34
H. P. GREGOR, L. B. LUTTINGER I N D E. M. LOEBL
moments of 0.47 D for pentene-1 and 0.54 D for 2-methylbutene-1 reported in the literature,6 which were also calculated from the Onsager equation, resulted from the larger dielectric constants reported by these investigators5along with their assumption that e'/% would be the same for unsaturated and saturated hydrocarbons. The dipole moment of 0.28 D for cyclohexene found in the present work is much smaller than the gas state value of 0.55 D." Beckett, et aE.,12have concluded that cyclohexene exists in two tautomeric forms similar to the chair and boat forms of cyclohexane. The chair form of cyclohexene is more stable a t low temperatures, but increasing contributions from the boat form (cis-like form) might be expected as the temperature increases. This tautomeric shift toward a more polar equilibrium mixture a t higher temperatures may partially explain the differences in dipole moments between the liquid state measurements a t 293°K. and the gas state measurements a t from 308 to 480°K. It may be noted also that the dipole moment of 0.55 D for cyclohexene is about 0.2 D larger than the dipole momenttj of straight chain alkene-1 compounds. This difference may also be due to the higher polarity of the boat form of the cyclic compound compared with the cis-form of the straight-chain alkenes. It is of considerable interest to attempt a semiquantitative interpretation of the molecular dipole moments arrived a t in this investigation and elsewhere" in terms of the bond moments which contribute to making up the molecular moments. If the simplest polar alkene, propene, is considered, it (12) C. W. Beckett. N. K. Freeman and Soc.. 70, 4227 (1948).
K. 8. Pitzer, J . Am. Chem.
VOl. 59
is clear that the molecule is made up of -C-H, =C-H, and =C-Cbonds which may be considered from the standpoint of orbital hybridization theory as C(sp3)-H, C(sp2)-H and C(sp2)-C(sp3) type bonds. The C(sp2)-H bond has a moment of around 0.G Dl3*l4with the probable direction
-++
C(sp2)-H. Experimental data for C(sp3)-H give a moment of 0.3 to 0.4 DI5-l7 and the probable direction C(spS)-H. This difference between the magnitudes of the C(sp2)-H and the C(sp3)-H bond moments is probably responsible for the values of the molecular dipole moments which are arrived a t from polarization measurements on propene and other alkenes. However, such additional factors as the small C(sp2)-C(sp3) bond moment of around 0.1 D, the effects of hyperconjugation of all three types of bonds, and the small contributions from induced moments also enter the picture. Conclusion.-The atomic polarizations of a number of alkene-1, cis- and tmns-alkene compounds have been determined. The dipole moments of the polar alkene compounds have been determined from the Onsager equation and satisfactory agreement with the dipole moments arrived a t from gas state polarization measurements has been found. A semi-quantitative explanation of the origin of the molecular dipole moments in terms of the bond moments contributing to the over-all molecular moment has also been given. (13) C. F. Hammer, Ph.D. thesis, Univ. of Wisconsin, 1948. (14) R. L. Kelley, R . Rollefson and B. 8. Schurin, J . Chem. Phya.. 19, 1595 (1951). (15) A. M. Thorndike, ibid., 16, 868 (1947). (16) R. P. Bell, H. W. Thompson and E. E. Vago, Proc. Roy. SOC. (London), A192, 498 (1948). (17) G . M. Barrow and D. C. kIoKean, ibid., A218, 27 (1952).
METAL-POLYELECTROLYTE COMPLEXES. I. THE POLYACRYLIC ACID-COPPER COMPLEX BY HARRY P. GREGOR, LIONELB. LUTTINGER'AND ERNSTM. LOEBL Cmtributim from the Department of Chemistry of the Polytechnic Institute of Brooklun, New York Received July 19, 1954
Formation constants for polyacrylic acid-copper complexes as obtained by a new adaptation of Bjerrum's method, are presented. Data taken at various ionic strengths indicate that in the presence of much neutral salt, the titration curve falls to a final, limiting value. The complexes formed are strong, and at least two carboxyl groups are involved simultaneously, as with simple dicarboxylic acids. The considerably greater stability of the polymer-copper complex as compared with that of a monomeric analog (glutaric acid) reflects the powerful field effect of the polyelectrolyte chain.
The potentiometric titration of polyacrylic and polymethacrylic acids has been studied by many investigators, principally Kern,2K a t ~ h a l s k yOver,~ beek4 and Doty6; an excellent review of the recent literature has been made by Doty and Ehrlich.6 (1) A portion of this work is abstracted from the Dissertation of Lionel B. Luttinger, submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Chemistry, Polytechnic Institute of Brooklyn, June, 1954. (2) W. Kern, 2.physik. Chem., Al81, 249 (1938); Biochem. Z . , 801, 338 (1939). (3) A. Katchalsky and P. Spitnik, J Polymer Sci., 2, 432 (1947). (4) R. Arnold and J. T. G. Overbeek, Reo. traw. chin., 69, 192 (1950). (5) A. 0 t h and P. M. Doty, THIB JOURNAL, 66, 43 (1952). (6) P. Doty and G. Ehrlich, Ann. Rev. Phys. Chem., 111, (1952).
In general, the experimental results for a single titration can be expressed by the modified Henderson-Hasselbalch equation, pH = pKa- n log (1 a ) / a , where a is the degree of neutralization, n a constant, and K a the apparent ionization constant. For the same polymer, it has been shown that: (1) the values of K a and n are independent of degree of polymerization (above a certain minimum) ; (2) the value of Ka increases with increasing ionic strength of neutral salt added; (3) the value of n, which is unity for a simple weak acid, viz., acetic acid, vanes from a value somewhat above 1to about 2, approaching the higher value with increasing dilution, approaching the lower value a t high ionic
THEPOLYACRYLIC ACID-COPPERCOMPLEX
Jan., 1955
0
0.5
1
a.
Fig. 1.-Titration of 0.01 +V PAA in absence of: neutral KC1 salt ( 0 ) ;in 0.1 M KC1 ( 0 ) ;in 1 M KCl (a);in 3 (A);in 2 M NaN03 (A). Dashed curve is for acetic acid.
strengths. Various statistical treatments of these systems have shown that the concepts of N. Bjerrum pertaining to the ionization of polybasic acids apply to the polymeric acids, and equations of the type of the Henderson-Hasselbalch relationship are obtained. This paper describes titrations of a polymeric acid in the presence of copper salts, under different conditions of polymer acid concentration, ionic strength, and copper(I1) concentration. The formation constants of the complexes formed are calculated using a modification of the methods of Bjerrum.' Experimental Methods.-A sample of pure polyacrylic acid (PAA) was first dialyzed using cellophane tubing. Only a negligible amount (
