1188
Vol. 66
NOTEA
3.00 L
greater for the anhydrous solvent than for the wet solvent. This implies that dissolved water forms hydrates with one or more of the acid species. ,4 comparison of solubility data for wet and dry solvents should yield values of equilibrium constants for the hydration reactions HA
+ inHz0 = HAmH20 and (HA)2 + pHzO
=
(HA)TpHZO
in various solvents, We are inclined to beliere that hydration of the monomer alone is important in many alkanoic acid-organic solvent systems.
THE STRENGTH OF BUTYRIC AND 0-TOLUIC ACIDS BY W. J. LE NOBLE,JAMES LUVALLE,AND ABALEIFEE 1.00
Department of Chsmistry State University of New York, Long Island Center, and Basic Research Laboratory, Fairchild Camera and Inetrument Corporation, Syossel, New York Received November 16, 1061
The curve of pK, in water vs. chain length of the straight chain aliphatic acids has a significant minimum at butyric acid. Similarly, the acid strength of o-toluic acid is known to be much greater than that of the m- and p-isomers.’ Two explanations have been advanced to account for these facts. Brown, Taylor, and Sujishi have suggested that the anomalous acid strengths are entropy effects related to a greater steric requirement for the carboxyl group than for the carboxylate2; this may make impossible certain conformations for the undissociated acids that can be accommodated in the anions. Alternatively, weak intramolecular H-bonds have been p o s t ~ l a t e dto ~ $ac~ count for the facts
i
0
II
0
In the latter case at least, the usual condition for intramolecular H-bonding (a six-membered, inflexible ring) is satisfied. This interpretation has not become popular, however; the H-bond usually is considered a dipole-dipole interaction and the carbon-hydrogen bonds in a methyl group are not strongly p01ar.~ Since 1I.m.r. spectroscopy provides an excellent criterion, it was decided to approach the question with this modern tool. Protons engaged in H-bonding appear at rather low applied magnetic fields in the n.m.r. spectrum, presumably because polarization by the base results in a net deshielding. The enol hydrogen of ethyl aceto( 1 ) For B recent cornpilation and discussion of acid strengths, see H. C. Brown, D. M. MoDaniel, and 0. Hafliger in “Determination of Organic Structurea b y Physical Methods,” by E. A. Braude and F. C. Nachod, Academic Preess, Inc., New York, N. Y., 1955, Ch. XIV. (2) H. C . Brown, M. D. Taylor, and S. Sujishi, J. A m . Chem. Soc., 75,2467 (1951). (3) J. F. J. Dippy, J . Chem. SOL, 1222 (1938). (4) H. B. Watson, “Modern Theories of Organic Chemistry,” 2nd Ed., Oxford University Press. Oxford. 1941.
0
0.5
1
N80l”te.
Fig. 1.-Plot
of CHCh resonance us. solute mole fraction.
acetate and the acid proton in certain succinate acid anions are examples of this behavior. Nor is it restricted to hydroxylic protons. We found, e.g., that the resonance of the hydrogen atom in chloroform, which is well known for its ability to form weak H-bonds, is shifted to drastically lower fields upon admixture of pyridine-N-oxide, to moderately lower fields by nitrobenzene, and is unaffected by nitrosobenzene (see Fig. 1); the strength of the H-bond is expected to be in this order in view of the decreasing charge on the oxygen atoms. Similar shifts have been observed with a number of aliphatic bases.6 The chemical shifts of the methyl hydrogen atoms of the acids and their anions are shown in Table I. Since water mas used as solvent for the salts (benzene as external reference) and benzene for the free acids, the differences in 6 between the two have only comparative meaning. I t is pointed out that in order to ascribe the abnormal acid strength of butyric acid t o H-bonding (Le., a much stronger carbonyl to methyl H-bond in the anion than in the undiseociated acid), one should expect a very much greater shift downfield from acid to anion than in propionic acid, the strength of which is not anomalous. A similar remark applies to the toluic acids. The data do not show the shifts described. The TABLEI METHYL RESONANCES dCHao (acid)
bCHi5 (anion)
6.33 Propionic acidb 6.46 Butyric acidb 4.63 0-Toluic acid 5.18 m-Toluic acid 5.24 p-Toluic acid In p.p.m. t o the benzene signal. center of the triplet. 0
AP
5.45 5.60 4.16
0.88 0.86 0.48
4.15
1.03
4.18 1.06 6 Measured to the
(5) The methyl hydrogen atoms furthermore must compete with water for bonding t o the carbonyl group; however, its favoiable steiic position may interfere with the solvation of this group by water. (6) G. Korinek and W. G. Schneider, Can. J. Chem., 56, 1157 (1957).
June, 1962
1189
NOTES
values of A6 (dC&cid - GCH3,nion) are identical for butyric arid propionic acid. Similarly, no support is found for the notion of a H-bond in o-toluate ion; A8 is actually smaller in that case than for the isomers. We conclude that these measurements are not consistent, with in trarnolecular H-bonding in butyrate and o-toluate anion, and that the strengths of the corresponding acids are best explained as by Brown, et aL2 It is intaresting to note that the methyl hydrogen resonance of o-toluic acid appears a t lower field strength khan that of its two isomers'; a similar difference has been reported for the cis- and transisomers of olefins hwing methyl and carbonyl groups on the 1- and 2-carbon atoms, respectively.8 Experimental Nitrosobenzene and plyridine-N-oxide0 were purified by sublimation; the other chemicals used were commercial materials of satisfactory purity. Nitrosobenzene in chloroform solution was found to obey Beer's law a t several wave lengths in the near infrared over nearly the entire concentration range used, so that this material presumably is monomeric under these conditions. The n.m.r. spectra of the chloroform solutions were measured a t 20" with tetramethylsilani. as internal reference, of the aqueous solutions a t 20" with benzene as external reference (one set of tubes being used for all measurements), and of the benzene solutions to the solvent eignal. All solutions of the acids and salts were 0.33 M . Since only the diferences in chemical shift mere considered of interest in this work, no susceptibility corrections were applied. (7) As the referee has kindly pointed out, the difference in chemical shift between the methyl protons of the undissociated toluic acids may be the effect of magnetic anisotropy of the carbonyl group. ( 8 ) L. M. Jackman arid R. H. Wiley, Proc. Chem. Soc., 196 (1958). (9) We are indebted to the Reilley Tar and Chemical Corporation for a generou8 gift of this compound.
SEPARA'I'IOK FACTORS IN THE NO-XOBr
SYSTEM' BY ALFREDNARTEN~ Chemistry Department, Columbza University, X e w York, A', Y. Recezved November 16, 1061
The exchange of the nitrogen, oxygen, and chlorine isotopes between the gas and liquid phases of the KL'O-SOCl system has been studied by Schaefiler andl Smith3 and by Y e a t t ~ . The ~ authors point out, that a general consideration of the factors affecting ;suitability of this system for isotope separation suggests that exchange reactions of KOCl might provide an interesting and useful means for the separation of the stable isotopes of nitrogen, oxygen, and chlorine in a simultaneous process. Whereas the chemistry of nitrosyl chloride is well understood, little is known about nitrosyl bromide. Xot even the boiling point of this substance is known with any accuracy.5 The thermodynamic properties of gaseous XOBr have been determined from spectroscopic data6 and from an (1) Supported by a grant from the U.6. Atomic Energy Commiesion. (2) Oak Ridge National Laboratory, Oak Ridge, Tennessee. (3) R. Schaeffer and H. Smith, contract W-740n-ens-82, Ames Laboratory, Ames, Iowa ISC-867. (4) L. B. Yeatts, Jr., J. Chem. Phya., 28, 1255 (1958). ( 5 ) M. Trauitz and V. P. Dalal, Z. anorp. U. allgem. Chea., 102, 149 (1918): 110,34 (1920).
+
inyestigation of the reaction 2x0 Brg = 2NOBr.7 The data obtained are in good agreement. At room temperature and atmospheric pressure, gaseous KOBr is 16% dissociated into NO and Brz. From the analogy with the NOZ-NZO~ system it is reasonable to assume that the degree of dissociation in thle liquid phase is small as compared with the degree of dissociation in the gas phase at and below room temperature. The efective separation factor is defined as
(X = or 0l8,Y = "4 or 0l6,c = concn. in moles per liter, 1 = liquid, g = gas) If we restrict the discussion to the exchange of the stable isotopes of nitrogen and oxygen between the two phases of the NO-NOBr system, and if we denote the concentration of the heavy isotope by c* and the concentiration of the light isotope by c, we obtain (14
Introducing the individual separation factors [ c y SO)/C(S O ) ]I IC*( SO)/c(SO)] I a2 = -
-~ [c*(SO)/c(NOlf, [e*(NOBr) /c( SOBrjx [c*( NOBr)/c( NOBr)] 1 IC*( NOBr)/c( NOBr)] I aa = [c*(NOBr)/c(NOBxg a4 = [c*(NO)/c(NO)I, (2)
(L.1=---
1
I
and the degree of dissociation pg(p;) of nitrosyl bromide in the gas (liquid) phase according to p = c(KO)/cO(NOBr) == [cO(NOBr) - c(NOBr)]/c(NOBr) (cobeing the concentxation a t p = 0), using definition (2) and the relation iaiffa
E
Oea4
we obtain the dependence of the effective separation factor on the individual separation factors and the degree of dissociation in the two phases (3)
(3) can be further simplified if me make the assumptions In ai E 6i s (ai - 1)
