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Vol. 59
NOTE T H E EXCHANGE REACTION BETWEEN DEUTERIUM AND METHYLAMINES ON IRON POWDER1
Results The exchange of methylamine and dimethylamine with deuterium at 242 and 289” followed a firstorder law. From this and from the distribution of BY JOEL R. GUTMANN the isotopes between the amines and the hydrogen Israel Atomic l n e r g y Commission, Rehouolh, Israel when isotopic equilibrium had been established it Received January 66,1966 appears that only the N-bound hydrogens of the Kinetic studies have established that the iron- amines exchange rapidly; methylamine giving CH3catalyzed exchange of hydrogens between ammonia N H D and CHaNDz and dimethylamine giving and deuterium comes about by the interaction of (CH3)2ND. A similar conclusion had been reached strongly adsorbed ammonia (or of the products of before by Roberts, Emeleus and Briscoe, for the rethe surface dissociation of ammonia, namely, M- action between methylamine and deuterium on a “2, M-NH or M-N) with either D atoms or nickel c a t a l y ~ t . ~ gaseous D2molecules. In the exchange run of dimethylamine and deuteThe rates of exchange between the methylamines rium a t 331” a continual slow change of the “D” and deuterium have now been measured on a pure content, at the practically constant rate of 6.6 X iron powder. By comparing these exchange rates min. -l, was superimposed on the fast first-order with the rate of exchange of ammonia and deu- change and measured after isotopic equilibrium for terium an attempt is made to identify the intermedi- the N-bound hydrogen of the dimethylamine should ates in all these exchange reactions. have been established. (The rate of exchange of the Apparatus and Materials.-The exchange rates were de- N-bound hydrogen in this run was 0.05 min. -l. termined in a slightly modified version of the apparatus This was evaluated from the reaction plot after previously described by the author .s Ammonia, hydrogen subtracting from each reaction point the amount and deuterium were prepared as described before. Methylamine, dimethylamine and trimethylamine were liberated due to the slow change.) The same slow dilution of from their respective hydrochlorides by concentrated po- deuterium was measured, unmasked by the faster tassium hydroxide solution, dried by passage through a initial change, in the interaction of (CH3),ND with column of potassium hydroxide pellets, distilled in vacuum from -80 into a receiver cooled in liquid air and finally deuterium. The slow change in “D” content was found to be due to hydrogen gas produced in the stored in bulbs covered with freshly distilled sodium films. Experimental Procedure .-Reduced iron (May and Baker, slight decomposition of dimethylamine on the cata65 mg.) was used as a catalyst, after initial exhaustive re- lyst a t this temperature and no exchange of hydroduction in a stream of dry hydrogen at 350’. Before each experiment the catalyst was flushed with deuterium and gens of the methyl groups was involved. The cataoutgassed at 350’ for about half an hour through liquid air lytic decomposition of dimethylamine, which was traps. The reaction mixtures of about 85 mm. deuterium found to give hydrogen with about 5% of methane with the same amount of either ammonia, methylamine or as the sole non-condensable products, is in contrast dimethylamine were introduced into the catalyst chamber to the homogeneous decomposition of dimethylamthrough an ice-cooled trap after the catalyst had been brought to the appropriate temperature. The exchange ine which yields mainly methane and some polymerreactions were followed by extracting small samples of as ized nitrogenous material.6 from the catalyst chamber, freezing out the condensa%le The rates of exchange of NH3, CH3NH2 a n d ‘ gases in a liquid air trap and measuring the atom fraction (CH&NH, respectively, with deuterium a t 242” of the heavy isotope in the hydrogen (the “D” content) in min.-’. The a t>hermal-conductivitygage. The rates of exchange in the were found to be 2.9, 3.0 and 2.5 different runs are given in terms of the initial rates of change corresponding rates a t 289” were 0.023, 0.022 and of the “D” contentsa ( L e . , the rate when the “D” content 0.021 min.-’. At 331” the rates for the NH3 is I ) , which were evaluated either from the first-order con- and (CH3)2NHexchanges, respectively, were 0.050 stant of the exchange and the “D” content when isotopic equilibrium had been established4 or directly from the slope and 0.042 min.-l. The rates are seen to be essenof the reaction plot a t zero time. I n order to detect vttria- tially equal, with the dimethylamine runs slower tions in catalyst activity from time to time, all exchange runs by 10-15% at all temperatures. The values for whose rates were compared were “bracketed” by standard each single temperature refer to the catalyst a t a exchange experiments between ammonia and deuterium. The sequence of runs where the rates of these control reac- constant activity as determined by control reactions. However changes in activity occurred betions were not reasonably constant, were rejected. tween groups of runs at different temperatures and (1) This work forms part of a thesis submitted to the Hebrew the values recorded should not be used to deterUniversity, Jerusalem, in partial fulfillment of the requirements for a mine activation energies. A separate determinaPh.D. degree. tion of the activation energy for the ammonia(2) K . J. Laidler, “Catalysis,” Ed. by P. H. Emmett, Vol. 1, Reindeuterium exchange on a different batch of the hold Publ. Corp., Inc., New York, N Y., 1954, p. 174. (3) J. R. Gutmann, THISJOURNAL, 61, 309 (1953) : for details of same catalyst, gave a value of 17.5 kcal./mole.2 modifications made see Thesis, 1954, The Hebrew University, No change of the “D” content was found on inJerusalem. (3a) Since the different runs were made in the same reaction vessel and at a fixed deuterium pressure, the number of hydrogen atoms exrlianged in unit time in the different runs are propoitional t o these rates. (4) J. Singleton, E. Roberts and E. Winter, T r a m Faraday R o r . , 47,1318 (1951).
(5) E. R. Roberts, H. J. Emeleus and E . V. A. Briscoe, J. Chem. S o c . , 41 (1939). (6) A. G . Carter, P. A. Bosanquet, C . G . Silcocks, &I. W. Travers and A. F. Wilshire, J. Ckem. Soc., 495 (1939); comgare, however, the
iron-catalyzed decomposition of monomethylamine, P. H. Emmett, and R . W. Harknesn, J. A m . Chem. SOC., 64,638 (1932).
b
May,-1955
NOTE
teracting trimethylamine with deuterium at low temperatures. At 331" a slow continual change of D content was observed (decomposition). The rate of this change was about one third of the corresponding rate measured with dimethylamine. The exchange rate of an ammonia-deuterium mixture a t 331" was unchanged by the addition of trimethylamine to it (equal rates of 0.055 f 0.002 min.-l (CHJ3N D, mixtures). for NH3 D2 and NH, The slow decomposition of trimethylamine on the catalyst which had been observed before at this temperature was not found in the presence of ammonia. Discussion The rates of exchange of methylamine and dimethylamine with deuterium have been found t o be essentially the same as those of ammonia with deuterium at 242,289 and 331". It follows that the Arrhenius parameters of the three reactions are similar and therefore the rate-determining steps are also similar. This fact enables us to obtain further insight into the nature of the reaction intermediates and their participation in the rate determining process. It is known7that the rate-determining step in the ammonia exchange is not associated with the activation or desorption of deuterium but rather involves the reaction of chemisorbed ammonia or one of its surface dissociation products. By analogy therefore the rate-determining step in the exchange of any of the amines will involve the reaction of the chemisorbed amine or one of its surface dissociation products. A further requirement is that the participating chemisorbed species in both the ammonia and amine exchange reactions occupy about the same surface area on the catalyst. We may now be more specific about the role of the surface dissociation products in these reactions. In the case of ammonia adsorption on iron it is knowns that extensive dissociation can occur on the surface, leading t o products M-NH2, M-NH and M-N. In the case of the adsorption of an amine the corresponding dissociative surface processes, involving the rupture of a C-N bond, cannot be a stage in an exchange reaction since such steps would be irreversible and lead to decomposition of the amine concerned. (Such decomposition was detected only a t the highest temperatures used.) It follows that the exchange reaction of dimethyl-
amine, for example, proceeds either through undissociated adsorbed molecule, M . . . NH(CH3)2, or through the chemisorbed radical M-N(CHa)2 but not through a more dissociated species. Since methylamine and ammonia exchange their Nbound hydrogens a t the same rate and by the same mechanism as does dimethylamine it follows that in both cases the reaction intermediates are the corresponding adsorbed molecules, M . . . NHzCH3 and M. . . NH3, or the radicals M-NHCH3 and MNH2, respectively (cf. also Kernball's conclusion on the NH3-D1 reactiong). A decision between the two alternatives may be reached on the basis of the experiment involving exchange in the presence of trimethylamine. As expected (CH3)3Nis strongly adsorbed on surfaces on which NH3 is adsorbed without dissociationlo and will partially displace ammonia from such sites. The fact that addition of trimethylamine has no effect on the rate of ammonia-deuterium exchange on iron suggests that the rate-determining step of this exchange does not involve adsorbed undissociated molecules of ammonia. We may therefore conclude that the exchange rate of ammonia with deuterium is determined by the rate of reaction of the M-NH, radical and analogously the rates of exchange of methylamine and dimethylamine, respectively, with deuterium are determined by the rate of reaction of the species M-NHCH3 and M-N(CH&. The results obtained furthermore show that no appreciable part of the catalyst surface is covered by the more dissociated mecies of ammonia (surface imides or nitrides) under the conditions prevailing during the exchange reaction of ammonia with deuterium. For the presence of these species, which have been found not to be intermediates, would inhibit the exchange reaction by blocking part of the catalyst surface and thus depress the rate of the ammonia exchange relative to that of the two amines. Acknowledgment.-The helpful guidance of Dr. I. Dostrovsky in the course of this work is gratefully acknowledged. The author wishes to thank the Research Director, ,Israeli Atomic Energy Commission, for permission to publish these results and the Weismann Institute of Science, Rehovoth, for its hospitality extended for the duration of this work.
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(7) A. Farkas, Trans. Faraday Soc., 38, 417 (193F). (8) Ch. Kernball and H. Wahba, zbid.,49, 1351 (1953).
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(9) CII. Kernball, Proc. Rou. S o c . (London), 8214, 413 (1952). (10) W. 8 . Felsing and C. T. Ashley, J . Am. Chem. Soc., 86, 2226 (1934).
