Conformational Equilibria of Ortho-Substituted Phenols
1157
Torsional Frequencies and Conformational Equilibria of Ortho-Substituted Phenols Gerald L. Carison" Melon lnstitute ot Science, Carnegie-Melon University, Pittsburgh, Pennsylvania 75213
and William G. Fateley Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 (Received January 8, 19731
1n.a recent publication [J. Phys. Chem., 76, 1553 (1972)], the use of torsional frequencies in determining the enthalpy differences between the cis and trans forms of the ortho halophenols was described. This study has now been extended to include several additional ortho-substituted phenols. Torsional frequencies and barriers to internal rotation are presented for o-methoxy-, -ethoxy-, -phenyl-, -cyano-, -tertbutyl-, -methyl-, -trifluoromethyl-, -2-tert-butyl-6-methyl-, -2,6-dimethyl-, and -2,6-diphenylphenols. For those molecules where cis and trans isomers are possible, the enthalpy differences between the isomeric forms are evaluated and compared.
Introduction In a recent paper, we reported on the value of far-infrared torsional frequencies in the study of intramolecular hydrogen bonding and cis-trans isomerism in o-halophenols.1 However, in addition to the halogens, there are many' other groups which also give rise to the possibility of cis and trans forms for ortho-substituted phenols. Puttnam2 and Krueger and Thompson3 have examined the OH stretching frequencies of a number of ortho-substituted phenols and have found that ortho groups, such as cyano, phenyl, ethoxy, tert-butyl, and hydroxyl, give rise to two OH stretching frequencies while for methoxy and all other alkyl substituents only one OH stretching frequency is observed. The number of OH stretching bands observed was generally taken to be indicative of the number of isomers present for a particular substituent. Ingold and Taylor have discussed the geometrical isomerism in a number of ortho-alkyl-substituted phenols and determined energy differences between the cis and trans forms of o-methylphenol and several o-tert-alkylphenol~.~ More recently, Allinger, Maul, and Hickey5 have calculated the conformational properties of o-tert-butylphenol from dipole moment measurements and have discussed the internal rotation barriers and conformational equilibria for other ortho-alkyl-substituted phenols. In the present study, we have observed the phenolicOH torsional frequencies for o-methyl-, -trifluoromethyl-, -tert-butyl-, -methoxy-, -ethoxy-, -cyano-, -phenyl-, -2,6dimethyl-, -2,6-diphenyl-, and -2-tert-butyl-6-methylphenol. These frequencies have been used to calculate internal rotation barriers as well as conformational equilibria for the cases where cis and trans forms are possible. Experimental Section Most of the phenols studied were available commercially in good purity; a few were obtained through the courtesy of Dr. A. W. Baker of the Dow Chemical Co. Authenticity of the samples was checked by comparison of their infrared spectra with published reference spectra. The omethylphenol sample was also analyzed by gas-liquid chromatography and found to be 98.9% pure. Low-frequency infrared spectra of the phenols and their phenol-OD derivatives as dilute (0.01-0.03 M ) solutions in cyclohexane were obtained on a Digilab FTS-14 Fourier
transform spectrometer employing a 3-p Mylar beamsplit ter to cover the range 650-100 cm-l. The experimental details were the same as described previously.1,6 Infrared spectra in the OH stretching region for a few of the compounds were obtained on a Beckman IR-9 spectrophotometer employing 5-cm quartz cells and CC14 solutions. Raman spectra and depolarization ratios for omethylphenol were recorded on a Cary 83 laser Raman spectrophotometer.
Results The low-frequency infrared spectra of the ten ortho-substituted phenols are given in Table I. The frequencies for the analogous OD compounds are included. The accuracy of these frequencies will be determined by the error in choosing the band maxima, for the frequency accuracy of the Digilab FTS-14 is better than f l cm-l. Far-infrared data for some of these phenols in the monomeric state have been reported previously; the torsional frequencies for o-phenyl- and o-methoxyphenol in CCll solution have been given by Nyquist;' torsional frequencies for o-methyl-, o-tert-butyl-, 2,6-dimethyl-, and 2-tert-butyl-6-methylphenol in polyethylene matrix by Jakobsen and Brasch;8 and the far-infrared spectra of omethylphenol and 2,6-dimethylphenol by Green, et al. 9 Our results, although in generally good agreement with this data, are considerably more detailed, Our assignments of the torsional frequencies were confirmed by their shift on deuteration of the phenolic OH group. The V O H / V O D ratios fell in the range 1.31-1.35. The torsions for o-cyanophenol-OH and o-trifluoromethyl-, G . L. Carlson, W. G . Fateley, A. S. Manocha, and F . F . Bentley, J. Phys. Chem., 76, 1553 (1972). N . A. Puttnam, J. Chem. SOC., 5100 (1960). P. J . Krueger and H . W. Thompson, Proc. Roy. SOC.,Ser. A, 250, 22 (1959). K. U. lngold and D. R. Taylor, Can. J. Chem., 39, 471 (1961); 39, 481 (1961). N. L. Allinger, J. J. Maul, and M . J . Hickey, J. Org. Chem., 36, 2747 (1971). G. L. Carlson. W. G. Fateley. and F. F . Bentley, Spectrochim. Acta, Part A, 28, 177 (1 972). R. A. Nyquist, Spectrochim. Acta, 19, 1655 (1963). R. J. Jakobsen and J. W. Brasch, Spectrochim. Acta, 21, 1753 (1965). J . H . S. Green, D. J. Harrison, and W. Kynaston, Spectrochim. Acta, Part A, 27, 2199 (1971); 28, 33 (1972).
The Journal of Physical Chemistry, Vol. 77, No. 9, 1973
1158
Gerald L. Carlson and William P. Fateley
v ) 3 N m a h
N
3
VI
0 0
Q,
m o
m
?
3 0 0
v)
r? m
z
YO
W
O 0)
N
E
0
.4d
.-
Y
m
N =
O . d
I
0
o
m
m
E P 0)
N
3 (D r
m
E h 0)
N
3
E m 0
3
E
3
(D
t
*
m
3 m
3
v)
3
3
(D
h
(D
N
t
0 d
0
m
N
E
E
3
r-
(D
8
v)
h
v)
0
m
N N
N
F.
E
v)
r
3
.-
m
t
m
N
3 u, N r
0
P
0
E
E
3
F1 m
m
V I 3
VI
VI
3
E
01
N
0)
m
0
3
3 m
r-
t
N
.r
v)
a
01
r
(D
The Journal of Physical Chemistry, Vol. 77. No. 9, 1973
v)
v)
v)
N N
h
(D
Q,
(D
m N
*m3 r
II v)
U
N
11ti9
Conformational Equilibria of Ortho-Substituted Phenols I
-r
LI
400
7
i
I
I
I
I
I
I
6.0 1
Frequency ( c m - f )
Far-infrared spectra of o-methylphenol (solid curve) and o-methylphenol-OD (dashed curve) in cyclohexane sohtion: sample concentrations -0.04 M, path length 5 mm. Figure 1.
2,6-dimethyl-, and 2-tert-butyl-6-methylphenol-ODappear as doublets apparently due to Fermi resonance with other nearby modes. In the case of 2-tert-butyl-6-methylphenol only one OH torsional frequency could be found, although the presence of two OH stretching bands indicates the existence of both cis and trans forms. The OH torsional region for o-methylphenol is abnormally complex and shows unusual behavior upon deuteration. The possibility that this complexity is due to impurity bands is ruled out because gas chromatographic analysis of the sample showed the purity to be 98.9%. Consequently, both the infrared and Raman spectra were studied in considerable detail in order to determine the number and positions of the torsional frequencies. The results of these studies are given in Figure 1and Table 11.
Discussion It is well known that the phenol molecule is planar with a twofold barrier to internal rotatiqn of about 3.5 kcal/ mol.10 For ortho-substituted phenols, the presence of the ortho substituent may introduce a VI term in the potential function for internal rotation resulting in an unsymmetrical barrier and the possibility of cis and trans conformers. If the ortho substituent is capable of hydrogen bonding with the phenolic OH group, the cis form is stabilized by an energy corresponding to the strength of the intramoleculrir hydrogen bond. This is the usual case and the form of the potential curve for o-cyanophenol is shown in Figure 2. However, in the case of o-tert-butylphenol, the steric effects of the bulky tert-butyl group favors the trans isomer although the cis isomer still exists apparently due to the fitting of the phenolic hydrogen into the tertbutyl group.4 In a previous paper, we reported our observations of torsional frequencies for both the cis and trans forms of the ortho halophenols and showed how these frequencies could be used to evaluate the enthalpy differences between the isomeric forn1s.l We have now used the method outlined in that paper to calculate barrier heights (VI and V2) and conformational equilibria for other ortho-substituted phenols which exist as mixtures of cis and trans forms. For
-90"
9 00
00
1800
2700
a Figure 2. Potential curve for internal rotation in o-cyanophenol: V ( a ) (kcal/mol) = 1.73(1 cosa)/2 -k 4.76(1 - cos 20()/2.
-
2,6-dimethyl- and 2,6-diphenylphenol, where VI must be zero, the calculation of VZ followed standard methods employing the Mathieu equation.ll The reduced momeint of inertia, F(a), for each phenol was calculated using the structural data for the phenol framework given by Pederson, et ~ 1 . 1 0As a consequence of the center of mass of the 0-H rotor falling on the internal rotation axis, F(a) was found to be the same for both the cis and trans isomers and virtually insensitive to the mass of the substituent group. A value of 22.47 cm-1 was used in all cases. The results of the barrier calculations will be discussed in the following sections. o-Methoxy-, -Ethoxy-, -Cyano-, and -Phenylphenol. A11 of these phenols except the o-methoxy compound show two OH stretching bands. Puttnam2 has discussed the assignment of these two bands to the cis and trans orientation of the 0-H group with respect to the ortho substituent for the cyano and ethoxy derivatives, and attributes the absence of the second, weaker OH stretch in the o-methoxy compound to an unusually low intensity in the trans isomer. o-Phenylphenol also exists in cis and trans forms with the cis form stabilized by an intramolecular hydrogen bond with the r electrons of the o-phenyl group.12 For all of these phenols, the major portion of the sample exists in the bonded (cis) form. Torsional frequencies attributable to both the cis arid trans forms were observed and confirmed by their shifts on deuteration for each of these phenols. By analogy with the o-halophenols,l the intense, higher frequency band is assigned to the cis form. The assignment of the trans torsion for o-phenylphenol is somewhat tenuous because the 319-cm-l band is considerably weaker than normal. HOWever, the fact that an analogous shifted band could be found in the OD compound and the absence of the barid in 2,6-diphenylphenol gives some support to its reality. (10) T. Pederson, N . W. Larsen, and L. Nygaard, J. Mol. Struct., 4, !i9
(1969). (11) D. R. Herschbach, J. Chem. Phys., 31,91 (1959). (12) A. W. Baker and A. T. Shulgin, J. Amer. Chem. SOC., EO, 5358 (1958).
The Journal of Physicai Chemistry, Voi. 77, No. 9, 1973
Gerald L. Carlson and William P. Fateley
1160 TABLE Ill: OH Stretching and Torsional Frequencies and
Internal Rotation Barriers for Ortho-substituted Phenol+
OH stretch, c m - l (CC14 soin)*
Phenol
o-Methoxy O-Ethoxy o-Cyano o-Phenyl 2,6-Diphenyl o-tert-Butyl
OH torsion, cm;'
(cyclohexane soln)
CIS
Trans
CIS
Trans
3557 s 3555 s 3559 s 3565 s 3558 s
-
428 s
3613 w 3595 w 3605 w
432 s
388 w 386 w 343 w
d
3647 w
3607 s
-382
sC
383 s
319 w?
389 s
i:;1
d
3649 w
or
o-Methyl 2,6-Dimethyi
3620 s
3617 s
or
3622 s 3614 s d
5.94 5.97
2.00
2.31 1.73 2.73?
4.76 4.52?
5.42
0 '
303 s
1 .38e
--
1 .05e
- -
307 s 297 s
1297s O-CF3
Vp, kcal/mol
341 w-m
264e
2-teff-Butyl-6-methyl
VI, kcal/mol
325~~
300 sf
g
297 s
g
d
0
-
3.34 3.29
3.41
a s = strong, w = weak. Frequencies taken from ref 3 and 4 except for O-CF3 and 2,6-diphenyl which were measured in this work. Weighted average of Fermi doublet. Not observable Since molecules exist as CIS form only. e Predicted values, see text. f Broad, asymmetric band. g See text.
The calculated values of VI and Vz for o-methoxy, -ethoxy-, -cyano-, and -phenylphenol in cyclohexane solutions are summarized in Table III. The magnitude of Vz, the twofold barrier, arises mainly from the overlap between the 7r orbitals of the phenolic oxygen and the aromatic ring, and the utility of this data in the study of r-electron densities in aromatic rings will be discussed in a future paper. In the present study, the value of VI is more pertinent because it is the energy difference between the cis and trans forms and is a direct measure of the strength of the intramolecular hydrogen bond. To our knowledge, the only previous data on intramolecular hydrogen bond strengths in these four phenols are AH values of 1.45 and 0.91 kcal/mol for o-phenylphenol in C C 4 solution given by Oki and Iwamura13 and Plourde,14respectively. A comparison of the bridge bond strengths for these four phenols with those determined previously for the ohalophenolsl is given in Table IV.l5 According to these data, in cyclohexane solution, the alkoxy, cyano, and phenyl groups all form stronger intramolecular hydrogen bonds than the halogens. o-Phenylphenol is- the only example we have studied ,which involves hydrogen bonding to a electrons. This type of intramolecular hydrogen bonding has been discussed by B a k e P and Oki and 1 ~ a m u r a . Our l ~ value of AH for this compound is two-three times that reported in previous studies.13.14 An effort was made to resolve this discrepancy by repeating the work of Oki and Iwamura on the temperature-dependent change in intensity of the OH stretching vibrations. Because of the great difference in intensity of the two bands, accurate intensity measurements are very difficult. Also, since CC14 was employed as the solvent, only a 50" temperature range was possible and over this temperature range the change in the intensity ratio was exceedingly small. This observation implies that AH for ths cis F? trans conversion is either very large (>2.5 kcal/mol) or very small (
