April, 1961
DECARBOXYLATION OF O X A M I C
ACID IN
IV. Triethylarnine-Iodine.-Of the many iodine complexes reported in the literature, the one formed with triethylamine is the most stable.30 It is not surprising, therefore, that use of approximate equations might result in appreciable error in the equilibrium constant, which for this system was found to be approximately 10% by.Drago and Rose." The point C in Fig. 1 was plotted using the mole l.-', Nagakura data" a t 25' of Kz, = 4.69 X CB = 3.90 X lo-* mole l-l, and CZ = 3.64 X 10-5mole1.-1. It may be seen from the graph that an error of this magnitude is possible for KS1and more important, that the direction for correcting the error can be predicted. Equations 23 and 24 represent a lower and an upper limit to K-'/Cz. Thus substituting the Nagakura result for KZIinto equation 23 gives the value K26 = 5.66 X 103 1. mole-'. This checks quite favorably with the value determined directly by the application of equation 17 to the original data, the result from a least squares treatment being 5.76 X l o 3 1. mole-'. Comparison is not strictly valid because in equation 17 the role of the slope and intercept is interchangcd from that of the Benesi-Hildebrand equation, which was the one employed by Nagakura. Different methods of plotting the same experimental data have been noted to give different results for the equilibrium c o n ~ t a n t . ' ~This is to be expected when a least squares treatment of data is employed, the differences being nil only when the experimental data all lie on an exact line, a point which has recently been menti~ned.~'The prediction that KI9, as determined by using equation 22, should be larger than 4.69 X lo3 1. mole-' and smaller than 5.66 X 103 1. mole-' is in accord with the result of 5.13 1. mole-' reported by Drago and Rose.' Only the constant a t 25' could be re-evaluated by Drago and Rose because detailed experimental data were available at this temperature ~ n l y . ~ oIn the absence of other information, a reasonable estimate may be made for the correction of K a t the other temperatures, thus permitting a re-evaluation of A H and A S . It is clear that the correction will increase with decreasing temperature, thereby increasing A H and AS. It was mentioned that it is not possible to select a priori that particular value of p between 0 and 1 which will permit proper conversion of equation 24 to 22. Using the Nagakura data and choosing a value of p = l/z gives K N = 5.13 X lo* 1. molo-l, exactly that obtained by Drago and Rose. (30) S. Nagakura, ibid., 80, 520 (1958). (31) M. Tamrea and Sister M. Brandon O.P., ibid., 83, 2134 (19601, see Table I.
QUINOLISEAKD
659
IN 8-hlETHYLQUINOLISE
Assuming that this value for p is also ap licable at the other temperatures,32 the KI9 column inTable f i s readily obtained. The last column establishes the expected upper limit, &.e.,for p = 1. At 40°, the percentage difference between the limits KZIand KZ0has been reduced considerably, rendering less critical a proper selection for p. The data for the corrected log K vs. 1 / T lie on a straight line. A least squares treatment gives A H = -12.9 kcal. mole-' and A S = -26.3 e x . , which may be compared to the Nagakura values AH = - 12.0 kcal. mole-' and A S = -23.5 e.u.
Acknowledgment.-The author wishes to acknowledge helpful discussions with Dr. M. Mayot, Dr. A. R. Bloemena, Dr. R. Albrecht and Miss C. Valdemoro during his stay a t the University of Paris and a t the University of Amsterdam. He wishes also to express his thanks to the John Simon Guggenheim Memorial Foundation for a Fellomshlp. TABLE I EQUILIBRIUMCONSTANT CORRECTIONS FOR THE SYSTEM TRIETHYLAMIXE-IODINE I N 12-HEPTANE h-21.a
t,
oc.
20.5 25.0 30.0 40.0 a Reference 30.
KIO,
x
Kza,
x 10-3 1. mole-'
x 10-3 1. mole-'
1. mole-'
6.32 4.69 3.31 1.74
7.14 5.13 3.52 1.80
8.21 5.66 3.76 1.86
10-3
(32) For any given concentration of base t o acid, the fraction of acid converted t o complex will be greater a t the lower temperature. B u t the 6 which is of concern here is the one value a t each temperature whiob will satisfy equations 2 2 , 23 and 24 simultaneously. I n the triethylamineiodine study where a constant concentration of Cz was used, and assuming only minor variation of Cz with temperature, the relation for B which applies a t two temperatures TI and Tz is
where the first and second terms in brackets tend t o compensate. However, appreciable and even random variation of with temperature may be found for a given set of experimental results, especially when there is greater scatter in the data.
THE DEC.ARBOXYLAT10~OF OXAR/IIC ACID IN QUINOLINE AXD IN S-&lETHYLQUIn'OLINE BY LOUISWATTSCLARK Department of Chemistry, Western Carolina College, Cullowhee, S o r t h Carolina Recezted A-owrnber 4, 1860
Kinetic data are reported on the decarboxylation of oxamic acid in quinoline and in 8-methglquinoline. The parameters of the Eyring equation are evaluated and compared with those for oxalic acid, malonic acid and the trichloroacetate ion. The data support the transition-complex mechanism previously proposed for the reaction. An interesting parallel is pointed out between the decarboxylation of oxamic acid and the acid oxalate ion in 8-methylquinoline.
Kinetic studies on the decomposition of oxalic acid into carbon dioxide and formic acid have been made in the solvents dioxane, glycerol,2 dimethyl sulfoxide, triethyl phosphate, aniline, S-methylaniline, N,N-dimethylaniline, quinoline, 6-methylquinoline and 8-methylquinoline.4 In weakly basic (1) A . Dinglinger and E. Schober, 2. p h u i s k . Chem., 8 1 1 9 , 401
(1937).
( 2 ) L. W. Clark, J . Am. Chem. SOC.,7 7 , 6191 (1955). (3) L. W. Clark, J'. P h y e . Chem.. 61, 699 (1957). (4) L. W. Clark, ibid., 62, 633 (1958).
solvents the rate-determining step of the reaction appears to be the formation of a transition complex, one of the carbonyl carbon atoms of the un-ionized diacid coordinating with an unshared pair of electrons on the nucleophilic atom of the solvent molecule. The results of kinetic studies on the decomposition of oxamic acid into carbon dioxide and formamide in the solvents aniline and o-toluidine indicated that, in this case also. the rate-determining
LOUISWATTSCLARK
680
step may be the formation of an intermediate compound between solute and solvent.6 Since more information on this reaction was needed in order to more firmly establish the proposed mechanism further kinetic studies have been carried out in this Laboratory on the decarboxylation of oxamic acid in two additional polar solvents, quinoline and 8methylquinoline. The results of this investigation are reported herein.
Vol. 65
sponding data for oxalic acid, malonic acid and the trichloroacetate ion. TABLE I APPARENT FIRSFORDER RATE CONSTANTS FOR THE DECARBOXYLATION OF OXAMIC ACIDIN QUINOLINE AND IN 8METIIYLQUINOLINE Temp
Solvent
Quinoline
Experimental Reagents.-( 1) The oxamic acid used in this investigation was analytical reagent grade, 100.0% assay. See reference 5 for analytical procedures. 2) The solvents were reagent grade chemicals. Each samp e of each solvent, before the beginning of each decarboxylation experiment, was distilled directly into the dried reaction flask at atmosphericpressure. Apparatus and Technique.-The kinetic experiments were conducted in a constant temperature oil-bath by measuring the volume of COz evolved at constant pressure, as described in a previous pa er.6 In each experiment a 0.1585-g.sample of oxamic acid [the amount required to produce 40.0 ml. of COz at STP on complete reaction) was introduced in the usual manner into the reaction flask containing a weighed sample of solvent saturated with dry GO2 gas.
I
Results
8-Methylquinoline
-
(5) L. W.Clark, J . Phvs. Chem., 65, 180 (1961). ( 6 ) L. W.Clark, ibid., 60, 1150 (1956).
No.of expta.
137.62 144.77 153.45 127.73 135.13 145.74 148.62
2 2 3 2 2 3 3
k X
l3‘,
sec.
1.33 3.65 12.08 1.27 2.92 9.21 12.53
Av. dev.
io.01 -1. .01
f .01 i .01 f .02
f .02 f .02
TABLE I1 KINETICDATAFOR THE DECARBOXYLATION OF OXAMIC ACID,OXALICACID,MALONIC ACIDAND TEE TRICHLOROACETATE IONIN QUINOLINE AND IN %METHYLWINOLINE Solvent
-Quinoline? AH+
Reactant
kcal.‘
Oxamic acid It was noted in the initial studies on the decar- Oxalic acids34 boxylation of oxamic acid in aniline and o-toluidine6 Malonic acid’ that there was a tendency for the reverse reaction to Trichloroacetateions
take place, namely, formamide and carbon dioxide combining to give oxamic acid. In further studies of this phenomenon the following experiments were performed (1) C o t was bubbled into formamideno reaction; (2) COSwas bubbled into a mixture of formamide and aniline-oxamic acid was produced. Evidently aromatic amines act as a catalyst for the reaction, the nitrogen probably coordinating with the aldehydic hydrogen atom of the formamide facilitating COt hation. It was found in the initial studies that the reverse reaction could be largely circumvented by employing a relatively large volume of solvent and using a gentle stirring action. Also no appreciable difference in the specific reaction velocity constant a t constant temperature was detected when the volume of solvent was varied between 50 and 135 g. Similar results were obtained in the present studies. No significant difference in the velocity constant was observed on carrying out the experiment in quinoline a t 153.45”,using 50, 100 and 135 g. of solvent. In the experiments in quinoline and 8methylquinoline there was a tendency for the reverse reaction to take place. However, on using large volumes of solvent (100-135 g.) and gentle stirring the forward reaction in most cases was found to go to 99% completion before oxamic acid crystals began to collect in the condenser. The rate of decarboxylation of oxamic acid was measured in quinoline and in 8-methylquinoline over a range of about 20”. The plot of log(V, Vt) vs. t was linear over nearly the entire experiment indicating that the reaction is first order. The average rate constants calculated in the usual manner from the slopes of the experimental logarithmic plots are shown in Table I. The parameters of the Eyring equation, based upon the data in Table I, are shown in Table 11, along with corre-
OC. 00’;.
47.0 38.9 26.7 24.0
8-Methylquinoline
AS*, e.u.
+37.5 I-15.8 2.3 - 2.4
-
AH*,
kcal.
36.0 37.7 24.4 22.3
AS*, e.u.
+12.2 I-13.7 -10.5 - 8.4
Discussion of Results In the decarboxylations of malonic acidgand the trichloroactate ionla as well as oxalic acidla intermediate compound formation between solute and solvent has been shown to take place. That the same is true for the oxamic acid reaction is clearly demonstrated by the data in Table 11. It will be observed in column I of Table I1 that, for the reaction in quinoline, the AH+ of the reaction decreases on passing from oxamic acid to oxalic acid, from oxalic acid to malonic acid, and from malonic acid to the trichloroacetate ion. This is the order that would be predicted on the basis of the principle that an increase in the attraction between two reactants lowers the activation energy of the reaction.1° The effective positive charge on the polarized carbonyl carbon atom of the reactant, involved in coordination with the solvent, increases on passing from oxamic acid to oxalic acid (since thc +M effect of the amide group is greater than that of the hydroxyl),ll from oxalic acid to malonic acid (since the methylene group tends to prevent the transmission of inductive effects between the two terminal carboxyl groups),12and from malonic acid to the trichloroacetate ion (due to the strong negative inductive effects of the three a-halogen atoms). It is well known that oxalic acid and malonic acid associate through hydrogen bonding to some extent, past the dimer stage, to form “supermole(7) L. W. Clark, ibid., 62, 600 (1958). (8) L. W. Clark, ibid., 63,99 (1959). (9) G.Fraenkel, R.Belford and P. E. Yankwich J . Am. Chem. Soc., P B , 15 (1954). (10) K. J. Laidler, “Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, N. Y., 1950,p . 138. (If) A. E. Remick, “Electronic Interpretations of Organic Chemis try,” John Wiley and SOM,Ino., New York, N. Y.,2nd Ed., 1949.P. 57. (12) R. Q. Brewster, “Organic Chemistry,” 2nd. ed., PrenticeHall, Inc., New York, N. Y.,1953,p . 581.
April, 1961
DEChRBOXYLATION OF OX2%hlICACIDI N
cules” composed of 3-4 single molecules each.13 However, oxamic acid, being a very weak acid, and having only one carboxyl group, probably does not associate thus to any appreciable extent. Since there is very little difference in size between oxalic acid and oxamic acid the A S values for their reactions in qilinoline would be expected to be approximately equal, .provided the transition complex in each case were composed of the same number of units. It is seen, however, in Table 11, column 11, that the AS* for the reaction in quinoline is considerably greater for oxamic acid than it is for oxalic acid. We have here additional evidence for the inference that a single molecule of oxamic acid is involved in the formation of the transition complex, whereas, in the case of oxalic acid, a “supennolecule” cluster composed of 3 4 single molecules is involved.6 The methyl group in 8-methylquinoline has two effects: (1) a positive inductive effect which releases electrons and increases the effective negative charge on the tertiary nitrogen atom, and (2) a steric effect which offers hindrance to the approach of an electrophilic agent to the nitrogen. If the rate-determining step of a reaction involves the coordination of an electrophilic agent with a nucleophilic agent, effect (1) will cause a decrease in the AH* of the reaction, whereas effect (2) will cause a decrease in the AS* of the reaction on going from quinoline to 8-methyl-quinoline. It will be seen in Table I1 that, on passing from quinoline to 8methylquinoline, a considerable decrease in both AH* and AS*: takes place for the decarboxylations of oxamic acid, malonic acid and the trichloroacetate ion. These results strongly substantiate the proposed transition-complex mechanism for the decarboxylation of oxamic acid in polar solvents. In the case of oxalic acid the decrease in AH and A S + is very slight in comparison with the behavior of the other three compounds. This anomaly has been expkned as being the result of the circumstance that, in tlhe reaction in quinoline, un-ionized oxalic acid is involved, whereas, in 8-methylquinoline, one of the acid hydrogens is ionized and the acid oxalate ion undergoes decarb~xylation.~ If, in 8-rnethylquinoline, the acid oxalate ion
*
*
(13) W. Hiickol. “Theoretical Principlea of Organic Chemistry,” Vol. 11, Elaevier Publiahing Co., New York, N. Y.,1958, p. 329 e l seq.
QUINOLINEAND
I N 8-METHYLQUINOLINE
661
suflers cleavage, it would be expected that the ion would form a transition complex with the solvent involving the un-ionized carboxyl group. Furthermore, having only one un-ionized carboxyl group, the ion would not be expected to form an association cluster or “supermolecule” through hydrogen bonding. The second ionization constant of oxalic acid being much smaller than the first, the effective positive charge on the carbonyl carbon atom of the ion involved in coordination with the solvent would be smaller than in the case of the diacid, resulting in the reaction’s requiring a higher activation energy. A single molecule of oxamic acid will have approximately the same size as a single acid oxalate ion. Since oxygen is slightly smaller than nitrogen, the acid oxalate ion will be actually somewhat smaller than oxamic acid. This will mean that the AS* values of the two reactions should be approximately equal, being somewhat larger for the acid oxalate ion than for the un-ionized oxamic acid. It will be seen in Table 11,lines 1and 2, columns 3 and 4, that all these deductions are verified completely by the experimental data. The fact that the AH* values of the two reactions are very nearly equal indicates that the two substances have very nearly the same acid strength, the oxamic acid being a slightly stronger acid than the acid oxalate ion. On the basis of the results reported herein it appears highly probable that (1) oxamic acid forms a transition complex with nucleophilic solvents ; (2) the complex consists of one molecule of solvent plus one molecule of un-ionized oxamic acid; (3) coordination takes place between the polarized, electrophilic, carbonyl carbon atom of the unionized oxamic acid and an unshared pair of electrons on the nucleophilic atom of the solvent molecule; (4) the acid oxalate ion, in 8-methylquinoline, behaves in a manner quite analogous to oxamic acid, and ( 5 ) oxalic acid and malonic acid, in weakly basic solvents, associate to form a cluster or “supermolecule” composed of 3-4 single molecules, one end of which coordinates with the nucleophilic agent. Further work on this problem is contemplated. Acknowledgment.-The support of this research by the National Science Foundation, Washington, D. C., is gratefully acknowledged.
