J. Phys. Chem. 1980, 84, 2498
2498
Oscillatory Nitrogen Evolution during the Decomposition of Aqueous Benzenediazonium Chloride Peter G. Bowers" and Yvette M. Dick Department of Chemistty, Simmons College, Boston, Massachusetts 02 115 (Received: July 30, 1980)
Periodic evolution of nitrogen during the decomposition of aqueous benzenediazonium ion is reported. Up to 20 oscillations, having a time period of about 10 s, were observed during the initial vigorous stage of the reaction. The oscillations are weak, irregular, and irreproducible compared to those in other chemical systems. It is suggested that the phenomenon is physical rather than chemical in origin. Observations of oscillatory carbon monoxide evolution in the decarbonylation of formic acid by concentrated sulfuric acid (the Morgan reaction) have been explained by both a physicall and a chemical2 mechanism. The differing opinions prompted us to search for similar periodic phenomena in other gas evolution processes, and we now report that such an effect occurs during the decomposition of benzenediazonium ion in aqueous solution. This well-known reaction PhN2+ H20 PhOH H+ Nz
+
-
+
r
+
has overall kinetics which have been thoroughly characterized, and indeed the reaction is often given as an example of a cleanly first-order process. In our experiments the diazonium ion was prepared in situ by the normal procedure at 0 "C, using aniline and nitrite ion in dilute hydrochloric acid ~ o l u t i o n .The ~ brisk gas evolution which resulted on heating the mixture to 60 "C was monitored by following photometrically the motion of copper sulfate solution in a U-tube manometer attached to the reaction vessel, as described in detail e1sewhere.l A representative rate curve is shown in Figure 1. Typically, a train of about 20 small and somewhat irregular oscillations occurs, superimposed on the steady release of gas. The pulses repeat at intervals of 8-10 s and damp away by about the second half-life. The effect is barely but definitely discernible to the naked eye. The oscillations are partially suppressed by stirring, and almost entirely suppressed in the presence of boiling chips. In some instances, for no clear reason, they do not occur a t all. Although the possibility of a chemical mechanism for the oscillatory nitrogen release cannot definitely be excluded, we believe that in this reaction, as well as in the Morgan reaction, the oscillations may arise for physical reasons involving a supersaturation-desaturation cycle the detailed mechanism for which is not yet understood. Certainly the known chemical oscillators (the Bray-Liebhavsky reaction, the Belousov-Zhabotinskii reaction, and their variants) are much more reproducible and relatively less sensitive to external influences such as degree of agitation. The slow and massive carbon monoxide oscillations in the Morgan reaction contrast with the rapid and
0022-3654/80/2084-2498$01 .OO/O
TIME ( m i d
Figure 1. Rate of nitrogen evolution from 0.45 M benzenediazonium chloride at 60 "C. The solution was unstirred. The ordinate is in relative nonlinear units.
feeble nitrogen pulses we observed during diazonium ion decomposition. Physically this is understandable if there is a lower degree of supersaturation in the latter reaction, and if the supersaturation is relieved more rapidly in the less viscous solution. A third kind of oscillatory gas evolution process has also been investigated by Noyes and Smitha4 This is the thermal decomposition of ammonium or methylammonium nitrite. While nitrogen is also evolved in these reactions, they are apparently complicated by the production of nitrogen oxides. It will be interesting to see whether oscillations occur, or can be made to occur, in other common reactions producing a gaseous product, and further work in this direction is continuing in our laboratory.
Acknowledgment. This research was supported by the Simmons College Fund for Research and by NSF Grant NO. SP1-7926902. References and Notes (1) P. G. Bowers and G. Rawji, J. Phys. Chem., 81, 1549 (1977). (2) R. M. Noyes and K. Showalter, J . Am. Chem. Soc., 100, 1042 (1978). (3) H. Hart and R. D. Schvetz, "Laboratmy Manual for Organic Chemistry", Houghton Mifflin, 1972. (4) R . M. Noyes and R. J. Field, Acc Chem. Res., 10, 273 (1977).
0 1980 American Chemical Society
