PHOTOENOLIZATION OF BIACETYL
fying corresponding states behavior. If we accept that the Kihara core model gives an adequate representation of the molecular interactions of the heavy rare gases, as a growing body of evidence now indicates, then the results of this paper indicate that the assump-
2653
tion of the additivity of the potential seems to be valid for thermodynamic calculations of the fluid phase within the range so far studied. Acknowledgment. The authors sincerely thank Professor I. Amdur for a critical reading of the manuscript.
Photoenolization of Biacetyl
by Jacques Lemsirel Department of Chemistry, The University of Texas, Austin, Tezas 78719 (Received February 9, 1966)
Irradiation of biacetyl either in hexane or in aqueous solution below 3100 A (to give the second excited singlet state) gives a product which is probably an enol. Biacetyl in a cis form could give an enol which would be stabilized by internal hydrogen bonding. The quantum yield of enol formation is about 0.12. In the vapor phase a similar product is formed. At 4358 A, enol formation at room temperature either in solution or in the vapor state is essentially zero but is appreciable at 100". The compound tentatively identified as the enol inhibits both the phosphorescence and the decomposition upon exposure to wavelengths which give the first excited singlet state. Since oxygen does not quench the fluorescence of biacetyl but quenches both the phosphorescence and the formation of the enol, it is suggested that the enol arises from the triplet state of biacetyl. If the triplet state is a precursor for enol formation, the data suggest that an activation energy is necessary and hence that high vibrational levels of the triplet state are involved.
Introduction The photochemistry of biacetyl has received much attention both in the vapor phase and in solution.2a The results of Porter2b and those of Backstrom and Sandros2c indicate that all initially formed singlet molecules at 4358 A which do not either dissociate or fluoresce cross over to the triplet state. Presently known processes do not account for all of the tripletstate molecules. Fluorescence from biacetyl when excited to the first singlet state either in the liquid or in the gas phase is very small, with a quantum yield of about 0.0025.a Thus, crossover to the triplet state at 4358 A occurs with a yield greater than 0.99. Phosphorescence from the triplet state has a yield of about 0.15. Decomposition at this wavelength is negligible at room tem-
perature, although there is some dissociation which results from interaction between two excited molec u l e ~ . The ~ dissociation yield increases rapidly with increase in temperature. Some unidentified product from biacetyl at 4358 A ~
(1) Address all correspondence to the author at the Ecole Nationale
Superieure des Industries Chimiques, 1 Rue Grandville, Nancy, France. (2) (a) W. A. Noyes, Jr., G. B. Porter, and J. E. Jolley, Chem. Rev., 56, 49 (1956): (b) G. B. Porter, J . Chem. Phys., 32, 1587 (1960): (c) H. J. L. Bilckstrom and K. Sandros, Acta Chem. S c a d . , 14, 48 (1960).
(3) G. M. Almy and P. R. Gillette, J . C h . Phys., 11, 188 (1943). The fluorescent yield in dilute solution may be as high as 0.01. F. Wilkinson and J. T. Dubois, ibid., 39,377 (1963). (4) W. A. Novas. Jr.. W. A. Mulac. and M. S. Matheson. ibid.. 36. 880 (1962): G. F. Sheats and W. A: Noyes, Jr., J . Am. Chem.Bot.; 77, 4532 (1955).
Volume 71, Number 8 July 1967
2654
at temperatures of 90" or more quenches the phosphorescence and inhibits diss~ciation.~JYang41s has suggested that this might be an enolic form of biacetyl. Attempts to identify this product so far have failed. Biacetyl vapor raised to the second excited singlet state does not emit and no emission is induced by addition of inert gas.7 Internal conversion from the second to the first excited singlet state is therefore not an important process unless dissociation immediately follows. The primary decomposition yield was found to be 0.39 a t 2654 A by Blacet, et al.,839and 0.64 a t 2700 A and 100" by Sheats and Noyes.l0 Primary dissociation does not account for all of the quanta absorbed. The complete problem of energy balence for excited biacetyl from both singlet states has not been solved. There is no self-quenching at 4358 and 4047 A and an increase in pressure leads to an increase in emission efficiency a t 3650 A.l1,l2 It has been suggested recently13 that isomerization may sometimes offer a path for radiationless deactivation. Since photoenolization has been reported for some ketones6 the possibility of such an isomerization for biacetyl has been explored.
Experimental Section Materials. The biacetyl was from either Matheson Coleman and Bell or from Fisher Scientific Co. For the experiments in the gas phase, biacetyl was introduced into the line, purified by bulb-to-bulb distillation, and thoroughly degassed. A conventional high-vacuum, grease-free line was used. Most experiments were carried out in a 50-cm3 T-shaped cell encased in an aluminum block which reduced the stray light and could be used as a furnace. The light source was a Hanovia 5-100, medium-prcssure mercury arc. The grating monochromator (Bausch and Lomb, Model 33-86-45) has 16 A/mm as reciprocal linear dispersion. The entrance and exit slits widths were set a t 4 mm, thus giving a maximum spread of about 120 A for each setting although probably 80% of the light lies in a range of about 60 A. With the 5-100 lamp set a t 2537 A, probably over 90% lies within a few angstroms of this wavelength. The light transmitted through the cell was monitored with an RCA 935 phototube in connection with a Keithley Micro-microammeter, Model 410. The emission was monitored with an RCA 1P28 photomultiplier tube placed opposite the side arm of the cell. In liquid-phase experiments, distilled water, methanol (Fisher Certified reagent), and n-heptane (Matheson Coleman and Bell Spectroquality reagent) were used as solvents. Samples were prepared by degassing The Journal of Physical Chemistry
JACQUES LEMAIRE
the solutions in an 8.5-cm3 quartz cell. The concentration of biacetyl was spectrophotometrically determined. For low-intensity irradiation (1013 photons sec-l in the entire cell) in a fairly narrow range of wavelengths, the monochromator equipped with a Hanovia 5-100 lamp was employed. An Osram HBO 500-w highpressure mercury lamp equipped with a Corning filter CS 7-54 (No. 9863) or CS 0-51 (No. 3850) supplied a high-intensity irradiation (lo15 to 10'6 photons sec-l in the entire cell). In Figure 1 the transmittance curves of the two filters are drawn. When the monochromator was used, the number of quanta absorbed by the sample was measured by the ferrioxalate a~tin0meter.l~An accurate measurement of the number of quanta absorbed by the biacetyl was difficult when the Osram lamp and Corning filters were employed. When the Osram lamp was equipped with a CS 0-51 filter an actinometric measurement would give the number of quanta absorbed by the ferrioxalate from 3600 A to the long-wave limit of absorption by ferrioxalate. The number of quanta absorbed by the biacetyl was obtained from the enol formed and from the quantum yield of photoenolization, previously determined under the same conditions with a monochromator. The same technique employed with the CS 7-54 filter gave only the magnitude of the quanta absorbed by biacetyl, since the enol also disappeared photochemically. The spectrophotometric measurements were made with a Cary spectrophotometer, 1CIodel 14. The absorption spectra of biacetyl in the solvents used are shown in Figure 1. The visible and ultraviolet absorption bands of biacetyl in n-heptane agree with values in the l i t e r a t ~ r e : ' ~Xmax 2750 (ernax 16); Amax 4210,4390, and 4470 (emax 20 for each). (5) D. S. Weir, J. Chem. Phys., 36, 1113 (1962). (6) N. C. Yang and C. Rivas, J. Am. Chem. Soc., 83, 2213 (1961). Dr. Yang also mentioned this to Dr. Noyes about 1958 and it is referred to in ref 4. (7) H. Ishikawa, Ph.D. Dissertation, University of Rochester, Rochester, N. Y., 1962. (8) W. E. Bell and F. E. Blacet, J. Am. Chem. Soc., 76,5332 (1954). (9) F. E. Blacet and W. E. Bell, Discussions Faraday Soc., 14, 70 (1952). (10) G. F. Sheats and W. A. Noyes, Jr., J. Am. Chem. SOC.,77, 1421 (1955). (11) F. C. Henriques, Jr., and W. A. Noyes, Jr., ibid., 62, 1038 (1940). (12) G. M. Almy and P. R. Gillette, J. Chem. Phyn., 1 1 , 188 (1943). (13) W. A. Noyes, Jr., and D. Phillips, Zh. Vses. Khim. Obshchestva im. D . I . Mendeleeva, 11, 141 (1966). (14) G. A. Parker, Proc. Roy. SOC. (London), A220, 104 (1953); C. G. Hatchard and C. A. Parker, ibid., A235, 518 (1956). (15) "Organic Electronic Spectra Data," Vol. 11, E. Ungnade, Ed., Interscience Publishers, Inc., New York, N. Y., 1953-1955.
PHOTOENOLIZATION OF BIACETYL
T
2655
transmittance
absorbonce
I 0 0 04
a0
2000
2500
3000
3500
mole 1.-1, 25O, Cary 14 Figure 1. Absorption spectra of biacetyl in different solvents (1.8 X spectrophotometer) and transmittance curves of Corning CS 7-54 and CS 0-51 filters.
In water the intensities of the visible bands decrease and their fine structures disappear, leaving a single maximum a t 4050 A. The intensity of the ultraviolet band increases and its maximum shifts t o 2820 A. An equilibrium between biacetyl and its fairly stable hydrate accounts for these changes. The equilibrium constant is close to the value reported by Bell and McDougall;16 75% of the biacetyl is in the hydrate form. In methanol the same phenomena are observed due to formation of a hemiacetal. About 80% of the biacetyl is in the hemiacetal form. The maximum of the ultraviolet band shifts to 2870 A. It is seen from Figure 1 that radiation from the Osram lamp equipped with a Corning CS 0-51 filter excited biacetyl only to its first excited singlet state. The use of the Corning CS 7-54 filter is not specific and both excited singlet states are formed. The enol is mainly formed from the second excited singlet state under such conditions.
Results in the Liquid Phase Experimental Evidence of a New Compound in Irradiated Solutions. When dilute solutions (0.23 mole 1.-') of biacetyl in n-heptane are irradiated with the Osram lamp equipped with a Corning CS 7-54 filter, the intensity of the ultraviolet absorption band increases (cf. Figure 2) with no shift of the maximum at 2750 A. In dilute soliitions of biacetyl in methanol
and in water irradiated under the same conditions there appears a very important absorption band with the same maximum at 2750 A (cf. Figure 2). New absorption does not appear in the visible region for the three solutions. A new compound (I) is therefore formed photochemically from biacetyl in solution. This compound does not absorb in the visible region but absorbs strongly in the ultraviolet region with a maximum at 2750 A. The position of this maximum does not depend on the solvent, but determination of the exact position of the maximum is difficult in the presence of a large amount of biacetyl. A rough estimate of the magnitude of the molar extinction coefficient of compound I was made from the consumption of biacetyl and the related increase of the absorbance at 2750 A on the assumption that compound I is the sole product of the photolysis of biacetyl. The extinction coefficient was found to be greater than 5000 mole-' 1. cm-I and could only be assigned to a r-r* absorption of an unsaturated carbonyl if the carbonyl group is conjugated to another double bond. The existence of such a conjugation in the new molecule was indicated also by addition of an alcoholic mole 1.-') to an solution of iodine monochloride irradiated aqueous (or methanolic) solution of bi(16) R. P. Bell and A. 0. McDougall, Trans. Faraday SOC.,56, 1281
(1960).
Volume 71, Number 8 July 1967
JACQUES LEMAIRPI
2656
t
:i R N
absorbance
Figure 2. Photoenolization of biacetyl; appearance of the 2750-A band of the enolic form: 1, 2, 3, absorption spectra of initial solutions of biacetyl (1.8 X 10-9 mole 1.-1, 25'); l', 2', 3', absorption spectra of irradiated solutions of biacetyl (1.8 X lo-* mole 1.-1, 25'). The conditions of the irradiation were: 2.2 X 10-1 mole 1.-1, 25', 20 hr; high-intensity irradiation set (Osram lamp CS 7-54).
+
acetyl. As shown in Figure 3, the new absorption band (maximum 2750 A) was completely suppressed since IC1 adds to the double bond and suppresses conjugation. With a standardized solution of IC1 in methanol, the actual extinction coefficient of compound I could be estimated. It was found to be QI,,,.~ 10,OOO mole-'1. cm-'. Addition of dilute aqueous NaOH of pH 10-12 to irradiated solutions of biacetyl produced a shift of the maximum from 2750 to 2940 A with an increase of the extinction coefficient ( B I , , , ~+ ~ 16,000). Neutralization by dilute acetic acid to pH 3 restored the initial absorption. Excitation to the First Excited Singlet State. The quantum yield of formation of compound I in dilute aqueous solution (0.023 mole 1.-') is $1 = 0.01 0.002 at 4030 A (using the Hanovin S-100 lamp and the monochromator). The mole fraction of compound I was determined spectrophotometrically based on the previously determined value of the extinction coefficient a t 2750 A. Two solutions of biacetyl in water and in n-heptane of about the same absorbance in the range 36004600 A were irradiated with an Osram lamp equipped with the
*
The Journal of Physical Chemistry
CS 0-51 filter (3600 < X < 4600 A). After the same irradiation time, the content of compound I was found to be 8 times larger in the aqueous solution than in nheptane. The hydrate (75% of the molecules a t room temperature) does not absorb a t these wavelengths. In both solutions, the quanta are absorbed by biacetyl. This undergoes photochemical transformation to compound I. Thus, compound I, which is formed a t the same rate in both solvents, appears to be stabilized in water. A high-intensity irradiation (10ls photons sec-l in the entire cell) was used. The cross section of the beam was larger than the cross section of the cell. The formation of compound I in n-heptane is completely inhibited when the solution is in contact with air. In aqueous solution, contact with air does not prevent the formation of compound I. The mole fractions of dissolved oxygen in water and in n-heptane at 25' are, and 0.3-2 X In aqueous respectively, 4 X solution, the concentration of oxygen is too low for efficient inhibition of compound I formation even at these high intensities. At low intensities (lola photons sec-l in the entire cell) oxygen inhibits compound I formation more effectively.
PHOTOENOLIZATION OF BIACETYL
2657
obrorbonce
/
2.0
\ \ \
\,\I
1
irrodiotrd solution
\
\ \ \ \ \ \ \ \
I
1.0
frradiotcd re1utton
\
1 .
0 2000
2500
3000
3400
h(A)
Figure 3. Effect of the addition of IC1 on the 2750-band of the enolic form of biacetyl: a, absorption spectrum of the irradiated solution (1.8 X 10-3 mole 1.-1, 25O, 20 hr, Osram lamp CS 7-54); b, absorption spectrum recorded after addition of IC1 methanol.
+
+
The determination of the quantum yield of disappearance of biacetyl in aqueous solution was carried out with high-intensity radiation. The number of quanta absorbed by biacetyl was calculated from the mole fraction of compound I after short irradiation by use of the quantum yield of formation of compound I (0.01). The disappearance of biacetyl was measured spectrophotometrically in the cell itself and found to occur with a quantum yield @B = 0.016 0.004. Most of the disappearance of biacetyl seems to be due to the formation of compound I. Excitation into the Second Singlet State. Quantum yields of formation of compound I have been measured with a low-intensity irradiation supplied by the Hanovia S-100 lamp and a monochromator.
*
A, A 2537 f 32 3000 f 32 3130 f 32
@I
* *
0.10 0.01 0.13 f 0.01 0 . 1 2 0.01
The variation of quantum yield of formation of compound I with wavelength may not exceed experimental error. The formation of compound I was totally inhibited by oxygen in aerated aqueous solutions. Further work on the behavior of the hydrate would be advisable since conceivably unhydrated bi-
acetyl would disappear with a much higher quantum yield.
Discussion of Results in the Liquid Phase The compound most nearly resembling the enol of biacetyl should be the well-known enol of acetylacetone." Acetylacetone is essentially in its enolic form in nonpolar solvents and the maxima of the ?r** band of the enol are at 2710 and 2740 A, respectively, in saturated hydrocarbons and in water (or methanol). The corresponding extinction coefficient is emax 11,100 mole-' 1. cm-l. In dilute sodium hydroxide solution an enolate is formed which produces a shift of the maximum to 2940 A and an increase of e (-. 20,000). The similarity between the enol of acetylacetone and new compound I favors an enol form for the latter. The mole fraction of enol in biacetyl has been determined by Schwarzenbach and WittneP and more recently by Gero.lgJ'I These two results do not agree with each other. Gero's technique uses the quantitative addition of (17) G. S. Hammond, W. G. Borduin, and G. A. Guter, J . Am. C h m . SOC.,81, 4682 (1959). (18) G. Schwarzenbach and C. Wittner, H d v . Chim. Acta, 30, 666 (1947). (19) A. Gero, J . Org. Chem., 19, 469 (1954). (20) A. Gero, {bid., 21, 1960 (1956).
Volume 71, Number 8 July 1067
JACQUESLEMAIRE
2658
iodine monochloride to the enol in methanol solutions. The mole fraction of enol was found to be as large as 0.011. Since this amount of enol would absorb more than biacetyl itself, the band of biacetyl would be due to the enol form and would not show any solvent shift. The ultraviolet absorption of biacetyl exhibits large solvent shifts and must be attributed to biacetyl itself. Gero’s work may not have been carried out in the dark and some side photochemical reactions may have affected his results. The fraction of enol in solutions of biacetyl in cyclohexane was found to be 5.6 X by Schwarzenbach and Wittner.lB This value appears to be reasonable. In long irradiation of aqueous solutions of biacetyl with the Osram lamp and the CS 7-54 filter there appears a second ultraviolet absorption band with a maximum a t 2335 A. This band does not appear when either the CS 0-51 filter or low-intensity irradiation is employed. The corresponding molar extinction coefficient is of the same magnitude as eenol. The position of the maximum, the high corresponding molar extinction coefficient, and possibly the acid character suggest that this compound might be a dienol. Further work on this point might be fruitful. The photolytic decomposition of biacetyl in solution has not been extensively investigated but the work of Greenberg and Forster21 indicates the quantum yields of disappearance of biacetyl to be very low (0.0015 at 25”, 3660 A in mineral oil, 0.006 at looo, 4358 A in a perfluoroether). These yields are just as low in the gas phaselo except at 3650 A a t pressures below about 15 mm and a t longer wavelengths at very high intensity. More than 80% of the triplet molecules in the liquid phase disappear by some process other than dissociation. Photoenolization may account for some of this lack of balance but the observed yield in the liquid phase is small (about 0.01). Photoenolization is totally inhibited in aerated nheptane but occurs in aqueous solutions in contact with air under high-intensity irradiation. Stevens and Dubois22observed the same effect of dissolved oxygen on the quenching of phosphorescence. They could regenerzite the phosphorescence of an aerated aqueous solution of biacetyl since the oxygen present in lom concentration can be photochemically consumed by the biacetyl. The concentration of oxygen in cyclohexane solutions is so much higher that no appreciable decrease could be obtained by photochemical reaction with the biacetyl. They were unable to regenerate the phosphorescence of an aerated solution of biacetyl in cyclohexane. Oxygen quenches the phosphorescence of biacetyl The Journal of Physical Chemistry
and does not affect significantly its fluore~cence2~ in the liquid phase. Oxygen inhibits the photoenolization under the same conditions as the phosphorescence and it is suggested that the precursor of the enol is the lowest triplet state. We have observed that the absorption curves of biacetyl and of its enol cover the same range of wavelength in the ultraviolet region and present the same maximum (2750 A) in n-heptane (i-e., in an inert solvent). It is probable that the first excited state of the enol lies a t about the same energy (3.9 ev) as the second singlet state of biacetyl. The triplet molecule in its lowest vibrational level (
