NMRSTUDY OF PROTON EXCHANGE
2515
Conclusions Spectroscopic evidence presented above indicates two types of positive solvation by electrolytes. (1) Small or highly charged ions, such as lithium and fluoride, are very effective in orienting the water molecules in their vicinity and tend to ion pair only with other ions having a high charge density. (2) Larger ions, such as cesium and tetraalkylammonium, tend to enhance the water structure around them; ie., the water is hydrogen bonded to a greater
extent. These ions tend to form ion pairs, especially with other large ions having a lower charge density. A type of negative solvation is exhibited by aromatic sulfonate anions which may be associated with a structure-breaking effect on the solvent. Nonelectrolytes are found to be both positively and negatively solvated, and, in all observed instances, the spectroscopic solvation evidence may be directly correlated with the direction of the deviation of their aqueous solutions from Raoult's law.
Nuclear Magnetic Resonance Study of Proton Exchange Involving Methyl-Substituted Pyridinium Salts in Methanol by Michael Cocivera Bell Telephone Laboratories, Inc., Murray Hill, New Jersey 07974
(Received January 6,1968)
The nuclear magnetic resonance line-broadening technique has been used to study proton-exchange reactions involving methyl-substituted pyridinium salts and methanol. For a reversible acid-dissociation reaction, ka BH+ MeOH F* B MeOHz+,changing the position of the methyl group from the para position to the k ortho position does not alter the value of either k , or k-,. In addition, the value of k-. does not depend upon the degree of methyl substitution, and this value is comparable to the value expected for diffusion-controlled kz reactions. The reaction BHf MeOH B -+ B MeOH BH+ was found to involve only one methanol molecule. For the monomethyl compounds, the value of kz does not depend upon the position of the methyl group. For the dimethyl compounds, the value of kz depends upon the position of the methyl groups and indicates that the rate for this reaction is retarded when both methyl groups are ortho to the nitrogen of the pyridine ring.
+
--&
+
+
+
+
+
Introduction Nuclear magnetic resonance studies of proton exchange involving substituted ammonium salts and such solvents as water and methanol have revealed considerable information concerning the mechanisms by which the exchange can occur.' However, little information has been obtained concerning steric hindrance in these reactions. The only available evidence that steric effects can be significant in these reactions comes from a study of proton exchange involving triethylammonium ion in water.2 I n that study, Ralph and Grunwald concluded that the ethyl groups were sterically hindering one of the exchange reactions. I n the present article, the results of a study of the exchange involving methyl-substituted pyridinium salts in methanol are presented. The salts which were studied were the hydrochlorides of 2-picoline (I), 4-picoline (11), 2,4-lutidine (111),and 2,6-lutidine (IV).
f;"3
I
31
y 3
m
llTL
These salts were chosen because of the possibility that the methyl group could sterically hinder one or more of the exchange reactions when it is ortho to the nitrogen of the pyridine ring. The advantage of using these salts is that the pK.4 of the salt does not change significantly when the methyl group is moved from the ortho to the para position on the pyridine ring. Consequently, the interpretation of the results is not (1) For a brief review, see E. Grunwald and M. Cooivera, Discussions Faraday Sac., 39, 105 (1965). (2) E. K. Ralph, 111, and E . Grunwald, J . Amer. Chem. Soc., 89, 2963 (1967).
Volume 7f2, Number 7 July 1968
2516
MICHAELCOCIVERA
complicated by changes in acidity or basicity of the compounds when the methyl is moved from the ortho to the para position. For each of the salts, the rates of the reactions given by eq 1-4 were measured. I n these equations, B signifies the amine (1-IV).
+ R/IeOH2+k+, XeOH2+ + RIeOH MeO- + MeOH k;MeOH + R4e0BH+ + RleOH J_ B + lfeOHz+ BH+ + O-H + B - % B + H-o + BH+ MeOH
(1) (2)
ka
k-a
I
Me
I
(3) (4)
Me
The values of le+ and le-, the rate constants for reactions 1 and 2, are known from previous studies.3 Consequently, the ~ K ofAeach pyridinium salt could be obtained from measurements of the rates of these reactions. The pKA for each salt in m.ethano1 is nearly the same as the value in water. Comparison of the ~ K values, A determined in the present study, with other methods is possible only for 4-picoline whose PKA in methanol was determined to be 6.090 by the differential potentiometric m e t h ~ d . ~The value obtained in the present study is 6.05. The ortho substitution of the methyl group was found to have little effect on k,. The value of k , for 2picoline is almost the same as the value for 4-picoline. Similarly, there is no appreciable difference between the value for 2,6-lutidine and that for 2,4-lutidine. I n addition, the position of the methyl group has little effect on k+. I n fact, le-& is essentially the same for all of the salts and is of the order of magnitude that one would expect for a diff usion-controlled reaction. For the reaction given by eq 4, the position of the methyl group has an effect on the rate. The value of lez for 2,4-lutidine is more than three times the value for 2,6-lutidine. Since 2-picoline and 4-picoline have almost the same value for Icz, one methyl group in the ortho position apparently is not sufficient to retard the rate.
Experimental Section Reagents and Solutions. The methyl-substituted pyridines were obtained from Aldrich Chemical Co. 2,6-Lutidine was purified according to the method of Brown.6 The other amines were purified by distillation, and the boiling points agreed well with values reported in the literature. The nmr spectrum of each compound is as expected on the basis of its structure. The hydrochloride of each amine was prepared by passing anhydrous HC1 into a methanol solution containing the amine. After removing the methanol, each salt was recrystallized twice from either isopropyl alcohol or n-butyl alcohol. Each salt was analyzed The Journal of Physical Chemistry
quantitatively for the chloride ion by using the Volhard method and for the methyl-substituted pyridinium ion by potentiometric titration. The equivalent weights determined by these analyses were in good agreement with the theoretical values. &lethano1 was obtained from J. T. Baker Chemical Co. It was purified as described previ~usly.~The solutions were prepared and analyzed by standard quantitative techniques. Nmr Measurements. From slow-passage spectra, the chemical shifts were determined at five temperatures, -75, -50, -25, -15, and 0.So, in the following manner. Using methanol solutions containing approximately 1 M salt and 2 M HCl, the chemical shift between the ISH proton of the salt and the CH, protons of the methanol was determined. The chemical shift of the OH proton of methanol relative to the CHa protons was determined using acidified solutions containing tetramethylammonium chloride at concentrations which were to be used for the pyridinium salts in the kinetic studies. Assuming that the NH-CH3 chemical shift is independent of the salt and HC1 concentration, the NH-OH chemical shift is taken as the difference between the NH-CHs and the OH-CH, chemical shifts. The value of the NH-OH chemical shift above 0.8" was obtained by extrapolation. At 25.1O for 2-picolinium, 4-picolinium, 2,4-lutidinium, and 2,6-lutidinium ions, this chemical shift is 625, 632, 621, and 628 Hz, respectively, at 60 MHz. The NHprot,on resonance occurs a t a lower field relative to the OH. The value of the transverse relaxation time, Tz, was obtained either by line-width measurements of resonance lines in slow-passage spectra or by means of a modified Carr-Purcell spin-echo scheme.6-8 The longitudinal relaxation time was measured using the null method.6
Results By means of nmr spectroscopy, exchange between the N H proton of the pyridinium salts and the OH proton of methanol was studied. The lifetime, T , for proton exchange involving the OH proton of methanol was obtained from the determination of the quantity (l/Tz) - (l/Tzo),for either the OH- or the CHa-proton resonance of methanol. T2 is the transverse relaxation time in the absence of exchange and Tz is the transverse relaxation time of the exchange-broadened line. A detailed description of the manner in which T can be obtained from the line shape of OH- and CH3-proton (3) E. Grunwald, C. Jumper, and S. Meiboom, 1.Amer. Chen. Soc., 84, 4664 (1962). (4) C.Ritchie and P. Heffley, ibid., 87, 5402 (1965). (5) H.Brown, S. Johnson, and H. Podall, ibid., 76, 5556 (1954). (6) H. Carr and E. Puroell, Phys. Rev., 94,630 (1954). (7) S. Meiboom and D. Gill, Rev. Sci. Instrum., 29, 688 (1958). (8) S. Alexander, ibid., 32, 1066 (1961).
NMRSTUDYOF PROTON EXCHANGE
2517
resonance has been given e l ~ e w h e r and e ~ ~will ~ ~not ~ ~be repeated here. The slow-passage CH3-proton resonance was analyzed in the manner discussed in ref 3. The Tz of CH, determined by spin-echo was analyzed in the manner described in ref 10, with the exception that the relation T$ = T1 was not used (TI is the longitudinal relaxation time), This relationship was not used because, for both the OH- and CH3-proton magnetic moments, the longitudinal relaxation rate depends upon the rate of OH-proton exchange.'l Instead, the Tzof the CH3of acidified methanol was used as Tzosince the CH3 is not exchange broadened under these conditions. Measurements of the T z of the CH, of acidified methanol containing various amounts of tetramethylammonium chloride indicate that the concentration effecton TzOis small and can be neglected over the range of concentrations used in the kinetic studies. For the OH-proton resonance, the equationB describing the line broadening in a system involving exchange between a dominant line and a weak one was employed because the OH-proton resonance was always much larger than the NH-proton resonance; i.e., the ITH-proton fraction, P", was always less than (PNHis defined as ["I/(["] [OH]), where the brackets signify molar concentrations). The analysis of the broadening of the dominant OH-proton resonance paralleled the analysis described in ref 10 and 12. Measurements of the CH3-proton resonance were made using buffered methanol solutions. Nine buffer to 1.982 ratios, [BH+]/[B], ranging from 8.36 X were used in the study of 2,6-lutidine. For 2,4lutidine, 2-pico1inel and 4-picoline, two were used. For all of the salts the concentration, [BHf], was M . Both 2,6- and varied from to 2 X 0.8, and 25.1". 2,4-lutidine were studied a t -24.5, 2- and 4-picoline were studied only at 25.1". Under these conditions, the NH-OH proton exchange was found to follow the rate law10~12-16 given by
+
+
+
R = k+[[1LleOH2+] k-[XeO-]
+
+
Since the values of k+a and k-3v16 and Kautol' are known at 25.0 and -0.7", the value of KAcould be calculated a t 25.1 and 0.8" using eq 6 and the values of the intercept, Rint, obtained from the least-squares fit.18 For each salt, the value of K A at 25.1" along with AH" for each of the lutidinium salts is listed in Table I. For comparison, the value of K Afor 4-picoline obtained by the differential potentiometric method4 is also listed. As can be seen agreement between the two methods is quite good. The slope of the plot of R vs. [BH+][B]obtained by least squares was assumed to be equal to nk2,where n is the number of solvent molecules involved in the reaction.14~15 The quantity nka is obtained from these measurements because the multiplet of the CH3- or OH-proton resonance makes it possible to distinguish between methanol molecules. Consequently, the lifetime for the OH-proton exchange measured from the line shape of the CH3- or OH-proton resonance is affected by exchange between methanol molecules. I n the present case, n was found to be nearly equal to 1 for 2,6-lutidine and is assumed to be the same for the other compounds. The manner in which this value was determined is discussed in the following paragraph. Measurements of the T z of the OH-proton resonance were made using buffered solutions of methanol for only one compound, 2,6-lutidine. The buffer ratio, 49.34, was chosen to make k+ [MeOHa+]/[JIeOH] large compared to the indirect spin-spin coupling constant between the OH and the CH3 protons. Consequently, the OH multiplet is collapsed to a single line, which cannot be broadened by exchange between methanol molecules. Thus under these conditions, the NH-OH exchange involves one weak line (the NH) and one strong line (the OH), and one can use the equationg describing the line broadening in a system involving exchange between a dominant line and a weak one. Furthermore, under these conditions, only the reaction given by eq 4 is important since eq 1 and 2 cannot affect the line shape and eq 3 is slow compared to eq 4. I n addition, because the OH multiplet is completely col-
nkz[BH+][B] (5) The brackets signify molar concentrations. For each buffer ratio, a plot of R vs. [BH+][B] was made. I n each case the plot proved to be linear, and the leastsquares intercept, Rint, was assumed to be equal to the k-[RiIeO-]. By substituquantity k+ [MeOHz+] tion, this expression is put in the form given by
+
In this equation
KA = [B][R'IeOH, and Kauto= [MeOHz+ [MeO-1
(9) S. Meiboom, J . Chem. Phya., 34, 375 (1961). (10) M. Cocivera, E. Grunwald, and C. Jumper, J . Phys. Chem., 68, 3234 (1964). (11) M. Cocivera, J . Chem. Phys., 47, 1112 (1967). (12) M. Cocivera, J . Amer. Chem. SOC., 88, 672 (1966). (13) The kinetic analysis follows closely the analysis given in a number of other and details will not be discussed here. (14) E. Grunwald, C. Jumper, and S. Meiboom, J . Amer. Chem. SOC.,85, 522 (1963). (15) 2.Luz and S. Meiboom, J . Chem. Phgs., 39, 366 (1963). (16) E. Grunwald, J. Phys. Chem., 71, 1846 (1967). (17) J. Koskikallio, Suomen Xemistilehti, B30, 111, 157 (1957). (18) In this fit, activity coefficients were not included in the term containing k- in eq 6 (as was done by Grunwaldll), since they introduce a correction which is negligible in the present case. This correction is negligible because low concentrations were used, and, more important, the contribution to R made by k - [ M e O - ] is small compared to other two terms in eq 5. Volume 72,Number 7 July 1968
MICHAEL COCIVERA
2518
Table I : Kinetic Parameters for Methyl-Substituted Pyridinium Salts in Methanol a t 25.1'
________--__1 0 7 ~ ~M 0,
lO-ak2, sec-l 10-10k-,0, M - l see-l - ~ B H C I , M-I
10-7nkz, M-1 sec-1 10-7kz, M-1 sec-' E,(k,O),' kcal mol-' E , ( J C ~kcal ) , ~ mol-' A H O ( K A ~kea1 ) , ~ mol-' E,( k-ao),s kcal mol-1
-----
Compound--
2-PiHCP
4-PiHClb
2,4-LuHClC
2,6-LuHCld
7.61
8.91 8.13* 10.6 1.2 0.96 34.4 f 1 . 2
1 . 3 6 f0 . 1
1.53f 0 . 2
1.32 0.97 0.95 29.7 f3 . 5
1.15 0.75 0.92 8.74 f 0 . 9 0 9.26 9.5 4.2 6.1 3.4
9.08 1.2 0.86' 28.9 f2 . 7
... 9.2
... ...
...
.
I
...
.
9.5 4.9 5.6 3.9
9.7
... ...
...
a 2-Picolinium chloride. 4-Picolinium chloride. ' 2,4-Lutidinium chloride. 2,6-Lutidinium chloride. Reference 4. I The value of b ~ ( a~t 25.1') l is -0.76. , a t 0.8, 25.1, and 39.8'. ti Calculated using values of kz a t Calculated using values of kO Calculated using values a t 0.8 and 25.1'. -23.3, 0.8, and 25.1'.
lapsed, the methanol molecules cannot be distinguished, and the lifetime, which is measured, is for exchange involving the KH and OH of only one methanol moleR/[BH+][B] was found to be c ~ l e . ' As ~ ~a ~result, ~ constant and was assumed to be equal to k2. The value of kz obtained in this manner for 2,6-lutidine is given in Table I. In addition, values of nk2 at 25.1' obtained from the CHs-proton resonance are listed in this table for each salt. From this table, it is clear that the ratio nkz/ke is very nearly 1 for 2,6-lutidine. Since the other compounds are similar, the ratio probably is close to 1 for each of them also. The OH-proton resonance also was studied using methanol solutions containing BH+C1- and HC1. Under these conditions the OH multiplet is collapsed to a single line. The concentration of BH+ was kept low so that the dominant line equation was applicable. Because of the excess HC1, the concentration of B is very low and the acid-dissociation step of reaction 3 is the main contribution to the exchange rate. The maximum contribution made by reaction 4 under these conditions was 38%, and usually it was much less than that. The rate constant for the acid-dissociation step of reaction 3 was found to be slightly dependent upon the concentration of salt at 39.8, 25.1, and 0.8". Values of k , a t various salt concentrations for 2,4- and 2,6-lutidine are listed in Table 11. This concentration dependence cannot be accounted for by assuming the formation of the ion pair BHfC1- which does not react as rapidly as the dissociated ion BH+. First, if ion pairs were to occur and the dissociation constant were small, R/ [BH+]"' would be constant. This ratio is not constant for any of the salts. Second, if the dissociation constant were not small, a plot of k , us. [BH+] would be asymptotic at low concentrations and would have a negative curvature. I n each case, the plots are not asymptotic at low concentration, and the curvature is positive. The Journal of Physical Chemistry
Consequently, this concentration dependence was considered to be a "salt effect," and the data in Table I1 as well as the data for 2- and 4-picoline were fit to the eq~ati0n.l~
log IC, = log k,'
+ b~~cl[BHCll
(7)
The values of ~ B H C and I k,O at 25.1" are listed in Table I for each salt. Also given in Table I is the value of E,, the activation energy, calculated using values of kao determined a t 39.8, 25.1, and 0.8". I n addition, using the data given in Table 11, the concentration effect of HC1 on the k, for 2-picoline was calculated using eq 7. The value -0.76 A4-l calculated for the bHC1 of this compound is very close to the values of ~ B H Cfor~ the salts. Finally, the value of k-a' is calculated as k,O/KA and is listed in Table I for each salt. As can be seen, the ~~
Table 11: Values of k , in Methanol a t 25.1' as a Function of Salt and HC1 Concentration [BH+I,
WC11,
Salt
M
M
5ee-1
2,4-LuHCl
0.1319 0.06595 0.03298 0.01649
0.02510 0.02510 0.02510 0.02510
0.990 1.09 1.23 1.39
2,6-LuHCl
0.09916 0.04958 0.02479 0.01240 0.00620
0.01480 0.01480 0.01480 0.01480 0.01480
0.930 0.995 1.04 1.12 1.13
2-PiHC1
0.03112 0.03112 0.03112 0.03112
0.2081 0.1040 0.0520 0.0260
6.29 7.49 8.21 8.16
(19) E. Grunwald, J. Phys. Chem., 67, 2211 (1963).
lo-%.,
NMRSTUDY OF PROTON EXCHANGE value of IC-, is large and is nearly the same for all of the compounds.
2519 I+
a+
Discussion For a study of steric effects on proton exchange, methyl-substituted pyridines are advantageous because a change in the position of the methyl group from ortho to para to the nitrogen does not change the acid dissociation constant K A significantly. This lack of dependence can be seen in Table I; 2-picoline (o-methyl) has essentially the same K A as 4-picoline (p-methyl), and the K A for 2,6-lutidine (two o-methyl groups) is very close to the value for 2,4-lutidine (one o- and one p-methyl group). Thus in comparing 2-picoline with 4-picoline and 2,6-lutidine with 2,4-lutidine, differences in acidity and basicity as measured by K A are negligible. As can be seen in Table I, the position of the methyl group has an effect only on the value of IC2, the rate constant for reaction 4. The value of kz for 2,6-lutidine is about one-third the value for 2,ii-lutidine. On the other hand, 2-picoline and 4-picoline have about the same value for k2. Obviously, two methyl groups ortho to the nitrogen can retard the rate of reaction 4, and one o-methyl group is not sufficient. Whether this retarding effect is a steric effect cannot be decided on the basis of the present data. A reduction in the rate due to steric hindrance should be reflected in the activatiorf energy. However, a factor of 3 in the rate constant borresponds to less than 0.6 kcal in the activation energy. This difference is comparable to the experimental error for E,(kz). Indirect evidence concerning the nature of the retarding effect can be obtained by considering the possible transition state for this reaction. As indicated by the data presented earlier, this reaction is termolecular, i.e., involves BH+, B, and one solvent molecule in the transition state. Most likely, in this transition state, the nonbonded p electrons of the oxygen of the methanol molecule are involved, and the oxygen is probably pyramidal as shown in Figure 1. The positive charge is probably spread over the three sites involved
Figure 1. Possible transition states for reaction 4. B signifies the amine. The dashed lines indicate partial bonds.
in the reaction. Since the present data are consistent with a symmetrical (Figure la) and an unsymmetrical (Figure lb) transition state, both possibilities are illustrated. For these transition states, models indicate that the two methyl groups of 2,Blutidine can cause steric hindrance. Furthermore, the hindrance appears to be due mainly to the interaction of the methyl groups on one pyridine ring with the other pyridine ring and its methyl groups. The methyl group of the methanol molecule seems to be able to fit in with little difficulty. This conclusion is supported by the results obtained from a study of water solutions, which is presented in the following article.20 The values of ka0 are consistent with this conclusion. The value for 2,6-lutidine is almost the same as the value for 2,4-lutidine, indicating that any steric hindrance due to interaction between the methanol molecule and the two o-methyl groups of the 2,6-lutidinium ion is negligible in reaction 3. The values of k-,O given in Table I are of the same order of magnitude as values expected for diffusioncontrolled reactions.l0Sl6 For comparison, the values of k-,O for p-toluidine'O and trimethylamine16 are 1.04 X 1Olo and 0.6 X 1O1O M-' sec-l, respectively. The fact that k-.O is essentially the same for all of these compounds is also consistent with a diff usion-controlled reaction. On the other hand, the activation energies for Lolisted in Table I are larger than might be expected for a diffusion-controlled reaction. Activation energies for viscous flow and for self-diffusion of methanol are about 2 kcal.16 (20) M. Cocivera, J. Phys. Chem., 72, 2520 (1968).
Volume 72, Number 7 July 1968
