NOTES
Oct., 1956 TABLE I HYDROQENATION OF n-PRoPYL FLUORIDE, 248" Hi Bow, moles/min. x 104
6.20 8.50 6.55 8.45 8.30 8.30 7.00
HF formation moles/min'. x 104
Energy rate, cal./min.
-AH, kcal./mole n-CaH7F
3.050 2.420 3.000 4.025 4.025 3.000 3.015
6.887 22.58 5,477 22.63 6,807 22.69 9,353 22.24 9.244 22.97 6.942 23.14 6.923 22.96 - A H av. 22.88 kcal./mole Twice the stand. dev. from the mean: f1 . 6 %
Five runs were made on isopropyl fluoride and they are given in Table 11.
1455
clave, we have since been able t o extend the investigation to solutions of higher acidity. The results of this work are reported here since they throw some further light on the mechanism of the reaction and on the nature of the activated intermediate which is formed in the rate-determining step. The procedure was the same as that employed earlier,' ie., the rate of reaction of H2 was calculated from the measured zero-order rate of reduction of Cr207-using the known stoichiometry of the reaction, L e . Crz07'
+ 3Hz + 8H+ +2Cr++* + 7Ha0
(1)
The decrease in rate as the HCIOI concentration was increased is shown in Fig. 1. At 1M HC104 the rate was only about 30% of its value a t low HC104 concentrations.
TABLE I1 HYDROGENATION OF ISOPROPYL FLUORIDE, 248" HZBOW
moles/min.
x
104
7.00 8.00 7.30 7.45 7.45
HF formation moles/mid.
x
Energy rate, oal./min.
104
3.215 4.52 3.50 3.50 3.013
-AH, koa1 mole iso-&F
6.692 20.85 9.563 21.16 7.419 21.19 7.445 21.27 6.346 21.06 - A H av. 21.11 kcal./mole Twice the stand. dev. from the mean: *O. 69% '
formation of n- and isopropyl fluoride, it is necessary to know their gaseous heat capacities in the range between 25 and 248". The data are not available. However, it is possible to calculate the heat of isomerization of normal to isopropyl fluoride a t 248". One finds that the is0 compound is more stable by 1.80 kcal./mole. The uncertainty in this value is about 2%.
THE NATURE OF THE ACTIVATED INTERMEDIATE IN THE HOMOGENEOUS CATALYTIC ACTIVATION OF HYDROGEN BY CUPRIC SALTS BY J. HALPERN, E. R. MACGREGOR AND E. PETERS Department of Mining and Metahwgy The Uniueraity of British Columbia, Vancouver: Canada Received A p d 12, lQS6
I n an earlier paper,l a kinetic study of the Cu++catalyzed reduction of Cr207= and other substrates by HS, was described. At low acidities, the kinetics were found to be of the form -d[Hz]/dt = k[C~++l[Hzl
(1)
suggesting that an Hz molecule reacts homogeneously with a Cu++ ion in the rate-determining step, leading to the formation of an "activated intermediate" which reduces Crz07- in a subsequent fast reaction. It was reported1 that the rate of this reaction decreased slightly as the concentration of HC10, was increased, but the limitations of the equipment which was available a t that time, precluded any studies with solutions containing more than 0.1 M HC10,. However, by using a titanium-lined auto(1) E. Peters and J. Halpern, THISJOURNAL, 69, 793 (1955).
0 01
0.5
1.0
0
[H+], mole/l. Fig. 1.-Dependence of the rate of reaction of Hz on the H + concentration a t 0.1 M Cu++, llOo, 20 atm. He: 0, rate us. [H+]; 0 , rate-'us. [H+].
An earlier suggestion1 that this apparent decrease in rate might be due to the reduction of Clod- competing with that of Cr2O7=appears untenable in the light of our failure to detect any C1- in the solutions and of our observations that the rate depends only on the concentration of H + and is insensitive to variations in the C104- concentration. An alternative interpretation is that the rate-determining step, in which Hz is converted to a reactive intermediate complex, releases an H + ion and that the decrease in rate is due to the fact that the reversal of this step, which regenerates H2, competes with the subsequent reaction of this complex, Le., with the reduction of CrzOs. The zero-order dependence of the rate on the Crz07- concentration, even a t high H + concentrations, suggests that the complex does not react directly with Cr207-. The most likely alternative is that it reacts with another Cu++ ion, reducing it to Cu+, which in turn reacts rapidly with C1-207~. These steps are incorporated in the mechanism
NOTES
1456
Vol. 60
Some further evidence, as follows, may also be advanced in support of the suggestion that CuH+ is k- 1 the activated intermediate in this system. k2 1. The available thermochemical data suggest CuH+ Cu++ +2Cu+ H + (4) that formation of other intermediate species which fast might be considered to result from the interaction 2Cu+ substrate (Le., CrzOl") + of an Hz molecule with a Cu++ ion, i.e., Cu+ or Cu", products 2Cu++ (5) is energetically inconsistent with the observed acApplication of the steady-state treatment to this tivation energy of 26 kcal./mole for the over-all resequence leads to the kinetics action. On the other hand, the formation of CuH+ appears plausible on energetic grounds.2 ki[C~++]~[Hzl -d[Hzl dt Rate = 2. A similar intermediate has previously been [Cu++]+ ( k - ~ / l c ~ ) [ H + ] postulated t o explain the homogeneous catalytic This becomes equivalent to equation 1 a t low H+ activation of Hz by cuprous salts in quinoline soluconcentrations. t i ~ n . I~n this system the kinetics demonstrate At constant [Cu++] and [Hz], as shown in Fig. 1, that two Cu+ ions are involved in the rate-deterthe experimental results conform to a reasonably mining step which may be most reasonably depicted good plot of rate-' vs. [H+], as required by equa- as tion 6. At 110", the values of kl and of the ratio (k-&.), obtained from the intercept and slope of ~ C U + Hz +2CuH+ (7) this plot, are 9.5 X 1. mole-' sec.-l and 0.26, respectively Similar considerations apply to the catalytic activaEquation 6 also predicts a linear relation between tion of Hz by silver salts in aqueous solution where [Cu++]/rate and [Cu++]-' whose slope, given by an analogous intermediate, AgH+, appears prob(k-l/kz) [H+]/kl [Hz1, should increase with [H+] able.214 while the intercept remains constant. I n effect, 3. The promoting influence of certain anions on this implies a shift of the kinetic dependence on the the catalytic activity of Cu++ can be explained in Cu++ concentration from first toward second order terms of the proposed mechanism by assigning to with increasing H + concentration. The results in them the role of stabilizing the H + ion which is reFig. 2 showing the dependence of the rate on [Cu++] leased in the initial step, i.e. a t 0.01 and 1.0 M HC104 are in quantitative accord CUX+ 4-Hz +CUH+ H X is prediction. The values of kl and of (8) kl + H, J_ CUH+ + H +
CU++
(3)
+
+
+
+
E
+
.
+
0
5 [Cu++]-II (mole/l.)-I.
10
Fig. 2.-Dependence of the rate of reaction of Hz on the concentration of Cu++ a t llOo, 20 atm. Hz: 0 ,0.01 M HC101; 0,1.0 M HC104. (k-&2) obtained from these plots are 9.4 X 1. mole-' set.-' and 0.25, respectively. These agree well with the values estimated earlier from the dependence of the rate on [H+].
Consistent with this interpretation, the catalytic activities have been observed6 t o decrease in the same order as the basicity of the anion (X), Le., butyrate, propionate > acetate > sulfate > chloride > perchlorate. Several examples have been reported earlier of homogeneous hydrogenation reactions in which the kinetic dependence on the catalyst concentration shifts from first t o second order with a change of solvent. For example, the catalytic activation of Hz by cuprous acetate is second-order kinetically in quinoline solution but first order in pyridine and dodecylamine.3~8 Similarly, in silver salt-catalyzed hydrogenation reactions, the kinetic dependence on the catalyst concentration shifts from first to second order in going from pyridine to aqueous solut i o n ~ . Some ~ ~ ~ insight into this phenomenon may be derived from the interpretation which has been proposed above for the Cu++ system, and which demonstrates how such a change in the apparent order of the reaction may occur, without a real change in the mechanism of the Hz activation process. Support of this research through a grant from the National Research Council of Canada is gratefully acknowledged. (2) J. Halpern, Proc. Int. Cone. CataEysis, in press (1956). (3) M. Calvin, J . Am. Chem. Soc., 61, 2230 (1939); 8. Weller and G. A. Mills, ibid., 76, 769 (1953); M. Calvin and W. K. Wilmarth, ibid., 78, 1301 (1956). (4) A. H. Webster and J. Halpern, THISJonRNaL,*60, 280 (1956). (5) E. Peters and J. Halpern, Can. J . Cham., 84, 554 (1956). (6) L. Wright, 8. Weller and G.A. Mills, T K IJOURNAL, ~ 69, 1060 (1955).
