Jan., 1960
DECARBOXYI,.4TION OF bi~4LONIC-4CID IN ACID
hf EDI-4
41
THE DECARBOXYLA4TIONOF MALONIC A4CIDIN ACID MEDIA BY LOUISWATTSCLARK" Contribution from the Department of Chemistry, Saint Bonaventure University, Saint Bonaventure, New York Received June 8. 1969
Kinetic data are reported on the decarboxylation of malonic acid in propionic acid, butyric acid, @-mercaptopropionic acid and monochloroacetic acid. The constants of the Eyring equation are evaluated. The inductive effect of the methyl group substituted in place of a hydrogen on to the propionic acid molecule results in a lowering of AHS on passing from propionic acid to butyric acid. I n monochloroacetic acid AH* of the reaction is lower than it is in propionic acid. This is attributed to an increase in electron density on the hydroxyl oxygen atom due to a +E effect of the halogen. The data for p-mercaptopropionic acid indicate that the electrophilic carbonyl carbon atom of malonic acid probably coordinates with an unshared pair of electrons on the S atom of the sulfhydro group and not on the 0 atom of the hydroxyl group. Steric effects are manifested by changes in AS* for the reaction between the different liquids,
Kinetic studies on the decarboxylation of malonic acid in various liquids'have established the essential validity of the mechanism of the reaction proposed by Fraenkel arid co-workers,2 namely, the formation, prior to cleavage, of a transition complex involving coordination of the electrophilic carbonyl carbon atom of the un-ionized malonic acid with an unshared pair of electrons on the nucleophilic atom of the solvent molecule. The tendency possessed by malonic acid to coordiiiate with an unshared pair of electrons and subsequently to split into acetic acid and carbon dioxide affords a convenient means of deducing the characteristics of the transition complex on the basis of the absolute reaction rate theory and obtaining thereby an insight into the electron structures of polar liquids in general. The method has been applied successfully, up to the present time, to the study of 34 non-aqueous solvents, comprising reprlesentatives of 9 homologous groups, namely, aromatic amines, alicyclic amines, aromatic nitro compounds, ethers, esters, phenols, thiophenols, sulfoxides arid polyhydric alcohols. The present, paper describes the results of kinetic studies carried out in this Laboratory on the decarboxylation of malonic acid in four monocarboxylic acids! namely, propionic acid, n-butyric acid, monochlorortcetic acid and P-mercaptopropionic acid. Experimental Reagents: (1) Reagent Grade malonic acid, 100.O~o assay, was used in this invrstigation. ( 2 ) Solvents: (a) the propionic acid, n-butyric acid and monochloroacetic acids used in this research were Reagent Grade chemicals. Each sample of each liquid was fractionally distilled a t atmospheric pressure into the reaction flask immediately before the beginnin:: of each experiment. (h) The 8-mercaptopropionic acid used in these experiments was obtained from Evans Chemetics Inc., 250 E. 43rd St., K . Y., h-. Y. Its physical chara'2teristics were: m.p. 16.8'; b.p. 110.5111.5' a t 15 mm.; sp. gr. 21/4, 1.218; assay, 99.6%. Samples of this liquid were taken directly from the container wit,hout any further purification. Apparatus and Technique.-The details of the apparatus and technique haye been described previously.lb I n these experiments a sample of malonic acid weighing 0.1857 g. (corresponding to 40 ml. of COz at STP on complete reaction) was weighed into a fragile glass capsule weighing ap-
* Department of Chemistry, St. lMary of the Plains College, Dodge City, Kansas. (1) (a) L. Tc'. Clark, THISJOURNAL. 60, 825 (1956); (b) 60, 1150 (1956); (e) 60, 1310 (1956); (d) 60, 1583 (195fi); (e) 61, 1009 (1957): (f) 61, 1575 (1957); (E) 62, 79 (1958); (h) 62, 368 (1958); (i) 62, 500 (1958): (j) 62, 1478 (1958). (2) G . Fraenkel, R . 1,. Belford and P. E. Ysnkwich, J . Am. Chem. Sac., 76, I d (19541,
proximately 0.1 g. blown from 6 mm. soft-glass tubing. A weighed quantity of solvent (saturated with dry C02 as) was placed in the 100-ml., 3-neck, standard-taper $ask immersed in the oil-bath. The temperature of the thermostat controlled oil-bath (maintained t o within rt0.05") was determined using a thermometer graduated in tenths of a degree and calibrated by the U.S.Bureau of Standards.
Results Decarboxylation experiments were carried out in each solvent a t three or four different temperatures over a 10-20" temperature range, two or three experimenti being performed a t each temperature in each solvent. In every experiment the plot of log ( a - z) us. t (where a is the maximum theoretical volume of COZ evolved, and 5 the volume evolved a t time t ) yielded a straight line. The reaction was generally carried out in approximately 50 ml. of solvent. However, as in the case of previous studies conducted in various types of solvents, variation in the ratio of solvent to solute had no appreciable effect upon the rate of reaction. The average values of the apparent first-order rate constants for the reaction in the four acids at, the various temperatures studied, obtained from the slopes of the experimental logarithmic plots, are listed in Table I . The parameters of the Eyring equation are shown in Table 11. Data for the decarboxylation of malonic acid in aniline and in thiophenol are included for comparison. TABLE I APPARENTFIRST-ORDER RATE CONSTANTS FOR THE DsCARBOXYLATION OF MALONIC ACIDI N SEVERAL LIQUIDS Temp. No. of k x 104 ("C. cor.)
runs
Propionic acid
128.05 133.77 138.44
3 2 3
n-Butyric acid
138.04 144.22 149.20 156.72
2 3 2
2.08 3.84 6.15 12.20
hlonochloroacetic acid
139.70 150.15 154.65 159.35
2 3 3 2
1.42 3.71 5.61 9.80
P-Mercaptopropionic acid
141.94 148.96 156.66 158.15
3 3 2
Solvent
a
Average deviation.
3
2
(sec. -1)
0.928 i 0.05" 1.74 i .07 2 . 8 2 i .06
i i i i
.05 .04 .06 .03
i .06 i .05 i .04 & .03
3.76 & .03 7 . 0 5 i .05 13.77 i .07 15.15 f .04
LOUISWATTSCLARK
42
Vol. 64
TABLE I1 chloroacetic acid. The halogens are potentially KINETICDATAFOR THE DECARBOXYLATIOS OF MALONIC capable of exerting either a -I effect or a +E effect. In the case of the trichloroacetate ion, ACIDIN SEVERAL LIQUIDS
the negative charge on the resonating carboxylate group probably acts effectively to repel the Solvent mobile electrons on the monochloroacetic acid (1) Propionic acid 33.6 + 6 . 1 31.1 3.4 molecule so as totally to prevent the +E effect (2) n-Butyric acid 32.3 + 2 . 5 31.3 2.5 from operating. The -I effect represents a (3) Monochloroacetic permanent state of the molecule, whereas the +E acid 3 1 . 7 -0.07 31.7 1.5 effect is temporary, and comes into play a t the (4) P-Mercaptopropiprecise moment of reaction on the demands of the onic acid 30.3 -1.9 31.3 3.1 reagent 26.9 -4.5 29.0 (5) Anilinelg 50.0 I n the case of the decomposition of the un(6) Thiciphenol’j 34.3 + 6 . 9 31.5 1.9 ionized, neutral malonic acid molecule, which, in this case, is the entity involved in the coordination Discussion of Results with the nucleophilic atom of the solvent molecule The hydroxyl oxygen atom of the monocarboxylic prior to cleavage, there is nothing to prevent the acids may act as a Lewis base, donating an +E effect from operating. It would be expected, unshared pair of electrons to an electrophilic therefore, that for the decomposition of malonic agent.3 I n the decarboxylation of malonic acid acid in monochloroacetic acid, the halogen would in these media the transition complex is evidently not be halted in its tendency to release electrons to formed by the coordination of the electrophilic the reactive center a t the precise moment of carbonyl carbon atom of the malonic acid with one reaction on the demands of the reagent-in other of the unshared pair of electrons on the hydroxyl words, for this reaction, a +E effect should operoxygen atom of the solvent molecule. An increase ate, producing an increase in the electron density in the effective negative charge on %he nucleo- on the hydroxyl oxygen atom, and result in a philic atom of the solvent molecule increases the lowering of A H * . Comparison of lines 1 and 3 of attraction between the reactants and results in a Table I1 reveals that the result anticipated is, in lowering of A H . 4 Aniline is generally regarded as a fact, actually obtained. On going from propionic weak base, propionic acid as a weak acid. Never- acid to monochloroacetic acid AH* decreases from theless these polar liquids are both amphiprotic, 33.6 to 31.7 kcal. Here, obviously, we are dealing a,cting either as a Lewis acid or Lewis base de- with a +E effect which makes the monochloropending upon the nature of the second reactant. acetic acid a stronger base than propionic acidI n this sense, therefore, propionic acid is evidently but only, of course, at the moment of reaction. a weaker “base” than aniline. The decrease in An interesting question arises in the case of the AH* from 33.6 ked. for the reaction in propionic acid to 26.9 kcal. for the reaction in aniline is in reaction in P-mercaptopropionic acid. In previous harmony with t’his difference in relative basicities studies on the decarboxylation of malonic acid in thiophenoll’ it was established that the electro(compare lines 1and 5 of Table 11). philic carbonyl carbon atom of malonic acid COSince t’he ethyl group exerts a larger +I effect ordinated with one of the unshared pairs of electhan does the methyl group it would be expected trons on the sulfur atom. Since it is possible for that the effective negative charge on the hydroxyl malonic acid to coordinate with the oxygen of the oxygen atom or butyric acid would be slightly hydroxyl group as \Tell as with the sulfur of the larger thsn that in the case of propionic acid. The sulfhydro group the question is: with which atom lowering of AH* from 33.6 kcal. for t,he reaction in mill it coordinate? Some insight into this puzzle propionic acid to 32.3 kcal. for the reaction in n- may be gained by considering lines 2, 4 and 6 of butyric acid (see lines 1 and 2 of Table 11) sup- Table 11. Since sulfur has the same electroports this expectation. The decrease in AS* from negativity as carbon,8 the effective negative charge +6.1 e.u. in propionic acid to S2.5 e.u. in n- 011 the hydroxyl oxygen atom of P-mercaptobutyric acid is evidence of the greater steric effect propionic acid should be approximately the same of the larger molecule. as that in the case of n-butyric acid. Therefore, if The mechanism of the decarboxylation of the the malonic acid attacked the hydroxyl oxygen trichloroacetate ion in polar media has been shown atom of p-mercaptopropionic acid, AH* for the to be analogous to that of ma,lonic acid.5 In reaction would be expected to be approximately studies on the deca,rboxylat,ion of the t,richloro- the same as that for n-butyric acid. Instead, acetate ion in acid media6 it m s demonstrated however, we find that AH” is 2.0 kcal. lower for that the halogen atom in monochloroacetic acid the reaction in P-mercaptopropionic acid than for exerts a pronounced -I effect, strongly reducing the that in n-butyric acid. In p-mercaptopropionic electron density on the hydroxyl oxygen atom, acid the sulfhydro group is linked to a carboxyresulting in an increase in AH* for the reaction from ethyl group, which would not be expected to have 35.5 kc:tl. in acet,ic acid to 48.9 kcal. in mono- any very appreciable negative inductive effect, inasmuch as the carboxy group is separated from (3) L. F . Fieser and M. Fieser, ”Introduction t o Organic CheniisAH* (kcal.)
AS* (e.u.)
AF*MO (kcal.)
kiw X 10‘ (sec.-1)
.’
try,” D. C. Heath and Co., Boston, Mass., 1957, p. 138. (4) K. J. Laidler, “Chemical Kinetics,” McGraw-Hill Book Co., Inc., New York, K, Y.,1950, p. 138. ( 5 ) L. W.Clark, THISJOCRNAL, 63,99 (1959). (6) L. \.v. Clark, ibid., 63, 1760 (1959).
(7) A. E. Remick, “Electronic Interpretations of Organic Chemistry,” 2nd E d , John Wiley and Sons, Inc., N. Y . ,N. Y . . 1949,p . 225.
(8) L. Pauling, “College Chemistry,” W. H . Freeman and Company, San Francisco, Cal., 1955, p. 236.
Jan., 1960
ELECTROSTATIC AND ELECTROMAGNETIC FORCE IN LIGAND FIELDTHEORY
the sulfur atoin by two methylene groups. I n thiophenol, however, the sulfhydro group is linked directly to the phenyl group which exerts a very strong -I effect. The decrease of 4.0 kcal. for the reaction on going from thiophenol to P-mercaptopropionic acid is in harmony with this large difference in the relative electron withdrawing tendencies of the phenyl group and the carboxyethyl group. Another argument which might be adduced for the choice by the malonic acid of the electrons on the sulfur atom rather than those on the oxygen atom is the fact that monocarboxylic acids are stronger acids, and therefore weaker bases, than thiols. The electrophilic agent mill tend to
43
combine with the stronger nucleophilic agent-in this case the sulfur atom. H-bonding and therefore association cannot take place between molecules of thiophenol, whereas in the monocarboxylic acids H-bonding leads t o dimerization. This increase in complexity of the molecule, leading to increased steric hindrance, is revealed by the decrease in AS* of nearly 9 e.u. on going from thiophenol t 3 P-mercaptopropionic acid. Further investigation of this reaction is contemplated. Acknowledgments.-This research was supported in part by the National Science Foundation, Washington, D. C.
RECIPROC.ATION OF ELECTROSTATIC AND ELECTROMAGNETIC FORCES I N LIGAND FIELD THEORY'p2 BY ANDREWD. LIEHR Bell Telephone Laboratories, Incorporated, Murray Hill,N e w Jersey Received J u n e 9, 1969
If ionic spin-orbital forces are weak, electronic motions in inorganic complexes are governed primarily by the electrostatic coercions of the surrounding ligands, whilst the presence of feeble coulombic directives, but robust spin-orbit correlations, dictates electronic trajectories which are only slightly modified over those characteristic of the free ion. Electronic itineraries of both types have been exhaustively discussed, in the past thirty years, within the framework of the BetheKramers-Van Vleck theory of crystalline fields. However, the rather more esoteric situation in which the spin-orbital and addend field potentials are of comparable magnitude does not seem to have received as thorough a consideration. I n this re ort an account will be given of the optical and magnetic properties expected of d", ( n = 1,9), molecular systems, in severafgeometries, which exhibit equi-energetic spin-orbital and augend field interactions, and will be applied, &There observational data is extant, to the transition metal complexes of the second and third group. An experimental prospectus s outlined in the hope that such will stimulate future research into this somewhat neglected domain.
Introduction current, as well as past, applications of the crystalThe renascent exploitation of the Bethe-Kra- line field theory have dealt with either of two mers-Van Vleck theory of ligand (crystalline) limiting behaviors: (1) the electronic motions are fields in spectro- and magneto-chemical investiga- governed predominantly by the electrostatic (couelectrotions of inorganic systems has led, of late, to an lombic) forces of the ligand field-the enhanced appreciation of the puissance of sym- magnetic subjection of spin and orbital magnetic metry considerations in the resolution of complex moments (spin-orbit coupling) acts solely as a electronic problem^.^ Indeed, the explicatory suc- minor irritant to these movements; (2) the eleccess of this theory may be traced directly to its tronic spatial excursions are preponderantly guided maximal utilization of molecular r e g ~ l a r i t y . ~It by spin and orbital magnetic coercions-the has not, however, been widely appreciated that coulombic constraints serve only as a small directive such geometrical regularity also imposes quite inducement on their regional jaunts. For interstringent correl.ations upon the orbital and spin pretive investigations of the spectral and magnetic segments of the electronic charge density distribu- properties of the iron group (the 3dn elements), tion.6J This {circumstance has prevailed since approximation (1) is normally sufficient (note through reference 5); while for the lanthanide (1) The Editors after careful consideration have decided t o accept (4fn) and actinide ( 5 f n ) groupings, description ( 2 ) this manuscript in itti present form. The scientific work is not quesis usually completely adequate (but see ref. 7 and tioned seriously b y the referees. T h e style of writing differs from t h a t customarily employeil in scientific articles. T o the extent t h a t this 8). style makes difficult the understanding of much of this article the Yet for similar researches on the palladium author disagrees. On the other hand i t seems best to publish this (4d") and platinum (5d") group metals, these article as desired by the author.-Ed. (2) Presented a t the Symposium on Molecular Structure and viewpoints are woefully I n these Spectroscopy, Columbus, Ohio, June 15-19, 1959. (3) Recent reviews of this theory have been given by the following authors: (a) W. E. Moffitt and C. J. Ballhausen, Ann. Reu.*Phys. Chem., 7 , 107 (1956) (b) J. 9. Griffith a n d L. E. Orgel, Quart. Rev., 11, 381 (1957); (0) M. H. L. Pryce, Nuouo Cimenlo, S u p p l . 3 [lo], 6, 817 (1957); (d) H. Hartmann, Z . Eleklrochem., 61, 908 (1957): (e) W. A. Runciman, Repta. Pros. Phys., 21, 30 (1958). (4) J. H. P a n Vleck, J . Chem.. Phys., 3, 803, 807 (1935). ( 5 ) A. D. Liehr and C. J. Ballhausen, Ann. P h y s . [ N . Y . ] ,6 , 134 (1959). (6) C. J. Ballhausen and A . D. Liehr, Mol. P h y s . , 2, 123 (1959).
(7) G. L. Goodman, ibzd., in press. (8) G. L. Goodman and M . Fred, J . Chem. Phys., SO, 849 (1959). (9) The secular determinants given in ref. 5, which are common t o all the kdn, (k = 3, 4, 5), configurations illustrate this point quite clearly, as the syatems kd", (k = 4, 5), are characterized by the stipulation A = Dq (n.b., ref. 10 and 11 in this regard). (10) W. E. hloffitt, G. L. Goodman, hI. Fred and B. Weinstock Mol. P h w . , 2, 109 (1959). (11) G. L. Goodman, J. Chem. Phys., in press: Doctoral dissertation, Harvard, 1959.
