ALVIN W. BAKER
744
Vol. 62
HYDROXYL STRETCHING FREQUENCIES AND CHEMICAL REACTIVITIES OF SUBSTITUTED PHENOLS BY ALVINW. BAKER The Dow Chemical Company, Western Division, Pittsburg, California Received February 18, lg68
The hydroxyl stretching frequencies of substituted phenols are shown to give a good linear correlation with Hammett u-values. Of the few substituents which do not fall on the linear plot, a para fluoro group is most anomalous because the spectroscopic data indicate that it donates electrons while the chemical data indicate that i t withdraws electrons. The correlation of spectroscopic and chemical data is used to show that the structure of p-nitrosophenol is p-benzoquinone monoxine.
The polar characteristics of various chemical groups have been determined in solution by familiar reaction rate studies.1-3 Hammett U-values have been assigned to these groups and have been shown to be transferable from one system t o another with good accuracy. Moreover, by a comparison between aromatic and bicyclohexyl systems,4 and judicious meta and para substituents, the relative contributions of inductance and resonance to the U-values have been calculated. Recently correlations were shown to exist between U-values and both the asyqmetric nitro stretching frequencies of para substituted nitrobenzenes5 and the 0-H stretching frequencies of substituted phenols.6 These correlations of physical and chemical data were quite satisfactory considering the fact that the physical data are applicable to the molecule essentially in the ground state while the chemical data are influenced by both the ground state and the transition state. Also, the chemical reactions were run in solvents of rather high dielectric strength, frequently involving hydrogenbonding, while infrared spectra are ordinarily obtained in non-polar solvents. This latter point may alter the correlation of individual compounds because of group solvation or dipole-dipole interaction. A recent theoretical approach' has shown that a good correlation also exists between U-values and the electron density a t the ring carbon meta or para to the substituent group. This conclusion can be extrapolated to include both of the above quoted papers6.6 except that the electron density will be drawn out into the nitro group because of the comparatively large amount of group resonance. Since the principal nitro resonance structures
nitrogen-oxygen single bonds, an increasing amount of resonance leads to a decreasing nitro frequency. Consequently, the nitro frequency will show an inverse relationship t o the electron density a t the adjacent ring carbon or a direct relationship to the U-values. I n contrast to the nitro group, the hydroxyl group is not stabilized by any resonance exterior to the aromatic ring and, therefore, the 0-H stretching frequencies should have a different dependence on the electron density a t the adjacent ring carbon. The only contributing resonance structures which need be considered for phenol are OH
6 r
6+
6
o-
(which contributes to the actual structure of the molecule)
8
1
other than the normal covalent structure, involve (1) L. P. Hammett, Chem. Revs., 17, 125 (1935). (2) L. P. Hammett, Trana. Faraday Soc., 34, 156 (1938). (3) R. W. Taft, Jr.. Fifteenth National Organic Chemistry Symposium of the American Chemical Society, June, 1957. (41 J. D. Roberts and W. T. Moreland, Jr., J . A m . Chem. Soc., 7 6 2167 (1953). (5) R. D. Kross and V. A. Fassel, ibid., 78, 4225 (1956). (6) L.L. Ingraham, J. Corse, G. F. Bailey and F. Stitt, ibid., 74,2297 (1952). (7) H. H.Jaffe, J . Chem. Phys., 20, 279 (1952); J. Am. Chem. SOC., 76, 5843 (1954).
.
0-H
The net effect is an increased electron density a t the ortho and para positions which become strongly activating for electrophilic reagents. The oxygen is more electropositive than the oxygen in aliphatic alcohols and the proton more acidic. It is obvious then that any substituent which increases the electron density a t the carbon adjacent to the 0-H will cause a reduction in the resonance interaction of the type illustrated or will, at least, partially compensate the S+ charge on the oxygen. This in turn will lead to an increased electron density in the 0-H bond with a concomitant rise in 0-H frequency. Therefore, the 0-H stretching frequency will increase with the electron density a t the adjacent ring carbon atom. This paper w ill extend the correlation between Hammett U-values and 0-H stretching frequencies and will discuss some of the apparent anomalies in the data. Due to the direct approach of the spectroscopic technique, it is perhaps possible that tlie frequencies of the 0-H stretching vibrations are more accurate measures of pure polar character than U-values assigned from chemical reactions.8 Experimental
6+a=N
