NOTES
July, 1957 must diminish significantly as the water content of the solvent falls to zero. The fact that the ultraviolet spectra of the tribromide species are relatively insensitive t o changes in solvent composition (see Table I) suggests that the tribromides formed in the anhydrous medium retain sufficient ionic character to display the spectrum typical of tribromide ion. The heat of reaction of bromine and sodium bromide in 75% acetic acid (AH" = -3.4 kcal./mole) has been calculated from the equilibrium constant for the reaction at 25 f 0.2" and a t 2.0 f 0.2". The K value for the reaction a t the lower temperature (100 l./mole) was determined by the same spectrophotometric procedure used in making measurements at the higher temperature.?
1009
3:
281-
'\
0
O
4r4 1 . 3 ~ H
I
1.2 e v
Y
1-14 1.o
(7) The values of t ~ ~ , -obtained in the measurements s t 2" were in good agreement with those obtained at 25O.
I 0.9
THE EFFECT OF TOLUIDINES AND XYLIDINE ON MALONIC ACID BY LOUISWATTSCLARE Contribution /Tom fhe Department of Chemistr , Saint Joseph CoZolleee, Emmitsburg, Marylana! Received March I d , iD67
The literature contains kinetic data on the decarboxylation of malonic acid in the following aromatic amines : aniline, quinoline, pyridine, 2-methylpyridine, 3-methylpyridine and 4-methylpyridine.'-8 The kinetics of this reaction have been investigated in this Laboratory in four additional aromatic amines, namely, 2-methylaniline, 3-methylaniline, 4-methylaniline and 2,6-dimethylaniliiie. The results obtained, which are reported herein, are consistent with the expected effects of ortho, meta and para substituents on the formation of the activated complex. Experimental Reagents.-( 1) Malonic acid, Analytical Reagent Grade, 100.0% Assay; (2) ortho-toluidine, Reagent Grade, b.p. 83-85' (15 mm.); (3) m-toluidine, Reagent Grade, b.p. 92-93' (15 mm.); (4) p-toluidine, Reagent Grade, m.p. 43-44'; (5) 2,6-dimethylaniline, Reagent Grade, b.p. 213-214' (760mm.). Apparatus and Technique.-The kinetic experiments were conducted in a constant temperature oil-bath (f0.1') by measuring the volume of COZevolved at constant pressure, as described in a previous paper in this ~ e r i e s . ~I n each experiment a 0.1857-g. sample of malonic acid (the amount required to produce 40.0 ml. of COZa t STP on complete reaction) was introduced in the usual manner in the reaction flask (a 100-ml. 3-neck standard taper Pyrex Brand flask containing 50 ml. of solvent saturated with dry COz gas). Temperature measurements were made using a thermometer calibrated by the Bureau of Standards, graduated in tenths of a degree, to which appropriate stem corrections and other necessary corrections were applied.
4
0.8
0
I1
0
50
100 150 200 250 300 350 Seconds. Fig. 1.-Experimental data, decomposition of 0.1857 g. of malonic acid in 50 ml. of m-toluidine a t 136.6' (cor.): I, ml. Cot at STP: 11,log ( a - 2).
of the reagent before decarboxylation could become effective. When the corrected volume of C 0 2 was plotted against time and graphs made of log (a - x) vs. t (where a is the maximum theoretical volume of C02 and x is the volume evolved at time t) from representative points on the smoothed experimental plots, straight lines resulted for the first 75% of the reaction. The primary decarboxylation reaction was therefore first order, and the rate at each temperature was calculated from the slope of the line. The data thus obtained are shown in Table I, and results for a typical run are shown in Fig. 1. TABLE I DECOMPOSITION OF MALONICACID IN TOLUIDINES AND XYLIDINE. RATE CONSTANTS AT VARIOUS TEMPERATURES Solvent
+Toluidine In-Toluidine p-Toluidine
Temp., O C .
k X 105 (seo.-l)
120.5 127,5 132.8 117.05 126.56 136.6 117.9 124.8 126.5 133.4 117.5 126.5 133.3
103 178.5 271 53 125 297 87 164 196 367 74.5 147.5 233
Results and Discussion 2,6-Dimethylaniline I n aniline solution each sample of malonic acid gave a 100% yield of C02 at the end of the experiment.2 In toluidines and xylidine the yield of COz The experimental data in Table I yielded straight was never more than 90%. Evidently in the latter lines when log k was plotted against 1/T (see Fig. solvents a slow secondary reaction consumed some 2). From the slopes of the lines in Fig. 2 activa(1) G. Fraenkel, R. L. Belford and P. E. Yankwich, J . Am. Chem. tion energies and frequency factors from the ArSoc., 76, 15 (1954). rhenius equation were obtained. The thermody(2) L. W. Clark, THISJOURNAL, 60, 1340 (1956). namic quantities in the Eyring equation were also (3) L. W.Clark, ibid., 60, 1583 (1986). calculated. These data are shown in Table 11. (4) L. W. Clark, ibid., 60, 1150 (1986).
1010
Vol. 61
NOTES TABLE I1 DECOMPOSITION OF MALONIC ACIDIN TOLUIDINES AND XYLIDINE.KINETICDATA E* (cal.)
Solvent
0-Toluidine m-Toluidine p-Toluidine 2,6-Dimethylaniline
24,650 29,600 29,300 24,300
A (sec. -1)
5.13 X 2.98 x 2.46 x 3.24 X
AH
*
(cal.)
1010 1012
1014 loLo
23,900 28,400 28,600 23,100
*
AS (e.u.)
-11.7 - 0.8 .3 -14.0
+
AFSI~O (cal.)
28,700 28,750 28,500 29,300
kl400
X IO6
(sec.
-1)
500 530 645 400
REARRANGEMENT PEAKS IN THE MASS SPECTRA OF CENTRALLY C13-LABELED NEOPENTANE’ BY C. PETER JOHNSON, JR.,A N D ALOISLANGER Weslinghouse Research Laboratories Chemistry Department, Churchill Borough, Pitisburgh, P a . Received August 9,1966
-2.4
/
~
247 249 251 253 255 257 l / T x 106. Fig. 2.-Effect of toluidines and xylidene on malonic acid: Arrhenius plots: I, m-toluidine; 11, 2,6-dimethylaniline; 111, o-toluidine; IV, p-toluidine. 245
The relative effects of the methyl group in the various possible positions on the benzene ring are clearly demonstrated by the data in Table 11. It would be expected that the reaction taking place in the solvent having methyl groups in the two ortho positions would, because of the positive inductive effect, have a lower enthalpy of activation than in the case of the solvent having but one methyl group in the ortho position. It would also be expected that methyl groups in both ortho positions would, because of the increased steric effect, lower the entropy of activation with respect to the solvent having but a single ortho position occupied by a methyl group.6 Lines 1 and 4 of Table I1 show that these expectations are completely confirmed. Very little steric effect would be anticipated by a methyl group in either the meta or para position.6 Again the experimental results are consistent with the expectations, as shown by lines 2 and 3 of Table 11. The results reported herein lend added support t o the hypothesis of Fraenkel and co-workersl that malonic acid forms an unstable intermediate compound with aromatic amines. Acknowledgment.-This research was supported by the Rasltob Foundation, W3iminrrton. Delaware. ( 5 ) L. P. Hatntnett, “Physical Organic Chemistry,” McGraw-Hill Book Co., Inc., New York, N. Y.. 1940, p. 206.
The results of the electron bombardment in the mass spectrograph of neopentane terminally labeled with one Cla-atom have been reported in a previous paper.2 Analysis of the Cla-content of the two-carbon fragments obtained in this process led to the conclusion that these fragments cannot be formed solely by simple bond rupture and transfer of hydrogen atoms, but that carbon-carbon bond formation between two methyl carbons must also be involved. I n neopentane, a t least, the evidence indicates that the two-carbon ions result from an almost random combination of any two-carbon atoms in the molecule. Observations made on C13-labeled propanes and butanes support this concl~sion.~Other work4 on the mass spectra of tetramethyl compounds of lead, silicon, germanium and tin showed that metal hydrides and also methyl and dimethyl metal hydride ions are formed in considerable abundance. Twocarbon fragments are found only in such quantities as might be explained by the presence of impurities in the original sample. Work on n-butyric acid,6 with the carboxyl group enriched with C13, showed that hydrogen transfer alone can explain its large rearrangement peak. Additional information on the occurrence of skeletal rearrangement was desirable, and therefore neopentane centrally labeled with GI3was synthesized. Preparation .-The reactions indicated below were chosen because they gave high yields of the desired products rather than because they produced the final compound in the least number of steps. All chemicals were of reagent grade. Seven grams (0.12 mole) of about 30y0 C%nriched potassium cyanide (Eastman Kodak) was covered with 25 ml. of methyl alcohol, and a 50% excess of methyl iodide in methyl alcohol was added. The cooled flask was shaken and allowed to stand for several days. The acetonitrile thus formed was mixed with a twofold excess of 6 N sodium hydroxide solution and refluxed for two days to convert it to sodium acetate. The mixture was distilled to remove methyl alcohol and other volatile compounds and caref111Iyneutralized with hydrochloric acid to a pH of 3.5. Then, 50 ml. of a 1 N sodium citrate-citric acid buffer solution of p H 3.5 was added. This solution was distilled until 500 ml. was collected, water being added to the flask from time to time (1) Presented at the ASTM meeting on mass spectroscopy in New Orleans, 1954. (2) A. Langer and P. Johnson, THE J O U R N A I61, ~ , 8’31 (1957). (3) D. P. Stevenson, J . Chem. Phys., 19, 17 (1951). (4) V. H. Dibeler, J . Research Noll. Bur. Nandards, 49, 235 (1952). (5) G. P. H a w and D. W. Stewart, J . A m . Chem. )Y*c., 74, 4404 (1952).
