April 5, 1952 PREPARATION AND PROPERTIES OF a-CYANOALKYLKETENE ACETALS l S l l change are compared. For deuteration, PO and P represent starting pressure and pressure a t time t ,respectively; for exchange po/P is equal to POD,/ PD,, wherein POD^ and PD, are initial pressure and pressure a t time t , respectively, of the Dz species. Since the analytical method takes no cognizance of species, it was necessary to consider two extreme cases for evaluating the pressure of DZ at time t:(1) that all the measured exchange is due to the conversion of DZ to Hz, and (2) that the measured exchange represents only the conversion of Dz to HD. Consideration of these cases gives two possible extremes for the disappearance of DZ with time, as indicated by the two broken lines of Fig. 4. The upper one coincides with the experimental points for deuteration. Since the utilization of Dz to form HD is preponderant in the early stages of the exchange, it is quite likely that the disappearance of Dz is represented more nearly by the upper one, so that consumption of Dz in deuteration proceeds a t about the same rate as the consumption of Dz in exchange with pure acetic acid. Therefore, if the two reactions were competing for deuterium in the ordinary kinetic sense, one should have had no difficulty in detecting the exchange process.The fact that the deuterium pressure falls off during the reduction reaction does not mean that the measured exchange, expressed as atomic per cent. hydrogen, would be correspondingly diminished. On the contrary, if both reactions were occurring a t the same rate, the atomic per cent. hydrogen should not change a t all, since this quantity is independent of the pressure. This statement follows directly from the data of Fig. 1. Since the exchange reaction is somewhat slower than the deuteration reaction, the atomic per cent. hydrogen might diminish somewhat, but should certainly be detected if the two reactions were competing.In the presence of nitroethane, the exchange of deuterium with acetic acid proceeds unhindered, since the acetic acid in this case can become chemisorbed on the catalyst surface and exchange with the deuterium.While the hydrogenation of nitrobenzene appears to be better explained by a mechanism of the Rideal-Eley type, the mechanism of the hydrogenation of nitroethane is not apparent from the exchange studies. I t does appear that hydrogenatioii and exchange in this case are taking place independently of each other, inasmuch as hydrogenation is independent of the gas pressure, while deuterium exchange is proportional to the first power of that pressure. This might appear to constitute an anomaly, since in the hydrogenation reaction the hydrogen may be presumed to be strongly adsorbed on the catalyst surface, while in the exchange reaction it is presumably weakly adsorbed. However, this may indicate that the reactions occur on different catalyst sites, or may be a consequence of the fact that acetic acid is much more strongly adsorbed than hydrogen while the hydrogen, in turn, is more str...
