KINETICS OF HALOGEN EXCHAXGE
Dec., 1963
The importance of the role of the co-ion in Donnan equilibria has previously been overlooked. This has been due in part to the fact that much attention has been given to the more obvious interactions of polyions with counterions, some studies of which have seemed to illustrate the lack of importance of coion interactions. The aforementioned study of Nagasama, et aZ.,21showed that, whereas the activity coefficients of the sodiurn ion decreased, activity coefficients of the chloride ion increased with decreasing KaC1
2557
concentration, except in the cases of 0.005 and 0.001 ycl- decreased markedly, particularly at high polyelectrolyte concentrations. Such observations are consistent with the coiicepl outlined above, wherein, at low salt concentrations. the degrrc of intcmction of co-ions increases significantly. Acknowledgment.-The author expresses his appreciation to Dr. Robert Kunin for many helpful discussions during the course of this work.
N solution. I n the latter cases, values of
KINETICS O F HALOGEN EXCHANGE BETWEEN IODIDE ,4ND IODOACETIC ACID AND BETWEEN BROMIDE AND BROMOACETIC ACID BY J. F. HINTOX ilKD F. J. JOHNSTON Department of Chemistry, University of Georgia, Athens, Georgia Received April 39, 1963 I n aqueous Bystems halogen exchange occurs between bromide and bromoacetic acid and between iodide and iodoacetic acid. Exchange rates are first order with respect to each of the exchanging species. Second-order rate constants for the bromide and iodide systems in 0.628 M nitric acid may be expressed as kx = k T / h exp[ - 13 4/ R] exp[ -18,98O/RT] and kx = kT/h exp[--14.6/R] exp[- 15,980/RT] 1. mole-' sec.-1, respectively. The rate constants are essentially independent of nitric acid concentration from 0.006 to 0.63 M . A comparison is made of these results with those for the chloroacetic acid--chloride system. Enthalpies of activation for the exchange reactions appear to depend linearly upon the C-X bond strength, while those for the hydrolysis reactions vary only slightly.
Introduction The kinetics of ha,logen exchange between chloride ions and chloroacetic acid' and between iodide ions and iodoacetic acid2 have been described previously. This report discusses in more detail the iodide-iodoacetic acid system and the corresponding bromide-bromoacetic acid system. These exchanges occur a t a measurable rate a t room temperature and have been studied over the range of 10 to 49.9'. Exchange rates increase in the order chloride, bromide, iodide. I n the chloride-chloroacetic acid system, exchange rates were conveniently measurable only above 70", and a significant hydrolysis occuirred simultaneously with the exchange. I n the bromide and iodide systems, no hydrolysis was detectable during the course of the exchange reaction. Reaction rates were evaluated in the usual way from plots of log (1 - 8') vs. t. A separation induced exchange prevented our obtaining satisfactory rate data for the iodide system a t total reactant concentrations above approximately 0.03 M . At the concentrations used in our experiments this exchange was always less than 10%. Prestwood and Wah13 have shown that if such background exchange is constant, the homogeneous exchange rate may still be obtained from the slope of a plot of log (1 - 8') vs. t with F being the apparent total fractional exchange. Experimental Eastman White Label iodoacetic and bromoacetic acids were used as reactant material. The iodoacetic acid was recrystallized from ether. The acid purified in this manner had a melting point range of 81 to 82.5'. The bromoacetic acid, with melting point range of 48 to 50" was not further purified. Labeled iodide for the iodide-iodoacetic acid system was prepared in the following manner. Sublimed, reagent grade iodine (1) R. A. Kenney and F. J. Johnston, J . Phys. Chem., 63,1462 (1959). (2) H. V. D. Stratten a n d A. H. Aten, J . Am. Cham. Hoc., 76, 3798 (1954). 13) R. Prestwood and A+C. Wahf, obid., 71, 3137 (1949).
dissolved in heptane was tagged by means of a heterogeneous exchange reaction involving equilibration with a dried sample of the sodium iodide-131 as obtained from the Oak Ridge Isotope Sales Department. An exchange of iodine between that in the liquid phase and the surface iodide ions occurs readily and produces labeled molecular iodine of controllable specific activity. This solution of iodine-131 in heptane was then equilibrated with reagent grade potassium iodide, producing the iodide-131 used as the iodide tracer in these experiments. For the bromide-bromoacetic acid system, potassium bromide82 as obtained from Oak Ridge, was used as a tracer without further purification. Experiments were carried out in 0.628 M nitric acid to ensure the presence of the haloacetic acid in the molecular form. Reactions were carried out over a temperature range of 10 to 49.9" with temperature control within &0.05". Aliquots of 10 cc. were withdrawn a t intervals from the reactant solution, placed on ice in a beaker to quench the reaction, and titrated potentiometrically with silver nitrate. All titrations were performed at 0" to reduce separation induced exchange. It was found that excessive background exchange occurred in the iodide-iodoacetic acid system unless the concentration of the iodoacetic acid was low, and unless the separation process was performed a t reduced temperature. For example, with 0.172 M iodoacetic acid and 0.089 M iodide, the exchange occurring during the silver iodide precipitation process amounted to 30%. It is felt that this background exchange occurs between iodoacetic acid and colloidal silver iodide formed during the early stages of the precipitation. Following precipitation, the solutions were filtered and diluted to a volume of 50 cc. Aliquots of 4 cc. of each solution were counted using a scintillation detector. The counting rate for a reactant solution was obtained by diluting 10 cc. of the original material to 50 cc. and counting 4-cc. aliquots. The fraction of equilibrium exchange a t time t was obtained from
Fg =
(specific activity of CH&COOH) (specific activity of total X ) t =
E
In every case the net counting rate was characterized by a standard deviation of less than 2%.
Results and Discussion Typical plots of log (1 - F ) vs. t are shown in Fig. 1
J. F. HINTON AND F. J. JOHXSTOK
2558
,
0
Vol. 67 TABLE I1
SUMMARY
REACTIOSRATECONSTANTS FOR ACID-IODIDESYSTEM
OF
THE IODOACETIC
[CHaI-
-0.1
[HNOsI,
COOHI, mole 1. -1
Temp., OC.
mole L-1
38.7
0.628
0.0206 ,0091 .0091
31.3
.628
,0150 ,0108 .0148
24.0
.628
.0177 .0199 .0090
14.0
,628
,0187 ,0105 .0208
26.1
.628 .0063
,0176 ,0226
-0.2 h
I L
[I-1,
kx,
mole 1.-'
1. mole-' hr.-1
0.0051 ,0051 ,0025 Av ,0025 ,0025 ,0051 Av. .0051 ,0025 ,0025 AV. ,0025 ,0025 .0051 Av. .0050 .0050
101.5 90.0 94.5 95.3
1
= -0.3 I
UI
3
-0.4
-0.5
i-
A\
\ 20 40 60 80 100 120
0
t (mid.
-
Fig. 1.-Typical plots of log (1 I") us. t for the iodide-iodoacetic acid and bromide-bromoacetic acid systems at 38.7" with molar concentrations: A, [CHJCOOH] = 0.0206, [I-] = 0.0051; B, [CH2ICOOH] 0.0091, [I-] = 0.0051; C, [CHp ICOOH] = 0.0025, [I-] = 0.0025; D, [CH2BrCOOH]= 0.1940, [Br-] = 0.0117; E, [CHZB~COOH]= 0.1271 M, [Br-] = 0.0218 31.
for both systems a t 38.7'. The linearity of the plots for the iodoacetic acid system indicates that for the concentrations studied, the separation-induced exchange is constant for a given series and, therefore, that calculation of exchange rates from the slopes is justified. I n Tables I and I1 are summarized second-order rate constants for the two systems a t the several temperatures studied. The consistency of these rate constants over wide ranges of reactant concentration indicates that both exchanges are first-order with respect to the halide and haloacetic acid concentrations and very likely occur through a bimolecular displacement reaction. TABLE I SUMMARY
OF
RATE CONSTANTB FOR THE RROMOACETIC ACIDBROMIDE SYSTEM [CHsBrCOOHI, mole I.-'
Temp., OC.
[HNOal, mole I.-'
49.9
0.6280
0.0606 .1574 .I548
38.7
.6280
.1940 ,2483 ,1270 .0218 .0398
26.7
.6280
.2940 .2960 .2960 .570
10.0 50.4
.6280 .6280 .0628 ,0063
,405 ,2100 .1995 ,2580
[Br-I, mole I.-'
0.0108 .0110 .0258 Av. .0017 ,0089 .0218 .0026 .0003 Av. .0691 ,0068 .0272 ,0284 Av. .0261 ,0197 .0197 .0197
kX, 1. mole-' hr.-I
4.26 4.20 4.32
kx = kt e A S * / R
0.393 ,396 ,390 ,376 0.390 ,058 4.41 4.40 4.42
25.5 10.2 8 83 10.4 9.S2
31.2 30.5
e - A ~ * / ~ ~
For the bromide-bromoacetic acid system the enthalpy of activation is AH* = 18,980 f 75 cal. rnole-l, and the activation entropy is AX* = -13.4 f 0.2 cal. mole-l deg.-l. For the iodide-iodoacetic acid the corresponding values are AH* = 15,980 i 380 cal. mole-l, and AS* = -14.6 i 1.3 cal. mole-I deg.-l. The variations given are standard deviations obtained in the same least squares program. These results are summarized in Table I11 along with those for the chloridechloroacetic acid system. Also included in Table I11 are pseudo first-order enthalpies of activation for hydrolysis of the three acids. TABLE 111 SUMMARY
1.40
53.8 25.3 22.5 28.6
Also listed in these tables are results of several series of experiments in which the effects of varying the nitric acid concentration was tested. It is seen that there is no significant change in the rate constants over a 100fold variation in the nitric acid concentration. This result is consistent with our observations of the chloroacetic acid-chloride system in which the HC1 concentration was varied over a wide range with no evident change in the rate constant. Enthalpies and entropies of activation were obtained by a least square fitting of the data for the two systems to the expression
4.26 1.41 1.40 1.42 1.40 1.33
54.5 55.3 51.5
OF
System
EXCHANGE AXD HYDROLYSIS COXSTANTS HALIDE-HALO ACETIC ACID SYSTEMS AH* (hydrolysis), cal. mole-1
AN* (exchange), cal. mole-1
FOR THE
AS
*
(exchange), oal. mole-' deg.-l
CH2ClCOOH-Cl" 24,150 23,850 zt 150 - 10.5 f 0.5 CHZBrCOOH-Br 23,070"' 18,980 f 75 -13.4 f 0 2 CH2ICOOH-I 22, 580b 15,980 i 380d - 14.6 f 1.3 F. Kunze and H. Merkader, Z. Physilc. Chem. a See ref. 1. (Leipzig), 187, 285 (1940). The activation energy for the hydrolysis of bromoacetic acid has been determined independently in our laboratory, and the value obtained is in good agreement with that obtained by Kunze and Merkader. d Stratten and Aten2 list, 16 kcal./mole as the activation energy for this eschange.
Dec., 1963
DISSOCIATIOX OF IMINODIACETIC ACIDGROUPS
2559
The small change in activation enthalpies for the hydrolysis reactions of the three acids, as contrasted to that for the exchange reactions, suggests that quite different mechanisms are involved for the two processes. A close relationship is evident between the enthalpy of activation for exchange and the corresponding C-X bond dissociatioii enthalpy. Our exchange enthalpies are very closely approximated by AH* = 0 . 2 9 0 ~ - x where Dc-x is the standard dissociation enthalpy a t 2Z0.4 (The use of the latter quantity, which is characteristic of a gas phase reaction, is partially justified in that the products and reactants of the exchange reactions are identical.) The virtual independence of the second-order exchange constants for b’othsystems of the nitric acid concentration over a 100-fold variation indicates a unique absence of ionic strength effects over the range studied. A variation of the effective activity coefficient of the transition state with ionic strength equivalent to that of the product of the activity coefficients of reactant species is implied by this result. The decreasing entropies of activation for exchange, in order chloride, bromide, iodide, are consistent with an increasing steric effect due to the increasing size of
the halide ion. The variation of the activation entropy with the mass of a reactant depends upon the configuration of the transition state and is not susceptible to a reliable prediction. I n any case we feel that such a mass effect would be too small to explain the observed change in entropy. In the classic work of de la Mare, Hughes, Ingold, and co-w~rkers,~ halogen exchange between halide ions and several series of alkyl halides were measured in acetone. Their interest lay primarily in the steric and ponderal effect of various substituted alkyl groups with regard to exchange or replacement of a given halogen. Their results, however, permit the calculation of exchange entropies, for several series of halides. For ethyl chloride, bromide, and iodide, the entropy change increases in that order. For methyl chloride and bromide, the change is also in this direction. These results are in contrast to ours in the aqueous systems and suggest that a very marked solvent effect must be considered as a part of the frequency factor in explaining the observed variation in the activation entropy.
(4) S. W. Benson, ”The l~oundationsof Chemical Kinetics,” McGrawHill Book Co., Inc., New Yoi k, N. Y., 1960, p. 670.
(5) P. B. D. de la Mare, L. Fowden, E. D. Hughes C. K. Ingold, and J. D. €1. Mackie, J . Chem. Soc., 3196 (1955).
Acknowledgment.-This work was supported by the Atomic Energy Coinmission Contract AT-(40-1) 2826.
THE DISSOCIATION O F IMINODIACETIC ACID GROUPS INCORPORATED IN A CHELATING ION-EXCHAKGE RESIN’ BY JOSEPH KRASXER~ AND J.ACOB A. R ~ A R I S S K Y ~ Department of Chemistry, State University of h-ew York at Bufalo, Buffalo, iYew York Received M a y .dl 1063 The electrolytic dissociation, in a limited pH range, of the iminodiacetic acid groups attached to a styrenedivinylbenzene copolymer matrix of a chelating resin has been investigated. The potentiometric behavior of the system during controlled neutralization of the acid form of the resin has been studied together with the variation of the osmotic properties of the differently dissociated forms for this purpose. An intrinsic disbociation constant of -2 X 10-3 is calculated for the monomeric unit. This value iv in good agreement with the first acid dissociation constant of 2.9 X 10-8 that has been reported for iminodiacetic acid.
Introduction Recently J. A. AI. has demonstrated that potentiometric and water absorption data that are obtained with relatively highly cross-linked ion-exchange resins can be interpreted quantitatively for evaluation of thermodynamic properties of the e~changers.~JThe intrinsic dissociation constant of cross-linked polymethacrylic acids of commercial origin6 was evaluated from such data. The pK value that was obtained was found to be independent of degree of cross-linking, degree of dissociation, and ionic strength and was consisteiit with the value that has been reported for the linear polymer analog.’ (1) Doivex Chelating Resin A-1, a product of the Dow Chemical Company, Midland Mioh. ( 2 ) This is an essential porlion of a thesis submitted to the Chemistry Department, State University oi’ Keiv York a t Buffalo, in partial fulfillment of the requirements of a M.A. dtgree. (3) Correspondence to be addressed to this author, Polymer Dept., the Weizmann Institute of Science, Rehovoth. Israel. (4) A. Chatterjer and J. A . Marinsky, J . Phys. Chem., 67, 41 (1963). (5) J. A. Xarinsky and A. (Chatterjee. zbzd., 6’7, 47 (1963). (6) Amberlite IRC-50, a product of Rohm and Haas Co., Philadelphia, Pa. (7) A. Katchalsky, X. Sharit, and H. Eisenberg, J. Polymer S e i . , 13, 69 ( 1954).
Equation 1 was used to analyze the data4,*and is based upon combination of the Gibbs-Donnan equation for a membrane equilibrium in the presence of a hydrostatic p r e s ~ u r e gwith ~ ~ ~ the theoretical treatment of polyelectrolytes due to Katchalsky and co-workers.lO*” 1-a pH = PKHR - log ___ a
+ log + 0.4343 -X kT &I+
-
all+
In this equation, a is the degree of dissociation a t any dissociation site, Feis the electrostatic free energy of the ionized polyanion carrying v negatively charged groups, K is the inverse Debye radius determined by the concentration of the small M+ ions in the resin phase, 1’ represents the partial molal volume of electrolytes MX and HX, II is the contractile pressure of the resin net(8) I. Michaeli and A. Katchalsky. zbzd.. 23, 683 (1957). (9) (a) F. G. Donnan and E. A. Guggenheirn, Z . physak. Chem. (Leiwig), 1628, 356 (1932); (b) F. G. Donnan, %bad., 16SA, 369 (1934). (10) S.Lifson and A. Katohalsky, J . PoZymeT Scz., 13, 43 (1954). (11) A. Katchalsky and S.Lifson, ibzd., 11, 409 (1953).
