M. M. Kreevoy and Y. Wang
1924
Kinetic and Equilibrium Acid-Base 5ehavior of Tertiary Amines in Anhydrous and Moist Dimethyl Sulfoxide' Maurice M. Kreevoy** and Yueh Wang3 Chemical Dynamics Laboratory, Depattment of Chemistry, University of Minnesota, Minneapolis, Minnesota 55455 (Received June 6, 1977) Publicatlon costs assisted by the National Science Foundation
The acid dissociation constants of a number of substituted tribenzylammonium ions and of the benzyldimethylammonium ion in anhydrous dimethyl sulfoxide (Me2SO)have been determined. The tribenzylammonium ions give a Hammet p of 4.12. In MezSOsolutions containing water and excess acid the rate of exchange of the N-bound proton of the substituted ammonium ions with the hydroxylic protons has been determined by a dynamic NMR technique. The exchange is strongly catalyzed by water, but also has a water-independent component. The water-independent exchange almost certainly involves dissociation of the proton followed by reprotonation, and the water-catalyzed probably does also. If this is assumed to be the mechanism, the reprotonation rate constants can be calculated. In spite of the spontaneity of the reprotonations none of them is diffusion limited, and a Bransted coefficient, P, around 0.5, is found for both the water-catalyzed and the water-independent reactions. Thus the use of MezSO as solvent gives these N-protonation reactions the characteristics of C protonation. Previous have pointed out the crucial role of solute-solvent interactions in the failure of bimolecular proton transfers, to or from carbon, to reach diffusionlimited rates, even when the reactions are strongly spontaneous. Other author^^,^ have stressed the delocalized electrons of typical carbon bases as the source of this behavior. Aliphatic amines are protonated at an unshared pair of electrons entirely localized on a single nitrogen atom. In the present paper it is shown that the spontaneous protonation of tertiary amine bases, mostly tribenzylamines, takes on the typical characteristics of a C protonation when the solvent is dimethyl sulfoxide (MeZSO). Thus, solute-solvent interaction, rather than electron delocalization, is suggested as the source of the special behavior of carbon bases.
Experimental Section Materials. The tertiary amines were either purchased, prepared by means of a Leukart r e a ~ t i o nby , ~ alkylating ammonia with an appropriately substituted a-halotoluene,1° or by alkylating a symmetrically disubstituted dibenzylamine with a similarly substituted benzyl bromideall Previously known materials were identified by means of their physical and spectroscopic properties. Tris(3-methoxybenzyl)amine,tris(3-methylbenzyl)amine, and tris(3-chlorobenzy1)amine do not seem to have been previously reported. For these, elemental analyses were obtained.12 The methods of preparation and physical properties of all the amines are reported in Table I, and, where possible, compared with values taken from the literature. Anal. Calcd for C24H2703N C, 76.37; H, 7.21; N, 3.71. Found: C, 76.39; H, 7.23; N, 3.56. Anal. Calcd for C24H27N:C, 87.49; H, 8.26; N, 4.25. Found: C, 87.61; H, 8.04; N, 4.13. Anal. Calcd for C21H18NC13:C, 64.55; H, 4.64; N, 3.58; C1, 27.22. Found: C, 64.48; H, 4.61; N, 3.33; C1, 27.36. Both 4-chloro-2,6-dinitrophenol and 2,6-di-tert-butyl4-nitrophenol, and also their respective tetraethylammonium salts, were gifts of Dr. M. K. Chantooni, Jr. Their preparation and purification has been described.13 2,6-Dinitrophenol was purchased from Aldrich Chemical Co. as a 50% slurry with water. It was purified by recrystallization from a 3:l water-ethanol mixture, and dried The Journal of Physlcal Chemistry, Vol. 8 1, No. 20, 1977
TABLE I: Preparation and Properties of Tertiary Amines
Compound Tribenzylamine Tris( 4-methoxybenzy1)amine Tris( 4-methylbenzy1)amine Tris( 3-methoxybenzy1)amine Tris( 4-bromobenzy1)amine Tris( 4-nitrobenzy1)amine Tris( 3-methylbenzy1)amine Tris( 3-chlorobenzy1)amine N, N-Dimethylbenzylamine 1,8-Bis(dimethylamino) naphthalene
Method of acquisition'" P L L L
Physical properties Mp 93-94 "C Mp 173-174 "C Mp 56-57 "C
Literature valueb 92 "CC 175 "Cd 55 "Cd
A
Bp 210 "C (0.008 mom) Mp74-75 C 76-78'Ce
Af,g
Mp 1 6 8 ° C
165"G
A
Bp 190 " C (0.4 m,m) Rp197 C (0.3 p m ) Bp28 C (0.2 mm) Mp 49.5 "C
66-67 'C ( 1 5 mm.)h 47-48 "C'
A' P P
P indicates purchase, L indicates Leukart reaction, A indicates alkylation of ammonia, and A' indicates Where none alkylation of a substituted dibenzylamine. is given the compound is not previously re orted. A. T. Mason, J. Chem. SOC.,6 3 , 1314 (1893). $P. Mostagli, M. Metayer, and G. Bierre-Gallin, Bull. SOC.Chim. Fr., 662 (1948). e Reference 10. f Under pressure in a glasslined autoclave at 100 'C for 3 h. g J. Strabosch, Berichte, 6, 1056 (1873). J. A. King and F. H. McMillan, J. A m . Chem. SOC.,6 8 , 1468 (1946). R. W. Alder, P. S. Bowman, W. R. S. Steele, and D. R. Winterman, Chem. Commun., 7 23 (1968).
under vacuum at 50 "C; mp 61 OC. Values between 61 and 63 "C have been reported.13 Trifluoromethanesulfonic acid was purchased from Aldrich Chemical Co., and purified by distillation within a few days of use: bp 162 "C (lit.I4 162 OC). Tetraethylammonium hydroxide was purchased from Aldrich Chemical Co. as a 20% solution in methanol and used without isolation or purification. MezSOwas purchased from Aldrich Chemical Co., with a specified minimum purity of 99%. However, typical batches were found to contain -0.3% water and no other
Acid-Base Behavior of Tertiary Amines
detectable impurity. It was dried as previously15and found to have a water content, after drying, ranging from 0.008 to 0.025 M. MezSO-d6(99.5 atom 5% D) was obtained from Merck and Co., Inc., and was used without purification. I t had a water content of 0.02-0.03 M. Acetonitrile was purchased from Aldrich Chemical Co. (99% minimum purity) and purified by the method of Coetzee.16 The purified material showed -0.02 M water and no other impurity detectable by IR or NMR spectroscopy. Preparation and Analysis of Solutions. All solutions were prepared and all transfers of nonaqueous solutions were made in a glove box under dry nitrogen. Concentrations of amines, water, and indicators were determined by weighing out the solutes, then making them up to known volume. Water concentrations of Me2S0 solutions were sometimes checked by analysis which was based on the water absorption at 3500 cm-l in the IR.15 The water concentration of acetonitrile was determined from the intensity of the water overtone at 1880 nm.17 Acids cannot easily be diluted to an exact volume with MezSO because of the heat and consequent expansion which attends such a dilution. The concentration of acid stock solutions in Me2S0 was determined by diluting with water, and then titrating with standard aqueous NaOH. These solutions were mixed with appropriate quantities of amine solutions to get the final compositions required. p K Measurements. The indicator method18 was used for all but one pK measurement. The ratio of anionic to protonated form of the indicator was determined from absorbances at two ~ave1engths.l~The measurements were made at 25.0 f 0.1 "C. The ionic strength, p, ranged from to M, and the Debye-Huckel limiting law,20 eq 1,was used to obtain the required activity coefficients.
In MezSO A is 1.116 and B was taken as 2.34, as suggested by Kolthoff, Chantooni, and Bhowmik.13 This corresponds to an interionic distance of 6 kZ1The indicator dissociation constants were known.13 The strong acid used to adjust the pH was CF3S020H,which was assumed to be completely dissociated at low concentration in MezS0.22 The pK of tris(p-nitrobenzy1)amine is too small to be obtained by the indicator method. It was determined from the change in the chemical shift of the methylene protons as a function of the CFBS020Hc ~ n c e n t r a t i o n . ~The ~ spectra were obtained with a Varian LX-100 spectrometer a t 25 f 1 "C. The total amine concentration, protonated and unprotonated, was 0.02 M. The chemical shift, 8obsd, was measured relative to tetramethylsilane. The chemical shift of the free base, &, was measured in the absence of acid; but, without excessive decomposition of the MezSO, , chemical it was not possible to directly measure ~ B H the shift of the protonated base. If eq 2 defines Asobsd, [B], "obsd
= 'obsd - 'B
is the total base concentration, protonated and unprotonated, and [HA], is the total acid concentration, and the small term, [BH']', is dropped, then eq 3 relate KBH
1/('BH - 6), (3b) and ~ B to H observable variables. Values of KBH (0.253) and the chemical shift differences (tiBH- 6 ~ ) 105.3 , Hz, were obtained from the slope and intercept of the linear plot
1925
80 N
-
=. 3 c-0
60 -
a
40-
0.0
0.4
0.8
1.2
1.6
( C F S~ O ~ O H ) M ,
Flgure 1. The agreement of experimental values of A6,,, for tris(4-nitrobenzyl)aminewith those predicted by eq 3a, with a KBH value of 0.253 and a of 105.3 Hz. Trifluoromethanesulfonic acid is HA. The amine concentration was 0.02 M. The line represents eq 3a, the circles are the measured points.
of l/Abobsd against l/[HA], (eq 3b). Figure 1 shows the fidelity with which these values permit eq 3a to reproduce Asobsd.
Exchange Rate Measurement. Exchange rates were measured by the dynamic NMR method.24 Spectra were obtained with a Varian XL-100 NMR spectrometer at 25 f 1 "C. For more dilute solutions a 12-mm sample tube was used. The benzylic protons were observed for the substituted tribenzylammonium ions and the methyl protons in the case of the benzyldimethylammonium ion. These protons generate a doublet, due to coupling with the NH proton, if exchange is slow; a singlet if exchange is fast. At intermediate exchange rates, the rate constant for exchange can be estimated from the shape of the band. The bands were simulated using an equation given by Gutowsky, McCall, and S l i ~ h t e r . Binsch's ~~ program, CLATUX, was modified for this purpose.26 The parameters of the Gutowsky, McCall, and Slichter equation are the in seconds, which effective transverse relaxation time, T2, is the inverse of the full line width at half-height in the absence of exchange; the coupling constant J in Hertz; and the average time between exchange incidents, r , in seconds. For all the amines other than tris(4-methoxybenzy1)amine T2was assumed to be the same for the ammonium ion as for the amine. This assumption is supported by two additional experiments. First, in anhydrous, acidified, acetonitrile the tribenzylammonium ion shows no measurable exchange and its Tzvalue is the same as that of the free amine. Second, monoprotonated l,&bis(dimethylamino)naphthalene, which shows very slow exchange rates, even in water,27in MezSO solution has the same T2value as the corresponding free amine. In the absence of acid, tris(4-methoxybenzy1)amineis insufficiently soluble in Me2S0 for its TZ to be reliably obtained. It was, therefore, measured in CDC13, as were T2 values for the tris(3chlorobenzyl)amine, tris(3-methoxybenzyl)amine, and tris(4-methylbenzy1)amine. For the last three compounds T2 was found to be smaller in CDC13by an average of 1.15 f 0.30 Hz. To get T z in Me2S0 for the tris(4-methoxybenzy1)ammonium ion, 1.15 Hz was, therefore, added to the measured T2for the amine in CDC13. This procedure undoubtedly adds to the uncertainties in the kinetic parameters for this compound. Having the T z values, spectra were simulated for ranges of 7 and J values until a good match was obtained for the experimental spectra reflecting the least exchange. These values of J were assumed to be independent of the exchange rate, and were used, along with the previously obtained T2values and smaller r values, to simulate spectra which could be The Journal of Physical Chemistry, Vol. 81, No. 20, 1977
M. M. Kreevoy and Y. Wang
1926
TABLE 11: Rate Constants and pK,, Values in Me,SO at 25 "C No.
Amine
1 2 3 4 5 6 7 8 9
N, N-DimethylLenzylTris(4-methoxybenzy1)Tris(4-methylbenzy1)Tris( 3-methylbenzy1)TribenzylTris( 3-methoxybenzy1)Tris(4-bromobenzyl)Tris( 3-chlorobenzy1)%is( 4-nitrobenzy1)-
IndicatoP
pKBH
k,, s-'
kKO, M-ls-'
104kH, M-1 s-l
CC
7.59 4.82 4.23 4.08 3.65 3.38 2.35 2.00 0.6
0.11 + 0.04 3.0 f 0.2 2.5 f 0.4 3.2 f 0.8 4.0 f 1.5 4.9 ?; 1.3 24+ 4 22 + 2
0.98 t 0.05 6.6 + 0.5 22.7 + 0.9 24.3 f 1.5 76+ 4 32f 4 50f 7 87 f 3 d
420 20 4.1 3.8 1.8 1.2 0.51 0.22 d
A A A A B
B B b
d
a Indicator A is 2,6-dinitrophenol, p K m of 3.51; B is 4-chloro-2,6-dinitrophensl, p K m of 4.85; and C is 4-nitro-2,6di-tert-butylphenol, pKm of 7.6. Determined by the NMR method, as described in the Experimental Section. The p K m of indicator C appears to be somewhat less reliable than the other two.' Limiting rates, independent of acid concentration, were not reached at accessible acid Concentrations because of the weakness of the base.
matched with experimental spectra reflecting faster exchange. All the matching was done subjectively. Simulation was continued until the fit could no longer be improved. The minumum change in T which produced a detectable change in the spectrum was shown to be -5%. Results Values of pKBH. The PKBHvalues obtained as described are shown in Table 11. The spectrophotometric values given are each obtained from the mean of five to eight determinations of KBH,with an average deviation from the mean of -15%. This implies a precision of f0.05for the PKBH values. In addition, their accuracy depends on the reliability of the indicator dissociation constants used. It seems reasonable to ascribe roughly 10% uncertainty to these.13 The overall probable error in the PKBH values is, therefore, kO.10. In the case of tribenzylamine it is possible to compare the present PKBH, 3.65, with a previously reported value of 4.1 f 0.2,28determined potentiometrically. If the rather large uncertainty assigned to the latter is realistic, the agreement is not unreasonable. The uncertainty in the listed PKBHfor tris(4-nitrobenzy1)amine is hard to assess, as it arises mainly from deviations from ideal solution behavior. Using the activity coefficients of neutral substances as a guide,29 f0.3 appears to be a reasonable estimate of the uncertainty in PKBH. When applied to solutions similar to those in which it was measured it will, of course, reproduce the degree of protonation much better than such an uncertainty might suggest. Exchange and Recombination Rates. In Me2S0 as a solvent the exchange of the proton on nitrogen is well within the NMR time scale. It is promoted by water, and, up to a limit, inhibited by excess acid. The typical response of 7 to changes in HzO and the excess CF3S020H concentration is shown in Figure 2. As the acid concentration is raised it has a diminishing effect on T , the latter eventually becoming independent of the acid concentration, Under those conditions 1 / is~a linear function of [H20]with a nonzero intercept. This situation can be represented by eq 4, in which ko is the first-order rate
1/7.= ko + h,2,[H201
(4)
constant for an uncatalyzed exchange and kHzO is the second-order rate constant for the water-catalyzed exchange. Tribenzylamine concentrations from 0.04 to 0.25 M were studied and no significant change in T was observed. The remainder of the study was, therefore, carried out with 0.04 M amine and the derived rate constants are thought to be appropriate for dilute solutions. It is hard to visualize a first-order exchange process, in an aprotic solvent, which does not involve dissociation and recomThe Journal of Physical Chemistry, Vol. 81, No. 20, 1977
It0 100 80 140
0.0 0.2, 0.4 0.b 0.8 1.0 WzO) M
1,2
1.4
Figure 2. The effect of water and excess CF,SOpOH on T at 25 OC. Open circles represent experiments in which the excess acid concentration was 0.08 M; closed circles, experiments in which the excess acid concentration was 0.40 M. At intermediate acid concentration intermediate points were generated, approaching the lower curve as the acid concentration was raised. The curve for 0.27 M excess acid is indistingulshable from that at 0.40 M excess acid.
bination. If that is the case, then kH, the rate constant for the second-order recombination process, eq 5, is given by k
R,N
+ H"(so1v)-ff, R,NH+
*
(5)
kO/KBH. Values of ko, kHz0, and kH obtained at 25 1"C are given in Table 11. The process which is inhibited by acid is, almost certainly, an amine-catalyzed exchange of protons, which has ample p r e ~ e d e n t . ~It~is, ~evident ~ from Figure 2 that this is also water catalyzed. The water-catalyzed, acid-independent, process may or may not involve dissociation. If it does, then ~ H P , the rate constant for the second-order reaction shown in eq 6 is R,N
-
+ H,O+(solv)k H 3 0
R,NH'
+ H,O
(6)
given by kH2&H40/KBF+. KH is the acid dissociation constant of H30 (solv) in Me2$0. It has the value Alternatively, recombination might be a ternary process involving R3N, H20, and H+(solv), or a binary process involving R3N-H20and H+(solv),but, a priori, the process shown in eq 6 seems more likely because of the substantial affinity of H20 for H+(solv). Discussion The Effect of Solvent and Structure on ~ K B HThe . dissociation constants for substituted tribenzylamines are
Acid-Base Behavior of Tertiary Amines
1927 3.0 2.0
3
0
2
3?
2.0
1.0 -z
I.o
0.0
0.0
-1.0
-c
-LO
y
-1.01 -9.0
I
I
I
-8.0
-7.0
-6.0
I
-5.0
-4.0
I
-3.0 -2.0
'
I
-1.0
0.0
BH
Figure 3. The Brhsted plots for k o (closed circles) and kHpO (Open circles). The numbers refer to Table I1 and identify the compounds. The slopes (avalues) are 0.41 and 0.35, respectlvely.
very well correlated by the Hammett equation.33 The correlation coefficient is 0.991. The average deviation of points from the line is 0.14 log units, only a little larger than their estimated uncertainty. The point for tris(4nitrobenzy1)amine fits about as well as the others. The p value is 4.1 f 0.2. No p value is available for the same reaction in water, but for benzylammonium ions in water p is 1.06.34 Substituent effects for substituents as well insulated from each other as those in a tribenzylammonium ion should be additive,35so that one would expect a p value of around 3.2 for the dissociation of tribenzylammonium ions in water. The value in MezSO is higher by a rather modest factor of 1.4. This may be compared with the factor of 2.5, by which p for dissociation of benzoic acids increases on going from water to Me2S0,36 and the factor of 2.0 for phenol^.^' These observations are consistent with the view that the increase in p for benzoic acid ionization, which is produced by changing the solvent from water to MezSO, is largely due to the less effective solvation of the anion by Me2S0. The neutral tribenzylamine would have less need for such solvation, and the charged tribenzylammonium ion would be relatively well solvated since Me2S0 is a good hydrogen bond acc e p t ~ r .An ~ ~ alternative explanation, suggested by a referee, is that a larger fraction of the electrostatic interaction between the charged group and the substituents passes through the solvent in the case of the oxygen bases. Rates of Proton Transfer. The rate and equilibrium constants given in Table I1 are all far short of the diffusion limit, even though the processes represented by eq 5 are strongly spontaneous. The rate constants can be correlated with the equilibrium constants by means of Bransted plots,39 as shown in Figure 3. These appear to be reasonably linear, particularly if the points generated by the benzyldimethylammonium ion are accepted as part of the family. In that case, a values of 0.41 and 0.35 are obtained from the values of ko and k H z O , respectively. In spite of the thermodynamic spontaneity of the reprotonation, the kH values generate a /? of 0.59. If the water-catalyzed exchange also involves dissociation, then the recombination has a /? of 0.65. Because of its smaller steric requirements the benzyldimethylammonium ion may not be a proper member of the family. In that case the points for the tribenzylammonium ions themselves would not exclude substantial curvature in the Bransted plots, such as have been observed in other system^.^-^ However, the a and /? values, which would, then, be the tangents to such curves, would not be substantially different from those cited. This behavior is very similar to that of carbon bases in hydroxylic solvents; that is, rates well below the diffusion limits and Brbnsted parameters around 0.5 even for
spontaneous reaction^.^ It is in sharp contrast with the behavior of tertiary amines in water. Their rates of protonation by H+(solv) are in the range (1-5) X 1O1O M-l s-l, and are not systematically related to their basicity.40 We believe that the origin of these differences lies in the nature of the solvents involved. Reprotonation of tertiary amines by H+(solv) in water involves at least two water molcules in covalent bond changes: the one to which the proton is primarily bound, to form H30+,and at least one water forming a short Grotthus chain41between H30+and the basic nitrogen.40 The total solvation enthalpy of H+(solv) in water is 261 kcal mol-l, as against 268 kcal mol-l in Me2S0,42They are similar and both are large. Even a very small fractional loss of such solvation enthalpies on forming the protonation transition state will result in rates well below the diffusion limit. In water, however, the disruption of solvation can be minimized and neither the H30+ unit nor the next layer of solvent molecules needs to be disturbed at all. The probable structure of H+(solv) in Me2S0,43i44 shown by 1, is much
/"
CH3
\
H3
1
less conducive to a facile transfer. MezSO cannot contrive a Grotthus chain mechanism for the transfer, and half the solvation shell must be replaced with the tertiary amine in order to establish a configuration in which proton transfer can take place. Not till this transformation is almost complete can proton transfer begin, and not till then can energy be recouped from the overall spontaneity of the reaction. Considering the discontinuous nature of the solvent, it is not surprising that there are stages in this process when solvent reorganization and interaction with the solute are not quite able to make good the loss of half the original solvation shell. The situation is similar to that which occurs in proton transfers from internally hydrogen-bonded species.& This, we believe, is the fundamental origin of barriers observed for the reactions, in Me2S0, symbolized by eq 5. Delpuech and his co-workers have observed that pro(2) in MezSO tonation of l,cis-2,cis-6-trimethylpiperidine
2
is relatively However, they also observed that the symmetrical transfer of a proton from the Tmonium ion to ammonia had a rate constant of 1.2 X 10 M-l s-l, and that several other proton transfer rate constants had quite high values. They concluded "that the presence of water, or more generally, of a hydrogen-bonded network, is not compulsory to promote fast proton transfer^."^^ The general conclusion is undoubtedly correct. Indeed, as shown by Ritchie and Ushold,4and confirmed by Delpuech and c o - w ~ r k e r s proton , ~ ~ abstraction by strong oxygen bases is faster in MezSO than in hydroxylic solvents and may approach the diffusion limit for spontaneous reactions. However, the structure of H+(solv) in Me2S0 imposes special barriers on its reactions. The detailed elucidation of the cooperative restructuring of the solvent (MezSO) and the amine, leading to proton transfer between them, requires further information about the shape of the Bransted plots, and the limiting value of The Journal of Physlcal Chemistry, Vol. 8 1, No. 20, 1977
Eastman et al.
1928
kH. It would also be very useful to know whether or not the water-catalyzed exchange involves dissociation. References and Notes (1) This work was supported by the US. National Science Foundation through Grants GP-31360X and CHE76-01181, to the University of Minnesota. (2) M. M. Kreevoy was a guest of the Physical Chemistry Laboratory, Oxford, while he was preparing this paper. He thanks the members of that laboratory, particularly Dr. W. J. Albery, for their warm hospitality. (3) Present address: Department of Biochemistry, The Medical School, University of Minnesota, Minneapolis, Minn. 55455. (4) C. R. Ritchie and R. E. Ushold, J. Am. Chem. Soc., 90, 3415 (1968). (5) A. I.Hassid, M. M. Kreevoy, and T-M. Liing, faraday Symp. Chem. SOC., 10, 69 (1975). Related work is cited in this paper. (6) M. M. Kreevoy and D. E. Konasewich, Adv. Chem. fhys., 21, 243 (1971). (7) A. J. Kresge, Acc. Chem. Res., 8, 354 (1975). (8) A. J. Kresge and G. L. Capen, J. Am. Chem. Soc., 97, 1795 (1975). (9) M. L. Moore, in “Organic Reactions”, Vol., V, R. Adams et al., Ed., Wiley, New York, N.Y., 1949, Chapter 7. (10) L. Jackson and W. Lowry, Am. Chem. J., 3, 251 (1881). (1 1) P. A. S. Smith and R. N. Loeppky, J. Am. Chem. Soc., 89, 1147 (1967). (12) Elemental analyses were performed by M-H-W Laboratories, Garden City, Mich. (13) I.M. Kolthoff, M. K. Chantooni, Jr., and S.Bhowmik, J. Am. Chem. SOC.,90, 23 (1968). (14) R. N. Haszeldine and J. M. Kidd, J . Chem. SOC., 4230 (1954). (15) L. M. Abts, J. T. Langland, and M. M. Kreevoy, J. Am. Chem. Soc., 97, 3181 (1975). (16) J. F. Coetzee, frog. fhys. Org. Chem., 4, 45 (1967). (17) R. F. Goddu in “Advances in Analytical Chemistry and Instrumentation”, C. N. Reilley, Ed., Interscience, New York, N.Y., 1960, p 347. (18) I. M. Kolthoff, Red. Trav. Chim. fays-Bas, 43, 207 (1924). (19) E. H. Baughman and M. M. Kreevoy, J. fhys. Chem., 78,421 (1974). (20) H. S.Harned and W. 8. Owen, “The Physical Chemistry of Electrolytic Solutions”, 3rd ed, Reinhold, New York, N.Y., 1958, p 66. (21) T. Kielland, J . Am. Chem. Soc., 59, 1675 (1937).
(22) T. Gromstad, Tidsskr. Kjemi, Bergves. Metall., 19, 62 (1959). (23) J. T. Edward, J. 6. Leane, and 1. C. Wang, Can. J. Chem., 40, 1521 (1962). (24) H. S. Gutowsky and G. Binsch in “Dynamic Nuclear Magnetic Resonance Spectroscopy”, L. M. Jackman and F. H. Cotton, Ed., Academic Press, New York, N.Y., 1975, Chapters 1 and 3. (25) H. S.Gutowsky, D. W. McCall, and C. P. Slichter, J . Chem. fhys., 21, 279 (1953). (26) G. Binsch, Top. Sfereochem, 3, 97 (1968). The modified program Is glven in full by Y. Wang, Ph.D. Thesis, University of Minnesota, 1977, Appendix. (27) F. Hibbert, J. Chem. Soc., ferkin Trans. 2, 1862 (1974). (28) C. p. Ritchie, J; Am. Chem. SOC.,91, 6749 (1969). (29) T. Skerlak, V. SiSlov, and B. Galie, BUN. SOC.Chem. Techno/. (Sarajevo), 15, 5 (1966-1967). (30) E. Grunwald and D. Eustace in “Proton Transfer Reactlons”, E. Caldin and V. Gold, Ed., Chapman and Hall, London, 1975, Chapter 4. (31) B. Binchin, J. Chrisment, J. J. Delpuech, M. N. Deschamps, D. Nicole, and G. Serratrice, in “Chemical and Biological Applications of Relaxation Spectroscopy”, E. WynJones, Ed., Reidel Publishing Co., Dordrecht, The Netherlands, 1975, p 373. (32) I.M. Kolthoff and T. B. Reddy, Inorg. Chem., 1, 189 (1962). (33) L. P. Hammett, “Physical Organic Chemistry”, 2nd ed, McGraw-Hill, New York, N.Y., 1970, Chapter 11. (34) L. F. Blackwell, A. Fisher, I.J. Miller, R. D. Topsom, and J. Vaughan, J . Chem. SOC.,3568 (1964). (35) H. H. Jaffe, Chem. Rev., 53, 191 (1953). (36) I.M. Kolthoff and M. K. Chantooni, Jr., J. Am. Chem. Soc., 93, 2343 (1971). (37) I. M. Kolthoff and M. K. Chantooni. Jr., J . fhvs. Chem.. 80. 1306 (1976). (38) J. F. Coetzee arid C. D. Ritchie, “Solute-Soivent Interactlons”, Marcel Dekker, New York, N.Y., 1969, p 223. (39) J. N. Brhsted and K. Pederson, 2.fhys. Chem., 108, 185 (1924). (40) E. Grunwakl and E. K. Ralph, 111, J. Am. Chem. Soc., 89,4405 (1967). (41) C. J. D. von Grotthus, Ann. Chem., 58, 54 (1806). (42) R. L. Benoit and S. Y. Lam, J. Am. Chem. SOC.,96, 7385 (1974). (43) M. M. Kreevoy and J. M. Williams, J . Am. Chem. SOC., 89, 5499 (1967). (44) R. A. Potts, Inorg. Chem., 9, 1284 (1970). (45) F. Hlbbert and A. Awwal, J . Chem. SOC., Chem. Commun., 995 (1976).
Electron Spin Resonance Studies of Atom Transfer Reactions Involving Crown Ether-Tight Ion Pair Complexes M.
P. Eastman,” Y. Chiang, G. V.
Bruno, and C. A. McGuyer
Chemistry Department, The University of Texas at E/ faso, El faso, Texas 79968 (Received July 19, 1976;Revised Manuscript Received July 15, 1977) Publication costs assisted by the University of Texas at €1 faso
Crown ether complexes of the potassium and sodium salts of tetracyanoethylene (TCNE) have been prepared in benzene and the atom transfer reaction from these complexes to neutral TCNE have been studied by ESR. The crown ethers employed in this work were 18-crown-6(18C6), 15-crown-5(15C5),21-crown-7,and the B isomer of perhydrodibenzo-18-crown-6.The transfer rate for the 18C6Naf-TCNE- complex is 4.2 X 10’ MI1 s-l at 15 “C with an activation energy of 3.3 f 0.2 kcal/mol while the rate for the 15C5Naf-TCNE- complex is 1.2 x lo9 M-l s? with an activation energy of 3.5 f 0.2 kcal/mol. In general, the results show that tight ion pairing per se does not lead to slow atom transfer in the TCNE system and that the crown ether present in the ion pair complex can affect the transfer rate. Details of a computationally efficient method for obtaining ESR spectral line shapes as a function of exchange frequency is presented. Introduction Studies of electron transfer reactions involving the tetracyanoethylene anion radical (TCNE)- in the free ion and in the loose ion pair forms have been reported in the literat~re.l-~ The studies of the electron transfer reaction between TCNE- and TCNE carried out in this laboratory have shown that in dimethoxyethane (DME) KTCNE exists as a loose ion pair with the second-order rate constant for electron transfer at 15 OC (k15) equal to 2.6 X lo8 M-l s-l and the activation energy (E,) equal to 5.2 The Journal of Physical Chemistry, Vol. 8 I, No. 20, 1977
kcal/mol. In acetonitrile KTCNE forms free ions with the parameters characterizing the electron transfer process being kI5 = 3 X lo9 M-l s-l and E, = 2.3 kcal/mol. The results obtained for KTCNE in acetonitrile and in DME are in good agreement with the results obtained by others in the same and related ~ y s t e m s . ~ - ~ A variety of ion pairs involving the TCNE- anion radical have been produced in benzene by dissolving MTCNE salts in benzene using crown ethers, cryptands, and the antibiotic valinomycin; however, there have been no re-
