3089
PHENOL INTERACTIONS WITH SUBSTITUTED PYRIDINES
alumina would suggest that the methyl group be attached to oxygen and hydrogen to the cation. While this chemistry is plausible, it is highly speculative and should be so considered.
Acknowledgment. This work was sponsored by the Gulf Research and Development Company as part of the research program of the Multiple Fellowship on Petroleum.
Hydrogen Bonding. 11. Phenol Interactions with Substituted Pyridines'"
by Jerome Rubinlb and Gilbert S. Panson Chemistry Department, Rutgers, The State University, Newark, New Jersey
(Received April 6,1966)
The hydrogen-bonding equilibrium constants of some substituted pyridines to phenol in carbon tetrachloride at 20 and 40" are reported. The constants are correlated with Hammett's and Tafts' substituent constants, the equation of the line having been calculated by the method of least squares. Thermodynamic data for the reaction are given. A steric effect caused by di-o-tbutyl groups is demonstrated, as well as a solvent effect which shows that the equilibrium constant value is dependent upon the solvent.
The hydrogen-bond formation of phenol to proton acceptors is well known and has been studied extensively.2 Investigations have been made from merely the molecular association of phenol to such electron-pair donors as carbonyl^,^ ethers,6 amides,6 and alkyl halides.' However, as indicated by the authorsJ2 comprehensive and complete studies are seriously lacking. The work of Gramstad is a noteworthy exception as can be seen by his contributions in this particular area. He has investigated the interaction of phenol and/or pentachlorophenol with organophosphorus c o m p o ~ n d s , amides,lO ~~~ nitrogen compounds,11carbonyls and ethers,12and sulfoxides and nitroso compounds.l 3 However, a complete and thorough study of the hydrogen bonding between phenols and pyridinw had not been done. It was with this intention that the project was undertaken. The previous communication had reported the equilibrium constants of some substituted phenols with pyridine in carbon tetrachloride." It is shown that a linear correlation exists for the logarithm of the association values vs. Hammett's substituent constants. Continuing this investigation, the equilibrium constants of phenol with substituted pyridines are now
reported. A steric effect is studied as well as the effect on the value of the equilibrium constant by changing the solvent.
Experimental The technique used for evaluating the equilibrium constants is the same as that reported previously.14 (1) (a) Parts I and I1 are excerpts of the thesis submitted to the Graduate School of Rutgers, The State University, by J. Rubin in partial fulfillment of the requirements for the Degree of Doctor of Philosophy. (b) E. I. du Pont de Nemours and Co., Richmond, Va. (2) G. C . Pimentel and A. L. McClellan, "The Hydrogen Bond," W. A. Freeman and Go., San Francisco, Calif.,1980. (3) (a) M.Iro, J . Mol. Spectry., 4, 125 (1960); (b) M. M. Maguire and R. West, SpSpedrochim. Acta, 17,369 (1961). (4) G. Akanes, Acta Chem. Smnd., 14, 1475 (1960). (5) R. West, etal., J . Am. C h . SOC.,86, 3227 (1964). (6) N.D. Joesten and R. S. Drago, ibid., 84,2696 (1962). (7) R. West, et al., ibid., 84,3221 (1962). (8) G. Aksnes and T. Gramstad, A d a C h a . Scand., 14, 1485 (1960). (9) T.Grsmstad, ibid., IS, 1337 (1961). (10) T. Gramstad and W. J. Fregledt, ibid., 16, 1369 (1962). (11) T. Gramstad, Spectrochim. A d a , 16,807 (1962). (12) T. Gramstad, ibid., 19, 497 (1963). (13) T. Gramstad, ibid., 19,829 (1963). (14) J. Rubin, et al., J . Phys. Chem., 68, 1601 (1964).
Volume 69,Number 9 Septembst 1066
3090
JEROME RUBINAND GILBERTS. PANSON
The concentration of phenol was 0.025 M as it was shown that no significant amount of self-bonding occurred at this dilution. The concentrations of the substituted pyridine ranged from 0.01 to 0.10 M . A minimum of six determinations, each with a different amount of pyridine, was made, the average value being taken as the equilibrium constant. The solvents were either spectroscopic grade or Fisher Certified. The chloroform was passed through a column of alumina before use. The compounds were obtained commercially, and all were purified by distillation in vacuo and/or recrystallization prior to use. The cells were matched fused silica, 10 em. in length. The measurements at 40' =k 1 were obtained by preheating the cell with the solution in a constant temperature bath. The thermodynamic parameters were obtained by solving the equation -RT In K = AH" - TAS" for two temperatures.
Results and Discussion A compilation of the results of the reaction of phenol with substituted pyridines is shown in Table I. Where previous work existed such as that of Halleux16and Gramstadll and comparisons could be made, the agreement is quite good.
1.8-
1.7
1.4 CT 1.5
Y
- 1.30
1.21.1
H
I
.3
40"
A F O , kcal.
AH', kcal.
A S o , cal.
59 82 65
29 41 31 40 45 8.4 8.3 9.2
-2.4 -2.6 -2.4 -2.5 -2.6 -1.5 -1.6 -1.6
-6.5 -6.4 -6.7 -5.9
-14.0 -13.0 - 14.5 -11.5 -12.0 - 6.0 -11.5 -10.0
14 15
I
.1
I
0
I
.1
I
.2
I
.3
I
.4
I
.5
I
.6
.7
Figure 1. Log k us. Hammett's u-values. Points are: (1) 4t-buty1, (2) 4methy1, (3) Cethyl, (4) %methyl, (5) hydrogen, (6) 3-bromo, (7) 3-cyano, (8) bcyano.
2.0
1
1.9 -
20°
84 12
I
.2
- 6 ' +
l./mole, K, l./mole,
77
-
1.0-
K,
4Methyl 3-Methyl 4-Ethyl 4t-Butyl &Cyano 3-Cyano 3-Bromo
-
1.6-
Table I: Thermodynamic Data for Some Substituted Pyridines with Phenol in Carbon Tetrachloride Substituent
-
1.9
-7.1 -3.2 -4.9 -4.5
Some relationships between the equilibrium con&ants a,nd other chemical or physical parameters have already been reported. Halleux had shown qualitatively and Gramstad quantitatively that there is a relationship between the log of the association constants and the pK of the proton donors. It was also reported that a linear correlation for the ionization constants of some substituted phenols with Hammett's u-values had been found.16 It thus seemed reasonable to expect a linear relationship between the logarithm of the association constants and the Hammett substituent constants for the system substituted phenols with pyridines and/or phenol with substituted pyridines. The Journal of Phyaical Chemistry
1.7 1.6 1.8
1.5-
m
0
1.4
-
1.3
-
1.2
-
1.1
-
1.0
.3
.2
.1
0
1 .
-
.2
do
.3
.4
.5
.6
.7
f
Figure 2. Log k us. Taft's u0-values. Points are: (1) 44buty1, (2) bmethyl, (3) 4ethy1, (4) 3-methyl, (5) hydrogen, (6) %bromo, (7) 3-cyano, (8) 4cyano. (16) A. Hdeux, Bull. 8oc. chim. Belges, 68, 381 (1959). (16) A. I. Biggs and R. A. Robinson, J. Chem. SOC.,388 (1961).
~
PHENOL INTERACTIONS WITH SUBSTITUTED PYRIDINES
As mentioned previously, the initial phase of this study experimentally vesified the former half of the assumption. Plots were then made to determine the linearity of the relationship for the substituted pyridine series. The results are shown in Figures 1 and 2. Although the agreement is good for both, the correlation is slightly better with Taft's values. This was also found to be so for the plots of phenols with pyridine. The regression line was calculated by the method of least squares and found to be y = -1.013~ 2.235 for the o-values and y = -1.063% 2.261 for the 8values. The pconstants here of -1.01 and -1.06 are slightly less than the pvalues of 1.14 and 1.21 for the substituted phenol series, the reason for the sign difference being obvious. Steric Effect. It was then decided to study the effect of large groups in the ortho positions of phenol. The equilibrium constant of o-t-butylphenol was determined first and found to be 31. This is quite similar to the value of 39 for p-t-butylphenol. The smaller value may be due to the greater inductive effect of the substituent in the ortho position &s compared to the further removed para position. The equilibrium constant of 2,6-di-t-butyl-4-methylphenol was found to be 0.65, which is considerably less than the value of 42 for 4-methylphenol. Thus, it can be safely said that, while a single large group in an ortho position does not inhibit the reaction, two large substituents surrounding the hydroxyl group of phenol definitely do. This result concurs with that of Coggeshall,l7 who studied simply the molecular association of substituted phenols.
+
+
3091
Solvent Effect. As the hydrogen bond is considered principally an electrostatic attraction, l8 it was thought that the value of the equilibrium constant for the reaction should be related to the dielectric constant of the solvent. The association of merely phenol to pyridine was then determined in other nonaqueous solvents to test this hypothesis. The choice of solvents was unfortunately restricted because of strong absorption in the near-infrared region. Table I1 shows how the equilibrium constant varies with the particular solvent.
Table II: The Equilibrium Constant of Phenol-Pyridine Solvent
Carbon disulfide Carbon tetrachloride Chloroform Methylene chloride
K,l./mole,
73 59 17 17
20'
U ,D.
e
0 0 1.02 1.54
2.64
2.24 4.81 9.08
It is obvious from this limited study that a linear relationship between the dielectric constant of the solvent and the equilibrium constant does not exist. More investigation must be made though, before it may be stated conclusively. Qualitatively, however, it may be said that the greater the dielectric constant, the smaller the value of the equilibrium constant. This, also, is as expected. (17) N. D. Coggeshall, J . Am. Chem. Soc., 69, 1620 (1947). (18) C.A. Coulson, Research (London), 10, 149 (1967).
Volum 69,Number 9 September 1966
