1468
NOTES TABLE I
VAPORPRESSURE OF METHYLSILANE t , "C. -29.0 0 24.0 35.7 40.5 47.2 P,mm. 2,340 6,310 11,110 14,800 16,670 18,920 Experimental Methylsilane was prepared by the reaction of methyltrichlorosilane with lithium aluminum hydride in di-n-butyl ether. "he crude product was analyzed by vapor-liquid partition chromatography and was found to contain approximately 0.5% each of dimethylsilane and trimethylsilane. It was redistilled through a low-temperature Podbielniak column rated a t one hundred plates and a center cut of the fraction boiling at -58" (700 mm.) was used in this work. No impurities could be detected in this fraction. The methylsilane was transferred to a small stainless steel cylinder equipped with two Bourdon gauges of different pressure ranges. The gauges were calibrated before and after the measurements were made. For measurements above room temperature the complete assembly was placed in a thermostated air chamber; for measurements below room temperature the cylinder was immersed in standard slush baths.
T H E DECARBOXYLATION O F MALONIC ACID I N PHENOL, THIOPHENOL AND ANISOLE BY LOUIS WATTSCLARK
Vol. 62
reaction in several basic type solvents containing nucleophilic elements other than nitrogen inasmuch as relatively few such compounds have been studied previously. *+ Preliminary experiments having revealed,that decarboxylation of malonic acid takes place readily in phenol, thiophenol and anisole, the kinetics of the reaction in these three solvents was investigated. Results of this investigation are reported herein. Experimental Reagents.-( 1 ) Reagent Malonic Acid, 100.0% Assay,was used in these experiments. (2) Solvents: (a) phenol, Reagent Grade, b.p. 181.8" (755 mm.); (b) thiophenol, b.p. 169' (755 mm.); (e) anisole, Reagent Grade, b.p. 153.8" (760 mm.). Each sample of each solvent was distilled a t atmospheric pressure directly into the dried reaction flask immediately before the beginning of each decarboxylation experiment. Apparatus and Technique.-The kinetic experiments were conducted in a constant temperature oil-bath (-lr0.05") by the technique previously described .5 Temperatures were determined by means of a thermometer calibrated by the U. S. Bureau of Standards. I n each experiment a 0.1857-g. sample of malonic acid (the amount required to produce 40.0 ml. of COS at STP on complete reaction) was introduced in the usual manner into the reaction flask containing 50 ml. of solvent saturated with dry COa gas.
Results and Discussion The decomposition of malonic acid in anisole was studied at four different temperatures between Contribution from the Department of Chemistry, Saint Joseph College, 134 and 151'; in phenol at six different temperaEmmitsburg, Maryland tures between 141 and 168"; in thiophenol at four Received June 89, 1968 different temperatures between 139 and 161". Kinetic data have been presented previously The experiments were carried out twice at each on the decarboxylation of malonic acid in thirty- temperature. The experimental data were conthree basic type solvents, including twenty-five verted to standard conditions and milliliters of aromatic mines, l s 2 one alicyclic amine, four aro- evolved gas was plotted against time for each matic nitro compounds,8 a s~lfoxide,~ a ~ 0 1 ~ 0 1 temperature. ,~ Values of ~t:(corrected volume of gas) and an alkyl phosphate.6 All the data presented corresponding to different values of t were obtained support the mechanism for the reaction proposed from the resulting isotherms. When log (a - x) by Fraenkel and co-workers,6 namely, that an TABLE I electrophilic carbonyl carbon atom of the malonic acid coordinates with a nucleophilic atom of a APPARENTFIRST-ORDER RATE CONSTANTS FOR THE DEmolecule of solvent forming a transition compIex CARBOXYLATION OF MALONIC ACIDIN PHENOL, THIOPHENOL which suffers cleavage. In order for the reaction AND ANISOLE Cor. temp. k x 10' to ensue by this mechanism, a solvent is needed, Solvent ("C.) (sec. -9 apparently, which is capable of furnishing a pair of Phenol 141.26 3.67 Z!Z 0.02 electrons to the electrophilic reagent, but which is 145.00 4.85f .05 not sufficiently basic to cause ionization of the 153.01 9 . 0 7 - .04 malonic acid. (It has been shown that in strongly 156.49 13.61 f .05 basic solvents in which one hydrogen atom of the 159.07 14.95 f .05 malonic acid ionizes decomposition of the acid 167.63 28.25 f .IO malonate ion is involved.6 If both hydrogen atoms Thiophenol 138.85 1.73 f 0 . 0 5 ionize no decomposition at all takes place.') The 147.46 4 . 1 3 f -05 reaction appears capable, therefore, of being of use 156.24 10.02 f .02 in the study of the structure and properties of 160.50 1 5 . 0 0 f .02 various basic type solvents, and also of contributAnisole 134.26 0.77 f 0.01 ing to a greater understanding of the principles 138.85 1 . 1 8 f .04 of kinetics. Additional motivation for further 147.46 2 . 5 4 f .04 investigation of the reaction is the importance of 151.45 3 . 6 3 f .03 the malonic acid decomposition in synthesis. It was thought worthwhile to investigate the (where a is the maximum theoretical volume of (1) L. W. Clark, THIS JOURNAL, 62, 79 (1958). CO,) was plotted against t straight lines were ob(2) L. W. Clark, ibid., 62, 500 (1958). tained in each case for the first 75% of the reaction. (3) L. W. Clark, ibid., 62, 368 (1958). (4) L. W . Clark, ibid., 60, 825 (1956). The average values of k obtained from the slopes ( 5 ) L. W. Clark, ibid., 60, 1150 (1956). of the logarithmic plots are shown in Table I. ( 6 ) G.Fraenkel, R. L. Belford and P. E. Yankwich, J . Am. Chem. When log k was plotted against 1/T for each solSOC.,76, 15 (1954). vent straight lines resulted. The parameters of the (7) G.A. Hall, Jr., ibid., 71, 2691 (1949).
t
Nov., 1958
1469
NOTES
TABLE I1 KINETIC DATAFOR THE DECARBOXYLATION OF MALONIC ACID I N METHYLANILINE, ANISOLE, PHENOL AND THIOPHENOL AH AS AF1400 klt00 x los E* A Solvent
(1) (2) (3) (4)
Methylanilinel Anisole Phenol Thiophenol
(cal.)
27,400 31,020 28,110 35 ,140
*
(cal.)
(sec. -1)
1.54 X 4.00 X 2.80 X 8.75 x
10l2 lOl2 10" 1014
Arrhenius and Eyring equations are listed in Table I1 with data for methylanilinel included for comparison. If we examine the data for anisole and methylaniline (lines 1 and 2 of Table 11) we see that H is higher for the reaction in anisole than for that in methylaniline. This we would predict since oxygen is more strongly electronegative than nitrogen and hence the unshared pair of electrons on the oxygen atom are less available than are those on the nitrogen. (It has been shown previously that an increase in the effective negative charge on the nucleophilic atom of the solvent molecule, causing an increase in the attraction between the two reagents, lowers the enthalpy of activation for this reaction.1) The entropy of activation AS* is slightly larger for anisole than for methylaniline. This could be attributed to the fact that in methylaniline the nitrogen has attached to it a methyl group and a hydrogen atom as well as the phenyl group, whereas in anisole the oxygen has a methyl group and a phenol group but no hydrogen. I n anisole one would expect that the malonic acid would have a greater probability of approaching and coordinating with the unshared electrons than it would in methylaniline. The higher value of AS* in the case of anisole is in accordance with this expectation. The mechanism for the decomposition of malonic acid in anisole appears, therefore, to be essentially the same as it is in methylaniline. At 140" malonic acid decomposes 32 times as fast in methylaniline as it does in anisole as a result of the higher basicity of the amine. Lines 2 and 3 of Table I1 reveals that both AH* and AS* are lower for phenol than for anisole. If phenol remained undissociated one would expect just the opposite of this, since the +I effect of the methyl group in anisole should increase the availability of the electrons on the oxygen atom. Furthermore, a methyl group should offer greater steric hindrance to the approach of the malonic acid than would a hydrogen atom, resulting in a smaller value of AS*. Now of all possible molecular species present in molten phenol near its boiling point, the only one which would have a greater effective negative charge on the nucleophilic atom than anisoig is the phenolate ion. It appears, therefore, that in the reaction in molten phenol the malonic acid coordinates with the phenolate ion. The lowering of AX* is undoubtedly attributable to the fact that extensive association obtains between phenolate ions and undissociated molecules of phenol. I n thiophenol hydrogen-bonding does not obtain and therefore association does not take place. Furthermore, in the absence of base, dissociation into thiophenolate ion cannot take place. Thiols are stronger acids than phenols, and are conse-
*
26 ,600 30,170 27,260 34,300
*
(e.u.1
-
5.33 3.67 8.90 +19.17
*
(cal.)
28 ,800 31,690 30 ,930 26,370
(sec. -1)
440.0 13.5 33.7 19.0
quently weaker bases. One would predict, therefore, that AH* would be larger for thiophenol than for phenol or anisole. The results are entirely in accordance with expectation as shown in Line 4 of Table 11. The higher value of AS* as compared with anisole corresponds with the fact that the sulfur atom is larger than the oxygen atom, and a hydrogen atom is smaller than a methyl group, hence there would be less steric hindrance in the formation of the activated complex. Acknowledgment.-The support of this research by the National Science Foundation, Washington, D. C., is gratefully acknowledged. The thiophenol used in this research was furnished by the Pitt-Consol Chemical Company, Newark, N. J. PROTON NUCLEAR MAGNETIC RESONANCE EFFECTS IN AROMATIC SOLVENTS BY R. E. GLICKAND D. F. KATES Department of Chemistry, The Pennsylvania State University, Universify Park, Pennsylvania Received June $3,1968
When aromatic substances are used as solvents in nuclear magnetic resonance studies, an anomalous shift in the position of the solute resonance frequency is observed as compared to that predicted by the magnetostatic modela2 This effect has been interpreted as due t o the field-induced magnetic anisotropy of the aromatic 7r-electrons8 and is the intermolecular counterpart of the intramolecular contribution to the chemical shift for hydrogen attached to aromatic rings as postulated by P ~ p l e . ~ Previously, Bothner-By and Glick3 and Reeves and Schneider,6examining the behavior of various substances in aromatic solvents, concluded, this intermolecular shift being relatively small for acetone and dioxane3 and large for chloroform,a-6that chloroform's hydrogen displayed an acid-like inter(1) This work was supported in part by the Office of Naval Research, Project NR055-328. Reproduction in whole or in part is permitted for any purpose of the United States Government. (2) The magnetostatic model is given by equation 1. Hi = 8iHv (1 - ad (1). For ( l ) , Ha is a reference resonant field (for a proton, Ho is the resonant field for the bare nucleus i n vacuo), si is a specific shielding factor and is a function of the electron environment of the ith nucleus, a is a shape factor, K is the magnetic susceptibility of the media, and Hi is the observed field for the ith nucleus. Using the Lorenz-Lorentz approximation for the molecular cavity and a cylindrical sample cell oriented transversely t o the magnetic field, a has the theoretical value of - 2 n / 3 (-2.09). The variations of proton resonance frequency with solvent have been found to follow equation 1 if a is given the value of -2.60.8 (3) A. A. Bothner-By and R. E. Glick, J . Chem. Phys., 3 6 , 1651 (1957). (4) J. A. Pople, ibid., 84, 1111 (1956). (5) L. W. Reeves and W. G . Schneider, Can. J . Chem., 36, 251 (1957).
