ENOLIMINE-KETOENAMINE TAUTOMERISM OF N-(0-
AND
~HYDROXYBENZYLIDENE) ANILS
2245
Spectroscopic Evidence for the Enol Imine-Keto Enamine Tautomerism of N-(0- and p-Hydroxybenzylidene) Anils in Solution
by John W. Ledbetter, Jr. Department of Chemistry, University of Kentucky, Lezington, Kentucky
(Received January 6,1966)
I n alcoholic and acidic solvents N-(0- and p-hydroxybenzylidene) anils exhibit an additional long wavelength band in the neighborhood of 400 mp. Evidence strongly indicates that this band is due to the absorption spectrum of the quinoid tautomer of the anil. First, isosbestic points in the cyclohexane-ethanol system demonstrate two absorbing species. Spectra of various molecular structures show that the N-(0- or p-hydroxybenzylidene) anil structure is required. It is also essential for the proposed tautomerism that hydrogen bonding occur between the solute (the imine nitrogen) and a solvent acidic hydrogen. Increasing the dielectric constant and the acidity of the solvent increases the tautomerism. It is believed that the intermolecular hydrogen bonding provides both a route for the mechanism through hydrogen transfer and the necessary stability for the quinoid tautomer.
Introduction Recently, an interest has been stimulated in the general spectroscopy of Schiff bases, mainly due to interest in the phototropism of solid N-(0-hydroxybenzylidene) anils. A ~ t h o r s l -have ~ explained this phenomenon as due to a reversible tautomerism with the quinoid structure of the anil. A requirement for the process has been shown to be an intramolecular hydrogen bond bridge between the o-hydroxy and the imine nitrogen. The tautomerism then occurs via an intramolecular hydrogen transfer. Visually, the tautomerism results in a color change of the solid from yellow to red. There has been little evidence forthcoming, however, on the proposed tautomerism occurring in solution. It was reported earlier6 that the phototropism did not occur in solution. The quinoid isomer of the benzylamine anil of 1-hydroxy-2-acetylnaphthalenein CDCL has been shown to exist at room temperature using nmr data.6 This same anil of o-hydroxybenzaldehyde did not exhibit a quinoid isomer under the same conditions. Recently, transient spectra observed in the flash photolysis of ethanol solutions of N-(0-hydroxybenzy1idene)aniline have been attributed to a shortlived quinoid isomer.? Azo-hydrazone tautomerism of 1-phenylazo-2-hydroxynaphthalene and the 1,4 derivative is more common and has been reported by a number of authors.8-11 Flashing of solutions of 2-
hydroxy-5-methylazobenzene has produced a shortlived species which is postulated to be the hydrazone tautomer. On observation by the author of a yellow color in ethanol solutions of M N-(0-hydroxybenzy1idene)o-aminophenol, the color not being present in cyclohexane, the phenomenon was investigated further. Cyclohexane-ethanol solutions of the above compound exhibit isosbestic points characteristic of two absorbing species. A thorough spectral study based on molecular structure and solvent strongly indicates that an enol imine-keto enamine tautomerism of the following type
(1) G. M. J. Schmidt, Acta Cryst., 10, 793 (1957). (2) M.D. Cohen, Y. Hirshberg, and G. M. J. Schmidt, “Hydrogen Bonding,” D. Hadzi, Ed., Pergamon Press, London, 1969, p 293. (3) M. D. Cohen and G. M. J. Schmidt, Symposium on Reversible Photochemical Processes, Durham, N. C., 1962, p 119. (4) M. D. Cohen and G . M. J. Schmidt, J . Phye. Chem., 66, 2442 (1962). (5) V. DeGaouck and R. J. W. LeFevre, J . Chem. SOC.,1457 (1939). (6) G. 0. Dudek and R. H. Holm, J . Am. Chem. SOC.,8 3 , 3914 (1961). (7) D. G.Anderson and G. Wettermark, ibid., 87, 1433 (1965). (8) R. Kuhn and F. Bar, Ann., 516, 143 (1935). (9) E.Sawicki, J . Org. Chem., 22, 743 (1957). (10)A. Burawoy and A. R. Thompson, J. Chem. SOC.,1443 (1953). (11) E.Fischer and Y . F. Frei, ibid., 3159 (1959).
Volume YO, Number 7 J u l y 1066
JOHN W. LEDBETTER, JR.
2246
exists in alcoholic and acid solvents capable of hydrogen bonding with the solute.
This phenomenon was also observed in the case of N-(phydroxybenzy1idene)aniline. The following tautomerism for this compound is proposed.
The tautomerism was found to increase with the dielectric constant and acidity of the solvent. The dependence of the tautomerism on intermolecular hydrogen bonding with the solvent leads to two conclusions. The hydrogen bonding provides both a route for the mechanism by hydrogen transfer and the necessary stability for the quinoid isomer.
Experimental Section The following compounds were prepared by the appropriate condensation of aldehyde and amine in ethanol or methanol and recrystallized from the same solvent : N-(o-hydroxybenzylidene) derivatives of oaminophenol (I),p-aminophenol (11), aniline (111), and m-aminophenol (IV) ; N-(phydroxybenzylidene) derivatives of paminophenol (V) and aniline (VI); Nbenzylidene derivatives of paminophenol (VII) and o-aminophenol (VIII) ; and N-(m-hydroxybenzy1idene)aniline (IX). The melting points agreed with those in the literature and the elemental analyses were in accord. Most of the solvents used for the spectral study were of spectral quality. Exceptions were t-butyl alcohol, 2,2,2-trifluoroethyl alcohol, acetic acid, formic acid, and sulfuric acid which were of reagent quality. The ethyl alcohol was absolute and the water was distilled. The spectra were recorded at room temperature with a Gary Model 15 recording spectrophotometer using 1.00-cm quartz cells. The wavelengths between 500 and 280 mp were calibrated according to a holmium oxide filter. Accuracy of the recorded spectra is to the nearest millimicron. Most solution spectra were recorded a t 5.0 X M. Results and Discussion It was initially observed that ethanol solutions of I exhibited an additional electronic absorption band at energies less than that required for the n* + ?r transiThe Journal of Physical Chemistry
Figure 1. Absorption spectra of N-( o-hydroxybenzy1idene)-o-aminophenolin cyclohexane-ethanol: (A) cyclohexane, (B) 97: 3 cyclohexane-ethanol, (C) 90: 10 and (D) 50:50 by volume. M = 5.0 X I
I
1
300
WAVE LENGTH, mp
Figure 2. Absorption spectra of
N-( o-hydroxybenay1idene)-p-arninophenol (11), N-( o-hydroxybenzy1idene)aniline(111),and N-( o-hydroxybenay1idene)-m-aminophenol(IV) in methanol. M = 5.0 X
tion. The r* + r transition for similar compounds has been assigned by a number of a u t h ~ r s . ' ~ - l ~The possibility of the additional long wavelength band being due to the n* + n transition was ruled out owing to the large intensity and its absence in a number of solvents (see Table 11). In the cyclohexane-ethanol solvent system of I (Figure l),isosbestic points a t 406, 352, and 280 mp definitely point out the existence of two molecular absorbing species in equilibrium. If the absorbance of the two long wavelength bands of I is determined as a function of the concentration in methanol (Beer's law), the 349-rm(band gives a linear (12) H. H. JaffB, 9. 120 (1958).
J. Yeh, and R. W. Gardner, J. Mol. Spectry., 2 ,
(13) W.F. Smith, Tetrahedron, 19,445 (1963). (14) V. I. Minkin, et al., Opt.Spedry., 18, [4]328 (1986).
ENOLIMINE-KETOENAMINE TAUTOMERISM OF N-(0-
AND
~HYDROXYBENZYLIDENE) ANILS
2247
Table I: Dependenct of Spectra in Methanol on Molecular Structure No.
Structure
Spectra, m p
(e)
OH
I
444 (1600)
349 (9800)
269 (8600)
228 s (13,700)
I1
435 (400)
349 (17 ,000)
269 (9100)
230 ( 17,300)
I11
432 (280)
338 (13,400)
270 (14,000)
225 (22,700)
IV
435 (240)
341 (9600)
268 (9600)
230 s (14,400)
420 (280)
333 ( 16,700)
284 (14,300)
230 s (11,500)
415 (150)
313 (16,500)
295 s (15,300)
226 (14 ,400)
VI1
336 (14 ,000)
264 (13,600)
VI11
345 (9500)
265 (14,600)
238 s (10,000)
IX
316 (10,300)
266 (14 ,100)
222 (20 ,800)
v
OH
OH
VI
HO'
c \
a4
I
Figure 3. Absorption spectra of N-( phydroxybenay1idene)-paminophenol (V) and N-( phydroxybenzy1idene)aniline (VI) in methanol. M = 5.0 X
plot while the 444-mp band plot is practically linear. It is, in fact, linear over the range 2.0 X to at least 10.0 X 10" M . For the plots to be linear, an equilibrium must exist between the absorbing species. The equilibrium, subsequently, depends on the solvent. To determine any structural requirements for the formation of such an equilibrium, several hydroxy-sub-
WAVE LENGTH,
mp
Figure 4. Absorption spectra of N-benzylidene-paminophenol (VII), N-benzylidene-o-aminophenol (VIII), and N-( m-hydroxybenzy1idene)aniline(IX) in methanol. M = 5.0 X 10-6.
stituted benzylidene anils were prepared. The spectra of the anils were recorded in methanol and are shown in Figures 2, 3, and 4, while Table I lists the critical spectral data. The small inset curves of Figures 2 and 3 demonstrate the formation of the longer wavelength band a t higher concentrations. The most obvious point, and the most significant, of the spectra is that an N-(0- or p-hydroxybenzylidene) anil structure is necesVolume 70, Number 7 July 1966
2248
JOHNW. LEDBETTER, JR.
Table I1 : Solvent Effects on the
Solvent
A*
c
A
Band System" N-(o-Hydroxybenzy1idene)o-aminophenol mfi (d
N-(pHydroxybenzy1idene)p-aminophenol mfi ( 4
Cyclohexane Carbon tetrachloride 1,4-Dioxane Ethyl acetate Acetonitrile Chloroform
358(10,800) 358 (13,400) 352 (8900) 349 (13,600) 348(11,400) 354 (10,800)
440 s (100)
Alcohols t-Butyl Isopropyl Ethyl Methyl 207, Ethyl (water)* 2,2,2-Trifluoroethyl
350 (9600) 348 (12 000) 350 (12,000) 349 (9800) 333 (5800) 342 ( 10,800)
432 (500) 446 (1000) 448 (1400) 444 (1600) 438 (3200) 437 (7800)
332 (17,300) 334 (17 700) 334 (17,500) 333 (16,700)
Acids" Acetic Formic Sulfuric
332 (6700) (Overlapped) 352 (8000)
402 (4000) 392 (11,200) 390 (7800)
381 380 (28,000) 365 (9900)
a Solvents in each group are in order of increasing dielectric constant. rapid hydrolysis. Acetic, 99.8%; formic, 90.0%; sulfuric, 95.5-96.5%. amounts of water present.
sary for the additional band to occur. Since the isosbestic study demonstrated two absorbing molecular species in equilibrium, the 0- and p-hydroxy substituents suggest that a quinoid structure is possible in solution. The equilibria would then involve enol-keto tautomerisms as shown previously. According to previous workers,I5 the trans isomer of N-(benzylidene)aniline is the more stable configuration a t room temperature. Also, it has been shown16 that intramolecular hydrogen bonding exists in N-(0-hydroxybenzylidene) anils. Hence, the tautomerism is thought to occur from the trans configuration of the benzenoid structure. It is reasonable to believe that the long wavelength band for the quinoid isomer would appear at longer wavelengths, since, for 1-phenylazo-2-hydroxynaphthalene and the 1,4 derivative, it appears at about 470 mp." This is about 80 mp longer than the azo isomer. The average shift in Table I is 93 mu. The compounds which exhibit the proposed tautomerism may be divided into three groups based on molecular structure and the intensity of the additional long wavelength band in methanol. This bahd intensity is taken as an approximate measure of the extent of tautomerism to the quinoid isomer. Compound I comprises the first group with the most intense band, four times greater than any of the others. Compounds 11, 111, and IV comprise the second group. The intensities of this group are found to decrease in The Journal of Physical Chemdstry
333 (14,300) 333 (14,100) 334(15,600) 330 (15,700) 329(16,000) 331 (11,600) 435 s (24) 428 s (48) 423 s (140) 420 (280) 408 (1500)
' Extinction coefficients are very approximate because of Extinction coefficients are approximate because of small
the order of decreasing imine basicity. The extent of tautomerism would be expected to decrease with the imine basicity if the tautomerism were by hydrogen transfer. The third group consists of the p-hydroxy derivatives, V and VI, whose band intensities are comparatively even less. The same relation of the intensity of the quinoid band with imine basicity is evident. It is reasonable that the increase in the tautomerism in going from the phydroxy to the o-hydroxy derivatives is due to intramolecular hydrogen bonding possibilities of the latter in the quinoid structure. Two important aspects in this tautomerism are the mechanism and the stability of the two forms. While it is difficult to define mechanisms, a solvent study of the phenomenon has pointed out that hydrogen bonding of the solute with the solvent is essential for the process. Table I1 tabulates the results of the spectra of compounds I and V in various solvents. The purpose of including V is to demonstrate clearly this property of the p-hydroxy derivatives and to eliminate intramolecular hydrogen bonding. The solvents can be divided into four groups. First, there are those solvents which do not allow the tautomerism. These are followed by chloroform, the alcohols, and last, the acids, in which occurs the most intense response. The spectra of V in (15) E.Fischer and Y. Frei, J . Chem. Phya., 27, 808 (1957). (16)S. B. Hendrioks, 0. R. Wulf, G. B. Hilbert,and U. Lidell, J. Am. Chem. SOC.,58, 1955 (1936).
PROTON NONEQUIVALENCE IN ORGANONITROGEN AND -PHOSPHORUS COMPOUNDS
the acids show only one band in the region of interest. It is reasonable to believe that it is a composite of both bands since the hypsochromic shift for I from ethyl or methyl alcohol tjo the acids is reflected in the spectra of V. The hydrogen bond conceived to be responsible is between the solvent acidic hydrogen and the imine nitrogen. It is believed that the hydrogen bonding provides a basis for hydrogen transfer to the imine nitrogen. Also, it was found that the formation of the additional long wavelength band with solvent was a reversible process. The strength of the intramolecular hydrogen bond between the o-hydroxy group of N-(o-hydroxybenzy1idene)aniline and the imine nitrogen16 is evidently not sufficient to initiate tautomerism in the first group of solvents. The possibility of an intramolecular hydrogen transfer with this compound in other solvents is being studied further.
2249
Two other facts are immediately evident besides the hydrogen bonding. They are that the tautomerism increases with the acidity and the dielectric constant of the solvent. The aforementioned azo-hydrazone tautomerism also increases with solvent polarity.l 1 The largest factor, however, seems to be the acidity. It is then proposed that the stability of the quinoid isomer is found through intermolecular hydrogen bonding with the solvent. The stability furthermore increases with the dielectric constant of the solvent. The enamine nitrogen of the quinoid isomer is expected to be more basic than the imine nitrogen. In addition, an increase in tautomerism with the ~ K N Hof+the corresponding aminophenol is observed. Increasing both the nitrogen basicity and the solvent acidity would increase a tautomerism of this type where solute-solvent hydrogen bonding is the critical feature.
Nonequivalence of Protons and Related Phenomena in Some Organonitrogen and Organophosphorus Compounds1
by T. H. Siddall, 111 Savannah Rivm Labaratmy, E. I . du Pont de Nemours and Co., Aiken, South Carolina (Received January 10, 1966)
Proton nonequivalence was rationalized in a variety of molecules in terms of the symmetry properties of the nonequivalent protons, groups, or radicals. The compounds that were studied included amides, an amine, a carbamylphosphinate (VIII) , carbamylmethylenephosphonates (IX-XIII), and diphosphonates (XIV-XVIII).
Introduction Nonequivalence Of magnetic nuclei, Or even of whole groups or radicals that contain such nuclei, has Observed* We have now Observed equivalence for protons in a still further variety of compounds. The purpose of this paper is to report these observations and to show how they can always be explained in terms of the symmetry properties of the protons, groups, or radicals.
Experimental Section The proton magnetic resonance (pmr) spectra were measured on a Varian Associates A-60 spectrometer fitted with the Varian variable temperature probe and dewar insert, Except where noted, measurements (1) The information contained in this article was developed during the course of work under Contract AT(07-2)-1 with the U.S. Atomic Energy Commission.
Volume 70, Number 7 July 1966
