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LOUISWATTSCLARK
Vol. 67
KINETIC STUDIES OK THE DECARBOXYLATION OF SEVERAL UNSTABLE ACIDS IN THE MOLTEN STATE BY LOUISWATTSCLARK Department of Chemistry, Western Carolina College, Cullowhee, N . C. Received July 2, 1962 Kinetic data are reported on the decarboxylation of anthranilic acid, p-aminobenzoic acid, benzylmalonic acid, and malonic acid in the molten state. The malonic acid study was undertaken in order to up-date previous data reported 42 years ago. An enthalpy-entropy plot of the data for these four acids plus those for picolinic acid and oxanilic acid reported previously yielded two straight lines. The isokinetic temperatures were calculated from the slopes of the lines and calculated AFo values were found to agree with the theoretical. Information regarding the nature of the activated states was deduced based upon the position of any acid of a given series on one of the lines. The malonic acid data were found to be at variance with those reported previously, indicating that older kinetic data in the literature are in need of rechecking.
Numerous kinetic studies have been carried out by many investigators on the decarboxylation of a large number of unstable acids in a variety of solvents.’ Although data on the decarboxylation of the free acids in the molten state or in inert solvents are needed also in order to obtain a more complete understanding of the effect of solvent and structure on the reaction, i t appears that relatively few such compounds have been investigated. These include malonic acid,2 trichloroacetic acid,a oxanilic and picolinic acid.6 The present paper reports results of kinetic studies which have been carried out in this Laboratory on the decarboxylation of three additional acids in the molten state, namely, anthranilic acid, p-aminobenzoic acid, and benzylmalonic acid. Efforts to correlate the data obtained in this research with those for the decarboxylation of malonic acid reported by Hinshelwood2 in 1920 were unsuccessful, and prompted a recheck of the behavior of this compound using the more up-to-date equipment now available. The data for the malonic acid reaction thus obtained are included in this report. Experimental Reagents.-The benzylmalonic acid and anthranilic acid used in this research were highest purity or reagent chemicals and were used without any further purification. The p-aminobenzoic acid was technical grade. However, before use, it was carefully recrystallized from ethanol, and a pure, dry sample of m.p. 187188’ was easily obtained. The malonic acid was reagent grade, 100.O~oassay, m.p. 134.0’. The benzylmalonic acid, anthranilic acid, and p-aminobenzoic acid used yielded the theoretical amount of COS on complete decarboxylation, but in the case of malonic acid t2he additive vapor pressure of the acetic acid released during decarboxylation caused a slight increase in the volume toward t,he end of the reaction. Apparatus and Technique .-The apparatus employed in the present investigation was the same as that which has been used in previous studies.6 It consists of a thermostated oil bath provided with a thermometer calibrated by the U. S. Bureau of Standards. The steam point of the thermometer is carefully rechecked a t frequent intervals to ensure reliability. The reaction vessel was the same as that used in studying the decarboxylation of molten oxanilic acid4 and picolinic acid.5 It was connected by standard taper joints to a condenser, the condenser being connected to the water-jacketed buret by a short (1) For reviews, cf., E. F. Gould, “Mechanism and Structure in Organic Chemistry,” Henry Holt and Company, New York, E’.Y., 1959, pp. 346 ff., S. L. Frioss and A. Weissberger, Ed., “Technique of Organic Chemistry; Volume VIII, Investigation of Rates and Mechanisms of Eeactions,” Interscience Publishers, Inc., Kew York, N. Y., 1st Ed., 1953, pp. 382 IT.: J. Hine, “Physical Organic Chemistry,” McGraw-Hill Book Co., Inc., New York, N. Y., 19.56, pp. 283 ff. (2) C . N. Hinshelwood, J. Chern. Soc., IlT, 156 (1920). (3) L. W. Clark, J. Am. Chem. Soc., 77, 3130 (1955). (4) L. W. Clark, J. Phys. Chern., 6 6 , 1543 (1962). ( 5 ) L. W. Clark, ibid., 66, 125 (1962). (6) L. W. Clark, ibid., 60, 1150 (1956).
rubber hose. A piece of flexible tubing joined the buret to a leveling bulb holding an entraining liquid. This liquid consisted of a solution 20% by weight of sodium sulfate and 5% by volume of sulfuric acid in which COXis insoluble. The vapor pressure of this solution was calculated using Raoult’s law and activity coefficients of the two solutes. It was found to be 91.0% of the vapor pressure of pure water. I n converting the observed gas volumes to STP, the vapor pressure of this solution was subtracted from the observed corrected barometric pressure. I n each decarboxylation experiment a sample of the required acid was used which would yield 40.0 ml. of COz on complete reaction.’ The weights of these samples in grams were as follows: malonic acid, 0.1870; benzylmalonic acid, 0.3489; anthranilic acid and p-aminobenzoic acid, 0.2464.
Results The decarboxylation of anthranilic acid, p-aminobenzoic acid, and benzylmalonic acid was studied a t three different temperatures over a 10-20’ temperature range. The experiments were performed two or three times a t each temperature. The evolved COz (converted to STP) was plotted against time for each experiment. The plot of log (Bm- Vt) us. time, taken from representative points on the smoothed experimental plots, yielded straight lines over most of the reaction in each case. The decomposition of molten malonic acid was studied a t four different temperatures between 140 and 150’. At higher temperatures the reaction was too rapid to be measured with precision and lower temperatures were impractical because of the slowness of the melting. Two or three experiments were performed a t each temperature. As in all rate studies, the beginning of the reaction was somewhat erratic, the first few milliliters of evolved COzoccurring prior to the complete melting of the sample. This necessitated an extrapolation back to zero time for the first 5-lOy0 of the reaction. I n a typical experiment with malonic acid, for example, the sample was added to the reaction flask at 148.03’ (cor.) and was completely melted in 5 min. No readings were taken before the melting was complete. At this time 1.4 ml. of COI (uncorrected) were evolved. To complicate the situation further, as acetic acid accumulated from the reaction, its vapor pressure became appreciable toward the end of the experiment so that a linear logarithmic plot was obtainable only over about the first 60-70y0 of the reaction. Figure 1 is a typical plot of the experimental data, showing the decomposition of molten malonic acid a t 150.93’ (cor.). A slight downmsrd trend of the logarithmic plot toward the end of the reaction is evident. (7) These weights were calculated, based not upon the ideal gas lawir, but upon the actual molar volume of COz, 22, 264 ml. a t STP.
DECARBOXYLATION OF UNSTABLE ACIDSIN MOLTEN STATE
Jan., 1963
1.5 1.4 1.3 1.2
< :
,
8
1.1
-6.3
s"
b bo
1.0 3
-66.2 -6.1
1.39
I-
c-
0.9
0.8
2
4
6
8 10 12 Time (min.).
14
16
18
Fig. 1.-Experimental data for the decarboxylation of 0.1860 g. of malonic acid a t 150.93' (cor.): I, volume of COZ at STP (ml.); 11, log (V, - VJ.
The average values of the apparent first-order rate constant for each compound a t each temperature were obtained from the slope of the experimental logarithmic plot. The values thus obtained are shown in Table I. An Eyring plot of the kinetic data for the decarboxylation of malonic acid a t the four temperatures studied is shown in Fig. 2. The parameters of the Eyring equation, based upon the data in Table I, are shown in Table 11, along with comparative data previously reported for oxanilic acid and picolinic acid.
236
Fig. 2.-Eyring
238 240 1/T X 10'.
242
244
246
plot for the decarboxylation of molten maloinic acid a t different temperatures.
TABLE I APPARENTFIRST-ORDER RATECONSTANTS FOR THE DECARBOXYLATION OF SEVERALUNSTABLE ACIDSIN THE MOLTENSTATE Temp., "C. (cor.)
Acid
Malonic acid
Benzylmalonic acid
Anthranilic acid
p-Aminobenzoic acid
x
k:
104,
8ec. -1
139.63 144.40 146.03 149.93 141.03 150.78 161.12 171.57 181.78 191.65 189.64 194.59 200.11
3.87 6.39 7.34 12.7
7.37 17.0 40.2 3.69 6.51 11.04 7.62 9.75 14.12
Av. dev.
10.03 f .04 f .04 f .05 f .04 .05 f .1 f .02 f .02 1 .05 1.04 f .04 rt: .05
TABLE I1 KINETIC DATA FOR STABLE
Acid
THE DECARBOXYLATION OF SEVERALUNACIDSI N THE MOLTENSTATE
AH*
AS*
(koal./mole)
(e.u./mole)
Oxanilic acid4 Picolinic acid' Malonic acid" Benzylmalonic acid p-Aminobenzoic acid Anthranilic acid The data of Hinshelwood (ref. AH* = 33.0 and A S * = $4.5.
$21.4 39.8 4-13.2 35.8 $11.9 29.4 - 2 6 24 9 -19.9 21.6 -26.5 2) for this reaction yidd
40.1
Discussion of Results It is not surprising to find that the data obtained in this present investigation on the decarboxylation OF molten malonic acid (see line 3 of Table 11) differ
- 20
-
10 0 A S * (e.u./mole).
+lo
+20
Fig. 3.-Enthalpy-entropy plot for the decarboxylation of several acids: I, amino acids (anthranilic, p-aminobenzoic, and picolinic); 11, keto acids (benzylmalonic, malonic, and oxanilic).
somewhat from that reported by Hinshelwood 42 years ago. This result points out the fact that many of the older kinetic data in the literature are in need of careful rechecking. It has been well established that the mechanism of the decarboxylation of malonic acid in basic type solvents is a bimolecular reaction, the rate-determining step being the formation of an intermediate complex between solute and solvent species.* It is logical to assume that the decarboxylation of the molten acid is bimolecular also, the complex in this case being formed by the coordination of two "supermolecules" of malonic acid-a polarized, electrophilic, carbonyl carbon atom (8) G . Fraenkel, €L L. Belford, and P. E. Yankwich, J . Am. Chem. Soo., 76, 15 (1954).
H. K. BODENSEH AND J. B. RAMSEY
140
of one molecule attracted to the unshared pair of electrons on the nucleophilic hydroxyl oxygen atom of another-both electrophilic and nucleophilic species existing as associated complexes composed of two or more molecules each.g There is no reason to doubt but that an analogous mechanism obtains in the case of the decarboxylation of the other acids listed in Table 1 1 . 4 Figure 3 is an enthalpy-entropy plot of the data shown in Table 11. It will be observed that the data for the two amino acids and the one imino acid fit on one straight line, of slope 46OOK. or 187OC., the so-called isokinetic temperatureIO (line I of Fig. 3). This temperature corresponds to the melting point of p-aminobenzoic acid. The zero intercept of line I in Fig. 3 is 33.9 kcal./mole. This is equal to AHo = AFO, or the free-energy change of the reaction a t the isokinetic temperature. The experimental values of AFOI8p for anthranilic acid, p-aminobenzoic acid, and picolinic acid turn out to be, respectively, 33.8, 34.0, and 33.8 kcal./mole, agreeing well with the theoretical values. Benzylmalonic acid and malonic acid may be regarded as types of p-keto acids, while oxanilic acid is analogous to an a-keto acid. The AH"-AS* plots of these three acids fit on a separate straight line (line I1 of Fig. 3) of slope 445'K. or 172°C. The zero intercept of line I1 is 30.5 kcal./mole, and the AF*1720 values for benzylmalonic acid, malonic acid, and oxanilic acid are, respectively, 30.4, 30.5, and 30.6 kcal./mole. These results suggest the possibility that other substituted malonic acids, substituted oxamic acids, and a- and p-keto acids should also fall on this same line. The position of any acid of a given series on one of the lines in Fig. 3 affords information on the nature of the activated state. We see that picolinic acid has a relatively high activation energy, and also a high probability of formation of the intermediate complex as shown by the large positive value of AS'". The high AH* may be attributed to the relatively weak electron attracting power of the a-imino group, on the one hand, and the weak basicity of the nitrogen due to resonance on the other. The large AX* indicates relatively little association of the picolinic acid molecules or (9) L. W. Clark, J . P l ~ y s Chem., . 64, 692 (1960). (10) J. E. Leffler, J. Org. Chent., 20, 1202 (1955).
Vol. 67
zwitterions. There is a very large decrease in both AH* and AS* on going from picolinic acid to p-aminobenzoic acid. The large negative value of AS* in this case may be ascribed to the association of the p-aminobenzoic acid to form long chain clusters. The low enthalpy of activation may be explained on the basis of the carbonyl group of a zwitterion coordinating with one of the amino nitrogen atoms of a cluster of un-ionized molecules. If an amino group captures a proton the resulti-
ing -XHa group will have an electron attracting effect which will increase the effective positive charge on the carbonyl carbon atom of the carboxylate ion, thus causing a lowering of AH*. A similar explanation undoubtedly holds in the case of the isomer, anthranilic acid. The lower AH* for this reaction is consistent with the closer proximity of the amino group to the carboxyl group, and the decrease in AS* is indicative of an ortho effect. -4comparison of the data for oxanilic acid and picolinic acid shows them to have very nearly equal enthalpies of activation. Picolinic acid is actually smaller than oxanilic acid, and the fact that the former has a lower value of AS* than the latter points to an ortho effect arising from the relative positions of the imino and carboxyl groups in the molecule. Although oxanilic acid has a more complex structure than does malonic acid a comparison of the AX'" values of these two reactions shows that the intermediate complex of the former has the simpler structure. This points again to the tendency of dibasic acids to associate past the dimer stage to form 'Lsupermolecule" clusters composed of an aggregation of molecules.ll The lowering of AS" on going from malonic acid to benzylmalonic acid reflects the large steric hindrance produced by the dangling benzyl group in the latter. The benzyl substituent also produces a rather large lowering of AH -I-as would be anticipated in view of the -I effect of the phenyl moiety. Acknowledgments.-The support of this research by the National Science Foundation, Washington, D. C., is gratefully acknowledged. Virgil Snell carried out the purification of the p-aminobenzoic acid. (11) W. Hilckel, "Theoretioal Principles of Organio Chemistry," Vol. 11, Elsevier Publ. Co., New York, N. Y., 1958, p. 341.
VARIATION I N THE K,-VALUE OF A SALT WITH COMPOSITION OF A BINARY SOLVENT BY H. I
