KIXETICS AND STOICHIOMETRY OF GAS-PHASEDECOMPOSITIOS OF OXALIC ACID
1863
Kinetics and Stoichiometry of the Gas-Phase Decomposition of Oxalic Acid
by Gabriel Lapidus,‘ Donald Barton,2 and Peter E. Yankwich Noyes Laboratory of Chemistry, University of Illinois, Urbana, Illinois
(Received February 6 , 1964)
The decomposition of oxalic acid vapor a t 0.9 mm. pressure has been studied over the range 127-157’. The only products found are equimolar quantities of carbon dioxide and formic acid, and the kinetics of the decomposition are first order with respect to oxalic acid. The observed hrrhenius parameters are: E = 30.0 f 1.3 kcal. mole-’, and log ( A , see. -I) == 11.9 =t 0.7. ‘The LLlow-iiorinal”pre-exponential factor, modest activation energy, and tentative evidence for absence of a major free-radical component to the mechanism are viewed as supporting assumption of a uniniolecular mechanism via a cyclic activated complex. Comparison is made to results on the solution decomposition.
Introduction Opportunities to study the rate of a given reaction in different phases are sufficiently rare as to be seized when discovered or hcreated wherever possible. There is a coiisiderable literature on the therinal deconiposition of solid oxalic a ~ i d , ~and - ~ solutions of the acid in water,6-8 concentrated and fuming sulfuric acids, 9-12 and various organic s ~ l v e n t s ’ ~ - l two ~ ; studies have been made of the carbon isotope fractionation accompanying the decomposition in ca. lOOyo sulfuric acid.I6l7 The solid acid and its solutions in organic solvents deconipose to carbon dioxide and formic acid; formic acid product is about half dehydrated to carbon monoxide where the acid is dissolved in water, this process being complete in concentrated sulfuric acid solvents. The rate of decomposition is kinetically first order with respect to oxalic acid in all the systems mentioned above. I n this considerable literature, there appears to be no mention of the decomposition of oxalic acid in the vapor phase except in a paper by Koyes and Wobbe‘* on the vapor pressure of the solid; a t temperatures near loo’, they observed the decomposition of the vapor to be sufficierttly rapid that a flow method instead of a static method had t o be employed for the vapor pressure measurements. This paper reports results obtained in the first part of a n investigation of the deconipositions of ordinary and isotopic oxalic acids in the gas phase.
Experimental Reagent. Fisher “Analytical” grade anhydrous oxalic acid was purified further by vacuum sublimation a t 110’; samples thus treated were stored in vucuo over magnesium perchlorate. Permanganate titrations showed the material to be pure within an analytical error of approxiinately 0.02770. Appamtus. The apparatus consists of four parts : (1) Research Associate, 1980-1963. (2) Visiting Assistant Professor, 1980-1962. (3) D. E. Wobbe and W. A. Noyes, Jr., J . Am. Chem. Soc., 48, 2858 (1928). (4) V. V. Korshak and S. V. Rogoahin, Dokl. A k a d . N a u k S S S R , 76, 539 (1951).
(5) V. V. Korshak and S. V. Rogoahin, Iznest. Akad. N a u k S S S R , Otd. K h i m . N a u k , 531 (1952). ( 8 ) A. Dinglinger and E. Schroer, 2 . physik. Chem., A179, 401 (1937). (7) A. Dinglinger and E. Schroer, ibid., A181, 375 (1938). (8) I. Liitgert and E. Schroer, ibid., A187, 133 (1940). (9) G. Bredig and D . M. Lichty, 2 . Elektrochem., 12, 459 (1908). (10) D. M . Lichty, J . Phys. Chem., 11, 225 (1907). (11) E. 0. Wiig, J . Am. Chem. Soc., 52, 4737 (1930); a particularly useful summary. C . 1%’. Deane, ibid., 67, 329 (1945). L. W. Clark, ibid., 77, 8191 (1955). L. W. Clark, J . Phys. Chem., 61, 699 (1957). L. M’. Clark, ibid., 62, 633 (1958). J. G. Lindsay, D . E. .McElcheran, and H. G. Thode, J . Chem. Phys.. 17, 589 (1949). (17) A. F r y and &I. Calvin, J . Phys. Chem., 56, 897 (1952). (12) (13) (14) (15) (18)
(18) W. A. Noyes, Jr., and D . E. Wobbe, J . Am. Chem. Soc.. 48, 1882 (1926).
Volume 68, S u m b e r 7 J u l y , 1964
1864
an injector, from which a sample of oxalic acid is sublimed into a storage section, held at a temperature at which decomposition of the acid is very slow; a reactor; and a collector into which the reaction products are frozen. Except for the collector trap, all parts of the apparatus are heated, the temperatures of each subsection being regulated carefully to prevent condensation of oxalic acid, to prevent unwanted decomposition of the reagent, to establish the desired decomposition temperatures, etc. The injector temperature was brought to 130 f 2", while the storage section was maintained at 115.5 i 0.5". Reactor temperatures were constant to about 0.1 ". The storage section was made from a Pyrex flask of about 22 1. volume, and was heated by an air furnace. The reactor, made from a Pyrex bottle of approximately 8.5 1. volume, was jacketed in aluminum 2.5 cm. thick, the ends being covered with asbestos paper and tape. All sections of this envelope were parts of a large number of controlled heaters. Procedure of Kinetics R u n s . Typically, 7 2 mg. of reagent was placed in the injector for sublimation into the storage section. Because a relatively high injection temperature was used to ensure rapid sublimation of the reagent, 1.2 f 0.3yo deconiposition occurred during injection. Correction is made for this decompositjon in treating the rate data. Partition of the charge between the storage section and the reactor delivered a n average of 19 mg. of oxalic acid to the latter. At the end of the preselected reaction period, the decomposition was quenched by freezing the contents of the reactor into the collector trap. The probable maxiinuni error in times recorded (due to the arbitrariness of allowed partition and trapping times) is 15 sec. or less. Procedure of Separations and AnaEyses. Products and undecomposed reagent are separated by diff erential distillation, formic acid product is burned over copper oxide to carbon dioxide for quantity deterniination, and oxalic acid is estimated by permanganate titration. Over-all material balance can be proved to f 1.4Cr,, the over-all analytical error. Results The formic acid product found averages 99.5 f 0.5y0 of the carbon dioxide produced; equivalence of the carbon dioxide produced and the oxalic acid decomposed is demonstrable to *l.Oyo, the mean composite analytical error. The same results are obtained when decomposition of the vapor is carried out at 180"; further, mass spectrometrjc examination of the product gases yields no evidence requiring inclusion of H, among the products. The Journal of Physical Chemintry
G. LAPIDUS,D. BARTON, AND P. E. YANKWICH
Twenty kinetics runs were carried out at temperatures between 126.6 and 155.6"; in each case the initial pressure of oxalic acid vapor was 0.88 f 0.02 nini. Examination of the data indicates that the rate is first order with respect to oxalic acid, the quality of the results being such that the accuracy of this order is approximately f0.1, Invividual and average values of the apparent first-order specific rate constant, kl (set.-'), are shown in Table I. Table I : Rate of Decomposition of Oxalic Acid Vapor Run temp., OC.
126.6
Time, see.
Degree of decompn., cor.
18,000 25,200 36,600
0.403 ,516 669
IC, (sec.-1) X 106
Av. 134.1"
4,200 14,400
21,600
146.4
1 800 4,200 7,200
11,400 155.6
1,715 2,015 3,350
2.86 2.88 3.02 2 , 9 2 1 t 0 07
,288 ,289 ,677 ,684 ,684 ,671 ,822 828 824
8.08 8.14 7.84 8.00 8.00 7.71 7.99 8.14 8.03 Av. 7.99 f 0.10
286 ,515 ,708 ,687 ,854 Av.
18.7 17.2 17.1 16.1 16.9 17.2f0.6
Av.
50.7 49.8 51.2 50.6f0.5
,585 ,634 817
a Three additional runs were made a t 134.1' which were carried only to 0.05-0.1 decomposition. The calculated k values were obviously deviant (7.19-8.89 X IO5), and exceed the limits for rejection by the Chauvenet criterion.
The magnitude and variation with temperature of = 30.0 f 1.3 kcal. inole-' and log ( 4 , sec.-l) = 11.9 f 0.7; these Arrhenius parameters for the vapor-phase decomposition, the transition state theory activation quantities, and related data for the various solution decompositions, are summarized in Table 11.
IC, correspond to E
Discussion The pre-exponential factor derived from these experiments lies in the lower part of the "normal" range
KIKETICSASD STOICHIONETRY OF GAS-PHASE DECOMPOSITIOS OF OXALIC ACID
1865
Table I1 : Calculated Activation Parametere kl (sf%-') X 105, at
E'"
This research: Gas phase Previous work; solvent: Aniline" N,N-Dimethylanilinec Dimethyl sulfoxide" Dioxaned Glycerine' N-Methylanilinec 6-Methylq~inoline~ 8-MethylquinolineB Quinoline' Triethyl phosphate' Waterd
log ( A , sec.-1)
4H*a
30.0 f 1.3
11.9 i 0.7
29.2f 1.3
41.45 33.10 40.65 32.30 27.2 35.85 33,14 38.45 40.13 29.95 37 57
18.16 13.53 17.48 13.45 11.79 14.76 13,77 16.36 17.00 12.24 15.47
40.28 32.40 40.60 31.5& 26.4 35.50 *32,26 37.65 38.90 28 85 36.8e
4F*a
4S*b
-6.6 f 3.0 +16.2 1.25 20.7 0.4 -7.8 +8.25 1.22 13,65 -15.75 -5.8 $9 6
140'
31.9 f 1.8
11.9
33.58 31.90 31.05 31.3 29.6 32.10 31.76 32,Ol 45.40 31.25 32.8
14.5 9.5 9.5 23.4 251 5.8 17.4 11
5.0 22.0 40
a Kcal. mole-'. Cal. mole-' "K.-l. Ref. 14. Ref. 8. e This and remaining quantities on this line calculated by us from data in the original article of Liitgert and Schroer. Ref. 13. Ref. 15.
of 1012-1014sec.-1,19~20 and the corresponding entropy of activation indicates only slight restriction of iiiotjoii in the transition state compared with the normal molecules. It is clear from examination of Table I1 that the decomposition behavior of oxalic acid in the gas phase is similar to that of the acid in those solvents (particularly triethyl phosphate, but including, perhaps, n-,S-diiiiethyIaniline, dioxane, glycerine, and 6-methylquinoline) expected or found14 to be kinetically inert toward it. We have not established the presence or absence of a heterogenous com~ponent in the gas-phase decomposition, nor conducted experiments specifically to demonstrate the preseiice or absence of a free-radical component in the decomposition inechaiiism, and our ability to give a detailed interpretation of our findings is thus limited. However, the relatively low actiSation energy for a reaction which ultimately involves both H-0 and C-C bond ruptures, the first-order kinetics, and the siniplicity of the stoichiometry between 127 and 180", place severe restrictions on the choice of mechanism. We believe the results a t hand lend mild support to the assumption of a unimolecular decomposition niechanism involving intramolecular hydrogen transfer through the formation of a cyclic activated complex such as
HO
0
\
//
/
\\
c . .. . c
0
0
the break-up of which is followed by rapid transfer of hydrogen to carbon in the iiforniic acid" fragment of the original molecule. I t may be that direct hydrogen transfer to carbon occurs, but the entropy of activation would seem to be somewhat high for the formation of a four-membered ring. To perinit more detailed exposure of the reaction mechanism, we have undertaken rate studies on hydrogen and carbon isotopic oxalic acids.
Acknowledgments. This research was supported by the U. S. Atomic Energy Commission.
~
~~-
(19) S. W. Renson, "The Foundations of Chemical Kinetics," McCraw-Hill Book Co., Inc., New York, N. Y., 1960, pp. 258264. (20) B. G. Gowenlock, Quart. Rev. (London), 14, 133 (1960).
Volume 68. I\'umber 7
J u l y , 1964
