DECARBOXYLATION OF METHYLMALONIC AND OCTADECYLMALONIC ACID
energy of formation, AG011'13, of - 232 f 15 kea1 is obtained for ThJ',. Acknowledgment. The authors are pleased to acknowledge the contributions of J. G. Davis in the per-
2523
formance of many of the emf measurements, of J. E. Mock in the preparation of the samples, and of M. A. VanCamp in the performance of the vacuum fusion analysis.
The Kinetics of the Decarboxylation of Methylmalonic Acid and Octadecylmalonic Acid in the Molten State
by Louis Watts Clark Department of Chemistry, Western Carolina College, Cullowhee, North Carolina (Receited Februarv 7, 1966)
Kinetic data are reported for the decarboxylation of methylmalonic acid and octadecylmalonic acid in the molten state at several different temperatures between 135 and 160". The activation parameters for the decarboxylation of these compounds are calculated and compared with those previously reported for the decarboxylation of malonic acid, n-butylmalonic acid, and n-hexylmalonic acid.
In the past, kinetic studies have been carried out in this laboratory on the decarboxylation of malonic acid' and several of its derivatives in the molten state, namely, benzylmalonic acid,1 n-butylmalonic acid, and n-hexylmalonic acid.a In order to try to establish a better insight into the effect of substituents on the reaction, the decarboxylation of two additional derivatives-methylmalonic acid and octadecylmalonic acidhave been carefully investigated in this laboratory. The results of this investigation are reported herein.
Experimental Section Apparatus and Technique. The decarboxylation of methylmalonic acid and octadecylmalonic acid in the molten state was studied in the same manner as was that of' picolinic acid14 etc. A fragile glass capsule containing a weighed sample of the acid was introduced in the usual manner into the reaction vessel connected by a standard taper joint to the reflux condenser. The condenser in turn was connected to a water-jacketed buret (calibrated by the U. S. Bureau of Standards) equipped with a leveling bulb filled
with the entraining liquid. The reaction flask was immersed in a constant-temperature oil bath, the temperature of which was controlled to within k0.005"by the use of a transistorized temperature control unit equipped with a sensitive thermistor probe. The thermometer used to measure the temperature of the oil bath was one which had been calibrated by the U. S. Bureau of Standards. A constant-temperature water circulator controlled the temperature of the water in the water jacket to *0.05". Reagents. The methylmalonic acid used in this research assayed 99.4% pure by neutralization equivalent. Based on the volume of COz evolved in the decarboxylation experiments, the purity of the octadecylmalonic acid was found to be 99.0%. Its melting point was 123.5". A sample of methylmalonic acid weighing 0.2122 g or a sample of octadecylmalonic (1) L.W.Clark, J. Phys. Chem., 67, 138 (1963). (2) L. W. Clark, ibid., 68, 587 (1964). (3) L. W. Clark, ibid., 6 7 , 2602 (1963). (4) L.W. Clark, ibid., 66, 125 (1962).
Volume 70, Number 8 August 1966
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LOUISWATTSCLARK
acid weighing 0.6406 g was used in each experiment. These are the weights of methylmalonic acid or octadecylmalonic acid, respectively, required to furnish 40.0 ml of C02a t STP on complete reaction, based upon the actual molar volume of C02 at STP, namely, 22,264 ml.
Results The decarboxylation of methylmalonic acid and of octadecylmalonic acid in the molten state was studied a t five different temperatures over a 20” range. Two or three experiments were performed a t each temperature. The rate constants for the various temperatures were obtained from the slopes of the experimental logarithmic plots. Values thus obtained in different experiments at the same temperature differed from one another by no more than 1%. Average values of the rate constants obtained in this research are shown in Table I. In Table I1 are listed the activation parameters in the absolute reaction rate equations
based upon the data in Table I. Activation parameters in the case of the decarboxylation of eeveral other acids, reported previously, are included in Table I1 for comparison purposes.
Table I1 : Activation Parameters for the Decarboxylation of Several Acids in the Molten State AH
*,
AF
Ass,
*Iso,
Reactant
kcal/ mole
eu/mole
kcal/ mole
Malonic acid“ Methylmalonic acidb n-Butylmalonic acidc n-Hexylmalonic acidd Octadecylmalonic acidb Oxanilic acid6 Oxalic acid (vapor phase)’
35.8 34.96 32.2 32.2 30.6 40.1 29.2
11.9 9.6 2.9 2.8 -1.3 21.4 -6.6
31.1 31.2 31.06 31.1 31.1 31.7 31.8
“ See ref 1.
* This research.
See ref 2.
W. Clark, J . Phys. Chem., 66, 1543 (1962). and P. E. Yankwich, ibid., 69, 1729 (1965).
See ref 3. * L. M. A. Haleem
the chain length reaches four carbon atoms. The parameters for the decarboxylation of n-hexylmalonic acid are very nearly equal to those for the decarboxylation of n-butylmalonic acid. On going from an alkyl group with four carbon atoms to one with eighteen carbon atoms (from n-butylmalonic acid to octadecylmalonic acid), the enthalpy of activation decreases by only 1.6 kcal/mole and AS* decreases by only 4.2 eu/mole. This is in marked contrast to the changes
a’
40
Table I: Specific Reaction Velocity Constants for the Decarboxylation of Methylmalonic Acid and Octadecylmalonic Acid in the Molten State
O C (cor)
k X 10’. 8ec -1
135.36 140.12 144.95 149.20 154.97 140.78 145.46 149.51 153.87 160.96
2.06 3.44 5.60 8.79 15.5 3.06 4.68 6.70 9.70 18.3
Temp, Reactant
Methylmalonic acid
Octadecylmalonic acid
-5
I
I
I
I
0
6
10
15
1 20
AS*, eu/mole.
Discussion In all four cases shown in Table 11, the alkyl moiety on the central methylene group of malonic acid lowers both the enthalpy of activation and the entropy of activation, pointing to the inductive and steric effects of the substituent. It will be noted, however, that there is a very rapid falling off of both effects after The Journal of Physical Chemistry
Figure 1. Enthalpy of activation vs. entropy of activation plots. Line I: Decarboxylation of oxalic acid in the vapor phase, and for molten oxanilic acid. Line 11: Decarboxylation of malonic acid and its n-alkylated derivatives in the molten state. Slope of both lines: 393’K. ~~
(5) S. Glasstone, K. J. Laidler, and H. Eyring, “The Theory of Rate Processes,” McGraw-Hill Book Co., Inc., New York, N. Y., 1941, p 14.
NMRSTUDY OF POLY(VINYL CHLORIDE)
produced by the first four carbon atoms (a decrease in AH of 3.6 kcal/mole, a decrease in Ah'* of 9 eu/mole). Stated in a different manner, the inductive and steric effects produced by the Jirst four carbon atoms appear to be more than twice those produced by the next fourteen carbon atoms. It will be observed in column 4 of Table I1 that the free energies of activation at 120" for the decarboxylation of malonic acid and its n-alkyl derivatives are fairly constant and equal to about 31.1 kcal/mole. This means that at this temperature the rate constant for the decarboxylation of malonic acid apparently is not affected by the presence of an n-alkyl moiet,y regardless of chain length. The free energies of activation at 120" for the decarboxylation of oxalic acid (vapor) and oxanilic acid (melt) are slightly greater than those for the malonic acid group and also fairly constant (average 31.75 kcal/mole). This means that the rates of decar-
*
2525
boxylation of these two compounds are approximately equal at 120" and less than those for the malonic acid series. Substituting the free energies of activation at 120" for these two different groups of compounds in the absolute reaction rate equation5 enables the calculation of the rate constants to be made. For the malonic acid group, k1200 in sec-I turns out to be 0.00042, for the oxalic acid group, 0.00017. In other words, at 120°, malonic acid and its derivatives will suffer decarboxylation 2.5 times as fast as will oxalic acid (vapor) or oxanilic acid (melt). Figure 1 is a plot of the enthalpy us. entropy of activation for the two groups of compounds listed in Table
11. Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research.
Nuclear Magnetic Resonance Study of Poly(viny1 chloride)
by Kermit C. Ramey The Atlantic Refining Co., Research and Development Department, Glenolden, Pennsylvania (Received February 14,1966)
The high-resolution nuclear magnetic resonance spectra of poly(viny1 chloride) yielded, upon examination at 60 and 100 Mc/sec, information on the various tactic forms present. The spectra are discussed in terms of the various conflicting analyses which have previously been proposed and an alternate analysis is suggested. The resonance of the 0 protons is interpreted in terms of a triplet centered at r 7.75, corresponding to isotactic diads, and two overlapping triplets centered at r 7.91 and 7.95, corresponding to syndiotactic diads, which in turn are attributed to the three possible tetrad configurations.
Introduction High-resolut,ionnuclear magnetic resonance spectrescopy has proved to be an effective tool for studying the stereochemical configuration of vinyl polymers.'-5 I n Some cases' that Of poly(methyl (PMM),' and poly(viny1 methyl ether) (PVME),2 the
spectra yield an unambiguous determination of the cases, such that Of Poly-
tactic content* In Other
(1) F. A. Bovey and G. V. D. Tiers, Fortschr. Hochpolymer. Forsch., 3 , 139 (1963). (2) K. C. Ramey, N. D. Field, and I. Hasegawa, J. Polymer Sci., ~ 2 865 , (1964).
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