly noted in the two isomers of chIoropheno1.
Acknowledgment. This work was supported by the United States Atomic Energy Commission. (32) M.Davies, Trans. Faraday floc., 36, 333 (1940). (33) M. Davies, ibid., 34, 410 (1938). (34) M.Davies, ibid., 34, 1427 (1938). (35) M.Charton and B. I. Charton, J. Org. Chem., 33, 3872 (1988).
Volume 74,Number 8 April 16, 1070
