GAVRIELAH GABORAND ERNST FISCHER
2478
TTol.66
TAUTOMERISM AND GEOMETRICAL ISOMERISM IN ARYLAZOPHENOLS AND NAPHTHOLS. PART 11.'" 2-PHENYLAZO-%NAPHTHOL. THE EFFECT OF INTERNAL HYDROGEN BONDS ON PHOTOISOMERIZATION.lb PART I BY GAVRIELAH GABORAND ERNST FISCHER Laboratory of Photochemistry and Spectroscopy, The Wimtann Instilute of Science, Rehovoth, Israel Received M a y $6, 1869
The above compound is the only ortho-hydroxy azo-compound in which tautomerism with the corresponding phenylhydrazone is improbable. Geometrical isomerization may therefore be investigated without the complication of tautomerisation. Comparison with the corresponding methoxy derivative thus shows the effect of the internal hydrogen bond on the isomenzation. The cis isomers of the above compound and of its 0-methyl ether were prepared, and the latter was isolated. Regarding the thermal cis + trans isomerization, the methoxy derivative behaves like the unsubstituted azo-c?mpoundo, (EA = 23 kcal./mole), while for the hydroxy derivative EA = 12-14 kcal./mole wm observed. Regarding photoisomemahr hv tion, the quantum yields for the cis --+ trans conversion were similar for both compounds, while those for the trans -+- cis conversion were smaller by 1-2 orders of magnitude for the hydroxy derivative. For both derivatives the quantum yields depend on temperature in the same manner as in the unsubstituted aro-compounds,eJ Le., the cis --r trans yield is practically constant down to -180", whereas the trans -c cis yield decreases sharply on cooling. For the hydroxy compound there is an indication of a minimum limiting value for aa the temperature is lowered to -160' to -180'.
Introduction I n Part I of this series1* it was shown that in 4phenylazo-1-naphthol (I) the thermal cis trans isomerization proceeds not directly but through the hydrazone1 (11), Le., cis 3 hydrazone -t trans, in two consecutive-reactions.
+.
t
OH
0
H I1
I
tent. It t,hen may be expected that any difference between the behavior of I11 and its 0-methyl derivative is due to the internal hydrogen bond indicated. In addition it is of interest to study the effect of this hydrogen bond on the cis @ trans photoisomerization and perhaps learn about the hydrogen bonds in the electronic excited states of the isomers. Finally, it was interesting to see whether the pronounced temperature dependence recently reported2J for azo-compounds also exlsts in 111. Experimental Irradiations, spectrophotometric and actinometric determinations, procedures of photochemical kinetics, and calculation of quantum yields were all similar to those described elsewhere.8 Compound I11 and its 0-methyl derivative were synthesized according to Fierz-David, el 01.6 The cis isomer of the 0-methyl derivative was prepared by ultraviolet irradiation of the trans isomer, followed by chromatographic separation on alumina, and served to measure its molar extinction coefficient. Its thermal stability is similar to that of azobenzene. cis-I11 is rapidly converted into the trans isomer at temperatures above -25", and therefore caflnot be separated under ordinary conditions. Ita absorption spectrum was estimated, making a reaeonable assumption regarding the absorption of the cis isomer in the region of the main absorption band of the trans isomer.**6
Q(XN5AO a\;$a O\H
111
IV
It was suggested that a similar mechanism may operate in cis --t trans transformations of orthohydroxy azocompounds, such as 0- and p-phenylazophenol or 1-phenylazo-%naphthol. This view is supported by the complex kinetics of these isomerizations,* as compared with the first-order reactions observed in the corresponding methoxy derivatives. In order to differentiate between the efiect of such tautomerization and that of hydrogen bonds in the ortho-hydroxy derivatives, 2phenylazo-3-naphthol (111) was investigated. I n accordance with the common view that 2,3naphthoquinones are highly unstable or do not exist a t all, it appears probable that its phenylhydrazone (IV) also does not exist, or at least does not appear as an intermediate, because of its high energy con(1) (a) Part I : E. Fischer and Y. F. Frei, J . Chrm. SOC.,3159 (1959): (b) a double title is given because the present paper is common t o two series. (2) E. Fischer. J . A m . Chem. Soc., Ba, 3249 (1960). (3) S. Rfalkin and E. Fischer. J . Phus. Chem.. 66, 2482 (19fi2). (4) E. Fischer and Y. F. Frei, unpublished results.
Results Photoequi1ibria.-The photoequilibria attained by irradiation with light a t various wave lengths are described in Fig. l a for compound I11 and in Fig. IC for 0-methyl-111. The spectra of the cis isomers given in the same figures were calculated or measured directly as described above. Thermal Isomerization.-The spontaneous cis + trans isomerizatiqn was measured for both cornpounds in isomer mixtures enriched in the cis isomer by irradiation at 365 mp. Kinetic measurements were carried out over a range of temperatures in order to determine the activation energy of the thermal isomerization. At lower concentra(5) H. E. Fiera-David, L. Blagney, and E. Rlerian, IIeZv. Chim. to, a4, 846 (1051). (6) W. R. Brode, J. €1. Could, and G . R'I. Wytnnn, J . A m . Chem.
SOC.,74, 4641 (1952).
EFFECT OF INTERNAL HYDROGEN BONDS ON PHOTOISOMERIZATION 2479
Dec., 1962
AI) the isomerizations were firstorder reactions flor 0-methyl-I11 and I11 up to at least 90% conversion, but only up to about 50% conversion for I11 at higher concentration (5 X M ) . The temperature dependence of this reaction in I11 was measured at two concentrations in methylcyclohexane, and also in propanol. The resulting values of EA for I11 (2 X M ) were: in methylcyclobexane, 12 kcal./mole; in alcohol, 14 kcal./mole. For the 0-methyl derivative E.4 was 23 kcal./mole in both solvents. Spectra.-A elomparison of the spectra of the two compounds, as given in Fig. l a and IC, shows the following differences between the two. (1) Frequency Shifts.-In the iihydroxy’’ compound the n--R* band is blue-shifted to about 433 mp from about 450 mp in the “methoxy” compound. The ?r P* band is red-shifted from about 350 mp in the “methoxy” to about 355 370 mp in the “hydroxy,” whereas the doublet at 270 280 mp is shifted. to 282 295 mp in the “hydroxy” compound. The difference between the spectra of the cis isomers is more marked, despite the common view that the hydrogen bond can be formed only in the trans isomer. However, no conclusion may be drawn from this fact because the spectra of the two compounds compared were obtained in different ways as described above. Moreover, the steric interactions of the two cis isomers are probably different. The solvent has only a small effect, as seen from comparison of Fig. la and l b or IC and Id. (2) Intensity Shifts.-These are very pronounced, as proved by the relative heights of the various peaks in the spectra of the two compounds. Both frequency and intensity shifts may be due to hydrogen bondir~g.~-’~ Temperature Dependence of Photoequilibria and Quantum Yields.-Photoequilibria were measured over a wide range of temperatures with light a t 313, 365, 405, 436, and 546 mp. Some results are summarized in Tables I and 11.
tions (2 X
+
-+
+
+
TABLE I
% trans ISOMER’
ATTAINEDAT VARIOUSTEMPERATURES BY IRRADIATION WITH
LIGHT
AT PHOTOEQUILIBRIUM
VARIOUS WAVE LENGTHS,I N A MIXTUREO F METHYLCY CLOHEXANE-ISOHEXANE The conccntrations of I11 and 0-methyl-I11 are 2.58 x 10-6 ill and 3.66 X 10-6 ill,respectively Temp., ‘C. 25 50 75
-
-100 -120
-125 -140 - 150 -160 180
0
OF
313 mp pia
pa”
365 mp PI“ paa
43 5 90 41.5 50 10 98 57 60 15 97.5 65.5 70 24 97 91 77 91 35 98.5 80 94 42 95 55 97 50 98 77
405 mp pi”
436 m p pio pan 85 89.5 75 86 90.5 80 88 92 84 96 94.5 96.5 96 97 98 93 98.5 98.5 97 5 100 100 100 100 100 100 ion
I
546 m p PP 38 38 40 48 52 (-110’) 51 71
pl refers to 111, pz to 0-methyl-111.
(7) E. Lippert, “Hydrogen Bonding,” ed. by D. Hadsi, Pergamon Press, New York, N. Y.,1959,pp. 217-257. ( 8 ) H. Baba and 5. Suauki, J. Chem. Phue., S5, 1118 (1961). (9) H.Babs, Bull. Chum. SOC.Japan, 31, 169 (1958). (10) G. C. Pimentd, J . Am. Chsm. SOC.,79, 3323 (lg57).
Wovelength w / i
Fig. 1.-Absorption spectra: (a) Compound I11 in methjlcyclohexane-iohexane (1 :l), 2.58 X lo4 M , at -75 before and after photoequilibration with light at the indicated wave lengths. The “cis” curve is explained in the text. (c) Ditto, 0-methyl-111, 3.7 X 10-6 M , in methylcyclohexane-isononane (1: 1 ) at - 125’. (b) Kinetics of photoisomerization cis 4 trans of 111, 2.2 X 10-6 M , in propanol-isopropyl alcohol (1 :1) with light a t 436 mp a t -150”. The irradiation times are indicated. (d) Ditto, 0-methyl-111, 3.4 X 10-6 M , in propanolisopropyl alcohol (1 :1) a t - 120’.
TABLEI1 % trans ISOMER OF I11 AT PHOTOEQUILIBRIUM IN NON-POLAR (p) SOLVENTS AT VARIOUS TEMPERA(np) AND HYDROXYLIC ATTAINEDBY IRRADIATION WITH LIGHTAT 365 mp FROM A MERCURY ARC
TURES,
Temp., ‘C.
- 50 - 75 - 100 - 120 - 150
np
P
50 60
31 41 56 78 93
io 91 95
From the photoequilibrium composition and the molar extinction coefficients the ratio between the quantum yields and &, for the cis -+ trans and trans -+ cis photoconversion, respectively, can be calculated, using the equation (bcEcXo = &Et& /L 4t/40 = EoX,/EtXt where X, and Xt denote the mole fractions of the two isomers. Application of this equation to the data of Table I results in the values of +t/4c given in Table 111. The most prominent feature of these results is the fact that qjt/dc is much smaller in the “hydroxy” compound. In order to determine the cause of this difference between the two compounds, absolute quantum yields were measured for the photoisomerization in both directions, again over a wide range of temperatures. I n view of the greater complexity of such measurements, they were performed only with light at 365 and 436 mp; light a t each of these wave lengths is absorbed in a different absorption
TABLE I11 RELATIVEQUANTUM YIELDS4pt/+oDENOTED BY ki AND k? FOR I11 AND 0-METHYL-111,RESPECTIVELY, AT VARIOUS w.4VE LENGTHS A S D TEMPERATURES Solvent: methylcyclohexane/isohexane ( 1:1). ConcentraM , C9 = 3.66 X 10-5M tions C1 = 2.15 X Wave length
J
Temp.,
-313
mp-
-366
OC.
ki
k2
- 50 - i5 - 100 - 125 - 140
0.07
0.6
- 150 - 160 - 180
,025
.25
,015
.12
-436
mp-
ki
0.06
ka
0.7 .04 .4 ,024 .24 ,005 .14 ,0032 .07 .003 .06 .002 .06 ,001 .02
mp-
ki
kn
0.08 .075 ,065 ,025 ,018 ,013 ,014 .015
0.26 .23 .03 .02 .04
.02 .02 .02
band. The results are shown in Fig. 2a and 2b for the two compounds, and are even more striking than those in Table 111. While the are all of the same order of magnitude for both compounds, the &'s of I11 are smaller than those of its 0-methyl derivative by 1-2 orders of magnitude. In both compounds the general trend of change of &C and @twith t'emperature is seen to be similar to that described for unsubstitut,ed azo-compounds and stilbene,3 i.e., decreases sharply on cooling, whereas I#I~is more or less constant, or starts falling off only a t the lowest t'emperatures investigated. Discussion The activation energy of the thermal isomerizat'ion cis + trans of 0-methyl-I11 is 23 kcal.,/mole, nearly t8he same as the activat'ion energy of t'his reaction in other azo-compounds, as found for instance by Halpern, et uL1l The activation energy of t,he so,me isomerization for I11 is much lower: 12 kcal.;'mole. There are two possible explanat'ions for this lowering of the activation energy. (a) Existence of Hydrazonic Intermediates.Ot,her o-phenylazonaphthols (1,2- and 2,l-) were found to exist in solutions,12as well as in t'he solid st'ate,13 as tautomeric mixtures of the azoic and quinoidic, or hydrazone forms. It may be postulated that the mechanism of t'he thermal isomerization of I11 is similar to that suggested for these other hydroxyazo-compounds,l i.e., it proceeds N" a hydrazonic int'ermediat'e. This assumption could account, for the lowering of the activation energy, but. is rather unlikely because the corresponding quinone, 2,3-naphthoquinone, is unknown, and probably unstable. Also, the hydrazone should have a high energy content and its formation as an intermediate would therefore involve a high act'ivation energy. (b) Direct Effect of Intramolecular Hydrogen Bonds.-The kinetic transition state may be st ahilized by hydrogen bonding, just like the ground &ate of the trans isomer, but not of the cis isomer. This is indicated in the scheme beyond, where tjhe dashed curve denotes the molecule stabilized by internal hydrogen bonds. (11) J. Halpern, G . K. Brady, and C. A. Winkler, Can. J . Res., 28, 140 (1950). (12) A . Hussaoy, .\. G . Salem, arid A . R. Thompson, J . Chem. S o c . , 4793 (1952). ( 1 3 ) 1). ITadai, ihid., 21-43 (195ti).
\ \
'..'
/
/
Thus the energy difference between the ground state of the c i s isomer and the transition state is diminished by the hydrogen bond in the latter. The trans isomer of I11 fulfils the conditions for intramolecular hydrogen bonding, which are, according to Pauling'*: (1) The hydrogen bond forming groups should be in conjugation with the aromatic system (planarity). ( 2 ) The formation of a hydrogen bond should give rise to a sixmembered ring (counting the hydrogen atom). The cis isomer of I11 does not fulfil condition (1) because of steric hindrance and the resulting lack of planarity. As shown in Fig. 2 , q4 is larger in a hydroxylic solvent (ethanol-methanol) than in a non-polar solvent, though it is still much smaller than that of 0-methyl-111. This may be due to partial weakening of the intramolecular hydrogen bonds through formation of solutesolvent hydrogen bonds. This also may be the reason for the somewhat higher activation energy observed for the thermal cis + trans conversion in alcoholic solvents. One may now examine the spectroscopic evidence for the existence of the hydrogen bond. According to Pimentel'O the stabilization energy due to hydrogen bonding affects the electronic excited state. Babag showed that the electronic excited state is less stabilized by the hydrogen bond than the ground state. This is caused by weaker electron migration from the hydroxylic oxygen to the ring in the excited state, which diminishes the stability of the excited state and also the electronegativity of the oxygen, and therefore weakens the hydrogen bond in the excited state. This causes a blue-shift of the n-A* transition. This blue-shift was found in the n-A* band of 111 as compared with the same band of 0-methyl-111. The red-shift of the A-A* band of 111 compared to that of 0-methyl-I11 also indicates the existence of the hydrogen bond. Similar red-shifts of A-A* transition bands caused by hydrogen bonds were found for both i n t e r m o l e ~ u l a lr5~ and ~ ~ intramolccular16hydrogen bonding. (11) I, C Prtuling, "'I l i p Nature of the Ciirriiicil H ( ~ n d'' Cornel1 I'm>. I'rrss, I t l i . l L % , N Y 1 " W
L
- - -.- __ singlet * irans
singlet'
HT sing,et
~~~~
-
CIS
istence of the hydrogen bond in the trans isomer. This appears to explain, albeit in a qualitative and naive way, the observation that the presence of an internal hydrogen bond affects primarily $t and not+,. pounds Similar areeffects being studied in othercurrently. ortho-hydroxy azo-comAcknowledgments.-The authors wish to thank 3Ir. M. Kaganowitch for synthesizing the compounds investigated, Mrs. N. Caste1 and Miss J.
