Acidity Function in Dipolar Aprotic Media

ACIDITY FUNCTION IN DIPOLAR APROTIC MEDIA

BX C. KALIDAS, D. LAKSHMANA RAO AND V. SRINIVAS RAO (Department of Chemistry, I.I.T., Madras) Received April 10, 1972 (Communicated by Prof. M. Santappa, F.A.SC.)

ABSTRACT The H o acidity function has been determined for hydrochloric and trichloroacetic acids in dimethyl sulphoxide (DMSO) and acetonitrile using three amine class of indicators. The variation of H o with solvent composition at a fixed concentration of 0. 1 M HCI in DMSO-water, acetonitrile-water and DMSO acetonitrile mixtures was also studied. Approximate medium effects on the proton in all these solvents were computed from the Ho data and the results are discussed.

THE acidity functions, Ho and H_ have received wide attention 1-5 in recent years, both in aqueous and non-aqueous media. However, very little systematic works on Ho in dipolar aprotic media such as acetonitrile and DMSO has been reported although considerable H_ measurements were carried out in the latter solvent. In view of the importance of dipolar aprotic medial , 8 and the usefulness of H function data not only in kinetic investi- gations but also in evaluation of the medium effect on the proton, the present work involving H0 measurements in DMSO and acetonitrile was undertaken. RESULTS

The Ho acidity function is defined by

CBH+ Ho = pk BH-'', W — log = — log aH+ .fB (1) CB fBH+ where pKBH+, W is the negative logarithm of the thermodynamic dissoia- tion constant of the conjugate acid of the indicator B, the activity coefficients being referred to a value of unity at infinite dilution in water and CBn+/CB is the measurable concentration ratio of the indicator in its two- coloured forms. The pKBH+, W values used in He calculations were those reported by Paul and Long. Al 51 52 C. KALIDAS AND OTHERS

The Ho data in the two solvents are given in Table I.

TABLE I Ho of hydrogen chloride in DMóO Acid (M) Ho

PNA ONA I DPA 111 .. 1 0.100 2.45

0.200 2.21 .. .. 0•400 1•91 .. 0.500 1.82 .. 1.4' 0•700 1.64 .. 1•000 1•54 .. 1.37 1.500 1•40 .. 2.000 1•31 .. 1.33 2.500 1.22 1.27 3.000 1•16 1•12 1.22 3.500 0.74 0.75 .. 4.000 0.41 0.41 ..

Ho of hydrogen chloride in acetonitrile

0•010 1.67 .. 1•71 0.020 1.45 .. 0•040 0.79 0.82 0•050 0.67 0.74 0•70 0•070 0•53 0.55 0.100 0.16 0•19 0•21 0.150 -0'14 -0.08 0.01 0.200 -0'48 --0.43 -0'31 0.300 .. -0.69 ..

Ho of trichloroacetic acid in acetonitrile

0•100 2.70 .. 0.300 2•25 .. ,.

0.500 1.58 .. .. 0.700 1.17 .. 0•600 1.04 1.05 .. 1.000 0.85 0•82 .. 1.400 .. 0.51 1.800 .. 0•32 .. 2.000 .. 0.19 .. 2.500 .. -0.05 .. 3•000 .. -0.26 .. 4•000 .. -0.47 ..

PNA = P-nitreaniline; DPA = Diphenylamine; ONA = O-nitroaniline.

The Ho values at 0, 20, 40, 60, 80 and 100 wt. per cent of DMSO in DMSO-water mixtures at fixed concentration of 0.1 M HCl are 0.98, Acidity Function in Dipolar Aprotic Media 53 1•45, 2•12, 2.37, 2.25 (within limits of error of the previous composition), and 2.46 respectively. The H° values at the same wt. percentages of acetonitrile in acetonitrile-water mixtures are 0.98, 1.51, 1.74, 1.82, 1.99 and 0.16 respectively. Similarly the H° values obtained at 0, 22.1, 35.2, 43 07, 57.4, 80.5 and 100 wt. percentages of DMSO in DMSO-acetonitrile mixtures are 0.16, 1.63, 1.87, 1.87, 1.98, 2.96 and 2.46 respectively.

DISCUSSION The validity of acidity function in any medium is governed by two conditions: (1) the activity coefficient ratio for a pair of indicators in the given medium must be equal; (2) the relative strengths of a pair of indicators must remain unchanged on transferring them from water to the non-aqueous solvent, i.e., the A pK values (A pK = pKSBH+ — pKscH+) for a pair of indicators B and C in the non-aqueous solvent must be equal to that in water. The parallelism of the log I plots (Fig. 1) in the two solvents indicates that the first condition is satisfactorily fulfilled in these media. Further, the average log I differences calculated between a pair of indicators in the over- lapping region for the two solvents from Fig. I are nearly equal to the ApK difference of the same pair in water indicating the validity of acidity function in these media. These results are surprising especially in acetonitrile, as it is known to be a dipolar aprotic solvent with weekly basic pro - perties having no capacity to donate or accept hydrogen bonds. In such solvents the indicator equilibria have been shown to be complicated by the extensive work of Kolthoff et al. 9 who proposed " homoconjugation ", i.e., the hydrogen bonding of the anion of the acid with the acid molecule itself. It is, however, pertinent to point out that results similar to our work were obtained by Arnett and Douty 1° in sulfolane and Hammett and Vanlooyll in nitromethane where the indicator equilibria are complicated as in acetonitrile. A comparison of H° for a given acid at definite concentration in these solvents with other media such as water, methanol, ethanol and ethylene glycol shows that acetonitrile is the least basic and DMSO the most basic compared to these amphiprotic media. Our conclusions regarding the basic strength of acetonitrile are in agreement with those of Bates 12 and Ritchie." The results on the basic strength of DMSO are, however, not in accord with the conclusions of King 14 obtained from a study of the proton exchange constants. The strongly basic nature of DMSO is most possibly due to the fact that it is a strong hydrogen bond acceptor. Thus the hydrogen

54 C. KALIDAS AND OTHERS ' ion may be expected to be extensively solvated in DMSO resulting in low proton activity in this solvent.

0 Solvent :CH3 CN Acid:CC13 -2 000H 1.0 2.0 3.0 4.0 2r

_ 0 c9 Solvent:CH3C'. 0 ^ _tAcid:HCl

0.1 0,2 0.3 0.4

Solvent: DMSO Acid: HCI

1.0 2.0 3.0 !.•0 Concentration of Acid (Moles/Litre) e p-Nitroaniline, o Diphenylamine, 0 0-Nitroaniline

Fta. 1. Plot of log I vs, concentration of acid in DMSO and CH 3CN.

A plot of Ho against log acid concentration with a slope of unity has- shown that hydrochloric acid is practically completely dissociated upto a concentration of about 3 M in DMSO. The pKS BH+, i.e., the pKSBH+ with the solvent standard state in DMSO obtained by a plot of log I — log C H + against acid concentration was found to be — 0.49 and — 0.68 for p-nitroaniline and diphenylamine respectively. Addition of acetonitrile to an aqueous solution of acid at 0.1 M concentration results (Fig. 2) in an increase of H o indicating an increase in the basicity of the medium. The basicity reaches a maximum at about 80% wt. of acetonitrile and further addition results in a sharp decrease of basicity A similar observation is noted (Fig. 2) for the addition of DMSO upto about Acidity Function in Dipolar Aprotic Media 55 60% wt. Further addition of DMSO results in negligible changes of the basicity of the medium. The absence of a maximum in the basicity of the medium is in striking contrast to the data in acetonitrile-water mixtures, or alcohol-water mixtures where such maxima have invariably been observedls in presence of water as one of the constituents. The H o variation in DMSO-acetonitrile mixtures under similar conditions does not show any maximum in basicity (Fig. 2) in agreement's with the results in other solvent mixtures involving two organic components. The existence of basicity maximum in acetonitrile-water mixtures can be explained in terms of the disturbance of water structure and the changing solvation characteristics of proton as proposed by Braude and Stern. 16 The absence of the basicity maximum in DMSO–water mixtures is most probably due to the higher stability of DMSO. H+ as compared to H3O+ in DMSO rich region. Appro- ximate values of the medium effect on the proton (log m7H) for DMSO- water and acetonitrile-water mixtures were computed according to the method of Popovych' 7 and are — 0.29, — 0.95, — 1.19, — 1.03 and — 1.11 at 20, 40, 60, 80 and 100 wt. % DMSO in DMSO–water mixturesa.

LU 41,) bU du Wight Percent of DMSO J.

0 I

0 20 40 60 80 100 Weight Percent of Water 1.Acetonitri le -Water Mixtures. 2. DMSO-Water Mixtures. 3 Acetonitnle-DMSO Mixtures (upper scale

FIG. 2. Variation of Ho (conc. of HCl = 0- M)1 with solvent composition usiq p-nitroaniline as indicator. 56 C. KALIDAS AND OTHERS Similarly, for acetonitrile-water mixtures' at the same wt. per cent of acetonitrile, log ml'H values are — 0.51, — 0•74, — 0.82, — 0.90 and +0.84 respectively. However, when a correction for the log m7BH+ (electrostatic) is made in pure acetonitrile, the log m^/H goes upto +2.30 .

[(a) log m' H = — Ho — log ma°;d — log a — logs'y H;

(b) log m'r H = — Ho — log:macid ; (c) using R+ (S) and R+ (W) as given in text and correcting for a as mentioned therein.] , In these calculations all other terms (not given above) have been ignored. While the salt effect activity coefficient, log (S'YBH+/S YB) is expected to be near about zero in both the solvent mixtures, the log m'YH values will be influenced by the log m-/BH+ (electrostatic) in both the media. However this will not affect the trend or the sign of the medium effects in DMSO- water mixtures and they might be expected to hold to within an order of magnitude on the basis of our approximate calculations using the modified Born equation. The results show that all DMSO-water mixtures including pure DMSO are considerably more basic than water in view of the negative sign of the medium effect in these mixtures. The results in acetonitrile, on the other hand, are considerably affected if they are corrected by incorporating the log m7BH+ (electrostatic) through the modified Born equation using 18 R+ (H8O) = 0.85 A and R+ (CH3CN) = 0.72 A and correcting for the degree of dissociation of the acid on the basis of the equilibrium 2HA = H+ + AHA - for which K = 2.52 x 10-7 (Ref. 9). Thus log myH in acetonitrile is + 2.30 which is of the same sign obtained from other methods but the value is not satisfactory when compared with Strehlow's value of 4.0. It is seen that log m11H in these mixtures passes through a minimum at about 80% wt. of acetonitrile. Thus acetonitrile-water mixtures are more basic than either of the pure solvents and the proton is in a higher energy state in acetonitrile compared to water. Other experimental approaches are necessary to confirm these observations.

EXPERIMENTAL Solvents.—B.D.H. L.R. grade samples of DMSO and acetonitrile were used throughout. DMSO was repeatedly frozen at about 5° C and after removing the supernatant liquid, was distilled under reduced pressure. The #'raction boiling at 63° C ul er 8 nom pressure was collected. AcetQ. Acidity Function in Dipolar Aprotic Media 57 nitrile was dried with anhydrous sodium sulphate, subjected to fractional distillation and the liquid boiling at 810 C was collected. Double-distilled water over an all glass apparatus was employed in measurements involving aqueous mixtures. Indicators. p-Nitroaniline (PNA), Diphenylamine (DPA) and o-nitroaniline (ONA) were recrystallised from aqueous alcohol and checked for purity by their melting points. Stock solutions (about 10-3 M) of the various indicators in the desired solvent were prepared by weight and used after suitable dilution.

Preparation and standardisation of acid solutions.—Hydrogen chloride gas, generated according to the method described 19 earlier, and trichloroacetic acid (E. Merck) were used as acids in these solvents. Solutions of the acids, in the concentration range studied, in the two solvents retained constant strength over the period of spectral measurements. Measurements in acetonitrile with HCl were restricted to about 0.3 M because of a gradual decrease in the strength of the acid with time at higher concentrations. Further details of preparation and standardisation of acid solutions are similar to those reported 19 earlier. A.R. hydrochloric acid was used in measurements in aqueous organic mixtures. Procedure.—All optical measurements were made with a P.M.Q. II

Carl Zeiss spectrophotometer a 30 0 C. The indicator ratio, C BH+/CB, was calculated from

CB,_ EB — Es CB Es — EBH+(2) where EB , Es, and EBH+ refer to the extinction coefficients of tha indicator in the basic form, in the test solution and in the form of its conjugate acid respectively at a definite wavelength. Details of measurement of the indicator ratio are described 19 elsewhere. Grateful thanks are due to Prof. M. V. C. Sastri for his encouragement during the course of the work.

REFERENCES

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4. Rochester, C. H. .. Quart. Revs., 1967, 21, 494. 5. Chattanathan, N. and Aust. J. Chem., 1971, 24, 83. Kalidas, C.

6. Dessy, R. E. .. J. Am. Chem. Soc., 1959, 81, 2683.

7. Parker, A. J. .. Quart. Revs., 1962, 16, 163. 8. Butler, J. N. .. Advances in Electrochemistry and Electrochenical Engineering, C. W. Tobias, Ed., Interscience, New York, 1970, 7, 77-161. 9. Kolthoff, I. M., J. Am. Chem. Soc., 1961, 83, 3927. Bruckenstein, S. and Chantooni, M. K. 10. Arnett, E. M. and Ibid., 1964, 86, 409. Douty, C. F. 11. Vanlooy, H. and Ibid., 1959, 81, 3872. Hammett, L. P. 12. Bates, R. G. .. Solute-Solvnut Interactions, J. F. Coetzee and C. D. Ritchie, Eds., Marcel Dekker, New York, 1969, p.90, 13. Ritchie, C. D. .. Ibid., p. 223. 14. King, E. J. .. Acid-Base Equilibria, Pergamon Press, London, 1965, p. 295. 15. Bates, R. G. .. Hydrogen Bonded Solvent Systems, A. K. Covington and P. Jones, Eds., Taylor and Francis, London, 1968, p. 52. 16. Braude, E. A. and J. Chem. Soc., 1948, _ p. 1976. Stern, E. S. 17. Popovych, 0. .. Crit. Revs. in Anal. Chem., L. Meites, Ed., Chemical Rubber Co., 1970, 1, 1. 18. Strehlow, H. .. The Chemistry of Non-Aqueous Solvents, A Lagowski, Ed.. Academic Press, Inc., New York, 1966, Chapter 4. 19. Kalidas, C. and Palit, S. R. J. Chem. Soc., 1961, p, 3991.