3254
Vol. 84
MANFRED EIGENAND EDWARD M. EYRINC [CONTRIBUTION FROM
THE
MAX-PLANCK-INSTITUT FUERPHYSIKALISCHE CHEMIE, GOETTINGEN, GERMANY]
Fast Protolytic Reactions in Aqueous Solutions of Aminobenzoic Acids BY MANFRED EIGENAND EDWARD X. EYRING’ RECEIVED JANUARY 29, 1962 The protolytic reactions in aqueous solutions of benzoic, 0-, m-, p-aminobenzoic, N-methylanthranilic and N-dimethyl anthranilic acid have been studied using the high field dispersion and the temperature jump method. The rates of recombination of H + with the acid anions are diffusion controlled, although certain structural influences are exhibited. The rates of the conjugate base reactions, i e . , the recombination of the acid molecules with OH-, are also of the order of magnitude to be expected for a diffusion controlled reaction with the exception of N-dimethylanthranilic acid. In this case, a strong internal hydrogen bond prevents the access of OH- to the acid proton, slowing down the reaction by several orders of magnitude.
Introduction The application of high electric field relaxation techniques to the study of rapid reactions has been described in a previous paper.2 In reaction systems of the type
and Experimental Method.-As in all relaxation studies of fast reactions,3 we slightly displaced the system 1 from equilibrium and measured the relaxation time T for readjustment t o the new equilibrium conditions. Our perturbing parameter is the electric field density E and the perturbation is measured as a change in the conductance A of the solutions, resulting-at sufficiently high field densities-in a deviation from Ohm’s law: the 2nd W e n 7 or Dissociation Field Effect. Onsagerg has shown this effect to be described quantitatively by the relation
(where R represents a weak acid, R - the corresponding anion), the specific rate constants k R of a number of such recombination reactions have been shown3 to approach the theoretical limiting ( A . i / A ) = [ i l - 01)/(2 - a ) ] b A E (1) values for a diffusion controlled reactions4 Small where the degree of dissociation = c i / c ~ , ci is the concenreproducible differences of rates of recombination tration of each of the recombining ions, and b is a known have been correlated with predicted steric or function of ion mobilities, ion charge, temperature and the electrostatic effects as, for example, in the case dielectric constant of the solvent. In the present application, the exact expression of b is not required since the disof phenols, carboxylic acids, etc.; We report here persion method depends only on the experimentally observed similar investigations of the rates of protolytic linear dependence of Ah/A on AE over the 30 to 100 kv./cm. reactions in dilute aqueous solutions of several range. The high voltage is applied as a pulse in the form of a aminobenzoic acids, which, in addition, exhibit critically damped, harmonic oscillation, the shortest of which has a duration of 0.2 psec. This voltage is dropped some peculiarities due to their ampholytic nature. across a balanced, symmetric, bifilar-concentric bridge. The Cohn and Edsal16 assembled the ionization con- sample and reference cells have the same dimensions (2.0 mm. stants for several of these acids and expressed the electrode distance). The reference cell contains a dilute related equilibria in a notation that we will adopt HC1 solution to cancel the 1st Wien Effect9 in the sample cell. The other two arms of the bridge contain NaCl soluhere : (i) the neutral molecule R=R2’NRCOOH tions, but in these the field density does not exceed a few where for the present purpose R’=H- or CHs- and kv./cm. hpproxinlately 30 sec. elapse between pulses so R in the formula is a benzene ring, (ii) the dipolar that changes in conductance due to heating are negligible. ion (zwitterion) R*= +HR2’KCOO-, (iii) the ions \Vhen the pulse length becomes comparable to the relaxation 7 of equilibration a dispersion of the field effect occurs R+=+HR2’NCOOH and R-=R2’YRCOO-. If time from which r can be deduced.*,3 the over-all concentration ce is known, the concenConditions and Results.-Our experimental results are trations of these species and of hydrogen ions are shown in Figs. 1-5. These are plots of the dispersion of the field effect resulting from the application of high calculable from the experimentally determined equi- dissociation voltage pulses of 2.0, 1.0 and 0.2 psec. duration. The aquelibrium constants K , = [H+] ([R*] [ R ] ) / ous solutions were prepared under a nitrogen atmosphere L2-’ cm.-]. R+I,Kz = IH+I[R-I/(,[R*I K z 7 [R*I/ from water having a conductance of about [R], and the conditions of charge neutrality and Concentrations were determined volumetrically and agreed coiiservation of matter. Once the relaxation n ~ l lnit11 tlic yalues from conductance and pH nieasure(error liiiiits c ~ fT i 205;) with the exception of t h e time r for the rccoiribination reaction 1 has been iiierits N-tliiiietliyl-o-:t~iii~iol~e~iz~~ic acid. 111 tliis case tlie ineastired determined, we can calculate concentrations of a n d calculated pH were identical, but an exes5 in conductIi+ and R- froin the above equations and then the ance (12 X 10-6 $2-1 cm.?, instead of 8 X 1 W s ) iiidicntetl specific rate constants k~ and k n according to the some ionic impurities, l%-liich,however, do not appreciably influence the T values (error limit 50 9).All measurerelations”.” inents were made a t 26 i 2‘. (Y
+ RI),
r
~
lkir(/H-
7 < ii
-C
A-eqjl ’
+
[2J
-
( I ) National Science Foundation Postdoctoral Fellow, 19b0-19Ul (2) M .Eigen and J. Schoen, Z . Elektrochenz., 69, 483 (19.55). (3) 31. Eigen and L. DeMaeyer, “Technique of Organic Chemist r y , ” Vol. V I I I , Investigation of R a t e and Mechanism of Reactions, 3rd Ed , A . Weissberger, editor, Interscience Publishers, Inc., lVew Y o r k . S . Y . . t o be published. I1 P. Ijebye, Timis. Elertiocheriz. S O L .82, , 265 (1942). L j j 51. Eigrn and K. Kustin. J . A m . Chem. SOL.,82, 5952 (1‘3dOj. it;) E . J. Cohn and J. T. Edsall, “Proteins, Amino -4cids and I’eptides as Ions and Dipolar Ions.” Reinhold Publishins Ciirp., N e w Yurk, S.Y.,1943, pp. 96-99.
An appreciable 2nd IX‘ien Effect is observed throughout for recombinations involving a neutralization of charge. Therefore, the equilibrium designated by K , = [H+][R-]/[KZl might be expected to show a somewhat smaller effect than the equilibrium K D = [H+][ R - ] / [ R ] since R* possesses a small residual separation of charge. We see then that the relaxation time 7 that we have measured represents a composite of two relaxation processes of the two different reactions proceeding concurrently. Two considerations lead 11q (7) 51. Wien and J. Schiele, Z . Physik,32, 345 (1531) (SI L. Onsager, J. Chem. Phys., 2 , .i99 [19;{4j. (9) hI. \Vien, Awn P h y r i k , 83, 327 (1!J27)
FASTPROTOLYTIC REACTIONS IN AQUEOUS SOLUTIONS
Sept. 5 , 1962
I 00
00
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10
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.\.I
...
IO'
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3255
lil
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Fig. l.-Experimental dispersion of the field effect in aqueous benzoic acid solutions Each set of three points corresponds to the three different pulse lengths (frequencies w1 = 5 X lo5 sec U J = lo6 sec.-l, and w 3 = 5 X IO6 reading from left to right). sec a
CO 261
Fig. 3.-Experimetital dispersion of the field effect in aqueous m-aminobenzoic acid solutions. CO I
14.
,
9C.
0
-3
.
Id'U
IC-iv 155 M
IZ
16'
M
2 6 . 10' M
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o
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1
99.IG'U
.-il__---
'
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1
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,
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Fig. 4.-Experiniental dispersion of the field effect in aqueous p-aminobenzoic acid solutions cn i:
1 5 r W'M
D
a1 H
N- rnelhylanlhrmil~cacid
N. dlmrlhyl.nlhrantlic acid
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Fig. 5.-Experimental dispersion of the field effect in aqueous solutions of N-dimethyl-o-aminobenzoic acid and of N-methyl-o-aminobenzoic acid X 10'" JI-' bec.-I. This diffeirnce is ne11 mithin our estimated ekperimental error of f 205;. n'e ctlso have deterinined the specific rate constants of the equilibrium
I
