CO Evolution in Formic Acid Dehydration (37) H. S. Johnston, "Gas-Phase Reaction Rate Theory", Ronald Press, New York, N.Y. (38) F. E. Saalfeld and H. J. Svec, J. Phys. Chem., 70, 1753 (1966). (39) K. J. Reed and J. 1. Brauman, J . Chem. Phys., 61, 4830 (1974). (40) K. C. Kim, D. W. Setser,and C. M. Bogan, J. Chem. phys., 60, 1837 (1974).
1549 (41) P.Potzinger, A. Ritter, and J. R. Krause, 2.hfatwforsch. A, 30,347 (1975). (42) R. Walsh and J. M. Wells, J . Chem. Soc., Chem. Commun., 513 (1973). (43) 1. M. T. Davidson and A. V. Howard, J. Chem. Soc., Farady Trans. 1, 71, 69 (1975).
Oscillatory Evolution of Carbon Monoxide in the Dehydration of Formlc Acid by Concentrated Sulfuric Acid Peter 0. Bowers" and Gulnar RawJI Department of Chemistty, Simmons College, Boston, Massachusetts 021 15 (Received March 14, 1977) Publlcation costs assisted by Simmons College
The periodic evolution of carbon monoxide from mixtures of formic acid and concentrated sulfuric acid has been studied. Oscillations are only observed under conditions favorable to good foaming. The composition of the foam differs from that of the liquid phase. A qualitative explanation of the oscillations is given in physical (rather than chemical) terms. Introduction The periodic nature with which carbon monoxide is evolved in the well-known reaction between formic acid and concentrated sulfuric acid was first reported over 50 years ago by Morgan1 Under rather specific conditions of mixture and temperature, Morgan observed that CO was released in gusts, which at their peak caused the whole mixture to rise and foam vigorously. The gusts were separated by quiescent periods with slow gas evolution, and little or no foam. Morgan was able to observe up to about 20 damped oscillations, with a time period of approximately 1 min. A t that time, the unusual kinetics were attributed to physical effects involving supersaturation, and indeed in later kinetic studies2carried out under conditions designed to prevent supersaturation, the oscillatory nature of the reaction was ignored, if indeed it was observed at all. Despite the upsurge of interest in oscillating reactions: there are still only two well-documented examples of true chemical oscillators (excluding biochemical systems and electrode phenomena). These are the Belouaov-Zhabotinski type and the Bray-Liebhafsky reaction (iodateperoxide). They are both redox processes. For this reason we undertook to reexamine the Morgan reaction, which superficially at least, appears to involve no redox process, and to be stoichiometrically simple. Experimental Section Chemicals were reagent grade and used without further purification. Batches of concentrated sulfuric acid and 88% formic acid from several different manufacturers gave substantially the same results. The best oscillations were obtained by cooling the two acids separately in ice before mixing in a large test tube immersed in a thermostat bath at 38 "C (11 mL of H804-3mL of HC02H). The acids were mixed vigorously for 30 s and subsequently agitated by gentle magnetic stirring. The test tube was connected to an adjustable capillary leak to the atmosphere, and to a U-tube manometer containing copper sulfate solution. By suitable adjustment of the capillary, periodic evolution of gas was reflected in oscillation of the levels of liquid in the manometer. A He-Ne laser beam was directed through the
length of Cu2+solution in the open arm of the manometer, and the oscillatory transmittance recorded via a conventional photocell-recorder arrangement. In a similar way, with the capillary leak completely closed it was possible to follow stepwise build up in the pressure of evolved gas. Analysis for formic acid was accomplished by measuring the absorbance (235 nm) of small samples which had been quenched with water, weighed, and appropriately diluted, after removal from the reacting mixture.
Results General Course of the Reaction. A typical curve obtained by the differential method described above is shown in Figure 1. Steady oscillations begin after an initial period of vigorous gas evolution, and during the first few of them the mixture foams to an extent that no separate liquid phase is visible. At each maximum the foam ruptures quite suddenly, just after the maximum rate of gas evolution, and drains back to form an almost quiescent liquid, before gas evolution builds up again. The periodic behavior damps away gradually, with the time period of the oscillation going through a maxium (Figure 2). Slow gas evolution continues for several hours after the oscillatory phase. When the reaction appears to be over, vigorous agitation (or pumping) results in the removal of much dissolved CO. Reproducibility. Appearance of oscillations in this system depend on a suitable choice of at least three critical variables-temperature, composition, and degree of agitation. Outside of fairly narrow limits for these, oscillations either do not occur at all, or are of low amplitude. Our composition and temperature are similar to that prescribed by Morgan. Mixtures too rich in either component react aperiodically, and without excessive foaming. Qualitatively, we observed oscillations in the temperature range 28-50 "C. At room temperature the mixture appears to be too viscous to foam well, while at high temperature gas evolution is too vigorous to permit the existence of a well-separated foam phase. The sharpest oscillations occur when the reactants are gently agitated. With no agitation, other than that naturally inherent in gas evolution, the amplitude is less The Journal of Physical Chemlstty, Vol. 81, No. 18, 1977
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Peter G. Bowers and Gulnar RawJi
r a 5,
-I
m
t (min.) FlgMre 4. Osciilations in the presence of a trace of detergent, using same conditions as in Figure 1.
t (rnin.) Figure 1. Oscillatory portion of differential rate curve for evolution of CC) from 11 mL of concentrated H2S04-3 mL of HCOPHat 38 O C . The ordinate is in relatlve nonlinear units.
T
le4I\
r
2
4
6 t (min.)
8
Figure 5. Relative absorbance (235 nm) of liquid (0) and foam (0) samples, taken at peaks of successive oscillations.
10
0
30
20 t(min.)
Flgure 2. Variation of the time perlod of the oscillation with time.
do dd
Yd
4
2
6
t (min)
Figure 3. Integrated rate curve for CO evolution showing the effect of vigorous agitation (- -) alternating with gentle agitation (-).
-
although the time period is little affected. Vigorous agitation resulb in smooth, nonoscillatory evolution of CO, and the mechanical production of foam. However, as Figure 3 shows, rapid agitation does not change the overall rate of gas evolution. Effect of Detergent. Figure 4 shows the effect of a trace of detergent added at the start of the reaction. Detergent increases the time period of the oscillation significantly and lengthens the total time of the oscillatory phase. The Journal of Physical Chemistry, Vol. 81, No. 16, 1977
Antifoam has the reverse effect, giving a train of rapid oscillations of low amplitude. Composition of Liquid and Foam Phases. The periodic behavior can be completely suppressed by removing most of the foam (by suction) just before it subsides. After several such removals during successive oscillations, the remaining liquid no longer oscillates. In related experiments, small samples of both liquid and foam were removed simultaneously for analysis at the peaks of successive oscillations. The composition of each phase, as indicated from their UV absorption, is shown in Figure 5. Since neither water, sulfuric acid, or dissolved CO absorb at 235 nm, the foam, when it becomes unstable, is evidently deficient in formic acid as compared to the liquid below it. Although the precision of this analysis is not particularly high, the same relative position of the curves in Figure 5 was reproducibly found in several trials. Similar analyses were carried out with mixtures in which the formic acid was replaced by either glacial acetic acid or formaldehyde. In these experiments, foam was produced using a stream of nitrogen through a fritted bubbler. Even after several separations and “refractionations”, no difference in composition between liquid and foam fractions could be detected. Discussion The results described above indicate the critical part played by foam production in this reaction. In particular, differences in kinetic behavior and in composition, between bulk liquid and liquid separated by removal as foam, suggest a basis upon which an explanation of the oscillations can be made. At the beginning of a cycle (time A in Figure 11, CO is being produced, but is supersaturating the solution, and not much escapes. Vigorous evolution begins at point B, resulting in the production of a foam phase in which the reaction proceeds more rapidly due to (I) significant foam fractionation, with a mole ratio in the foam more favorable
Photolsomerization of Substituted Cyclic Compounds
to reaction, and/or (11) inherent kinetic effects possibly involving more advantageous orientation and juxtaposition of the reactants at a surface than in bulk. At point C gas evolution ceases and almost simultaneously the foam ruptures. Factors influencing the rupture point might include (a) natural drainage of the foam, (b) instability of foam caused by composition changes due to reaction, (c) excessive CO pressure in the bubbles, and (d) depletion of reactants in foam causing cessation of gas evolution. In any case, at C, on the basis of our experiments, there is a composition difference between the foam and bulk liquid. Between points C and D the foam drainage liquid runs back into the bulk. The drainage liquid is probably not, at this stage, supersaturated with gas, because of its history as foam with a high surface area. Thus the newly re-formed bulk liquid is less than supersaturated, and the cycle begins again. It is difficult to decide which of the factors I or I1 above is the more important in producing a composition difference between the two phases at maximum amplitude. The fact that foam is produced at all indicates a surface excess of one of the components, and intuitively we would expect this to be the organic acid. However the reverse is observed. This, together with the fact that little, if any, fractionation occurs on foaming similar but nonreacting mixtures, leads us to favor explanation I1 as the predominant cause of the composition difference. That is, the acids react more rapidly at an interface than in bulk, and this results in faster depletion of formic acid in the foam. Gentle agitation, by promoting good mixing between supersaturated bulk liquid, and the less saturated foam-drainage liquid, helps to produce sharp oscillations by making the slow stage more marked. Indeed, were it not for the composition and kinetic differences seen on separating the foam, a simple saturation-desaturation sequence could explain the periodicity, with the foam acting to “catalyze” the desaturation. Vigorous agitation, on the other hand, prevents natural periodic separation of the two phases; rapid mechanical interconversion of the mixture between foam and liquid, with an approximately constant amount of each, results in constant average rate
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Changes which are noted under different conditions (notably temperature, composition, and the presence of detergent or antifoam) are all explicable qualitatively in terms of the adverse or favorable effect they have on the Stability of the foam. The pertinent physical factors which determine this stability are viscosity (relating to the drainage rate) and surface tension (relating to rupture of the bubbles): The maximum exhibited by the time period of the oscillation (Figure 2) is also understandable in terms of two opposing factors. Aa the reaction proceeds, the formic acidxoncentration decreases. This at first results in a decrease in the rate of gas production and hence in the formation and rupture of foam, which therefore increases the time period for a cycle. Eventually, depletion of the organic component adversly affects the stability of the foam for physical reasons, and the cycle speeds up before osciliations damp away entirely. Concluding Remarks Perhaps the chief point we wish to make from this work is that carbon monoxide evolution in the Morgan reaction is periodic in nature for reasons we believe to be physical in nature, and associated with the production of foam. The fact that it may not be a true chemical oscillator does not make it any the less interesting, and we are at present studying a number of other gas-evolution reactions which might, under the proper conditions, show periodicity for the same reasons. Acknowledgment. One of us (P.G.B.) wishes to thank Professors C. A. McDowell and G. B. Porter of the Chemistry Department, University of British Columbia, where part of this research was carried out. The work was also supported in part by NSF Grant SMI76-03651. Pieferenqes and Notes J. S. Morgan, J. Chem. SOC. Trans., 109, 274 (1916). 0.A. Ropp, A. J. Weinberger, and 0. K. Neville, J. Am. Cbem. Soc., 73, 5573 (1951), and references cited therein. G. Nlcolls and J. Portnow, Chem. Rev., 73, 4 (1973). Varying the external pressure also changes the frequency of the oscillations In a regular way (R. M. Noyes and K. Showatter, personal communlcatlon). This Interesting effect constitutes a rapid and reversible method of “tuning” the frequency of gas evolutlon.
Cis-Trans Photoisomerization of P-Styrylnaphthalene and 3-Styrylquinoline G. Gennarl, G. Caurzo,’ and G. Galiarzo Institutes of Physical Chemistty and Organic Chemistry, University of Padova, 35100 Padova, Italy (Received January 21, 1977) Publication costs assisted by the Consigiio Nazionale delle Ricerche (CNR)
The direct and anthraquinone-sensitized cis-trans photoisomerizationof P-styrylnaphthalene and 3-styrylquinoline has been investigated in benzene at 25 O C . The effect of azulene on the photostationary states is consistent with a triplet mechanism for direct photoisomerization. The influence of oxygen and of the substrate concentration on the reaction pathway is also discussed. Introduction Whereas the photosensitized cis-trans isomerization of ethylenic compounds in solution was shown to proceed by a triplet mechanism,l singlet or triplet pathways have been proposed for their isomerization induced by direct irradiation. For stilbene, quenching experiments of Saltiel and
co-workers2led to the conclusion that the direct cis-trans photbisomerization occurs via a single route. Recently, the singlet mechanism of stilbene has been confirmed by both theoretical calculations3and photophysical measurements: A theoretical state model for the singlet mechanism has been proposed6 where a S2 (‘Ag*) excited singlet has a The Journal of Physical Chemistry, Vol. 81, No. 16, 1977
