3234
M. COCIVERA, E. GRUXWALD, AND C. JUMPER
Acid Dissociation and Proton Transfer of p-Toluidinium Ion and Dimethylphenylphosphonium Ion in Methanol
by M. Cocivera, Ernest Grunwald, and Charles F. Jumper Bell Telephone Laboratories, Incorporated, M u r r a y H i l l , N e w Jersey
07971
(Received J u n e 19, 1964)
Acid dissociation constants, K,, and rate constants, k , and k-,, in methanol have been measured over a wide temperature range; K , = k , / L a . For p-toluidinium ion a t 25O, k , = 2.92 X 103sec.-', k-, = 1.04 X 101Osec.-lM-l,K,= 2.8 X (AI);AH,' (-28') = 13.7 kcal. mole-l, AS,' (-28') = 16 e.u. For dimethylphenylphosphonium ion at 25O, k , = 3.7 X lo3set.-', k-, = 1.3 XIOBset.-' M-I, K , = 2.8 X 10-5 (M) ; AH,' (-22') = 10.2 kcal. mole-', AS,' (-22') = 13 e.u., AH,* (0') = 11.3 kcal. AS,* (0') = -4 e.u. Kinetic data for the symmetrical proton exchange, BHf CH30H* B ---t B HOCH, H*Bf, are as follows. For B = p-toluidine, kz (25') = 8.1 X lo7 set.-' M-l, AHz* (-28') = 2.33 kcal. mole-', AS2*(-28') = - 14.4e.u. ForB = dimethylphenylphosphine, kz (25') < lo4sec.-'AP1. Thedata provide a further example that proton transfer involving phosphorus acids and bases is appreciably slower than that involving nitrogen acids and bases.
+
+
For acid and base dissociation in hydroxylic solvents BH+
B
+ ROH
+ ROH
ka
B k- s
+ ROHz+
(A)
+ RO-
(B)
kb
k- b
BH+
the rate constants for reaction in the direction of decreasing standard free energy are of considerable current interest. If B is an oxygen or nitrogen base and ROH is water, data are available for a nunibei of substrates and suggest that such reaction takes place at each encounter of suitable geometry.'-4 On the other hand, if B is a phosphorus base, this conclusion may not be justified; however, kinetic measurenients have been reported for only one system, triniethylphosphine in water.6 To be specific, let us compare rate constants for triniethylphosphine with those for the nitrogen analog, trimethylamine. The following data rcfer to reaction in mater in the direction of decreasing. standard free energy: for trimethylamine, k-, = 2.7 X l o L o set.-' M-1 at 25", and kb= 2.1 x 1O1o see.-' AI-' a t 20°3r4; for trirnethylphosphine, k-, = 5 X lo9 sec.-l M-l a t 22" arid k-b = 4.6 X lo7 sec.-l at T h e Journal of Physical Chemistry
+
+
22 o.5 For comparison, the specific rates for encounters between reactant molecules6 in these processes are about 4 X 1010sec.-l M-l. On the basis of these data, if reaction were taking place at each encounter of suitable geometry, the geometrical factors would be about 0.5 for the processes measured by IC-, and k-b in the case of trimethylamine, but only 0.1 or less in the case of trimethylphosphine. While geometrical factors of the order of 0.5 are plausible for diff usion-controlled processes, lb values much below this figure are not. Therefore, for trimethylphosphine the process measured by k-, is probably,
(1) (a) hf. Eigen and L. DeMaeyer, Proc. R o y . Soc. (London), A247, 505 (1958); (b) M. Eigen and K. Kustin, J . Am. Chem. Soc., 82, 5952 (1960). (2) W. J. Albery and R. P. Bell, Proc. Chem. Soc., 169 (1963). (3) (a) M, T. Emerson, E , Grunwald, and R. A. Kromhout, J . Chem. P h y s . , 33, 547 (1960); (h) E. Grunwald, J . P h y s . Chem., 67, 2208 (1963). (4) A useful tabulation of data has been given by M. Eigen and L. DeMaeyer, in "Rates and Mechanisms of Reactions," S. L. Friess, E. S. Lewis, and A. Weissberger, Ed., Part 11, Interscience Publishers, New York, N . Y . ,pp. 1031-1050. (5) B. Silver and Z. Luz, J . Am. Chem. Soc., 83, 786 (1961). (6) Calculated f:om eq. 18 and 19 of ref. 3a, assuming a collision diameter of 4.8 h.
ACIDDEMOCIATION AND PROTOX TRANSFER OF ~-TOLUIDINIUM ION
and that measured by k-b is almost certainly, activation-controlled. When phosphonium ion and phosphine both participate in a proton transfer process, the reduction in reactivity relative to that of the nitrogen analogs appears to be very striking indeed. Thus, for the symmetrical exchange reaction C, which involves a solvent molecule (ROH) BH+ 3. ROH'
+ H ---%B + HOR + H'B+
(C)
the values of kz in water at 25" are 3 X lo8 sec.-I M-' and 5 1.2 X 102 set.-' M - 1 , respectively, when B is trimethylamine3band trimethylphosphine.5 In order to extend the rate nieasurements for reactions A-C to other hydroxylic solvents and to extend the comparison of phosphorus with nitrogen bases to other structures, we now report a kinetic and equilibrium study of acid dissociation and proton transfer for p-toluidinium ion (I) and dimethylphenylphosphonium ion (11)
T+ CHs I
(HaC)d'H+
8 I1
over a wide tempeirature range in buffered and unbuffered methanol solutions. The measurements were made by nuclear magnetic resonance techniques. The kinetic data demonstrate again the high reactivity characteristic of nitrogen acids and bases and the rdatively low reactivity of the phosphorus analog. The K , measurements enable us not only to evaluate both k , and k-, in (A) but also to elucidate a discrepancy of long standing concerning the absolute value of K , for p-toluidinium ion.
Experimental MateriaEs. Commercial reagent grade methanol was dried by treatment with magnesium' and distilled, the first and final quarter being discarded. A small quantity (ca. 50 nqg./l.) of benzoic acid was added to the middle fraction, and the resulting solution was redistilled, only the middle portion being collected. Dimethylphenylphosphine was prepared in the following manner. Dichlorophenylphosphine (obtained commercially) was added to an ether solution of methylmagnesium iodide under a nitrogen atmosphere and a t Dry Ice-methanol temperature. The crude product was fractionally distilled at reduced pressure in a nitrogen atmosphere on a spinning band column. The fraction with b.p. 65.0" a t 8 mm. was collected.
3235
This fraction was redistilled a t reduced pressure under a nitrogen atmosphere in an all-glass system. The fraction with b.p. 70.0" at 12 mm. was collected. Eastman Kodak White Label p-toluidinium chloride was recrystallized first from pure methanol to which a small amount of HCl was added, and finally from pure, neutral methanol. The resulting crystals were pure white and were stored over anhydrous Mg(C10& in an evacuated desiccator. For experiments involving acid-base ratios in excess of 100, this recrystallized material was further purified by zone refining in a nitrogen atmosphere. Solutions. Solutions of dry HC1 in methanob; NaOMe in methanol, and the various buffers in methanol were prepared using standard quantitative methods. The dimethylphenylphosphine buffers were in outgassed solutions while those for the toluidine buffers were air-saturated. Whenever possible, those concentrations that were made up by weight were checked by titration with standard acid or base. N.m,r. Measurements. Measurements were made on Dr. S. Meiboom's n.m.r. spectrometer, using spinecho and slow passage techniques. For further details, see previous papers.8-10 Chemical Shifts. Pertinent n.m.r. line positions a t constant field and at a nominal resonance frequency of 60 Mc./sec. for p-toluidinium ion in methanol at 25" were found to be as follows: CH3 protons of methanol (internal reference), 0.0 c.P.s.; OH protons, 94.1 c.P.s.; NHB+ protons, 405.4 C.P.S. In the presence of up to 8.5 M HC1, the OH line position was 94.1 44.7[HCl] - 0.13[HC1I3C.P.S. The chemical shift of the NH3+ protons was measured directly a t -80" (407.5 c.P.s.) and a t -60" (407.1 c.P.s.). Because of proton exchange, the NH3+ protons are not directly observable a t 25", except in very strong acid. The value given above was obtained from the low-temperature data by linear extrapolation; direct measurement at 25" in 6.4 N HC1 gave a value of 390 C.P.S. Under all conditions where direct measurement was possible, the NH3+ proton resonance was a single line; that is, spin-spin interaction with N14 was effectively averaged out to a value approaching zero by TI relaxation of the nucleus. Pertinent n.ni.r. line positions for dimethylphenylphosphonium ion and the conjugate phosphine are
+
(7) N. Bjerrum and L. Zeohmeister, Ber., 56, 894 (1923). (8) (a) E. Grunwald, C. F. Jumper, and S.Meiboom, J . Am. Chem. Sac., 84, 4664 (1962); (b) E. Grunwald, C. F. Jumper, and S.Meiboom, i b i d . , 85, 522 (1963). (9) S.hleiboom and D. Gill, Rev. Sci. Instr., 29, 688 (1958). (10) S. Alexander, ibid., 32, 1066 (1961).
Volume 68, Number 11
November, io64
3236
-+
M. COCIVERA, E. GRUNWALD, AND C. JUMPER
54.6
C67.9
(ii)
Table I : Sample Kinetic Data for Proton Exchange between p-Toluidinium Ion and Methanol
F-
e I
.
I 1
I 1
R REFERENCE
(CH3I2 i P H CHJ (MeOH) OH
$PH
41 500 CPS
Figure 1. Pertinent n.m.r. line positions, in c.P.s., a t -42.7' and 60 Mc./sec. ( i ) 0.3 M dimethylphenylphosphine in methanol. (ii) 0.3 M dimethylphenylphosphonium chloride in methanol containing 1 ~ 1 fhydrochloric acid. Precise line positions, relative to CH3 of methanol, are as follows: al, -74.5; a2, -59.6; J P - c E ~ , 14.9; J C E ~ - P 4.4; E , bl, -24.6; bz, 491.7; e, 107.8 - 0.5331 - 0.00096tZ; t is temperature in "C.
shown schematically in Fig. 1, and precise values for these line positions a t -42.7' and 60 Mc./sec. are stated in the caption. All positions were measured directly under these conditions in methanol solution, except for the P H line labeled bl. Its position was determined by assuming that the phosphorus-hydrogen spin-coupling constant has the same value in methanol as in 6 M aqueous HCl, where it could be measured directly a t - 1'. The chemical shifts (relative to the CH3 protons of methanol) were measured also a t -63.0' and a t -21.8' and, with the exception of that for the OH protons, were independent of the temperature.
Results p-Toluidinium Ion. Proton exchange rates of the OH protons of methanol were measured by means of the CH, proton resonance of methanol, as described previously.8 Measurements were made at 25 ' and a t -81.6 ', in buffered solutions containing p-toluidinium ion (BH+) and ptoluidine (B). Data a t both temperatures are represented accurately by the rate eq. 1, where R denotes the rate of exchange of OH protons, in g.-atoni/l. sec., and [BH+] and [B] denote molar concentrations. The latter were varied over wide ranges.
[CIH~NH~I-I
0.545 0.439 0.330 '0.188 0.133
Other solutes
or R',
--g.-atom/l. (obsd.)
s e c . 7 (oalcd.)
1. From CH3 proton resonance a t -81.6' 2260 1690 [C~H~NHI+] 1110 780 [ C7H7NHzI 490 280
2. 0.00999 0.00999 0.00999 0.00999 0.00999 0.00999
2550a 1720 1060 470 290
From OH-NH proton system a t 24.9" [HCl] = 0,000463 31.7 0,00139 30.5 0.00417 29.0 0.0125 29.6 0,0375 28.7 0.1126 24.6
33.8' 30.4 29.3 28.7 27.8 25.5
+
a R = 6.2 X 106[BH+][B] 8.9 X 10-3[BH+]/[B]. R' 2890[BH+] antilog (-0.48[HCl]) f 22.8[BH+I2/[HC1].
=
Table I1 : Kinetic and Thermodynamic Results for Acid Dissociation and Proton Transfer of p-Toluidinium Ion in Methanol Temp., "C. kueOBz+, see.+
&,
Mb K,, M kB,sec.-'M-l kz', see.-' M-'
kl' = k,, 8ec.-lC k- sec. -1 M - I d
24.8 8 . 7 9 X 1010" 2.47 x 1 0 4 ~ . 2 . 8 1 x 10-7# 8 . 1 X lo7' 9.0 x 1 0 7 ~ 2.92 x 1 0 3 ~ 1.04 X
AH," (kcal., a t ca. -28") AS," (e.&, a t ca. -28") AHz* (kcal., a t ca. -28') A&* ( e x . , at ca. -28")
-81.6 1 . 2 x 1o1ou 8 . 9 x 10-3' 7 . 2 x 10-13g 6 . 2 X lo6" ... , . . , . .
13.7 I0 . 7 16.1 f2 . 5 2 . 3 3 f.0 . 1 5 -14.4f 0.6
See eq. 1. Extrapolated to zero ionic strength. a Ref. 8a. Actual values: [BHCl] = 0.01, kl' = 2890; [BHCll = 0.11, kl' = 2640. kl'/K,. e Accuracy I t l o % or better. Accuracy =k10-20y0. Accuracy below f20%.
On that basis, Q = K , ~ M ~ O H where ~ + , K , is the acid dissociation constant of BH+ and ~ M ~ o Hthe ~ + rate constant for proton exchange of CH30H2+. Values R = h [ B H + l [ B l Q[BH+l/[Bl (1) for the latter have been reporteds"; hence, K , can be evaluated. The results are included in Table 11. Sample data are shown in Table I, and values of the The accuracy of K , is limited by that of I C M ~ O H ~ + . parameters k z and Q are given in Table 11. These We estimate that a t 25', K , is accurate to better than values of the parameters reproduce 15 data sets a t io%, but a t -81.6' the value is probably only semi-81.6' to i s % , and 7 data sets at 25' to I S % . quantitative. Our interpretation of eq. 1 is that ICz is the rate While the CH3 proton resonance measures the exconstant for reaction C, and that Q[BH+]/[B] is the rate of proton exchange catalyzed by CHaOH%+. change rate of OH protons of methanol in a com-
+
T h e Journal of Physical Chemistry
3237
ACIDDISSOCIATION AND PROTOX TRAXSFER OF ~-TOLUIDINIUM ION
pletely nonspecific way, the rate of exchange of 013 protons specifically wiith S H 3 +protons of p-toluidinium ion can be derived from resonance measurements on the “,+-OH proton system under acidic conditions where the CH, resonance is a sharp, fully exchangenarrowed single line. The method is analogous to that described previously for solutions of benzoic acid in methanoLgb Let R’ denote the rate of exchange, in g.-atom/l. see., ol’ OH protons with SH3+ protons. We measured R’ at 2 5 ” , either in the presence of excess HC1 or a t buffer ratios in excess of 50. The data are reproduced by thle empirical equation
R’
=
kz’[BH+][B]
+ kl’[BH+]
(2)
Some sample data are included in Table I, and values of the parameters k2’ and k,’ are listed in Table 11. Strictly speaking, the presence of a term kl’[BH+] in (2) is inconsistent with the absence of an analogous term in (1). However, we must remember that eq. 1 and 2 are empirical rate equations. Under the conditions of relatively low [RH+] and high [B] under which R is measured at 25”, the kinetic term k1’[BH+] would be smLall compared to the experimental error. Examination of Table I1 shows that, within experimental error, k2’ (which is based on R’) is equal to k2 (which is based on 12). This equality completes the proof that the underlying process is reaction C. The concentration depenldence proves participation by one molecule each of BH.+ and B. The appearance of this kinetic term in the expression for R proves participation by one or moire molecules of methanol. The equality, k2’ = k2, proves that the number of methanol molecules is 0ne.l’ The term k,’[BH + ] most probably measures the rate of acid dissociation, that is, ICl’ = k,. It has been found that in water, k,, in contrast to k 2 , is subject to rather large salt effect^.^^,^^ Within the limited scope of the present data, the same appears to be true in methanol. I n the range of ionic strength covered by this investigation, 0
