1043
PMRSPECTRAOF ortho-SUBSTITUTED PHENOLS
The Proton Magnetic Resonance Spectra of Five ortho-Substituted Phenols by James L. Roark and William B. Smith Chemistry Department, Texas Christian Untversity, Fort Worth, Texas
76139
(Received September 1 3 . 1 9 6 8 )
The proton magnetic resonance spectra of five ortho-substituted (methoxy-, chloro-, bromo-, iodo-, and nitro-) phenols have been determined in carbon tetrachloride and in dimethyl sulfoxide solutions. The spectra were completely determined for the first solvent and for the chemical shifts in the latter. The values of the chemical shifts are compared with results for other ortho-disubstitutedbenzenes.
Recently we have examined the proton magnetic resonance spectra for a large number of ortho-disubstituted benzenes,192 and a summary of the various factors thought to influence aromatic proton chemical shifts has been given.2 In particular, we have observed that substituent effects at the ortho position can be correlated with the parameter Q1-3 and that empirical values for Q may be derived from chemical shifts for protons in known systems.2 Concurrently, Traynham and coworkers4 have noted that the chemical shift for the phenolic proton in a series of phenols in dimethyl sulfoxide (DMSO) correlated with up- and that the relation held for orthosubstituted phenols. Thus, it has been possible for them to establish a uo- scale which measures the electronic effects of the substituent at the phenolic proton and, presumably, the ortho carbon. The method employed suggests that no steric effects of the substituents were being measured. Because of these observations, it seemed of interest to determine the ring proton chemical shifts in the same system. It was hoped that a comparison of the H3 chemical shift with Q and with the chemical shift of the phenolic proton might be instructive as to the mode of operation of substituent effects on proton chemical shifts at the ortho position.
Experimental Section The various phenols were studied in both carbon tetrachloride and in DMSO solutions (10% v/v) with tetramethylsilane as an internal standard. All samples were thoroughly degassed before measurement. Spectra were determined on a Varian A-6OA spectrometer a t resolutions between 0.1 and 0.2 cps as measured by the width at half-height of the narrowest line in the spectrum. Spectra were recorded on the 50-cps chart width a t a sweep speed of 0.02 cps. All spectra were calibrated by the usual audio-side-band technique, and measured line frequencies were the averages of four to six determinations. The spectra in carbon tetrachloride were completely analyzed as before.1~2~6 Lines for nonadjacent protons were assigned by application of the additivity rules of Martin and Dailey.6 Substituent parameters for the
phenol hydroxyl group were taken from the appropriate chemical shifts in phenol' as do,0.55 ppm; d,, 0.18 ppm; and d,, 0.47 ppm. The chemical shifts for H3 were assigned using the previously developed Q relationships.2 The spectra changed only slightly in the solvent DMSO. The chemical shifts of the various protons could be estimated quite well (we feel to at least 0.01 ppm) by comparison with the completely analyzed spectra in carbon tetrachloride. The spectral parameters are given in Table I. The spectrum for 2-methoxyphenol is given in Figure 1 as an example of a tightly coupled unsymmetrical four-spin system. In the spectrum of 2-nitrophenol in carbon tetrachloride the line assigned to H5 appears as an octet of doublets due to a long-range splitting with the phenol proton (J = 0.42 cps). This has been noted previously.s In DMSO this coupling disappears, and the HElines appear as a sharp octet.
Discussion The chemical shifts of aromatic protons ortho to a varying group of substituents have been found to corThis parameter relate well with the parameter &.'-3 has been defined by Schaefer and coworkers3 as P/r31 where P is the polarbability of the C-X bond, r is the bond length, and I is the ionization potential of the element X. The concept of Q arose from consideration of the paramagnetic term of the Ramsey shielding equation which contains an average excitation energy, here approximated as I-', and which arises from mixing of excited states of the substituent with the ground state (1) W. B. Smith and G. M. Cole, J . Phys. Chem., 69, 4413 (1965). (2) W. B. Smith and J. L. Roark, J. Amer. Chem. Soc., 89, 5018 (1967). (3) (a) F. Hruska, H. M. Hutton, and T. Schaefer, Can. J. Chem., 43, 2392 (1965): (b) T . Schaefer, F. Hruska, and H. M . Hutton, i b i d . , 45, 3143 (1967). (4) J. G. Traynham and G. A. Knesel, J . Org. Chem., 31, 3350 (1966); J. G.Traynham and M. T. Tribble, private communication. We wish to thank Professor Traynham for purifled samples of several of the phenols used in this study. (5) W. B. Smith and J. L. Roark, J . Chem. Eng. Data, 12, 587 (1967). (6) J. S. Martin and B. P. Dailey, J . Chem. Phys., 39, 1722 (1963). (7) 9. Castellano, 0. Sun, and R. Kostelnik, Tetrahedron Lett., 51, 5205 (1967). ( 8 ) S. Castellano and R. Kostelnik, ibid.. 51, 5211 (1967).
l'olume 73, Number 4 April 1969
1044
JAMES L. ROARKAND WILLIAMB. SMITH ~~
~
~
~~
~~~~
Table I: Parameters for the ortho-Substituted Phenolso ortho-Substituent
TS
74
7 6
76
Jar
Jaa
Jaa
J4s
CHIO
3.27 3.15 2.75 2.63 2.60 2.47 2.41 1.92 2.08
3.28 3.15 3.21 3.16 3.27 3.22 3.39 3.05 2.98
3.22 3.11 2.89 2.80 2.85 2.76 2.83 2.45 2.42
3.16 3.11 3.03 2.96 3.03 2.98 3.06 2.88 2.81
8.12
61 Br
I NO*
J4a
... 8.11 ...
1.48
0.36
7.67
1.68
1.59
0.35
7.50
8.10
1.56
0.30
7.97 8.64 8.36
1.55 1.71 1.71
0.27 0.40 0.33
...
...
...
...
...
...
...
J4
Rmsb
a
8.14
0.031
1.52
8.25
0.019
7.40
1.55
8.22
0.027
7.35 7.16 7.29
1.50 1.31 1.29
8.17 8.48 8.37
0.020 0.029 0.044
... ...
...
..*
...
...
...
...
...
...
...
...
a Coupling constants are inops. The first entry for each compound is for the carbon tetrachloride solvent; the second is for DMSO. mean-square error in fit.
of the electrons in the C-H bond. Suffice it to say, Q can only be calculated from the definition for hydrogen and the halogens. However, in our work we have noted that Ha in a series of 2-X-halobenzenes followed QX very well with no dependence on the nature of the halogen.2 Consequently, empirical values of Q for other functional groups were derived and applied to other systems from the parent halobenzene plot. Among these groups were methoxyl, cyano, and nitro.
‘I
Root-
In their original work, Schaefer and coworkersa¬ed the correlation of the ortho protons in the halobenzenes (and benzene itself) with their calculated Q values. We have recently found that while the values for methoxyl and cyano fit this plot for their appropriate ortho-proton chemical shifts, the point for nitrobenzene is far off the plot if the Q derived from the ortho-disubstituted benzenes (4.00) is used.9 The value of Q required for nitrobenzene itself is 6.33. Furthermore, the Q relation has been found to work very well for Ha in a series of para-disubstituted benzenes where only the para substituent is varied, and Q N O ~is 6.33 in this situation also. In future discussions we will refer to Q ( 2 ) as the value obtained from the ortho-disubstituted benzenes and to Q(l)as the value obtained when the substituent is flanked by hydrogens.1° The most reasonable explanation for the variation in Q N Ois~ the change in molecular geometry on going from nitrobenzene to an ortho-substituted nitrobenzene. In nitrobenzene the nitro group is presumed to be planar with the aromatic ring, With a large ortho substituent the nitro group is forced out of the ring plane. In Figure 2 is shown the plot of the Ha chemical shifts for the phenol series us. the appropriate Q values. For o-nitrophenol in carbon tetrachloride the chelate ring structure ensures the planarity of the nitro group, and the Q(1) value clearly applies. Evidence €or the planar ring structure is given by the long-range coupling of the phenolic proton to Hs.ll Polarized infrared studies by FranceP also indicate the planar structure of o-nitrophenol. In DMSO some intermediate value of Q N O ~ is required. Reasonably, DMSO competes with the nitro group for the phenolic proton. The (9) Unpublished results. (10) We have found that
the amino and hydroxyl groups also show different Q values when flanked only by hydrogens on the benzene ring as opposed to when there is a large ortho substituent [Q(1) 0.67 E
and 0.38; Q(2) = 0.11 and 1.19, respectively]. (11) Such couplings are well known in other, more highly
411 C.P.S.
401
Figure 1. Experimental and calculated 60-Mcps spectra of 2-methoxyphenol. The Journal of Physical Chemistry
C.P.S.
substituted phenols and benzaldehydes. They are of maximum value when operating through the “straightest zig zag path:” see for instance 9. Borsen and R. A. Hoffman,J. Mol. Spectry, 20, 168 (1966). (12) R. J. Francel, J. Amer. Chem, Soc., 74, 1266 (1952).
PMRSPECTRA OF ortho-SUBSTITUTED PHENOLS
1045
. OT
Br c1
0 .
Oe4l
Q
t
m3°
-
*\
0. 0 .
0
Figure 2. HS chemical shifts us. Q for a series of 2-substituted phenols in carbon tetrachloride (upper) and DMSO (lower). Dashed line shows the chemical shift for H a for o-nitrophenol in DMSO over the range of values for Q N O ~ ,
chelate ring structure is broken, and the nitro group is forced out of the plane of the ring. Presumably some time-averaged situation exists. In DMSO the longrange coupling of the phenolic proton to H5 is no longer apparent. As shown in Figure 3, there is a correlation of sorts between the Ha chemical shifts and uo- though the quality of the fit is indeterminant from the limited amount of data. It has been known for some time that ortho-proton chemical shifts roughly follow uD in the monosubstituted benzenes.I3 The halogens are anomalous in behavior, a fact that has been attributed to large magnetic anisotropy effects.I3 The Q relation provides a more satisfactory correlation for ortho-proton shifts, but it is not completely clear to us whether the definition of Q encompasses a mode of substituent interaction with the ring comparable with the resonance and inductive effects measured by u or whether these electronic effects represent another term in the Q expression, imposed by the empirical way in which our Q's are determined; outside the original definition, e.g., a plot of Q vs. upgives a fair correlation with the halogens again off the plot. In any event, it is evident that for groups such as nitro there is a geometric dependence for the Q relation which may be useful in solving stereo-
0.8
I
Figure 4, Hg chemical shifts us. a, (upper plot) and Ha chemical shifts us. urn(lower plot) for a series of 2-substituted phenols. Solid lines are for carbon tetrachloride solutions while dashed are tar DMSO solutions.
chemical problems. As shall be seen below, this does not seem to arise because of magnetic anisotropy. It has been observed previously that the He chemical shifts in a series of ortho-disubstituted benzenes vary It with the alteration of the C-2 substituent as may be seen in Figure 4 that the same relation holds for the phenols in both carbon tetrachloride and DMSO solution (the exception of o-iodophenol in DMSO is real and unaccounted for). Similarly, plots of the Ha chemical shifts (Figure 3) are seen to follow up very well indeed. The point of interest here is that the rotation of the nitro group from the plane of the benzene ring does not influence the electron density a t H6 or HB in any way different from the in-plane nitro group. Yet there is evidence that structures such as I are of more importance in carbon tetrachloride than in
I"\" I
DMSO by virtue of the enhanced values of Jar and Jse and the decreased value of Jd6in the former as compared to the latter s01vent.l~
7
Figure 3. Ha chemical shifts for the 2-substituted phenols us. Traynham'e uo- values."
(13) H Spiesecke and W. G. Schneider. J . Chem. Phys., 3 5 , 731 (1961). (14) The influence of resonance-contributing structures and ?r-bond orders on ortho coupling constants have been subjects of previous investigations: W. B. Smith, W. H. Watson, and S. Chiranjeevi. J. Amer. Chem. SOC., 89, 1438 (1967); W. B. Smith and T. Kmet, J . Phys. Chem., 7 0 , 4084 (1966). Volume Y9,Number 4 April 1969
JAMESL. ROARKAND WILLIAMB. SMITH
1046 The change at Hs on rotating the nitro group out of the plane of the ring is not due to magnetic anisotropy as might be supposed.16 The change a t He on altering solvents still follows urn (Figure 4) for o-nitrophenol. There is a similar change at Hi. The slight change at Hs is uninformative since HS is on the symmetry axis of the nitro group. If the alterration in chemical shifts at Ha in o-nitrophenol on changing solvents were
due to the magnetic anisotropy of the nitro group, one could reasonably expect the effect to appear also at H4 and Ha.@
Acknowledgment. We wish to acknowledge the generous support of The Robert A. Welch Foundation, (16) I. Yamaguchi, iw0i.mys..6, 106 (1963).
The Nuclear Magnetic Resonance Spectra of Six ortho-Substituted Fluorobenzenes by James L. Roark and William B. Smith Department of Chemistry, Texas Christian University, Fort Worth, Texas 76189
(Receioed September 1 8 , 1968)
The proton and fluorine nuclear magnetic resonance parameters have been determined for six ortho-substituted fluorobenzenes. The chemical shifts for the protons adjacent to the ortho substituents were found to follow the Q parameter correlation quite well, and there was also a fair correlation of Q with the fluorine chemical shifts. The nitro group provides an exception in both series which is taken to indicate that the nitro group is somewhat out of the plane of the aromatic ring on the average of the nmr time scale for o-nitrofluorobenxene. During the course of our studies of substituent effects on proton chemical shifts in aromatic systems, we have found that the chemical shifts of protons adjacent to the substituents on the ring are well correlated by the parameter &.Ie3 This parameter, originally defined by Schaefer and coivorker~,~ is presumably a measure of the paramagnetic shielding induced by the mixing of excited states of the substituent with ground states of the electrons in the ortho C-€I bo11da4 While the definition of Q is not restated here, it is true that the parameter may be calculated only for hydrogen and the halogens. Experimental methods for determining Q for other functional. groups have been developedI2J and it has been noted that groups without cylindrical symmetry along the C-X axis may show multiple Q values depending on the average orientation of X with respect to the ring.3 T‘alues of Q for groups coplanar with the ring are characterized by Q ( l ) while “out of the plane’’ values are called Q(2). Several sets of these have been given in the preceding paper,3 and an argument was presented against magnetic anisotropy as an explanation of the effect. We wish to report here the parameters for a series of ortho-substituted fluorobenzenes. This series was attractive in that it completed our earlier work with the halobenzeneslJ and it has been noted that ortho fluorines for the ortho fluorohalobenzenes also follow the Q relation.4a We wished to know if this was true for other substituent groups. The Journal of Physical @hemistr#
Experimental Section The fluorobenzenes were all commercially available compounds. They were run in 10% v/v solutions and degassed as before. The general technique has been des~ribed.‘-~>jSpectra were determined on a Varian A-60h instrument. The fluorine decoupled spectra were taken with the aid of a Nuclear Magnetic Resonance Specialties SD-60B heteronuclea,r decoupler operating through a modified A-60 probe. Fluorine spectra were taken on a Varian HA-100 operating at 94.1 Mcps. Chemical shifts were taken with reference to 1,1,2,2-tetrach1oro-3,3,4,4-tetrafluorocyc1obutane (TCTSB) as an internal standard. All spectra were calibrated by the usual audio-side-band technique. The fluorine decoupled spectra were used to obtain a preliminary set of parameters for the ring protons. These were then used to fit the normal spectra. The fluorine decoupled, normal, and calculated spectra for o-cyanofluorobenzene are shown in Figure 1. The various chemical shifts are given in Table I and coupling constants in Table 11. The values for 2-bromoand 2-iodofluorobenzene have been determined in (1) W. B. Smith and G . M. Cole, J. Phys. Chem., 69, 4413 (1905). J. Amer. Chem. Soc., 89, 5018
(2) W. B. Smith and J. L. Roark,
(1967). (3) J. L. Roark and W.B . Smith, J. Phys. Chem., 7 3 , 1043 (1969). (4) (a) F. Hruska, H. M. Hutton, and T.Schaefer, Can. J . Chem., 43, 2392 (1965); (b) T. Schaefer, F. Hruska, H. M. Hutton, i b i d . , 54, 3143 (1967). (5) W. B . Smith and J. L. Roark, J . Chem. Eng. Data, 12, 587 (1967).
