K. C. Chang and E. Grunwald
1422
n-PrdNNO3 is perhaps related to the similarity in size with the leaving ethyl group. References a n d Notes (1) (2) (3) (4) (5) (6)
8.Perlmutter-Hayman, frog. React. Kinet., 6, 240 (1971).
G. Scatchard, Natl. Bur. Stand. (U.S.), Circ., No. 534, 185 (1953). A. indeili, Gazz. Chim. /tal., 92, 365 (1962). A. indeili and R. De Santis, J. Chem. fhys., 55,481 1 (1971). J. I. Hoppe and J. E. Prue, J. Chem. SOC.,1775 (1957). A. Indeiii, V. Bartocci, F. Ferranti, and M. G. Lucarelli, J. Chem. fhys., 44, 2069 (1966); A. Indelli. J. fhys. Chem., 65, 972 (1961). (7) A. R. Olson and T. R. Simonson, J. Chem. fhys., 17, 1167 (1949). (8) A. Indelii. Ann. Chim. (Rome), 43, 845 (1953); 46, 717 (1956): 47, 588 (1957); T.Moeller, lnorg. Synth., 5, 98 (1957).
(9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
R. F. Nielsen, J. Am. Chem. SOC.,58, 206 (1936). A indelli and J. E. Prue, J. Chem. Soc., 107 (1959). J. E. Mayer, J. Chem. fhys., 18, 1426 (1950). J. C. Rasaiah and H. L. Friedmann, J. Chem. Phys., 48, 2742 (1968). A. indelli and F. Malatesta, Gazz. Chim. ltal., 103, 421 (1973). A. lndelli and F. Malatesta, Gazz. Chim. ltal., 103, 435 (1973). R. M. Heaiy and M. L. Kilpatrich, J. Am. Chem. SOC.,77, 5258 (1955). V. Carassiti, C. Dejak, and I Mazzei, Ann. Chim. (Rome),50, 979 (1960). A. lndelli and E. S.Amis, J. Am. Chem. SOC.,82, 332 (1960). A rationalizationof such mcdei can be done by taking into account that the activated complex is a species intrinsically different from the other ions. it is not surprising therefore that its "radius" cannot be assumed to be identical with that of the other ions, even in a gross approximation which neglects the individual difference between "normal" ions. Obviously, no treatment of this kind can be made using IPBE. (19) J. N. Butler, "Ionic Equilibrium. A Mathematical Approach", Addison-Wesiy, Reading, Mass.. 1964, p 469.
Water Participation in Proton-Transfer Reactions of Glycine and Glycine Methyl Ester' K. C. Chang and Ernest Grunwald* Chemistry Department, Brandeis Unlversity, Waltham, Massachusetts 02154 (Received January 15, 1976)
NH-proton transfer reactions with water participation of glycine and methyl esters of glycine, alanine, and phenylalanine were studied by dynamic NMR methods. For glycine and its methyl ester, kinetic analysis (and comparison with total rates of NH-proton exchange due to Sheinblatt and Gutowsky) reveals that processes which are second order in substrate proceed partly with and partly without water participation. There is evidence for intramolecular proton transfer between the NH3+ and C02- groups in glycine zwitterion, and for bifunctional proton transfer between the zwitterion and the uncharged amino acid. Rate constants are reported for proton transfer between ammonia and a series of carboxylic acids. A precise and convenient pulse sequence for NMR T1 measurement is described. The kinetics of "3-proton exchange of glycine in aqueous solution has been studied comprehensively by Sheinblatt and Gutowsky (SG).2These authors examined the CHz-proton resonance and thus measured the total rate of NH-proton exchange. Because of the insight one can gain into solvation phenomena by studying proton exchange with water particip a t i ~ n we , ~ now report a complimentary study of "3-toHOH proton exchange. Total rates of NH3-proton transfer between glycine and water have also been studied by 15N NMR,4s5and by relaxation ~ p e c t r o m e t r y . ~ , ~ In the present work, exchange rates were deduced from measurements of (l/Tz - 1/T1) of the H2O NMR or, for fast exchange, of the collapsed H20-NH3 NMR. The technique and rate calculations are familiar from previous publications.8 Rate measurements were made at five glycine concentrations ranging from 0.04 to 0.20 M, and in the pH range 3.9-6.3. All in all, -60 independent solutions were measured at 25 " C and subjected to kinetic analysis. The kinetics is fully consistent with that established by SG. The rate law is shown as follows: 3/7"
= kA + kg[R*]
+ kc[OH-] + k ~ [ R - l
(1)
where 7" = mean time a proton resides on -NHs during one cycle of proton exchange between R* and water; R+ = H3N+CHzCOzH;Rf = H3NfCH&02-; Ro = HzNCHzCOzH; R- = HzNCHzC02-. The factor 3 allows for the fact that there are three NH protons per molecule of the reactant, R'. The Journal of Physical Chemistry, Vol. 80, No. 13, 1976
The new rate constants for proton exchange with water, and comparable rate constants reported by SG2 for total NHproton exchange, are listed in Table I. The ratio in each case measures the fraction fw of reaction with water participation. These rate constants will now be discussed briefly. kA. As pointed out by SG, the major contribution to kA is made by an intramolecular reaction, R* Ro. Our rate constant is in good agreement with that of SG. Unfortunately, fw is indeterminate in this case because the COzH proton in Ro is in rapid exchange with water proton^.^^^ kg. Table I shows three possible reactions which might account for the observed kinetics. A fourth possibility, 2R* s 2R0, has a pK of 10.8 and would be undetectably slow. Reactions I11 and IV, which yield R- R+, involve just one functional group in each of the reactants, while the symmetrical process V involves two functional groups. Reaction I11 involves the functional groups NH3+ and "2. Rate constants for such processes in the direction of negative AGO are rarely greater than 2 X lo9 s-l M-1.9J0 Thus a plausible upper limit to the contribution of 111to kB is estimated to be 100 s-l M-l. Reaction IV involves proton transfer between NH3+ and COz-. Rate constants for the analogue of the reverse reaction have been measured11,12and are listed in Table 11. The values are generally less than lo9 s-l M-l, and a plausible upper limit to the contribution of IV to k B is thus estimated to be 50 s-l M-1. Thus reaction V appears to make a significant contribution. Jencks and Hand13 recently con-
+
Proton-Transfer Reactions of Glycine
1423
TABLE I: Kinetic Results for "-Proton
Rate constant, s-l or s-1 M-1
Term (es 1) hA
kB(Ri)
Exchange of Glycine in Water at 25 "C
Reaction (I) (11) (111) (IV) (V) (VI)
+
R* G R- H+ (ha) R* G Ro (kintra) Ri Ro e R- R+ R* + R* R- R+ R* RO ' e Ro Rti R* OH- ~t R- HOH
+ + + R* + R'-
+ + +
PK"
with H2O
Total (23 & 1 O C )
fw
9.60
(10)b
5.39
160 f 20
17OC
Indeterminatef
1.87 7.26
390 f 100
64OC
0.61 & 0.16
1.00
0.00 -4.40
1.5 f 0.6 + (2.6 f 1.0) X 1O1O 1.4 X 1O1O 1.9 x 1010 ~D(R-) (VII) R- + R'* 0.00 (7.9 f 0.2) x 107 3.8 X los 0.21 0.01 Based on data in ref 2 and 12. h - , for GME+ was multiplied by a factor of 1.88 to allow for the effect of the negative charge, to give h - , = 4.1 X 1O1O s-l M-l for R- + H+. Reference 2. Reference 7 . Reference 6. Because the CO2H and water protons are kc(OH-)
e
F?
z!=
a
e
f
in rapid exchange.
TABLE 11: Rate Constants for Proton Transfer in the System RCOzH + NH3 G RCOz- NH4+ in Water at 25 "C"
+
8.0 X lo8 2.5 x 103 HCOzH 6.5 X lo8 2.1 x 104 CH3C02H 4.7 x 108 2.0 x 104 CH3CH2C02H 6.1 X los 3.8 x 104 (CH3)3CCOzH a Data of V. K. Anderson and E. Grunwald. Rate constants refer to total proton exchange.
sidered the formation of RC02-.H02CR hydrogen-bonded complexes in aqueous solution and presented evidence that the association constant is 0.25 M-l for HC02-.H02CH. Accordingly, reaction V could be rationalized as bifunctional proton transfer within a hydrogen-bonded complex, as follows:
I - -
_L
be unity. However, the fact that the actual value is >1, and that a control experiment for CH3NH3+ (Table 111)gives good agreement between NMR and relaxation results, leaves open the possibility that "2-proton exchange of R- with water is significant. That is to say, at the higher pH's more than one NH3+ proton exchanges per cycle of reaction. For reaction VII, f w is accurately determined as 0.21 f 0.01. Thus symmetrical proton exchange in this case proceeds largely by direct bimolecular reaction, without water participation. For the analogous symmetrical proton exchange of aliphatic amines, f w is considerably
