246
N. F. HEPFINGER, R. P. T. TOMKINS, AND P. J. TURNER
A Nuclear Magnetic Resonance Study of Acid-Base Interactions for the
Chloranil Electrode System in Acetonitrile by N. F. Hepfinger, R. P. T. Tomkins,” and P. J. Turner Department of Chemistry,REnSSElaEr Polytechnic Institute, Troy, N . Y . 1.9181 (Received July 19, 1971) Publication costs borne completely by The Journal of Physical Chemistry
In the hydrochloranil-chloraiiil electrode system in acetonitrile, specific hydrogen bonding iiiteractioiis between solutes and solvent and between the two solutes are sufficiently weak to indicate that the activity coefficients of the electrode materials should be fairly constant. Chloraiiil behaves as a weaker base than either the solvent or small amounts of water in the solutions and should not interfere with acid solutioiis except a t high dilution. Hydrochloranil has weak acid properties which may become significant in basic solutions although its restricted stability to air oxidation limits its application to acid and weakly basic solution. Protoii exchange in this system is rapid.
The use of emf methods for the study of acid-base interactions in nonaqueous solutions is occasionally impeded by difficulties in obtaining a satisfactory electrode reversible to hydrogen ions. In acetonitrile, the hydrogen electrode on platinized platinum has been found satisfactory in some cases;l however, it is subject to interference by the ~ o l v e n t . ~ -Although ~ the glass electrode shows some stability in acetonitrile,6the proposed mechanisms, whereby the potential of the glass electrode depends linearly on the pH, require empirical terms to account for the asymmetry potentials; i e . , each electrode must be calibratedae Electrodes of the quinhydrone type offer a possible solution to this problem; however, some basic disadvantages of these electrodes do arise from the additional activity terms in the Nernst equation and interference of the acidic and basic electrode materials with the solution under investigation.’ Both of these difficulties are aggravated by the generally greater solubilities of the electrode materials in nonaqueous solvents.8 The chloranil-hydrochloranil (tetrachlorobenzoquinone-tetrachloroquinol) system has been found satisfactory in acidic solvents where comparison with the hydrogen electrode is possible.9 The smeller susceptibility of this latter system to base-induced side reactions found with the benzoquinone systems suggests that the chloranil electrode would be more suitable for use in less acidic solvents, e.g., acetonitrile. The use of dilute electrode systems of this type is recommended to minimize anomalous solvent effects and requires some understanding of the acid-base interactions of chloranil and hydrochloranil. It is to be expected that the most significant specific interactions in quinhydrone systems in aprotic solvents are protonation and hydrogen bonding, between dissolved species. Both should sharply influence proton activities and hence the adequate functioning of the The Journal of Physical Chemistry, Vol. 76,No. 2, 1972
electrode. Since two separate activity coefficients appear in the Nernst equation for the quinhydrone electrode, the emf method provides only the relative values. It is possible to use hydroxyl proton chemical shifts t o evaluate protonation of the materials independently rather than as the mixture used for emf measurements.’O The chemical shift Avi of a proton whose concentration is Ci,in environment i, if exchanging rapidly with a set of other environments, will give a resultant chemical shift which is the mean of all the exchanging protons Av =
CCt i
If a strong interaction takes place between two sites, e.g., the case of hydrogen bonding, a further indepen-
dent environment is added and must be included in eq 1, which now contains an additional parameter. To examine these interactions, solutions of chloranil and hydrochloranil in acetonitrile containing perchloric (1) I. M . Kolthoff and F. G. Thomas, J . Phys. Chem., 69, 3049 (1965). (2) G. J. Hills, in “Reference Electrodes,” D . J. G. Ives and G. J. Janz, Ed., Academic Press, New York, N. Y., 1960, p 446. (3) C. Papon and J. Jacq, Bull. Soc. Chim. Fr., 13 (1965). (4) J. N. Butler, Advan. Electrochem.Electrochem.Eng., 7 , 8 7 (1970). ( 6 ) I. M. Kolthoff and M. K. Chantooni, J . Amer. Chem. Soc., 87, 4428 (1 965). (6) G. Mattock and D. M. Band, in “Glass Electrodes for Hydrogen and Other Cations,” G. Eisenman, Ed., Marcel Dekker, New York, N. Y . , 1967, Chapter 2. (7) D . J. G. Ives and G. J. Janz, ref 2, p 289. (8) P. J. Turner, M.S. Thesis, Rensselaer Polytechnic Institute, Troy, N. Y . , 1970. (9) B. 0. Heston and N. F. Hall, J. Amer. Chem. Soc., 56, 1462 (1934). (10) See, e.g., E. J. King, “International Encyclopedia of Physical Chemistry and Chemical Physics,” Val. 4, I.U.P.A.C., Topic 15, “Acid-Base Equilibria,” p 110.
NMRSTUDYOF ACID-BASEINTERACTIONS acid as the more strongly acidic, and acetone as the more strongly basic, species have been examined a t conM up to saturation. The centrations from about use of such a weak base as acetone is preferred, as strong bases still tend to cause some polymerization of hydrochloranil in presence of 0xygen.l' The use of an aliphatic ketone can also give qualitative indications of hydrogen bonding between hydroquinones and quinones by making use of concentration ranges above those accessible with the quinone as solute.
247 saturation, as shown in Table I. This implies that hydrogen bonding t o the solvent, vix.
Hog o,H---N==C-
A1
Hz.
Results and Discussion Chloranil itself is pmr inactive but two to four weak resonances appeared about 5 Hz downfield of the main solvent peak and are assigned to solvent impurities, possibly water, of concentration in the region of M . The shift of these relative to solvent was essentially independent of chloranil concentration, suggesting that chloranil was not very strongly solvated by the impurities. The -OH protons of hydrochloranil appear about 314 HZ downfield of the solvent and are concentrationindependent over the concentration range M to
c1
c1
dominates the interactions, rather than a hydrogen bonded self-association of the type
Experimental Section Materials. Chloranil (Eastman Organic Chemicals) was recrystallized from ethanol and dried for 12 hr under vacuum a t room temperature. Hydrochloranil (Eastman Organic Chemicals) was not improved by sublimation in vacuo or by recrystallization from 70% aqueous ethanol and was used as received, mp 236-238' (lit. mp 232", 236").12 The purification of acetonitrile (Fisher reagent grade) has been described previously. l 3 Solvent peak asymmetry in the nmr spectra indicated very slight traces of water in the acetonitrile after three distillations probably as a result of access of air t o the distillate during sample preparation. Samples were therefore condensed rapidly from batches of about 50 ml which were refluxed continuously over -5 g calcium hydride (Metal Hydrides Inc.) and led directly into the nmr tube. Acetone (Fisher ACS reagent grade) was used as received. Perchloric acid (Baker AR grade) was used as a -0.25 M stock solution in acetonitrile, prepared by weight. Analysis of the acid as received by potentiometric titration with aqueous sodium carbonate showed that the reagent contained 61.0y0 HC104, i.e., could be formulated as HC1O4.3.6H20. Nmr Spectra. Spectra were measured using a Varian Associates T-60 60 MHz analytical spectrometer a t a sample temperature of 34". Chemical shifts for each sample were measured relative to the main solvent band, the center of which was located by averaging the four strongest spinning bands. Uncertainty in chemical shifts obtained by this procedure is estimated as
CH,
HO
OH
c1
c1
Table I: -OH Proton Chemical Shifts for Hydrochloranil Solutions in Acetonitrile Hydrochloranil, M
Au,
Hz
0.02 0.04 0.05 0.13 (sat.)
312.0 314.8 314.3 312.5
In solutions containing both chloranil and hydrochloranil (see Table 11),there is a slight tendency for the -OH peak t o move upfield as the relative concentration of chloranil is increased, but this movement is only about 0.5 HZ per tenfold increase of [C/CH2], which is considerably less than the general scatter of points (-2 Hz) and can be neglected. This invariance is a strong indication that any specific interaction in solution analogous to the formation of quinhydrones is weak by comparison with solvation, and the two components do not significantly interfere with each other in dilute solution in acetonitrile. Table 11: -OH Proton Chemical Shifts for Solutions of Chloranil (C) with Hydrochloranil (CH2) i n Acetonitrile ----Conan,
M---
AY,
C
CHz
0.015 0.015 0.020 0.020 0.030
0.050 0.050 0.120 0.025 0.040 0 * 080
0
C:CHz
0
0.30 0.125 0.80 0.50 0.375
Hz
314.3 314.0 313.1 313.5 314.3 312.5
(11) R. P.T. Tomkins and P. J. Turner, unpublished results. (12) "Beilsteins Handbuch der Organischen Chemie," 4th ed, Springer-Verlag, New York, N. Y.,1923,Vol. 6,p 861. (13) R. P. T. Tomkins, E. J. Andalaft, and G. J. Janz, Trans. Faraday Soc., 65, 1906 (1969).
The Journal of Physical Chemistry, Vol. 76, No. 2, 1972
248
N. F. HEPFINGER, R. P. T. TOMKINS, AND P. J. TURNER
The spectra of chloranil and hydrochloranil solutions containing perchloric acid are presented in Table 111. The system HC1O4-H20-hydrochloranil-acetonitrile gives a broad, concentration-dependent resonance which is assigned to the sum of -OH protons from all the solutes. These must therefore be participating in an exchange equilibrium which is fast relative to the nmr time scale (-lou3 sec).14 A second, sharp resonance, 24.0 f 1.0 Hz downfield of the solvent, is concentration-independent and may be attributed to proton solvation by acetonitrile, i e . , to methyl protons of the species CH3-C+NH. The chemical shifts for the -OH protons in a 0.016 M HC10, solution follow an equation of the form Av =
(2%
+ ~ ) A ~ H c ~ o ~ ' ~ +I I ~c AI oV~C H , * C C H ~ @n + + 1)CHCIOd
2CCE12
where n is the effective degree of hydration of a solution of HCI04.nHz0, AVHCIO~ = 130 i 2 Ha, CHCQ = 0.016 M. It is found that the fit is not particularly good, f-10 Hz, with the curve of best fit having n close to 1.5, indicating that only about half the water is apparently taking part in rapid exchange. The sharpness of the protonated acetonitrile peak suggests that a second fast exchange may be taking place which involves the remaining water but not hydrochloranil, although no resonance corresponding to NH or water could be located in these systems. The poor fit may be a result of slow oxidation of hydrochloranil by perchloric acid. These results are consistent enough to indicate that (a) perchloric acid undergoes proton exchange with aqueous impurities, and these protons exchange rapidly with those of hydrochloranil; (b) dissociation of hydrochloranil is relatively weak, as estimated from the line width which is increased somewhat on addition of hydrochloranil to the acid solution, but still sufficient to promote proton exchange. Addition of chloranil to perchloric acid solutions in acetonitrile had a very small effect on the -OH protons, causing a 1-2 Hz downfield movement. Some interaction with acid protons is therefore to be expected, which may cause difficulties with very dilute acids. These interactions are apparently considerably weaker than those of the proton with water or comparable bases. A greater downfield movement is seen when appreciable amounts of undissolved solid are present. This may be due to adsorption of the acid by the solid phase. If this is the case, the unsaturated electrode is more suitable for precise work. The effect of acetone on hydrochloranil solutions is shown in Table IV. The small downfield shift as acetone concentration is increased indicates a very slight increase in hydrogen-bonded interactions, either of hydrochloranil to acetone or by self-association as the solvent dielectric constant is lowered. This is approximately linear with acetone concentration and indicates The Journal of Physical Chemistry, Val. 76, No. 8, 1.978
Table 111: Chemical Shifts of -OH Protons and Protonated Solvent for Chloranil (C) and Hydrochloranil (CH2) Solutions in Acetonitrile Containing Perchloric Acid and Water" ---Av7
C
0.016 0.016 0.016 0.016
0.058 0.058 0.058 0.058
0.016
0.058
0.016 0.016 0.016 0.016
0.058 0.058 0.058 0.058
0
0
0.095 0.190 0.057 0.019 0.190
0.34 0.68 0.205 0.068 0.68
0 0 0
0.0256 (sat) 0.0256 (sat) 0 0
0 0 0 0
0.022 0.022 0.020 0
OH
CH2
Kz-Solvent
0 0 0 0
132 129 133 134
24 24 24 24
0
146
24
0.0081 0.0149 0.0493 0.178 (sat) 0.13 0.13
166.5 201 233 273
24.5 25 25 25
312.5 246.3 248.8 202.0 148.8 250.0
23.3 23.3 24.0 23.8 23.5
0 0 0 0
Reference frequency is the methyl resonance of the bulk solvent in the same solution.
that hydrochloranil hydrogen bonds to aliphatic carbonyl groups about as well as to acetonitrile, hydrogen bonding is dictated on a probabilistic basis, and no significant shift occurs a t concentrations of acetone comparable with those of hydrochloranil, or with those attainable with chloranil.
Table IV: -OH Proton Chemical Shifts for Hydrochloranil (CH2) in Solutions in Acetonitrile Containing Acetone" %
----Conon, MezCO
2.0 2.0 20 33 50
0.276 0.276 2.76 5.55 6.90
MezCO,
0
0
M--
OrVOH,
CH2
He
0.034 0.090 0.11 0.20 0.12 0.05
. . .b 314 335 354 368 314
a Reference frequency is the methyl resonance of the bulk solvent in the same solution. Signal below the limit of detection.
Acknowledgment. Helpful suggestions by Professor G. J. Janz are acknowledged with pleasure. This work was supported by the U. S. Department of Defense, Project Themis. (14) L. M. Jackman and S. Ster:hell, "Applications of NMR Spectroscopy in Organic Chemistry, Pergamon, Elmsford, N. Y., 1969, p 56.
