NOTES
position of methyl cyclobutanecarboxylatee are considered also, it is possible to compare the relative influence of four substituents containing a >C=O adjacent to the cyclobutane ring. The cyclobutane derivatives (CIHTR)may be arranged in the following order of decreasing rate constant at 390" according to t h e nature of R : HCO > C2H5C0> CH&O > CH30C0. The pyrolyses of the two ketones do not differ greatly in rate; but the rate constant for the ester a t 390' is only about one-fifth that of the ethyl cyclobutyl ketone. The same order (aldehyde > ketone > ester) representa the effectiveness of the substituent in decreasing the activation energy below that for ryclobutane itself. As has been discussed previously, the rate relationship [(aldehyde or ketone) > ester] would seem to be in ac.. cord with the mesomeric effect'" of the whole substituent group upon the activated complex. The decrease in the resonance effect of the carbonyl group by replacement of an adjoining alkyl group with an alkoxy1 group was attributed to cross conjugation by Branch and Calvin." The lowering of the C=C infrared frequency is less for a conjugated ester than for a conjugated aldehyde or ketone, l 2 but the frequency shifte observed for acrolein, methyl vinyl ketone, and ethyl vinyl ketone probably do not differ enough from one another to be useful for predictions about differences in the effects of the substituents HCO, CH3C0, and C2HBCO upon the eyclobutane pyrolysis. The mesomeric effects of aldehyde, ketone, and ester groups seem to be in accord with the sequence of optical exaltations (HC=O > RC=O > ROC=O) observed for olefins with these ~ u b s t i t u e n t s . ~As ~ a measure of a composite inductive and mesomeric effect of a substituent a relationshjp involving n.m.r. data has been proposed, l4 but a subsequent analysisI5 of the n.m.r. data for substituents of interest here has not confirmed the numerical values of the n.m.r. parameters used in the proposed relationship. The question of whether the transformation of an activated complex from cyclobutane (or a derivative) can lead to a biradical and/or directly to two molecular products has not been resolved experimentally.16 Resonance stabilization by a carbonyl group would be expected for either the biradica12sa*9*17 or molecular products and might cause a decrease in the energy OS the activated complex. Likewise the lower entropies of activation for the cyclobutane derivatives with >C=O in the side chain in comparison with alkyl derivatives could be accounted for by the effect of resonance upon the stereochemistry of the activated complex. With respect to the lowering of the activation energy and frequency factor of the pyrolysis there is a
1609
similarity between the carbonyl derivatives and isopropenylcyclobutane. l8
Acknowledgment. The authors wish to thank Mr. Carl Whitemari for his assistance and the General Railway Signal Co. for the use of an IBM computer. (9) If. Zupan and W. D. Walters, J . Am. Chem. Soc., 86, 173 (1964). (IO) C. K. Ingold, "Structure and Mechanism in Organic Chemistry," Cornel1 University Press, Ithaca, N. Y., 1953, pp. 76-78. (11) G. E. K. Branch and M. Calvin, "The Theory of Organic Chemistry," Prentice-Hall, New York, N. Y., 1941, p. 238. , (1953); (12) W. H. T. Davison and G. R. Bates, J . Chem. ~ o c . 2607 R. Mecke and K. Noack, Chem. Ber., 93,210 (1960). (13) See ref. 10, p. 128. (14) W. Brugel, T . Ankel, and F. Krtickeberg, 2 . Elektroehem., 64, 1121 (1960). (15) 9. Castellano and J. S. Waugh, J . Chem. Phys., 37, 1951 (1962). (16) This question has been considered recently by R. W. Vreeland and D. F. Swinehart, J . Am. Chem. SOC.,85, 3349 (1963), who discuss a suggestion of B. 8. Rabinovitch. (17) P. Nangia and S.W. Benson, {bid., 84,3411 (1962). Moreover, the importance of recycliration vs. decomposition of the biradical will influence the observed rate of formation of the final products. (18) R. J. Ellis and H. M. Frey. T r a n s . Faraday Soc., 59, 2076 (1963).
The Contraction of Polyvinyl Alcohol Films in Aqueous Mediala
by Hajinie Noguchi'b and Jen Tsi Yang Cardiovascular Research Institute and Department of Biochemistry, University of California Medical Center, B a n Francisco, California (Received February 3, 1964)
Recently, we studied the contraction of poly+ glutamic acid film as a result of conformational change (coil to helix).a By chance, we found that added salt affected the coiled form of a film previously crosslinked with polyvinyl alcohol (PVA) more than one that had been cross-linked with glycerol. This led us to believe that the conformation of PVA, an uncharged polymer, depends on the added electrolyte or nonelectrolyte in the solvent. Thus, in this note we present the results of experiments using PVA film under various conditions. The contraction of the films in the presence of certain monovalent ions or sucrose might be termed "osmotic" and can probably be attributed to (1) (a) This work was supported by grants from the U. S. Public Health Service (GM-10880, GM-K3-3441, and HE-06285) ; (b) San Francisco Heart Association Researrh Fellow; on leave from AichiGakugei University, Okaraki, Japan, 1962-1963. (2) H. Noguchi and J. T . Yang, Biopolymers, in press.
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the dehydration from hydroxyl groups inside the film. On the other hand, LiBr, divalent cations, and the “denaturing” agents (urea and guanidine hydrochloride) dissolved the films; the mechanism is probably related to the solubility of these compounds in alcohol.
Experimental PVA was a gift of the DuPont Company. Two fractions having the molecular weights of 84,000 and 47,000 (as determined from their intrinsic viscosities in aqueous solutions; [17] dl./g. = 14 X 10-4M0.6’J)a were used ; their contractile properties, however, were the same. The PVA films were prepared by spreading a 2% solution on a chromium-plated metal plate and heating to dryness for about 2 hr. at about 80’. The film strips peeled off the plate were further heated under a small load (2-3 g.) a t 110’ for 90 to 120 min. The dried films (about 0.03 X 4 X 40 mm.9 swelled isotropically when suspended in water and reached the equilibrium state within a few minutes, the length increasing by about 30y0. Kuhn and his co-workers4f5suggest the possible existence of cross linking in the film through etherification of the hydroxyl groups in the PVA during heating. The film prepared for this study was insoluble in water a t room temperature. However, the dried film would dissolve in hot water at about 65’ within 10 to 20 min. Thus, there is no evidence of such cross linking inside the film, and its insolubility in water might be attributed to the entanglement of the polymer chains and the formation of some crystalline regions during drying. Results and Discussion Reversible Contraction of P V A F i l m in N a C l Solution. Figure 1 shows the relative length, L/Lo,of the PVA films when they are suspended alternately in 50 ml. of double-distilled water (Lo) and 5 M NaC1. The 5min. periods represented about 98% of the maximum contraction and elongation at the equilibrium state, which could be reached in about 30 min. This process is completely reversible, as is that observed for the polypeptide film.2 As expected for an un-ionized polymer, the pH of the solvent media did not affect the length of the films; for example, length was coilstant between pH 2 and 11.5. E$ect of Solvent Composition. Figure 2 shows the L / L , of the PVA film in various solvent media. ( a ) Monovalent Salts. Both NaCl and K F caused the film to contract. The L / L o changed linearly with increasing salt concentration up to 5 M and it leveled off at higher concentrations of KF. On the other hand, LiBr expanded the film at concentrations up to 3 M ; in 4 M LiBr the film broke up after about 15 min. The Journal of Physical Chemistry
1.00
4 0
5
IO
IS
2s
PO
TIME. MIN.
Figure 1. Reversible contraction of polyvinyl alcohol film in alternating water and 5 M NaC1.
0.70
0.80
0.00 GUANIDINE
H CI
L/L, 1.00
1.10
i‘ t
\
i
II
Figure 2. Relative lengths of polyvinyl alcohol films in various solvent media. Each point (but not the broken arrows) represents the equilibrium state.
and finally dissolved in about 9 hr. (see section c on divalent ions). ( b ) Sucrose. The film shrank in sucrose, but the contraction was nonlinear (Fig. 2 ) . The fact that salts and small organic compounds have the same effect on the PVA film leads us to believe that “osmotic contraction” is a plausible explanation for these results. Raising the osmotic pressure in the solvent causes the water of hydration to diffuse out of the film. This does not imply that the added salt or sucrose molecules cannot penetrate the film. Indeed, we observed that when sucrose was added to the solvent, the film contracted in the first few minutes and later elongated slightly to reach the equilibrium state. This seems to indicate that the rapid dehydration of the film was followed by a slow diffusion of sucrose molecules into the film. The extent of dehydration, however, outweighs the diffusion of the sucrose or salt molecules. To substantiate this explanation further, we also (3) H. Dieu, J . Polymer S e i . , 12, 417 (1954). (4) W. Kuhn. Ezperientia, 5, 318 (1949).
(5) W. Kuhn, B. Hargitay, A. Katchalsky, and E. Eisenberg, Nature. 165, 515 (1950).
NOTES
measured the intrinsic viscosities, [ q ] , of the PYA (fraction I) solution in various YaCI and sucrose concentrations. From pure water to 0.5 M NaCl or sucrose, [ q ]remained virtually constant a t 1.3 dl./g. At higher salt or sucrose concentrations, it dropped to 0.97 at 1 X ,clearly indicating a coiling up of PVA. Beyond 1 M NaC1 or sucrose, [VI began to increase, probably because of aggregation; at 1.5 M it was 1.5 in sucrose and 1.15 in SaCl; at concentrations higher than 1.5 dd, P V A began to precipitate. We also found that KC1, KF, and CsC1, but not LiBr (see section c), are all poor solvents for PVA, as is NaCl. (c) Divalent Cations. In contrast to the monovalent salts (except LiBr), the divalent cations, Ca2+, Mg2+, and Cu2+ swelled the PVA film. The film elongated about 1 and 5% in 2 M CaClz and MgC12, but dissolved in 4 M CaClz or 3 M RlgC12. The film length increased about 3% in 1 M C:uC12, about 50% after 20 hr. in 2 X CuC12, and about 400% in 3 A4 CuC12. Furthermore, with Cu2+ the elongation was irreversible. For comparison, we found that the intrinsic viscosity of the PVA (fraction 11) solution increased from 0.89 dl./g. in water (plH 6.4) to 1.18 in 3 %’A hlgC12 (plI 5.7) and 1.08 in 4 M LiBr (pH 8.1). Thus, the chlorides of the three divalent cations and LiBr studied ha’ve a strong affinity for the PVA molecules. They penetrate into the film, probably break up the crystalline regions, and expand the polymer coils. This in turn prevents the “osmotic contraction” observed previously for some monovalent salts and small organic compounds. In this respect we note that LiBr, CaCl,, llgCl2, and CuCI, are all soluble in ethanol, whereas NaCl and KCl are only slightly soluble, and KF is insoluble in ethanol. Perhaps the different behavior of PJ’A with different salts is closely related to the aEnity between the hydroxyl groups of PVA and these salts. ( d ) Urea and Guanidine Hydrochloride. The PVA film elongated by about 7% in 8 III urea (Fig. 2). Guanidine hydrochloride is a more effective dispersing agent than urea; in 5 M salt the film dissolved after several hours of exposure. These two compounds are well known as denaturing agents for globular proteins, although the mechanism of denaturation is still unsettled. The effect of urea on a neutral polymer, polyvinylpyrrolidone, was studied by Klotz and R u s ~ e l l . ~They found a slight decrease in the intrinsic viscosity when the polymer was dissolved in 8 114 urea instead of pure water; this they attributed to a possible decrease in the degree of hydration of the macromolecule. We found, however, that the intrinsic viscosity of a I’VA solutioii (fraction 11) increased from 0.89 dl./g in water to 1.02 in 8 M urea, Thus,
1611
-
the PVA molecules can be swamped with urea or guanidine hydrochloride molecules, although the mechanism of such interaction is still unknown.
Acknowledgment. We thank Professor 31. F. Morales for his stimulating discussions. Technical assistance by Miss K. Graham is gratefully acknowledged. ~
~~~
(6) I. M. Klotz and J. W. Russell, J . Phys. Chem., 6 5 , 1274 (1961).
Common Ion Effects on the Solubility of Silver Chloride and Thallous Bromide in Fused Nitrate Solvents
by Ralph P. Seward and Paul E. Field’ Department of Chemistry, Pennsylvania State University, University P a r k , Pennsylvania (Received February 10, 1964)
The authors2 have recently calculated solubilities for salts of type AC in fused salt solvents BD from the thermodynamic properties of the two components and those of the reciprocal salt pair AD and B e . I n the systems investigated, where the solubilities were small, reasonable agreement of calculated and experimental values was found. It was thought to be of interest to extend the measurements to include the changes in solubility produced by the addition of salts having an ion in common with the solute to see to what extent similar thermodynamic calculations would serve to predict the observed effects. For this purpose the solubilities in potassium nitrate of thallous bromide in the presence of added thallous nitrate and potassium bromide and the solubility of silver chloride in sodium nitrate in the presence of added silver nitrate and sodium chloride have been measured. In an earlier note one of the authors3 showed how the solubility of silver chloride a t 300” expressed as mole fraction of AgCl increased from a value of 7 X 10-4 in pure potassium nitrate on addition of silver nitrate to a mole fraction of 0.55 in pure silver nitrate. This behavior is qualitatively reasonable. When silver chloride dissolves in silver nitrate the number and kind of nearest neighbor, anion-cation interactions must be essentially equal to those existing in the separate com(1) From the Ph.D. Thesis of Paul E. Field, Pennsylvania State University, August, 1963; supported by the U. S. Atomic Energy Commission under contract AT(30-)-1881. ( 2 ) R. P. Seward and P. E. Field, J . P h y s . Chem., 6 8 , 210 (1964). (3) R. P. Seward, ibid., 63, 760 (1959).
Volume 68, N u m b e r 6 J u n e , 1964
