972
ANTONIO INDELLI
negligible chain-to-chain crosslinking during exposure. This is known to be true during PMMA degradation by ionizing radiations. 33 Moderate extents of concurrent crosslinking would not sensibly affect the viscosity-molecular weight obs e r v a t i o n ~ . ~A ~ ' ~certain ~ parallelism between effects of ionizing radiations and ultraviolet light on poly-alkyl-methacrylates might be remarked in passing. Increasing length of alkyl side chains in the homogeneous poly-n-alkyl-methacrylates favors increasing chain-to-chain crosslinking relative to backbone scission under ionizing radiation.35J8,39 Beyond poly-n-hexyl methacrylate (36) B. H. Zimm and R. W. Kilb, J . Polymer Sci., 37. 19 (1959). (37) R. W. Kilb. ibid., 38, 403 (1959). (38) R. K. Graham, rbid., 38, 209 (1959). (39) A. R. Shultr, P. I. Roth and J. M. Berge, Paper presented at the 135th National American Chemical Society Meeting, Boston, Mass., April, 1959.
Vol. 65
crosslinking predominates over scission arid infinite networks are formed. A similar progression occurs in poly-alkyl-methacrylate films undergoing ultraviolet light irradiation in air; infinite network formation occurs a t shorter side-chain lengths than under ionizing radiation conditions.40 Quantitative investigations of polymer reactions under light and ionizing radiations should permit comparison beneficial to basic interpretations in both areas. Acknowledgments.-The assistance of Rlr. James B. Watrud in executing a large portion of the experimental work is gratefully acknowledged. The aid of Mr. Floyd C. Vincent in performing the ultraviolet spectrophotometric measurements is also appreciated. (40) R. L. Feller, Mellon Institute of Industrial Research, privste communication, May, 1957.
KINETIC SALT EFFECTS BY THE TETRAALKYLAMMOYIUNT. IONS BY ANTONIOINDELLI Chemistry Department of the University of Ferrara, Ferrara., Italy Received November 6, 1960
The salt effects produced by tetraalkylammonium salts have been measured in five reactions involving anions. For the bromate-iodide and the iodate-iodide reactions the effects are in agreement with the Brflnsted-Debye theory. For the ethyl oxalate-hydroxide reaction the effects are lower than normal. For the ferricyanide-iodide reaction the effects are opposite to those predicted by the Br~nsted-Debyetheory. The activated complexes have charges 0, - 1, -2 and -5, respectively. These results are interpreted in terms of short range interactions, which are more important when the negative charge of the activated complex is high and when non-electrostatic factors are involved. The results for the bromoacetate-thiosulfate reaction are closer to the predictions of the BrGnsted-Debye theory than those for the ethyl oxalate-hydroxide reaction. This fact is discussed on the basis of the absence of non-electrostatic factors, and of the organic character of the tetraalkylammonium salts.
Differences between the salt effects of the tetraalkylammonium and the alkali-metal salts have been reported in several occasions. I n some cases these differences have been attributed to the failure of the tetraalkylammonium ions to form ion pairs with the reacting anions. l s 2 Association phenomena have been considered by Kurz and Gutsche too.* In some other instances the micelle formation by these large organic ions seems to be releant.^ Previous work on kinetic salt effects in reactions between anions seems to indicate that the higher the negative charge of the activated complex, the more anomalous the effects of the tetraalkylammonium salts. I n the reaction of potassium ethylmalonate with sodium hydroxide tetraethylammonium nitrate has a smaller effect than sodium or potassium nitrate.6 In the reaction of persulfate with iodide tetramethylammonium bromide has a much smaller accelerating effect than sodium or potassium chloride, and tehaethylammonium bromide has a still smaller effect, and has a retarding effect in a certain range of concentrations.6 In the reaction of trimetaphosphate (1) R. M. Healy and M. L. Kilpatrick, J. A m . Chem. Soc., 77, 5258 (1955). (2) J. R. VanWszer, E. J. Griffith and J. F. McCullough, ibid., 77, 287 (1955). (3) J. L. Kurr and C. D. Gutsche, ibid., 82, 2175 (1960). (4) E. F. J. Duynstee and E. Grunwald, i b i d . , 81, 4540, 4542 (1959). (5) A. Indelli, G. Nolan, Jr., and E. S. Amis, ibid., 82, 3237 (1960).
with sodium hydroxide tetramethylammonium chloride has a retarding effect, jn qualitative disagreement with the Rrgnsted-Debye theory on the salt effects.' Therefore we thought it interesting to study the salt effects of some tetraalkylammonium salts on a series of reactions of different charge type. In all cases solutions as dilute as possible were used, to minimize the differences in the activity coefficients arising from the different ionic radii.
Experimental Materials.-Most of the chemicals were Erba R P products and were recrystallized from conductivity water. Tetramethylammonium nitrate was pre ared and purified as described in a previous paper.8 '%etraethylammonium perchlorate was an Erba RS product, specially purified for polarographic use; it was found t o be ash-free, neutral and with no buffer action. Tetra-n-butylammonium perchlorate was prepared from tetra-n-butylammonium bromide in aqueous solution, by precipitating with perchloric acid, and was recrystallized three times from conductivity water. Potassium ethyloxalate was prepared from the diethyl ester by the Nielsen's method,g and was recrystallized from absolute ethanol. Sodium bromoacetate was prepared by neutralizing an aqueous solution of monobromoacetic acid Erba RP, which had been standardized with sodium hydroxide; it was kept in a refrigerator and diluted just before use. It contained only a barely detpct16) A. Indelli and J. E. Prue. J . Chem. Soc., 107 (1959). (7) A. Indelli, Ann. C h i n . (Rome), 48, 332 (1958). (8) A. Indelli, {bid., 48, 345 (1958). (9) R. F. Nielsen, J. Am. Chem. Soc., 58, 306 (1936).
LHM i E ' l K A A L K Y L A M M U N l U M l U N S
able trace of bromide ions. All the water used for the solutions and tlie runs \vas prepared as described in previous papers.6 Procedures.-The experiments on the reactions between bromate and iodide and between iodate and iodide were performed as described in previous papers.lO,ll The reaction between ferricyanide and iodide was followed by adding some starch to the reaction mixture and by titrating the iodine with a Na2S203solution delivered from an "Agla" microsyringe. Very small and almost constant amounts of titrating solution were added and the end-point was taken when the last amount failed to give any discoloration. The sharpness of the end-point was satisfactory and ten of such titrations were performed during each run, the total amount of added Na,S20a corresponding to about 0.4% of the ferricyanide. Preliminary experiments showed that no appreciable reaction of the ferricyanide with the thiosulfate or the tetrathionate did occur. The reaction between potassium ethyloxalate and sodium hydroxide was followed with a method similar to that used for the reaction of potassium ethylmalonate with sodium hydroxide,&only minor modifications being introduced. The samples and the quenching solution of potassium hydrogen phthalate were measured by volume, whereas the titrations were performed with a weight buret. The color of the indicator during the titrations was compared with that of a buffer solution, freshly prepared, having exactly the same volume and the same indicator concentration. In this way more accurate end-points were obtained. The reaction of sodium bromoacetate with sodium thiosulfate was also followed with a sampling technique. At suitable intervals 100-ml. samples of the reacting mixture were withdrawn from a polyethylene reaction bottle and were titrated with a 0.00032 M solution of iodine, the polarized platinum electrode being used as an indicator. The reaction was followed for eight to eleven days, which correR onded to about 30 to 40% reaction. $he temperature was kept to 25.0" with a water thermostat in all cases, but for the reaction bromoacetatethiosulfate where an air thermostat was used. The accuracy waa about 2 to 4% for the reactions iodate-iodide and bromate-iodide, 5 to 8% for the reaction ferricyanide-iodide, 2 to 3% for the reaction ethyloxalate-hydroxide and 1 to 2% for the reaction bromoacetate-thiosulfate.
Y1.3
tween ethylmalonate and hydroxyl iamb I t seems therefore that another important factor in determining these salt effects must be non-electrostatic in nature, and in fact activation energy measurements show that prominent non-electrostatic effects are present in the persulfate-reaction and absent in the bromoacetate-thiosulfate reaction.14-" On t'he other hand the presence of nonelectrostatic effects in the ethyloxalate-hydroxide reaction is also suggested by the abnormal salt effects by multivalent cations,'* and by the geometry of the activated complex.14 The same considerations suggest that the non-electrostatic effects should be more importoant in this reaction t,han in the ethylmalonate-hydroxide. The tetramethylammonium nitrate has a remarkable accelerating action on the bromoacetate-thiosulfate reaction. This could be correlated with the organic nature of the cation, as it is known that this reaction is accelerated by organic substances, l9 and shows also abnormal salt effects in the presence of large organic anions.20 On the other hand tetraalkylammonium and other large organic ions have abnormal salt effects on some first-order reactions.21#22
Discussion
The simplest interpretation of the above results is that the greater the negative charge of the activated complex, the greater is the importance of the short range actions on t'he part of the cations. The presence of non-electrostatic effects should enhance the importance of the short range actions because the decrease in the repulsion between the reactants, or the increase of probability of formation of the activated complex, is accompanied by a decrease of the intrinsic energy barrier which must Results Table I reports the rate constants for the five be overcome to form the products.14 A polarizareactions in the presence of tetraalkylammonium tion of the reactants or of the activated complex salts, and for comparison the reaction rate con- seems to be a reasonable explanat,ion for these nonstants obtained in the presence of potassium nitrate. electrostatic effects. A similar phenomenon has The latter data are taken from previous papers.1°-13 been postulated to account for some salt effects in The correlation with the charge of the activated the aromatic nucleophilic substitution reactions.23 complex is satisfactory, except in the case of the The tetraalkylammonium ions, because of their thiosulfate-bromoacetate reaction, and in fact the large size, cannot have a great short range action, difference between potassium nitrate and the tetra- and therefore cannot have an accelerating action alkylammonium salts increases when the charge comparable to that of the smaller sodium or potasof the activated complex increases. For the ferri- sium ions. Moreover, due Do their polarizability, cyanide-iodide reaction, where the charge of the they can cluster around a highly charged activated activated complex is -5,12 the salt effect is re- complex and prevent the smaller ions to get near versed. The results obtained by Kurz and Gutsche3 enough the activated complex to exert any appreciand by Duynstee and Grunwald4 are also in agree- able short range action. A rather similar mechament with this general trend. The charge of the nism has been postulated by Davies and Dwyer to activated complex, however, is not the only in- explain the retarding action that large anions and (13) A. Indelli, ibid., 1, 143 (1960). fluential factor. Actually the effects of the tetra(14) A. Indelli and E. 9. Amis, J. A m . Chem. Soc., 82,332 (1960). alkylammonium salts on the bromoacetate-thiosul(15) H. G. Davis and V. K. LaMer, J . Chem. Phye., 10,585 (1942). fate reaction are more normal than the correspond(18) P. A. H. Wyatt and C. W. Davies, Trans. Faraday Soc., 45,774 ing ones in the persulfate-iodide reaction,6 where (1949). (17) C. W.Davies and 1. W. Williams, ibid., 54, 1547 (1958). the activated complex has the same charge of -3, (18) J. 1. Hopp6 and J. E. Prue, J . Chem. Soc., 1775 (1957). and then those on the ethyloxalate-hydroxide (19) V. K. LaMer and M. E. Kamner, J . A m . Chem. Soc.. 57,2669 reaction where the activated complex has charge (1935). -2. The effects on this last reaction are also less (20) J. A. Erikson and E. C. Lingafelter, J . Colloid Sci., 4, 591 normal than those on the very similar reaction be- (1949); 10, 71 (1955). (10) A. Indelli, G. Nolan, Jr., and E. S. Amis, J . Am. Chem. Soc.. 3233 (1960). ill) A. Indelli, J . l'hus. Cheni., 65, 240 (1961). (12) A. Indelli, Ann. unto. Ferrara Nuovo Serze, 1, 129 (1960).
(21) J. P. Hunt and R. A. Plane, J . A m . Chem. Soc., 76,5960 (1954). (22) A. Jensen, F. Baa010 and 11. M. Neumann, ibid., 80, 2354 (1958). (23) J. D. Reinheimer, W. F. Kieffer. S. W. Frey, J. C. Cocliran and E.W. Barr, J . A m . Chem. SOC.,80, 164 (1958).
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h T O X I 0 INDELLI
TABLEI REACTION RATECONSTANTS IN THE PRESENCE OF TETRAALKYLAMYONIUM SALTS Added salt con cn .a
Reactants concn.“ and units of k’
KBrO3 = 8.3; KI = 25; H S 0 3 = 25; k’ = k equiv.-+ sec.-l); k’o = 7 X b KIO3 = 0.83; KI = 3.75; HSO3 = 5.0; k‘ = lo-@ k ( L 4 equiv.? sec.-’j; kf0 = 6.4b
100 200 20 40 100 200 200
KN03
6.1 5.6 5.8 5.4 4.81 4.17 K3Fe(CX)&= 100; KI = 400; k‘ = 104k (l.z eyuiv.+ sec.-l); 2.G9 k’o = 2.45b 400 3.31 3.97 800 1600 5.3 Sa2S2O3= 1.5; BrCHzCOOXa = 1.5; 12‘ = lo3 k (1. equiv.-’ 5.62 16 see.-’); = 5.21b 6.28 40 80 6.86 7.92 160 EtCzOaK = 8.3; NaOH = 8.3; k’ = 10 k (1. equiv.-’ sec.-’j; 53 5.61 k’ o - 5.21b 107 5.97 213 6.52 7.02 427 5 Concentration units = mole 1.-’. ko’ = Iz‘ in the absence of added salt.
k’ in presence of MerNNOs EtrNClOa
5 5 4 4 2
8 3 72 14 40
2 44 2 06 1 99 5 94
6 7 8 5 5 5 6
43 21 06 63 79 91 36
7
BurNC.04
6 14 5 45 5 7
54 4 77 4 1-1
2 45 1 92 1 43
0 5 6 6 7 5 5 5 6
56 81 25
83 49 60 72 86 16
5 84 6 17
5 46
cations have on trhe racemization of tris-dipyridyl-
These results do not seem to support the O l w i and Simonson claim that either the Brinsted origiThe short range actions can, t’o a certain extent nal equation or the Debye-Hdckcl theory is unbe considered as due to ionic a s s o ~ i a t i o n , ~ J ~sound.28 * ~ ~ ~ ~ They show once more, honcrcr, that the and if this model is assumed the action of the tetra.- combination of the two leadi t o :t rathw poor apalkylamnionium salts can be considered as a ((sec- proximation, which in many caqes, and particularly ondary salt effect.” In fact one can say that the for reactions between ions of the same sign, caiinot “dissociation constant,” of the “ion pairs” has been be useful. The treatment of Scatchard can prohincreased by the presence of the tet,raalkylammo- ably account for part of the unusual salt ef?wt\ ilium ions. However, t’he consistent difference be- exhibited by the tetraalkylammonium ion$, I)r tween the act’ion of the tetraet,hyl- and the tet’ra- cause of the great importance that the ionic radii , ~ ~a consideration of the methylammonium salt’s is conflicting with this hare in this ~ a l c u l a t i o nbut non-electrostatic effects seems also necessary. interpretation. It might be interesting to observe that tetraa,lkylammonium and perchlorate ions Therefore the salt effects can depend both upoii the are often used in kinetic work to avoid the forma- structure and the charge of the activated complex, tion of ion pairs with reacting anions and cations, so that they can probably give useful indications respectively: both of these kind of salt’shave often a on the mechanism of many ionic reactions. Acknowledgments.-Thanks arc’ due to “salting in” effect on non-electrolyte brogio Giacomelli for valuable u4stancc with the (24) N. R. Daries and F. P. Dwyer, Trans. Faraday Soc., 50, 24 experiments. (1954). ( 2 5 ) E. A . Guggenlieiiii. Disc. Faraday Soc., 24, 53 (1957). ( 2 6 ) G. Scatchard, N a t l . Bur. Standards (U.S . ) , Circ. -Yo., 534, 18:
(1963).
127) r. 1 Lona dlld IF r. h 1 c l ) r ~ l t C h n Ire i\ 51, 119 (1952) ( 2 8 ) h. R. Olson and T. R Siinonson, .I C i e m Phijs 17, 1167 ( l‘j49).
