HYDROGENATION O F CUPRIC ACETATE
April 5 , l9.X
cannot occur or the hydrogen carrying intermediate is unable t o react with the cupric salt; parahydrogen conversion experiments will distinguish between these two alternatives. The conclusion t h a t steric effects are important is also consistent with the over-all activity of the carboxylic acid salts as compared with the salicylaldehyde derivatives. Here, if our explanation is applicable, the more favorable basicity of the sali-
[COSTRIBUTION FROM
THE
1.705
cylaldehyde derivative is almost completely compensated for by the steric hindrance irnposetl by the extensive chelation of the cuprous atom and the carbonyl oxygen. The inactivity of the o-nitro derivatives may also be of steric origin but an explanation based upon acidity considerations would be equally applicable. BERKELEY, CALIFORMA
LOSASGELES,CALIFORSIA
DEPARTMENT O F CHEMISTRY, UNIVERSITY O F
SOUTHERN
CALIFORNIA, L O S
;iSGELES]
Homogeneous Catalytic Hy~drogenation. VI. The Rate Law and Temperature Coefficient for the Hydrogenation of Cupric Acetate' BY W. K. WILMARTII A N D MAXK. B A R S ~ I ? RECEIVED JULY 5, 1955 T h e homogeneous hydrogenation of cupric acetate has been studied in quinoline over the temperature range 25-100 ". A t all temperatures the rate of hydrogenation was proportional t o the concentration of hydrogen arid the square of the
- 13,700
(mole/l.) -* scc.-I. \{-hen cuprous acetate concentration. T h e third-order rate constant has the value 3.31 X lo3 e-IiTthe hydrogenation was carried out using deuterium gas the rate was somewhat slower; over the temperature range 25-60" t h e decrease in rate was accompanied b y a n increase in the activation energy of approximately '700cal./mole. absorbed v s . the time, and tlie slope of the curve, taken a t selected cuprous acetate concentr;itions, provided a measure of the rate of hydrogenation. Tliis rate TV:IS corrected for the inhibitory effect of acetic acid liy extrapolating all rxtes t o zero acetic acid concentration.4 The magnitude of the correction was sinall a n d about the w i n e ;is thxt m:i& a t 100'; the average correctibn a t tlie tiyo temperatures averaged only about 5-1070. At temperatures below GO" the time rcquirctl to perf(Jrm a complete hydrogenation was inconvc~iiciitl!. loiig a n t l ; I I I alternate procedure for measuring rates \V:LS devi>ed. A t a given temperature and a t known conccntr;ition o f all rvagents, this W:IS achieved by frillon.iiig tlic rate of hytlrc 1genation for a period just sutlicieiit t o ensure tliat tlie V O ~ U I I I ~ ~ Experimental of hydrogen absorbed could be measured with the rcquirrll Materials.-Quinoline solutions of cuprous and cupric accuracy; a V(JluIIie [ J f 1 - 3 ml. usu:~ll!~sutlicctl. T h e d ( 1 1 1 t of a plot of volulne of li!.drrigen :ihSorbed i ' s . time over t i l l acetate were prepared in the manner previously described .5 segment of thc Ii!.(lrr)gcnatioii curve j),rovidcd a nic:i%ire < 11 Commercial electrolytic hydrogen was purified by passing it the rate of li~~drogenatiiin.After such a rate ~ ~ C : L S U ~ ~ I I I C ' I I I slowly through a deoxo unit to remove oxygen and dried had been completed, tlie concentr:itioti of c u p r m s :cct:Lic by passage through two phosphorus pentoxide drying towers was increased to SOIIIe nciv desired value by rapidly a h 1r11and a liquid nitrogen trap. Deuterium gas of 99.9% purity was obtained from the Stuart Oxygen company of San Fran- ing the required amount of liydrogcii a t some Iiiglicr tcinpcr:iture. The reactioii vessel JWS then returiicd t o the origi11;11 cisco b y arrangement with the Atomic Energy Commission. temperature and a value ( j f tlic rcactioii r:tte a t tlie ncw C I I I I Hydrogenation Apparatus.-A magnetic stirrer was used in a reaction vessel similar t o that described p r e ~ i o u s l y . ~ centration of reagents iititained by repeating the procctlurc outlined above. I I I view of the fxct that the acetic :icicl I t was attached to a vacuum line which was constructed of correction did not cli:ingc apprecialil!, from ii0 to l f J O o , : i l l ( ] capillary tubing in order t o minimize the volume of the sysin the absence of data that would provide n direct c r ~ r r e c t i < ~ ~ i , tem. T h e vacuum line also had a mercury manometer dethe 60" correction curves were used to estr:ipolatc tlie r a t e signed to maintain automatically a constant hydrogen presdata to zero acetic acid concentration, Siiice tlic correcsure in the reaction vessel hy changing the mercury level tions were a11 small, no significant error cilulil be iiitrr~tlucctl in the gas buret. The vcilume of hydrogen absorbed a t any by this procedure. time could he determined with an accuracy of 1 0 . 0 2 ml. The solubilities of hydrogen and deuterium were dctcrby direct reading of the gxs buret. The gas buret was maintained a t 25' and the reaction ves- mined a t various temperatures between 0 and 100°. l ' h v sel a t any desired temperature by pumping liquid from a results were obtained by direct measurement of tlie a n i o u ~ ~ t of hydrogen absorbed from a gxs buret 1)). a tleg:iswtl ~ L I ~ I I C I constant temperature bath through the glass outer jackets line solution. For high accuracy by this nictiiotl, the gai surrounding these portions of the apparatus. volume in the system must he as smnll :IS possihle. 'l71ib Hydrogenation Procedure.-At 60" the hydrogenation was achieved in our s>-stc.inb y almost completely filling tlie was rapid enough SO that the reaction could be followed t o thermostated nieasuring vessel with quinoline m i d using :I completion. A plot was made of the volume of hydrogen _ _ -~ ~ ~ small gas buret attached to the manometer and mcusuriiig vessel with a miiiimum length G f capillary tubing. 'l'li(, (1) This research was sponsored b y t h e Office of Naval Research. Reproduction in whole or in part is permitted for any purpose of the initial reading of the buret was somewhat uncertain 5i11cc a small amount of hydrogen was absorbed b y the ~ ( I ~ L I ~ ~ ~ I I Cnited States Government. ( 2 ) Taken in part from t h e Dissertation submitted by X f a x K . during the 10-20 seconds required t o balance tlie h u r c t . Barsh in partial fulfillment of the requirements for t h e Degree of DocHowever, a correction for this unknown volume of h y d r o g c ~ ~ tur of Philosophy in Chemistry. University of Southern California, was obtained by following the very slow absorption of liyJune. 1955. drogen by the unstirred solution for a few minutes and extrapolating the volume readings hack t o zero time. Undcr (3) hl. Calvin, Traiis. F n r a d o y S o c . , 34, 1181 (1938). the conditions of the experiment, where care was taken to ( 4 ) hI. Calvin and W. I;.Wilniarth. THISJ O U R N A L , 78, 1301 1195G). avoid d l possible convection anti vilirational stirring of tlie (3) W. K. W i l m a r t h a n d M a x K . Barsh, i b i d . , 76, 2237 ( 1 9 3 ) .
Introduction It has been shown previously t h a t in quinoline solution cuprous salts catalyze the homogeneous hydrogenation of the cupric salts of various organic a ~ i d s . ~X. ~recent paper has dealt with the catalytic properties of cuprous acetate a t 100°.4 The present work extends this kinetic study to lower temperatures in order to confirm the general validity of the rate law and to obtain an activation energy for the hydrogenation process.
~~~
I
W. K. WILMARTH AND MAXK. B A R S ~ I soliltion, the corrections proved to be negligibly small. After the initial buret reading was obtained, saturation of the solution was rapidly achieved by starting the magnetic stirrer. T h e final buret reading was taken after repeated reading of the buret indicated t h a t saturation had been reached, The solubility d a t a for both hydrogen isotopes are listed in Table 11; because of the improved design of the measuring apparatus, t h e results are believed t o be considerably more accurate than those previously rrported.6
Results and Discussion The data for the hydrogenation of cupric acetate TABLE I RATEOF HYDROGEXATION OF CUPRICACETATE
T. OC. ti0
50 40
--Deuterium(CuAc)
0,028 ,042
--
Ratea
0.033 064
029
,020
,041
,037
,030 ,041
,0098
--Hydrogen-(CuAc)
Rateb
0 0i8
0,040 ,040
019
,040
025
,0161
.042 ,0164 ,030 ,0033 ,040 0080 ,042 ,0047 ,042 0043 Rates reported as ml. of H ? (measured a t 25') observed per minute in 50 ml. of quinoline solution at temperature T . Rates for hydrogen are smoothed values calculated from the straight lines in Fig. 1.
25
0
1'01. 78
are collected in Table I. In order to permit a ready comparison with the results in the literature, the rates are reported as the ml. of hydrogen (measured a t 25") which would be absorbed per minute in a reaction vessel containing 50 ml. of solution. At BO, 50 and 40' the results were entirely reproducible and the general kinetic behavior resembled that observed a t 100'. However, a t 25" it was found that reproducibility could be obtained only if 0.10 g. or more of solid cupric acetate was prcsent in the reaction vessel; in the presence of a suitable excess the rate was independent of the exact amount of the solid. Qualitative studies a t 2;' showed that the solubility of cupric acetate was very small and saturation only very slowly achieved. I t would appear that the rate of solution of the solid can be partially rate determining a t this temperature. The two points lying below the 25' curve in Fig. 1 illustrate the order of niagriitude of this effect. - i t 0" the rate proved to be so dependent upon the amount of solid that meaningful results could not be obtained. At each temperature the results obtained in this study are in agreement with the rate law adopted on the basis of the 100" d a t a 4 However, the rate law has been established with much greater certainty by the present experiments. I n particular, the 60' data plotte'd in Fig. 1 constitute the strongest evidence in favor of the strict second-order dependence of rate upon cuprous acetate concentration. At present it is our feeling that there is n o compelling evidence, either a t 100' or a t lower temperatures, which would justify the postulate that cuprous acetate exists in a dimeric form in kinetically detectable amounts.4 The hydrogen solubility data listed in Table I1 are required in the calculation of the rate cor,stant, kl,for the rate-determining step of the hytlrogenation process.
+
ki
~ C U ' HP --+ 2CuI.11
Calculated values of kl for either hydrogen or deuterium can be obtained by substituting the appro0
TABLE I1 .-c E
T , OC.
\ N
I
-
E
24.6
40.1 50,0
C
60.8 80.8 100,s 100
Hydrogenn solubility
Hydrogen 0 0300 0333 ,0355 0376 ,0422 0467 ,0467
1 0 ? k l (rnolc/l.) sec. - 1
-2
56, 15 4 28 8 43 3 138 510 43@
Deuterium 1 15
30
45
60
,
(Cu'fx lo:
Fig. 1.-Rate of hydrogenation of cupric acetate: a t 60' 0, direct measurement; 0,calculated from parahydrogen conversion. Points at all other tempera h r e s represent hydrogenation experiments. See text for the significance of the squares lying below the 25" line.
24.6 40.1 50 60.8
,0314 ,0345 ,0366" ,0386
3 0 9.7 20 6 33 3
a Tabulated as ml. of Hz (measured a t 25") per ml.. of quinoline. b Calculated from parahydrogen conversion measurements. Aside from the difference in the unit of time, the symbol k" of reference 5 is t o be identified with k , . 6 Obtained by interpolation.
April 5 , 19.iB
HYDROGENATION OF CUPRICACETATE
constants are much smaller than the ones reported in this paper.*
priate values in the equation k I
rate
1307
(mole/l.)+ see.-’
- 60V8S(Cu1)2
Here Vsis the volume of the solution, S is the solubility of hydrogen in the units of ml. of H2 (measured a t 25’) per ml. of solution, and the rate is expressed, as described above, in ml. of HZabsorbed per minute in 50 ml. of solution. The factor 60 in the denominator is required to convert the units of time from min.-l to set.-'. The rates for hydrogen were evaluated from the straight lines in expanded second-order plots similar to those of Fig. 1. Similar graphs (not included here) with about the same scatter of points were used to evaluate the rate constants for the deuterium data of Table I. The calculated values of kl for both hydrogen isotopes are collected in Table 11. The Arrhenius plots of the rate constants of Table I1 are presented in Fig. 2. A straight line has been drawn through the points for the hydrogen isotope Hz, but a slight concave curvature in the line would provide a better representation of the data. h t e constants consistent with the straight line can be calculated from the equation kl = 1.62 -13,000
X lo8 e R T (mole/l.)-2 sec.-l .6 The corresponding equation for the straight line through the -13,700
deuterium points is k = 3.31 X lo8 e R r (mole/ 1.) -2 set.-'. The activation energy for the hydrogenation process using normal hydrogen is in good agreement with the value of 13,000 cal./mole reported by Calvin for the cuprous acetate-catalyzed hydrogenation of benzoq~inone.~Since his experiments were less extensive than the present ones the agreement may be said to add support to his early measurements. Our present value for the activation energy may also be comparecl,with the value “in the neighborhood of 14-15 kcal./mole” reported by Weller and Mills, but any agreement here would seem to be somewhat fortuitous since their rate (6) Calculation of an activation energy from the parahydrogen data would lead t o a somewhat larger value, since both the 00 and 80’ points lie below the line.& The value of 14,500 cal./mole obtained by using only these data would be lower than that previously reported because of the correction imposed through use of the improved hydrogen solubility data. (7) M . Calvin, THISJ O U R N A L , 61, 2?30 (1939).
00-
I
0
0 -
-I
0
-
- 2 01 2 50
I
I
3 00
3 50
(I/T)xlO?
Fig. 2.-The temperature dependtnce of kl: 0, direct measurement using H2; 0,calculated from parahydrogen conversion; a, direct measurement using Dz.
When the reaction is carried out with deuterium replacing normal hydrogen a marked decrease in rate is observed. The rate data are accurate enough to establish an approximate difference in the activation energy for the two hydrogen isotopes, but no significance can be attached to the relative values of the pre-experimental frequency factors. However, there is no doubt that a frequency factor of the order of magnitude of 10* (mole/l.)-2 set.-' is required for both isotopes. So few third-order reactions have been studied in a condensed phase that a “normal” value for such a frequency factor cannot be given, but our observed value seems to be of a plausible order of magnitude. Los ANGELES,CALIFORNIA ( 8 ) S. Weller and G . A . Mills, ibid
, 76, 769 (1953).
