NOTES
Jan., 1957 TABLE I 3,4-DICHLOROBENZOICACID PH
Temp OC.a"
pKb
PH
Temp., "C.0
pKb
3.39 26.2 3.62 3.73 26.0 3.68 3.47 25.3 3.64 3.73 25.4 3.63 3.54 25.4 3.63 3.82 25.4 3.66 3.61 25.3 3.63 4.71' 28.0 3.64d a Temperature of solution during measurement of pH. During absorbance measurements the temperature was Average result using spectral data for the 25.00 f 0.05'. 8 wave lengths 242, 243, 244, 246, 248, 250, 252 and 254 mp (exce t where otherwise indicated) to calculate the term log ([SIAAI). T he deviation from the average value of this term was in all cases 0.01 or less. c N o buffer was present. Exclusive of results for 242 and 244 mp. I n this case the deviation from the average value for log ([S]/[A]) was 0.017. I n the attempt to determine experimentally the limiting spectral absorption curve for the non-ionized form of 2,4dichlorobenzoic acid,l6 for which high concentrations of acid are needed, the absorption maximum of the principal band was found to shift gradually toward longer wave lengths, and the isosbestic point (which was near 229 mp) was not sharp, just as others have frequently found for analogous cases. An absorption curve for the non-ionized acid was calculated, using eq. 2 and spectral data, taken a t 2-mp intervals in the range from 236 to 254 mp inclusive, for each of five solutions which were maintained a t different p H values by the addition of either sodium acetate plus acetic acid (of ionic strength about 5 X lo-' M ) or of hydroM to 4 X lo-* M ) . Values of pK chloric acid (6 X over the range 2.71 to 2.79 were substituted in the equation, and the best fit was found when the pK value assumed was 2.76. The calculated absorption curve differs very little from curves that were obtained for 5 X lo-' M 2,4-dichlorobenzoic acid in the presence of about 0.48 to 0.96 M hydrochloric acid.16 A few illustrative data and calculated p K values are given in Table 11. In calculating the results given in the table the absor tion curve obtained experimenM 2,4-&hlorobenzoic acid in the prestally for 5 X
TABLE I1 2,PDICHLOROBENZOIC ACID PH
2.43 2.78 2.92 3.23 3.38
X = 24Omra Absorbance per cm. PK
X = 250 mpb Absorbance per cm. PK
0.389 .349 .32G .286 .270
2.74 0.250 2.71 2.80 .212 2.78 2.77 .189 2.76 2.76 .151 2.75 2.72 2.75 .134 Av. 2.76 Av. 2.74 a The limiting absorbance found for the completely ionized acid, 5 X 10-6 M ,was 0.223. The absorbance (0.470) of a solution approximately 0.48 M in hydrochloric acid waa assumed to be that of completely non-ionic 2,4-dichlorobenzoic acid in com uting the pK values given in this table. b T h e values used &r completely ionic and non-ionic acid are 0.090 and 0.334, respectively. (15) The authors thank Maya Paabo for assistance in the spcctrophotometric determination of p K for 2,4-dichlorobenzoic acid. (10) Our spectral data for ionized and non-ionized 2.4-dichlorobenzoic acid are not in good agreement with results reported by Doub and Vandenbelt (see ref. 14). I n our experiments, 5 X 10-6 M s o h tions of the acid had almost identical absorption curves in the pH range about 0 t o 9 (the solutions being buffered with sodium acetate, borax, or borax plus boric acid). These curves, which were considered t o be caused by the completely ionized acid, showed a flat plateau or step-out in the wave length region 224 to 230 m p . instead of a band near 229 mp, as reported by Doub and Vandenbelt. For non-ionized 2,4-dichlorobenzoic acid, instead of their "first primary band" a t 232 mp, with a molar absorbance index of about 7400, our calculated curve exhibits a n absorption band near 240 m r , having the approximate molar absorbance index 9500.
125
ence of approximately 0.48 M hydrochloric acid was assumed to be the correct curve for the non-ionized form. The best experimental value of pK for 2,4-dichlorobenzoic acid, considering all the potent,iometric and spectrophotometric data, appears to be 2.76, but because of the experimental uncertainties this value cannot be regarded a8 differing significantly from the calculated pK value." (17) There appears t o be a similarly close agreement between the actual and calculated ionization constants for 2,4-dibromobennoic a d d (see footnote 2).
T H E IONIC DISSOCIATION OF 2,G-DIMETHOXYBENZOIC ACID I N WATER' BY MARIONMACLEAN DAVISAND HANNAH B. HETZER National Bureau of Standards, Washington Received September 17, 1966
D. C.
'
In view of the marked deficiency of published ionization data for aromatic acids with substituents in both the 2- and 6-positions1 it seems worthwhile to report the pK value, 3.44, recently obtained for 2,6-dimethoxybenzoic acid at approximately 25" by a titration procedure.2 An ionic dissociation constant for the acid can be calculated by applying a generalization of Shorter authors pointed out that the and S t ~ b b s . These ~ change in the free energy of ionization of benzoic acid, A( -RT In K ) ,upon the introduction of two or more substituent groups is usually nearly the same as the algebraic sum of the effects of individual substituents, except for cases of substitution in both the 2- and 6- or the 2- and 3-positions, when marked discrepancies between the calculated and experimental values generally appear. They observed, moreover, that improved agreement between observed and calculated values of A(-RT In K ) can be achieved by using in the computation the value of A(-RT In K) obtained experimentally for the corresponding di-ortho-substituted acid. Using ionization data for benzoic acid and for o-methoxybenzoic acidT4the calculated pK value for 2,6-dimethoxybenzoic acid is 3.99; a lower pK value (3.31), which is in closer agreement with our experimental value, results when the calculation is based on a published value for the acidic ionization of 2,4,6-trimethoxybenzoic acids in conjunction with the ionization constant for p-methoxybenzoic acid. In the case of 2,G-dichlorobenzoicacid we found2 that the experimental value of p l i (1.82) exceeds the calculated value (l.GS), indicating that the first (1) This research was supported in part by the United States Air Force, through the Air Force Office of Scientific Research of the Air Research and Development Command, under contract No. CS0-67055-21. (2) The equation used in the calculation of p K , was
[B-I
pK
=
pH
+ [H+l
0.5094i
- log [HB] - [H+] 4- 1 + 1.324,ii
See also M. M. Davis and H. B. Hetzer, THIBJOURNAL, 61, 123 (1957), footnotes 10 and 11. (3) J. Shorter and F. J. Stubbs, J. Cham. Soc., 1180 (1949). (4) The ionization data used were values for the thermodynamic dissociation constants determined by Dippy and associates and summarized in Table I (3)-(a), J. F. J. Dippy, Chem. Rev., 2 5 , 151 (1939). (5) W. M. Schubert, R. E.Zahler and J. Robins, J . A m . Chern. Soc.. 77, 2293 (1955).
126
NOTES
ortho-substituted chlorine atom is more effective than the second ortho-chlorine in enhancing the strength of benzoic acid. I n contrast to this, 2,6dimethoxybenzoic acid appears to owe its enhanced strength more to the second than to the first ortho-methoxy group. Experimental To prepare 2,6-dimethoxybenzoic acid, m-dinitrobenzene was converted successively to 2-nitro-6-methoxybenzonitrile and 2,6-dimethoxybenzonitrileby essentially the procedures described in references 6 and 7. The latter compound was then saponified.?& The over-all yield by these procedures is low. After crystallization from benzene using decolorizing charcoal, followed by heating in a vacuum oven to about 80°,the acid melted at 187-188O.8 The purity by potentiometric weight titrations was 99.1%. The most probable impurities are +,racesof 2,6-dimethoxybenzonitrile or the amide that results from its partial saponification.70 Either of these compounds would be inert during titrations. I n determining pK two independently prepared 0.01 M solutions (100-ml. portions) were titrated with standard Rodium hydroxide approximateIy ten times as concentrated, using glass and saturated calomel electrodes,e and with precautions to exclude carbon dioxide. The temperature was in the range 25 to 26". Preliminary adjustments of the apparatus were made usin NBS standard potassium hydata used in computing pK drogen phthalate. The were those recorded at 0.5-ml. intervals from 0.5 to 5.0 ml. inclusive. Both titrations yielded the average pK value 3.44 f 0.01.
p8
(6) A. Russell and W. G. Tebbens, Org. Synthsser, 22, 35 (1942). (7) (a) N . J. Cartwrigbt, J. I. Jones and D. Marmion. J. Chem. Soc., 3499 (1952); (b) C. A. Lobry de Bruyn, Rec. trau. ohim., 2 , 205 (1883); (c) F. Mauthner, J. prakt. Chem., i21, 259 (1999). (8) Others have reported the melting points 186-18f0 (ref. 7a,c) and 187.5-188.5O (A. Kreuchunas, J. Org. Chem., 21, 368 (1956)). In the latter case the starting material was 2-methylresorcinol. (9) The titration apparatus wa8 similar to that described by C. J . Penther and F. B. Rolfson, Ind. Enp. Chem., Anal. Ed., 16,337 (1943).
MAGNETIC STUDIES OF SOME COBALT COMPLEXES OF AMINO ACIDS AND PEPTIDES1 BY JAMES M. WHITE, THEODORE J. WEISMANNAND NORMAN C. LI Department of Chemielry, Duquesne Uniuersitv, Pittsburgh, P a . Received August 88, 1966
It has been pointed out2 that the assignment of configuration and state of coordination of complexes can be obtained from knowledge of the magnetic moments. A search of the literature shows however that of the cobalt complexes of amino acids and peptides, the magnetic moments of only the histidine complexes2&~b have been reported. As part of an extensive program on studies of metal complexes of amino acids and peptides carried on in this Laboratory,8 therefore, this note presents the results on magnetic studies of some cobalt complexes of aniino acids and peptides. (I) This investigation was aupported by Grant No. NSF-G1928 from the National Science Foundation. (2) (a) J. A. Hearon, D. Burk and A. L. Schade, J . Natl. Cancer I n s t . , 9, 337 (1949); (b) L. Michaelis, Arch. Biochem., 14, 17 (1947); (0) L. Pauling, "The Nature of the Chemical Bond," Corneil University Press, Ithaca, N. Y., 1940. (3) (a) N . C. Li, T. L. Chu, C. T. Fujii and J. M. White, J . A m . Chem. Soc., 77,859 (1955); (b) N . C. Li and R . A. Manning, ibid., 77, 5225 (1955); ( c ) J. M. White, R. A. Manning and N. C. Li, ibid., 78, 2367 (1956).
Vol. 61
Experimental Materials.-Oxidized glutathione was a Schwarz product and contained about 14% associated alcohol. The material was dried in vacuo a t 56" to constant weight. The hexamminecobaltic chloride was prepared and recrystallized according to the method of Bjerrum and McReynolds.4 All other chemicals were of reagent grade, and were used without further purification. Magnetic Measurements.-The magnetic susceptibilities of the solutions were measured by the Gouy method a t room temperature. The apparatus, consisting of a General Electric Isthmus Electromagnet and an Ainsworth microbalance, has been described previously .616 The procedure and the method of calculating magnetic moments are similar to those used by Li, et al.,aa in their magnetic study of the nickel complexes of imidazole.
Results Table I summarizes the results of the magnetic studies. It was found that cobalt solutions containing cysteinate and tris-(hydroxymethy1)-aminomethane became diamagnetic rapidly in the presence of sir. For this reason, the cysteinate and tris- solutions were carefully degassed and measured under vacuum. TABLE I MAQNETIC MOMENTS IN AQUEOUS SOLUTION Concn. of metal ion, M
COG12 or Co(N08)2,0.057 coc12, .057 Co(NOJ2, .007 COCl,, .01 ,007 .007
Concn. of ligand, M
Glycylglycinate, 0.224 .136 Triglycinate, .05 Methionate, .12 Oxidized glutathione, .03 Cysteinate, .12 Tris-, .30 Ammonia, 1 Glycinate, 0.10 Imidazole, .25
Magnetic moment, B.M.
5.02 4.63 4.62" 4.85 4.64
5.05 .04 3.86b .06 4.80b .05 4.92 .07 4.94 .05 5.04 CO(NHa)oCla, .05 0 NiC17, ,005 Cysteinate, .05 0 After passing air or oxygen through the solution overnight, the magnetic moment drops to zero. t. See text.
Discussion It is seen from Table I that all the cobaltous complexes listed therein are paramagnetic and the moments indicate the presence of three unpaired electrons per molecule. The calculated spin moment for three unpaired electrons is 3.88, and this value is obtained for the cobaltous cysteinate complex only. The bonds in the cobaltous complexes listed in Table I therefore are all predominantly ionic. Li and Whiter have found from ion-exchange CXperiments that the highest order complexes of glycylglycinate and triglycinate are of the COA:, type, and have concluded that the coordination number of Co++ toward these peptides is four. Since the position of minimum potential energy for four ionic bonds is the tetrahedral arrangement, the configuration of the cobaltous complexes of the peptides is tetrahedral. On the other hand, it has (4) J. Bjerrum and J . P. MrReynolds, "Inorganic Syntheses," Vol. 11, McGraw-Hill Book Co., Inc., New York, N. Y., 1946, p. 216. (5) T . L. Chu and 8. C. Yu, J . A m . Chem. Soc., 7 6 , 3367 (1954). (6) T. L. Chu and T. J. Weismann, ibid., 78, 23 (1956). (7) N . C. Li and J. M. White, unpublished data.
