J . Phys. Chem. 1986, 90, 1659-1663
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CHEMICAL KINETICS Proton Kinetics and Thermodynamics of Carboxylic Acid Crown Ethers. Influence of Hydrogen Bond Formation in Water, 80 % (w/w) Methanol-Water, and Chloroform-d Raymond J. Adamic, Barry A. Lloyd, Edward M. Eyring,* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112
Sergio Petrucci, Department of Chemistry, Polytechnic Institute of New York. Farmingdale, New York I I735
and Richard A. Bartsch Department of Chemistry, Texas Tech University, Lubbock, Texas 79409 (Received: June 30, 1985)
Rates of protonation-deprotonation of the acid crowns sym-dibenzo-16-crown-5-oxyacetic acid (1) and 2,6-dimethylbenzoic acid- 18-crown-5 (2) have been determined in water and 80% (w/w) methanol-water by the electric field jump relaxation method. The rates of proton recombination with ionized 1 and 2 are diffusion controlled in both solvents; the lower mean rates observed for 2 are ascribed to steric effects in the vicinity of the polyether cavity. In addition, direct proton-transfer rates between 1 or 2 and the indicator, bromocresol green, were determined in 80% (w/w) methanol-water. The proton-exchange rates substantiate the role of steric effects in proton removal and recombination. Furthermore, the thermodynamic behavior of 1 and 2 was studied by evaluating the acid dissociation constants in water and 80% (w/w) methanol-water, and by far-infrared spectroscopy in deuterated chloroform. Acid crown 2 may form an intramolecular hydrogen bond in 80% (w/w) methanol-water as suggested by the increased pK, (+2.96 units) over that in water. Such a bond in 2 is strongly evident in deuterated chloroform as a broad far-infrared band near 190 cm-'. Acid crown 1 apparently forms hydrogen bonds with hydroxylic solvent molecules and does not form internal hydrogen bonds in deuterated chloroform. The implications for acid crown ethers which can hydrogen bond with solvent molecules and/or internally hydrogen bond are discussed in terms of the reaction kinetics occurring at the interface of liquid membrane systems.
Introduction The rate of proton transfer in hydroxylic media depends on the extent and the type (inter- vs. intramolecular) of hydrogen bonding. Intermolecular hydrogen bonding resulting from interaction of the solvent with an acid and its conjugate base produces rates of recombination approaching the diffusion-controlled limit predicted by the Debye-Smoluchowski equation. However, when the proton is internally bonded to a nearby electronegative or negatively charged atom, energy is required to break the bond before proton transfer can occur and correspondingly slower deprotonation rates are observed. Crown ethers1q2with pendant carboxyl groups are possible candidates for intramolecular hydrogen bonding. The acid proton of the pendant sidearm may effectively interact with the oxygen atoms of the macrocyclic cavity. Whether an intramolecular hydrogen bond forms or not is dependent upon the proper orientation of the acidic proton to the polyether ring atoms and on the thermodynamics of the solute-solvent environment. Knowledge of these interactions is paramount to understanding the complexation properties of acid crowns in solution, especially in situations in which acid crowns are utilized for carrier-mediated cation transport across liquid membra ne^.^,^ Liquid membranes5 in which the aqueous and organic phases are vigorously stirred may have separation rates which depend not only on the diffusional (1) Newcomb. M.: Cram. D. J. J. Am. Chem. SOC.1975. 97. 1257. ( 2 j Bartsch, R'. A.;'Heo, G. S.; Kang, S. I.; Liu, Y.; Strezelbicki, J. J. Org. Chem. 1982, 47, 457. (3) Frederick, L. A.; Fyles, T. M.; Gurprasad, N. P.; Whitfield, D. M. Can. J . Chem. 1981, 59, 1734. (4) Charewicz, W. A.; Bartsch, R. A. J . Membr. Sci. 1983, 12, 323. ( 5 ) Choy, E. M.; Evans, D. F.; Cussler, E. L. J . Am. Chem. Sot. 1974,96, 7085.
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processes but also on the chemical reactions occurring at the interface. In liquid membranes, the carrier molecule or acid crown normally resides in the organic layer which is usually an aprotic solvent of low polarity, such as chloroform. Transport of a cation between two aqueous phases, one acidic, the other basic, is facilitated by countertransport of a proton. The reaction of a proton with the macrocyclic carrier is rapid5 compared to the actual complexation of the metal ion and the subsequent transport of the cations. Microscopically, the proton-transfer reactions are occurring at the interfacial region where the acidic proton can interact with nearby water molecules to form hydrogen bonds. A medium simulating the interfacial region would have the physical and chemical properties of both the organic solvent and the aqueous phase. In the present investigation an 80% (w/w) methanol-water solvent mixture is used to model the interfacial region. The effect on the thermodynamic and kinetic protonic behavior of acid crowns 1 and 2 on going from a very polar 0
1
N
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medium, water, to an 80% (w/w) methanol-water mixture is 0 1986 American Chemical Society
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The Journal of Physical Chemistry, Vol. 90, No. 8, 1986
examined in conjunction with possible intramolecular hydrogen bond formation. Proton recombination rates with ionized symdibenzo- 16-crown-5-oxyaceticacid (1) and 2,6-dimethylylbenzoic acid-18-crown-5 (2) were determined by the electric field jump relaxation technique in water and in 80% (w/w) methanol-water. The deprotonation rates of 1 and 2 were deduced from the acid dissociation constants measured by potentiometric titration and the ion recombination kinetic data. In addition, the rate of proton exchange between either 1 or 2 and bromocresol green near pH 7 in 80% (w/w) methanol-water is reported. Finally, far-infrared spectral measurements of 1 and 2 in deuterated chloroform were made to evaluate the extent of internal hydrogen bonding in this solvent medium.
Experimental Section Chemicals and Solutions. Acid crowns 1 and 2 were synthesized by published methods'-* and recrystallized from ethanol/water and methylene chloride/pentane-acetonitrile, respectively. Anal. for 1: Calcd for C21H,,0s: C, 62.38; H, 5.94. Found: C, 62.41; H , 6.02. 2: Calcd for C17H2407:C, 60:OO; H, 7.10. Found: C , 60.14; H , 7.03. The precursor of crown 2, the methyl ester, was also synthesized by methods listed in ref 1. Bromocresol green (Aldrich) was recrystallized from glacial acetic acid and dried under vacuum. A solution of 0.1 M nBu,NOH in 80% (w/w) methanol-water was used as titrant for the potentiometric titrations and was standardized with recrystallized benzoic acid. Methanol (acetone free) and triply distilled water were boiled prior to use. Deuterated chloroform (SIC) was stored over 3A molecular sieves. Stock solutions were prepared and diluted prior to each experiment. Thermodynamic Measurements. Potentiometry. pH measurements and potentiometric titrations were carried out with a Sargent-Welch N X pH meter. The glass combination electrodes were conditioned in water or 80% (w/w) methanol-water and calibrated with the appropriate buffers.6 All titrations were conducted under a nitrogen atmosphere (boil off of liquid nitrogen). The mixed acid dissociation constants, pKaM,were corrected to the thermodynamic dissociation constants, pK2, with the following equation:' pKaT = pKaM4-
Az21,'i2 1 4- Balm'/*
where A and B equal 1.265 and 0.446, respectively, for 80% (w/w) methanol-water,* a, the distance of closest approach, is taken to be 5 A, and I , is the ionic strength at the midpoint of titration. Spectrophotometric Titration. The pKa of bromocresol green in 80% (w/w) methanol-water was determined by pH titration using a Beckman DB UV spectrophotometer at a wavelength of 615 nm with the cuvette thermostatted at 25 f 0.1 "C. Far-Infrared Spectra. Spectra (4-cm-I resolution) were recorded with a Nicolet 71 99 FT-IR spectrophotometer equipped with a 6.25-pm Mylar beam splitter and a room temperature TGS detector with a polyethylene window. The optical bench was evacuated except for the sample compartment which was purged with nitrogen. All sample preparation and filling of high-density polyethylene cells were carried out in a drybox. Kinetic Measurements. Electric field jump (E-jump) relaxation data were obtained with an apparatus previously described9~l0 except for the changes noted below. Relaxation traces for the proton recombination reactions were observed on a Tektronix 7834 storage oscilloscope with 7A22 (bandwidth 1 MHz), and 7A13
TABLE I: Acid Dissociation Constants, pK,, of Acid Crowns 1 and 2 and Various Functional Analogues in Water and 80% Methanol-Water 80% (w/w) K*,,,,l acid water MeOH-water K,,, M,oH-water 4,590 5.90 20 1 4.Sb 7.76 912 2 acetic acid 4.76 6.71' 129 methoxyacetic acid 3.57d ethoxyacetic acid 3.87e 4.92e benzoic acid 4.26 6.46 I82 "Reference 2. bReference I . 'pK, from Shedlovsky, T.; Kay, R. L. J . Phys. Chem. 1955, 60, 151. "King, E. J. J . Am. Chem. Soc. 1960, 82, 3575. '50% (v/v) ethanol-water. Reference 12. JLange, N . A,, Ed. Lunge's Handbood of Chemistry, 10th ed; McGraw-Hili: I i e w York, 1967.
( 5 and 100 MHz) vertical amplifiers and 7B50A time base. Four to seven traces were stored, photographed, enlarged, and digitized with a Tektronix 4662 digital plotter. Relaxation times less than 300 ns were corrected for an electronic response time of approximately 30 ns. Pulses with widths from 0.5 to 5 ps and amplitudes from 40 to 50 kV were applied to a thermostatted sample cell with a 0.5-cm interelectrode spacing. For the direct proton-transfer reactions near pH 7, a C W Radiation, Inc. 1.0mW H e N e laser was used in place of the pulsed xenon arc lamp to provide an intense, stable source of light at long pulse lengths. In addition, relaxation traces were digitized with a Tektronix 7D20 programmable digitizer. Samples were degassed (under vacuum) prior to filling the cell. The pH of the solutions was measured before and after each experiment, with the average pH recorded. Solutions for which the pH varied by more than f0.03 pH units in the course of the experiment were discarded. Reactions were monitored spectrophotometrically with bromocresol green at wavelengths in the 600-633-nm range. Rate constants were corrected to zero ionic strength with the activity coefficients given , by the Debye-Huckel term in eq 1. Results and Discussion Thermodynamic Behauior. Acid dissociation constants of crowns 1 and 2 and benzoic acid determined in water and 80% methanol-water are given in Table I. In addition, the pK,'s for some functional analogues of 1 and 2 are listed. The extent of ionization of acid crown 1 in water (dielectric constant t = 78.54 at 25 "C) decreases in 80% methanol-water ( t = 42.60 at 25 "C)" by a factor of 20: 1. A similar decrease for a functional analogue of 1, ethoxyacetic acid, in water vs. 50% (v/v) ethanol-water ( t = 53.44)12 has been reported.I3 The decrease in acidity can be explained in large part by ion-solvent interactions; a less polar medium is not as effective in stabilizing the anion. However, ion-solvent interactions do not explain why 1 should be less acidic than either methoxyacetic acid or ethoxyacetic acid. A closer analysis of 1 and its functional analogues in water and 80% (w/w) methanol-water suggests an inductive mechanism is operating. The acidity of 1 in aqueous solution lies between that of acetic acid (CH,COOH) and ethoxyacetic acid (CH,CH,OCH,COOH). Therefore, replacement of hydrogen atoms on the acidstrengthening methoxy group (Taft constant, u*, = 1.81)14 with atoms or molecules which can weaken the electron-accepting ability of CH,O- will result in a weaker acid. A decrease in acidity becomes apparent when a hydrogen atom (u* = 0.49) on the ( 1 I ) Oiwa, T. I. J. Phys. Chem. 1956, 60, 754.
( 6 ) DeLigny, C. L.; Luykx, P. F. M.; Rehbach, M.; Wieneke, A. A . R e d . Trau. Chim. Pays-Bas 1960, 79, 7 1 3 . (7) Albert, A.; Sarjeant, E. P. Ionization Constants of Acids and Bases; Metheun: London, 1962; p 58. (8) Nancolas, G. H. Interactions in Electrolyte Solutions; Elsevier: New York, 1966; p 13. (9) Olsen, S. L.; Silver, R. L.; Holmes, L. P.; Auborn, J. J.; Warrick, P.; Eyring, E. M . Reo. Sci. Instrum. 1971, 42, 1247. (10) Olsen, S . L.; Holmes, L. P.; Eyring, E. M . Rec. Sei. Instrum. 1974,
45, 859.
(12) Wyman, J. J . A m . Chem. SOC.1931, 53, 3292. ( I 3) Minamida, I.; Ikeda, Y . ;Uneyeama, K.; Tacaki, W.; Oae, S . Tetrahedron 1968, 24, 5293. (14) u* values taken from Perrin, D. D.; Dempsey, B.; Sargeant, E. P. p K a Predictions for Organic A d d s and Bases; Chapman and Hall: London, 1981. u* values are Taft constants and measure, relative to CH,, the inductive effect
of various substituents. Positive u* values are good electron acceptors and
as the values become increasingly negative the substituents represent good electron donors. Thus, CH, for which u* = 0 is a better electron acceptor than C,HS, where u* = -0.10.
Carboxylic Acid Crown Ethers
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SCHEME I
CH,O- group of methoxyacetic acid ( C H 3 0 C H 2 C O O H )is replaced with a better electron donor, CH,- (.* = o). Likewise, a reduction in ionization can be expected when a second methoxy hydrogen is substituted with another methyl group to form 2isopropoxyacetic acid ( (CH3)2CHOCH2COOH). The acid proton of crown 2, which does participate in the formation of an intramolecular hydrogen bond in the solid state,15 shows a significant decrease in acidity in 80% methanol-water MeOH-wa,er for 2 is 912. By compared with water. Kwater/Keo% comparison, benzoic acid, only four times more acidic than 2 in water, is twenty times more so in 80% methanol-water. Such a large decrease in acidity for 2 in 80% methanol-water implies a much greater interaction between the proton and transannular oxygen, and any similar interaction in water is probably quite weak. As far as solvation of the anion is concerned, solvation of the anion of 2 is sterically inhibited as compared to benzoic acid, but water molecules, not methanol, of the 80% methanol-water mixture are expected16 to solvate the carboxylate anion. Thus solvation should be similar in both solvent systems. Far-Infrared Spectra. It has been inferred12that ethoxyacetic acid may form an intramolecular hydrogen bond between the acid proton and the a heteroatom (in this case, oxygen) with the probability of interaction being the highest in less polar solvents. For 1 a similar type of hydrogen bond may exist besides a transannular hydrogen bond to the macrocyclic cavity. Chloroform, in addition to its importance as a liquid membrane component, has a low dielectric constant ( t = 4.81 at 2 0 "C) and should enhance the probability of H bonding. This is not the case, however, for acid crown 1. The far-infrared spectrum of 1 (0.05 M) in chloroform-d as reproduced in Figure lb, is essentially identical with that of the solvent, Figure la, with no evidence of a low-frequency hydrogen-bonding mode. Not unexpectedly, the infrared spectra of acid crown 2, Figure IC, do show the presence of a broad peak ranging from 150 to 235 cm-I. Assignment of this band to a H-bond interaction that is intramolecular in nature was based upon the following: (1) H-bond interactions are weak (compare its relative intensity to that of the chloroform-d peak at 365 cm-' and note the decreasing intensity of absorption as a function of concentration), broad, and typically lie in the range of 50-250 cm-I. ( 2 ) The peak intensity decreases to a small hump when the H atom is replaced with -CH,, as indicated in Figure Id. ( 3 ) In the crystal structure15 of 2 the coplanar arrangement of the acid proton to the polyether oxygen promotes the formation of an internal bond. (4) There are no peaks for 1, which, if present might be suggestive of an intermolecular hydrogen bond. Thus, based upon the far-infrared and potentiometric results, it appears that acid crown 1 is not capable of forming hydrogen bonds with itself or nearby acid crown molecules in solvents of wide-ranging dielectric constant, but proper solvent conditions can induce or control the strength of an intramolecular hydrogen bond in 2. Relaxation Kinetic Behavior. Proton transfer involving acid crowns 1 and 2 was investigated under acidic and neutral conditions and was monitored by couplirlg the reaction of interest to a pH indicator as illustrated in Scheme I in which CR- and In2represent the anion and dianion of acid crown and bromocresol green, respectively. The ionic recombination and dissociation reactions, k I 3 ,k3l and k 1 2 ,k 2 , , become important at pH
