862
JOHN 0. EDWARDS AXD RICHARD J. SEDERSTROM
5'01. 65
+
+
(kO.5) ~ f O 2 9 8 , 1 5 ( s ) , kcal. mole-', in simple alkane disulfides and in &(g).' Evidently, the S-S thermochemical bond energy is affected only slightly by the molecular environment of the diThe heats of sublimatioll of TJITS arid TJITDS sulfide bond. The fact that the s-s dissociation are unk11oTv-n. but the difference in heat of subli- bond energy in TAITDS is unusually low2 must be mation i.; estimated to be about 2.5 + 0.5 kcal. due to resonance stabilization of the radical prodmole-'. bahed on results of this Laboratory for ucts of dissociation. Acknowledgment.-The authors gratefully acother wlfide-disulfide pairs. With this estimate aiid the heat of formatioii valuec. in Table 111, knowledge the assistance of Dr. C. Howard ShoE(S-S) = 14.ci(* 1.0) AHf02g8.15(s), kcal. mate, U. S. Naval Ordiiance Test Station, China mole-l, n-here A I l f 3 ~ g B . 1 5 (is S ) the heat of atomiza- Lake, California, who furnished the program for tioii of sulfur, which i:, controversialg and herein data reduction by high speed digital computer and unspecified. t'or comparison, E(S-S) = 12.7 arranged for the necessary computer time. A
~ = o
R-d-S-R'(g) = R-S-R'(g) Y(g) AHf"(R-S-R/) + AHf" (s) - AH;TO(R-S-~;-IT~)= E( S-S)
+
THE THERMODYSAJIICS OF IONIZATIOS OF BESZESEBOROSIC ACID BY JOHS0. EDWARDS ASD RICHARD J. SEDERSTROM Jf etculf Chewzzcal Laboratoraes of Brown Vnaversity, Pron'dence, R. I . R e c e n e d Dcctmber lcib1960
-1stud\ of the thermodynamic quantities for the ionization of benzeneboronic acid has been made. The enthalpy and entropy nere found to be 1.9 kcal. mole-' and -31 cal. mole-' deg.-' a t zero ionic strength. The results are discussed in referenrr to the structure of the anion 11hich presumably contains a four-coordinate boron.
It has beeii pointed out by E'itzer' that the ent'ropy of ionization for neutral acids is usually about - 2 2 cal. mole-' deg. -I. Certain exceptions are known, however; in the case of boric acid at 2 3 " , the entropy is reported? t o be -30.9 cal. mole-l deg.-l. In order to see how another boron acid would compare, u-e have measured thermodynamic quantities for benzeneboronic acid. The interest in these values stenis from the fact that the entropy of ionization should differ from the usual figure of - 2 2 jf H coordination number change occurs during the ioiiizatioii process. Such a change is kiiowii to occur in boric acid for the anion has a tetrahedral bor011.~ There is evidence1 t o indicate that the anion of heiizeneboronic acid also has a tet'rahedral configuratioi~,~and t'he present, evidence agrees with such a roiiclusion as may be see11helow. Experimental
pH meter \\-as standardized with coninierclal buffer at pH 7 before each run and checked after each run. A specidlvdesigned, double-walled, glass-jacketed beaker6 was used to permit the measurements to be carried out in an atmosphere of nitrogen and at a constant temperature fixed by an outside bath. Procedure.-A sample of appro.;imately 0.15 J l benzeneboronic acid stock solution was titrated against a 2.489 Jf sodium hydroside solution in the presence of mannitol to determine the exact amount of base necessar? to neutralize the acid sample. From these data, the amount of base necessarv to half-neutralize the remaining benzeneboronic acid stock solution v a s computed and added to it. The resulting solution had therefore equal amounts of benzeneboronic acid and benzeneboronate ion. Solutions of varving ionic strength were then prepared from the half-neutralized acid solution. The ten solutions of different ionic strength now were brought t o thermal equilibrium a t various temperatures and the pH data takrn.
Results With one set of solutions. pH data were obtained at five temperatures. X second set of solutions ~ r a 5studied a t four temperatureq such that the Reagents.~--Bcnzeriet)oronir acid was piirchused from ranges would overlap to provide some idea of the 11ann Research Laboratories, Inc.; the melting point and other properties checked sat,isfactorily with the literature experiniental error. The data are presented 111 values, the samples were employed without further puri- Table I which contains the pH rewlts for all runs fication. Reagent grade sodium nitrate was used t o con- plus the extrapolated values at zero ionic strength trol the ionic. strength. Thc water used in making solu- for each temperature. tions was freshly boiled, distilled r a t e r . CarbonateAt zero ionic strength, all the activity coefficients free sodium hydroside was prepared and standardized become equal t o one, so pII is equal to -log[I-I+]. according to the procedure of Kolthoff and Sandell.6 Equipment.--X11 pH measurements were made using a Using brackets to denote conceutrations and the I3cckni:in \lode1 GS inctc,r arid Rrc~krnanelectrodes. The iynihols HR and R - t o dpiiote beiizenehoronic acid aiid ljciizeriel.)oroiiutc 1011. rehpectivcly, wc liavc , J . .Lm. C/tem. Soi:., 59, 28CiTr (l9:37). .I. Rssenrr.h .Ya/l. ( 2 ) (;, Q. Mano\-, N. .I. i)elmllis and S. F. .ICI.PP, H I L T .S t n i i d n r d s . 33, 287 (19.14). C. Rro\vn, ibid., 77, 37:iR (19.53); ( h ) K . Torssell. A r k i u Kemi, 10, 541 (1937); (cj .I. P. Lorand and J. 0. Edwards. J . O w . ('hem., 24, 769 (19.59); id) R. L. Letsinger a n d .J. R . h-aap, J . .4m. Chem S o c , 81. 3013 (1959). , Tiolthoff a n d . :1 R. Rnndell. "Textbook of Inorganic The 3Iaciiiillan Co., N e w Y o r k N. Y., re\.. e d . , p . ,550-554, 1948.
The experimental conditions were bet up bo that [HB] = [B-1, therefore Ka = [H+] and pK, = pH a t zero ionic strength. The quantities AH,,, and ASlon nere evaluated from n plot of pKa ngaiiiqt rcciprocal ahsolutc (bj kI R Lllison 1'11 U thesis R i o \ \ n U n l \ e i s l t j
1% 0
ACID-BASEPROPERTIES OF SOME PYRAZINES
863
TABLE I DEPENDEWE OF p K , pl/3
00 087 166 "8 ,260 296 328 358 ,384 109
.433 All valiies of p K , in
19.5'
20.0'
25.5'
OK
TEMPERATURE AND IOSIC STRENGTH 25.9'
33.0'
8.855 8.850 8.833 8.823 8.805 8.82 8.81 8.80 8.78 8.76 8.79 8.79 8.76 8.75 8.73 8.76 8.76 8.71 8.73 8,71 8.75 8.74 8.73 8.71 8.70 8.73 8.73 8.71 8.71 8.60 8.72 8.71 8.70 8.69 8.68 8.71 8.70 8.69 8.68 8.67 8.70 8.70 8.68 8.68 8.66 8.70 8.69 8.68 8.67 8.65 8.68 8.67 8.67 8.66 8.63 this horizontal row are extrapolated values ohtained
temperahre. The final values obtained (using all nine temperatures) are AH,,, = 1.9 kcal. mole-] and A s i o n = -34 cal. mole-' deg.-'. The results from the tlvo sets of solutions individually agreed with the above combined result t'o within a few hundred cal. for t'he enthalpy. Using the above values of enthalpy and ent'ropy, a comparison of observed values of pKa with calculated values was made; t'he maximum difference u-as 0.010 pH unit and thc average difference \vas only 0.005 p H unit. The pKa value for 25" and zero ionic strength is 1.46 x IO-!). This agrees well with the values reported in the literature: 1.9 X a t 20°j7 at 25',* and 2.38 X a t 25" and 1.37 X 0.1 ionic strength.4h
33.5"
8.795 8.76 8.73 8.70 8.69 8.68 8.66 8.65 8.64 8.63 8.62 from plots
36.ZS
J 8 00
38.5'
8.788 8 788 8.74 8 73 8.71 8 71 8.69 8 69 8.68 8 67 8.66 8 67 8.65 8 66 8.63 8 61 8.62 8 63 8.61 8 62 8.60 8 61 of pH against p I ':
8 767 8 72 8 70 8 68 8.66 8 66 8 63 8 62 8 62 8 60 8 50
negative than the usual is expected as the coordination number change< from three in the acid to four in the anion. Thi., of nereshity, requires that one more nater molecule be lost i n thi5 ionization process than for a normal ioiiization. Other things (such as secondary solvation) being equal, the entropy for the proces RB(OH),
+ 2H20
ItB(OH),-
+ H,O-
should be more negative than for the process RB(OH)?
+ Hi0
RB02H-
+ H30+
which is about - 2 2 cal. mole-1 deg.-'. The absolute difference should be somewhere between the entropy of fusion of water (f5.5 cal. mole-' deg.-') and the whole entropy of water in the liquid state a t 25" (+16.75 cal. mole-1 deg.-l). The Discussion observed values of 9 and 12 for boric and benzeneboronic acids, respectively, are in good agreement The ioiiizatioii of henzeiieboronic acid is only slight,ly endothermic, thus the reason it is so weak xvith the rough prediction. The fact that the enthalpy is less positive than an acid lies in the entropy of ionization. This entropy is considerably more negative than the normal for such a n-eak acid is also in agreement usual value for an oxyanion' but is close to that with the coordinatioii number change. For the ohuerved for boric acid which suggests t'hat a similar ionization process going to RB(OH)3-, there is one ionization process occurs for these tn-o hron-con- more sigma bond in the products than in the reactants. The exothermicity resulting therefrom taining acids. Thf fact that these tn-o entropies are more will partially overcome the endothermicity of thp ionization process. We conclude that the thernio(7) L). I,. Yahroff. G. E. I
