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Do protic ionic liquids and water display analogous behavior in terms of Hammett acidity function?
Chemical Physics Letters 566 (2013) 12–16
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Do protic ionic liquids and water display analogous behavior in terms of Hammett acidity function? Shashi Kant Shukla, Anil Kumar ⇑ Physical & Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India
a r t i c l e
i n f o
Article history: Received 31 October 2012 In final form 16 February 2013 Available online 5 March 2013
a b s t r a c t We address an issue whether the strength of carboxylic acids in water is linear with respect to that in ionic liquids. Strength of carboxylic acids in water and different PILs using Hammett function (Ho) has revealed interesting linear correlation between the Ho function for all acids in PILs and PIL-water in spite of large structural and electronic differences. These observations suggest that different structural and electronic features of PILs and water behave analogously towards Ho. This linearity in Ho functions between PILs and PIL-water systems can be used to develop predictive method to calculate Ho values. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Acidity of a medium is an intrinsic number. However, the strength of an acid in a medium varies with (i) the nature of the acidic species and (ii) the ability of the solvent to dissociate it. Strength of an acid can be used for establishing the reaction mechanism and also in fine tuning the yield and selectivity of acid catalyzed reactions. Determination of strength of an acid in non-aqueous medium by using the same procedure as in aqueous medium cannot be employed satisfactorily due to some practical limitations [1]. The common methods that have been used for the precise determination of acid strength beyond pH range in aqueous/non-aqueous medium are potentiometry and ultraviolet spectroscopy. Measurement of acidic strength using spectral technique depends on the isolation of spectral changes due to the protonated and non-protonated forms of indicator. Ionic liquids (ILs) are molten organic salt which contain asymmetric organic cation and symmetric/asymmetric organic/inorganic anion [2]. Last few decades have witnessed the stupendous increase of ILs for a variety of reactions [3] because of their unique solvation behavior and the presence of electrophilic (cation) and nucleophilic (anion) sites. ILs have been divided into two sub-groups viz. protic ionic liquid (PIL) and aprotic ionic liquid (AIL). The prefixes protic and aprotic designate that quaternization step during synthesis of IL involves proton (H+) and alkyl (–R) group, respectively. IL serves as a potential reaction media for a variety of polar and apolar reactant because of their amphiphilic nature. They have wide applications in electrochemical and biochemical processes [4,5] because of wide electrochemical window, high thermal stability, insignificant vapor pressure, wide liquidus range, non-flammability, recyclability, etc. [6–11]. PILs contain dissociated proton ⇑ Corresponding author. Fax: +91 2025902636. E-mail address: [email protected] (A. Kumar). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.02.029
therefore, act as efficient reaction media for a number of acidcatalyzed reactions [12]. However, the measured acidity level are usually low, suggesting small degree of dissociation (a) of PILs [13]. The efficiency of PILs as catalyzing media can further be improved by adding Brønsted acids. Strength of added acid in a medium depends on the dissociation (a) of acid and the stabilization of the conjugate base [14]. Since different solvents solvate proton (H+) up to different extent therefore, strength of an acid vary with medium. Because of the various specific and non-specific interactions, ILs have unique solvation behavior. However, it is difficult to correlate the solvation behavior of ILs with its solvent properties such as polarity (ENT ) [15] and relative permittivity (er) [16]. Thomazeau et al. for the first time measured the strength of HNTf2 (Bistriflimide) and HOTf (Triflic acid) in alkylimidazoliumbased ILs using Hammett acidity function (Ho) [17]. They observed that C2–H has no influence on the strength of HNTf2 and HOTf. They also noted similar Ho values for HNTf2 and HOTf in [BMIM][NTf2] (1-butyl-3-methylimidazolium bistriflimide) due to the leveling effect of the medium. Higher acidity level for HNTf2 in [BMIM][BF4] (1-butyl-3-methylimidazolium tetrafluoroborate) than that in [BMIM][NTf2] was attributed to the lesser solvation of proton (H+) by BF 4 than by NTf 2 . MacFarlane et al. showed that the relative basicity of IL anion and water is the only criterion in deciding the strength/acidity level of an acid in these media [18]. Though PIL mimics water in a sense of extensive hydrogen bonding [19], they have different structural and electronic features. In our laboratory, we have attempted to correlate the structure– activity relations for different ILs by measuring their physicochemical properties [20–22] and their implications on various chemical reactions [23–25]. Prompted by the similarity between PILs and water in terms of hydrogen bonding, we now address an issue whether PILs and water display analogous behavior with regard to their Hammett acidity functions. For this purpose, we investigate the strength of carboxylic acids (HCOOH, CH3COOH
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S.K. Shukla, A. Kumar / Chemical Physics Letters 566 (2013) 12–16
and CH3CH2COOH) in water and in different PILs. The PILs of current interest are [HMIm][HCOO] (1-methylimidazolium formate), [HMIm][CH3COO] (1-methylimidazolium acetate), [HBIm][HCOO] (1-butylimidazolium formate), [HBIm][CH3COO] (1-butylimidazolium acetate), [HPyrr][HCOO] (1-methylpyrrolidinium formate) and [HPyrd][HCOO] (1-methylpiperidinium formate). The structures of PILs are shown in Figure 1.
1.5
Absorbance
1.2
2. Experimental section The spectroscopic indicator dye 4-nitroaniline was obtained from M/s Sigma–Aldrich and used as obtained. 1-Methylimidazole, 1-butylimidazole, 1-methylpyrrolidine and 1-methylpiperidine were all purchased from Sigma–Aldrich and were distilled prior to use. Formic acid, acetic acid and propionic acid were provided by Merck and used as obtained. Deionized water having specific conductance <0.055 106 was used wherever needed during experiment. The water content in PILs was measured before the start of each experiment and it did not exceed beyond 50 ppm. PILs employed in the current work were synthesized by the procedures used by Ohno and Yoshizawa [26]. PILs were further purified and freed from other volatile impurities by placing them under high vacuum (102 bar) for 10 h at 60 °C. 1H NMR data of these PILs were found in good agreement with those reported in the literature. Since these PILs were synthesized by halide free pathway they were found devoid of any halide impurities. A stock solution of 4-nitroaniline (104 M) was prepared in methanol and was used for measurement of strength in water and PILs. All the acids were mixed in equimolar proportion with deionized water. A small volume of indicator solution was added in 1 ml of PIL/water after removing methanol. The sample was filled inside the quartz cuvette capped with septum and further protected by parafilm to avoid any contamination. The cuvette was placed inside the UV chamber and absorbance was recorded at 25 °C. The absorbance without any acid content was taken as reference. The reference absorbance was due to only the non-protonated form of indicator. In all the experiment reference value was kept constant around 1.45. Now the calculated amount of acid was added inside the cuvette and absorbance was recorded. These absorbances at different concentrations of acid were compared with reference value to get the relative amount of protonated ([I]s) and non-protonated ([HI+]s) forms of indicator. Strength of an acid solution then calculated by using Hammett equation:
Ho ¼ pKðIÞaq: þ log ½Is =½HIþ s
ð1Þ
where pK(I)aq. is the protonation constant of indicator in aqueous solution. For 4-nitroaniline pK(I)aq. = 0.99. [I]s and [HI+]s indicates the relative population of unprotonated and protonated forms of indicator. From Eq. (1) it is clear that Ho will be lower for strong acids and vice versa. A typical absorption spectra for a series of experiment for an acid/PIL couple is shown in Figure 2.
0.9 0.6 0.3 0.0
320
360 400 440 Wavelength (nm)
Figure 2. A typical absorbance spectra of 4-nitroaniline at various concentrations of an acid in PIL.
Eq. (1) can also be written as,
Ho ¼ log aðHþaq: Þ log ½cðIÞ=cðHIþ Þ log ½CðIÞ=CðHIþ Þ
3. Results and discussion Strength of an acid in ILs using Hammett function is not much discussed. Only few reports are available on the measurement of strength in ILs using Hammett function and their correlation with the variation of cation and anion [17]. Other report includes the measurement of strength and its correlation with catalytic activity for acid-catalyzed reactions [13,27]. Current work includes the determination of strength for weak carboxylic acids in PILs and water using Hammett function. Unlike water, PIL offer different interactions for a released proton (H+) and hence can affect the solvation of H+. In water, solvation of proton is precisely known [28]; however in PILs it is ambiguous. Similarly, the relative tendency of proton solvation of PIL and water cannot be compared due to the unavailability of enthalpic data. Since solvation of proton affect
Anions O
N
N H
N H3 C
C
N H
ð2Þ
where c is the activity coefficient and U is the transfer activity coefficient of protonated (HI+) and unprotonated (I) forms of indicator. In a very dilute solution of indicator log[c(I)/c(HI+)] remain constant. The ratio log[U(I)/U(HI+)] arose during the exchange of indicator from one medium to other medium. To keep this ratio constant, it was considered that the different forms of indicator have the same solvation shell in different medium. However, it is difficult to meet this requirement since different solvent solvates these two forms of indicator up to different level. Therefore, it is assumed that for structurally similar indicators the ratio log[U(I)/ U(HI+)] will remain constant.
Cations
R
480
H3C
H
R
O
R = H, CH3, CH3CH2
R = CH3, C 4H9 Figure 1. Structures of PIL employed in study.
S.K. Shukla, A. Kumar / Chemical Physics Letters 566 (2013) 12–16
ðHoÞ2 ¼ m ðHoÞ1 þ c
2.25
(a)
(b)
2.10
2.3
2.2
(b)
3
2.2
2.0
2.1
2.0
1.8 1.8
1.9
2.0
Ho[HMIM][HCOO]
2.1
1.8
1.9
2.0
2.1
Ho[HMIm][HCOO]
Figure 4. Linear correlation in acidity function of HCOOH between (a) [HMIm][HCOO] vs [HPyrr][HCOO] and (b) [HMIm][HCOO] vs [HMIm][CH3COO].
strength of acid between PILs and PIL-water can be interpreted up to certain extent. The different values of slopes and intercepts for HCOOH, CH3COOH and CH3CH2COOH between different PILs and PIL-water are shown in Tables 1–3, respectively. For convenience, influence of PILs on Ho have been discussed in terms of (1) effect of cation, (2) effect of anion and, (3) influence of water on Ho compared to that of PILs. 3.1. Effect of cation From Tables 1–3 it is evident that the variation of cation has more effect on the strength of carboxylic acids. However, the relative tendency of different cationic core with [HCOO] differs largely in promoting the strength of HCOOH, CH3COOH and CH3CH2COOH. The order of different cationic core in promoting the strength of HCOOH and CH3COOH is in the order,
½HMImþ > ½HBImþ > ½HPyrdþ > ½HPyrrþ whereas, for CH3CH2COOH the order reverse as,
½HPyrrþ > ½HPyrdþ > ½HBImþ > ½HMImþ These observations suggest specific interaction between the cationic backbone and conjugate anion of acid. Similar trend of PILs with [CH3COO] anion for all carboxylic acid was observed. These trends of cationic backbone were estimated from the slope of linear correlation between PILs. The order of strength for HCOOH in different PILs having [HCOO] is shown in Figure 5. 3.2. Effect of anion Unlike cation, anion has a weaker influence on the strength of carboxylic acids. The difficulty in synthesizing PILs with different anions made our work complicated. However, between [HMIm][HCOO] and [HMIm][CH3COO], [HCOO] favor strength of CH3CH2COOH more than that of [CH3COO], while for HCOOH and CH3COOH, [CH3COO] promote acidic strength more compared to [HCOO]. 3.3. Effect of water
2
2
HoH O
HoH O
1.65
1.50
1.35
(a)
ð3Þ
The subscripts 1 and 2 have been used for medium at x and yaxis, respectively. In short m > 1 indicate higher strength of an acid in 1 compared to 2 and vice versa. With the aid of slope and intercept values, possible effects of cation and anion variation on the
1.80
2.4
Ho[HMIm][CH COO]
the strength (which in turn affect Ho value), the relative Ho values in water and in PILs can be used as a measure of solvation of medium. Carboxylic acids (HCOOH, CH3COOH and CH3CH2COOH) have different strength in PILs and water as suggested by different Ho values. The different Ho values in PILs and water for carboxylic acids may be due to the combined effect of the structural and electronic variations of these media on the solvation of protons. The relative influence of structural and electronic differences between PILs and PIL-water can be easily seen by plotting their Ho values against each other for carboxylic acids. Surprisingly a linear relation in Ho values was observed for all carboxylic acids in different media. A linear correlation between the Ho values of HCOOH and CH3COOH in [HMIm][HCOO] and water is shown in Figure 3. Similarly, a linear relation was observed for Ho values of HCOOH in different PILs despite of their different cationic and anionic counterparts (Figure 4). Though the linear dependency between Ho values for carboxylic acids in PILs and PIL-water is surprising, these observations cannot be explained satisfactorily due to the limited knowledge about solvation behavior of PILs. This analogous behavior of PIL and water suggest that though the molecular environment around proton remains different for PIL and water, overall response on the strength of weak carboxylic acids remain same in these media. Similar observations on the protonation constant (pKa) of different carboxylic acids in AILs have been noted by D’Anna et al. [29] and Deng et al. [30]. Optical basicity scale is developed for determining the Lewis basicity of metallic oxide and it primarily depends on the polarizability of the base [31,32]. Madden et al. have calculated the optical basicity (k) for different protic solvents (HF, H2O and NH3) and their binary mixtures [33]. They have measured the Hammett acidity function (Ho) at different compositions of protic solvents and found a linear relation with optical basicity. This correlation can be used to develop the understanding between variation in polarizability of the mixture, optical basicity and Hammett acidity. However, in the present work, linearity in Ho function has been used to develop simple equation between PILs and PIL-water that would be helpful in predicting the Hammett acidity (Ho) at a different composition of acid only by using the Ho value in any of them. A general form of linear equation for Hammett function (Ho) is as,
Ho[HPyrr][HCOO]
14
1.95
1.80
1.8
1.9
2.0
Ho [HMIm][HCOO]
2.1
1.65
1.50 1.65 1.80 1.95 2.10
Ho ([HmIm][HCOO])
Figure 3. Linear correlation between Ho of [HMIm][HCOO] and H2O for (a) HCOOH and (b) CH3COOH.
Unlike PILs, where the relative interactions between cation and anion vary, water differs a lot from PIL in both structural organizations and the absence of distinct charge. Solvent parameters such as polarity (ENT ) and relative permittivity (er) therefore cannot be accounted for this unusual behavior, since both water and PILs have different values. Therefore, at a moment it seems improbable to explain these observations by using a suitable solvent scale. The slope of linear correlations between PILs and water for HCOOH exhibit higher strength in [HMIm][HCOO] and lower in
15
S.K. Shukla, A. Kumar / Chemical Physics Letters 566 (2013) 12–16 Table 1 Linear correlations between the Hammett function for HCOOH between PILs and PIL-water. Ionic liquid
Intercept (c)
Slope (m)
R
r
[HMIm][HCOO] vs [HPyrr][HCOO] [HMIm][HCOO] vs [HPyrd][HCOO] [HPyrr][HCOO] vs [HPyrd][HCOO] [HMIm][HCOO] vs [HBIm][HCOO] [HMIm][HCOO] vs [HMIm][CH3COO] [HMIm][CH3COO] vs [HBIm][CH3COO] [HMIm][HCOO] vs H2O [HPyrr][HCOO] vs H2O [HPyrd][HCOO] vs H2O [HBIm][HCOO] vs H2O [HBIm][CH3COO] vs H2O
0.962 0.551 0.250 0.332 0.216 0.515 0.848 0.057 0.298 0.497 0.561
1.54 1.31 0.850 1.276 1.011 1.256 1.282 0.827 0.973 0.996 0.991
0.994 0.993 0.994 0.991 0.990 0.990 0.995 0.993 0.998 0.995 0.996
0.022 0.013 0.018 0.021 0.018 0.022 0.015 0.019 0.010 0.016 0.014
Table 2 Linear correlations between the Hammett function for CH3COOH between PILs and PIL-water. Ionic liquid
Intercept (c)
Slope (m)
R
r
[HMIm][HCOO] vs [HPyrr][HCOO] [HMIm][HCOO] vs [HPyrd][HCOO] [HPyrr][HCOO] vs [HPyrd][HCOO] [HMIm][HCOO] vs [HBIm][HCOO] [HMIm][HCOO] vs [HMIm][CH3COO] [HMIm][CH3COO] vs [HBIm][CH3COO] [HMIm][HCOO] vs H2O [HPyrr][HCOO] vs H2O [HPyrd][HCOO] vs H2O [HBIm][HCOO] vs H2O [HBIm][CH3COO] vs H2O
0.417 0.388 0.026 0.117 0.008 0.048 0.480 0.747 0.731 0.564 0.511
1.268 1.250 0.984 1.142 1.096 1.021 0.812 0.640 0.650 0.710 0.725
0.995 0.997 0.999 0.996 0.998 0.997 0.995 0.999 0.999 0.999 0.999
0.026 0.021 0.007 0.023 0.015 0.018 0.017 0.007 0.008 0.008 0.007
Table 3 Linear correlations between the Hammett function for CH3CH2COOH between PILs and PIL-water. Ionic liquid
Intercept (c)
Slope (m)
R
r
[HMIm][HCOO] vs [HPyrr][HCOO] [HMIm][HCOO] vs [HPyrd][HCOO] [HPyrr][HCOO] vs [HPyrd][HCOO] [HMIm][HCOO] vs [HBIm][HCOO] [HMIm][HCOO] vs [HMIm][CH3COO] [HMIm][CH3COO] vs [HBIm][CH3COO] [HMIm][HCOO] vs H2O [HPyrr][HCOO] vs H2O [HPyrd][HCOO] vs H2O [HBIm][HCOO] vs H2O [HBIm][CH3COO] vs H2O
0.218 0.101 0.152 0.224 0.375 0.063 1.006 0.854 0.946 0.866 0.796
0.818 0.922 1.174 0.914 0.831 1.014 0.573 0.700 0.600 0.627 0.677
0.999 0.999 0.999 1.0 0.997 0.998 0.998 0.998 0.999 0.991 0.998
0.009 0.013 0.017 0.004 0.017 0.015 0.009 0.009 0.006 0.010 0.009
2.4
Ho
2.2
2.0
1.8
1.6
0.6
0.9
1.2
1.5
CH+ (M) Figure 5. Ho vs C Hþ plot for HCOOH in [HMIm][HCOO] (h), [HPyrr][HCOO] (.), [HPyrd][HCOO] (s) and, [HBIm][HCOO] (N).
[HPyrr][HCOO] compared to water, whereas the same strength was noted for HCOOH in [HPyrd][HCOO] and [HBIm][HCOO] when compared with water, probably due to the structural compensation (larger size of [HPyrd]+ and presence of –C4H9 in [HBIm]+).
Similarly, linear correlation for Ho values of HCOOH in [HBIm][HCOO] and [HBIm][CH3COO] with water indicated that basic anion [CH3COO] promote strength of an acid more than less basic [HCOO] anion. These conclusions were drawn from the slope value of linear correlations between PILs and PIL-water. However, the effect of PIL and water for CH3COOH cannot be explained on the similar ground as the observed slope values were unexpectedly very low (m < 1). The lower m value indicates higher strength in water than that in PILs. These dubious observations based on slope value for CH3COOH in water and PILs are in contradiction to that indicated by Ho value. Thus the lower strength of CH3COOH in water than in PILs can only be explained by using unusually large intercept (c) values. Intercept of a linear correlation indicates the starting point at y-axis when the value at x-axis is zero. According to Eq. (1), the value of intercept represents the non-assistance of medium 2 compared to 1. Therefore, the enhancement in the strength of CH3COOH by various PILs compared to that of water is in the order,
½HMIm½HCOO > ½HBIm½CH3 COO > ½HBIm½HCOO > ½HPyrd½HCOO > ½HPyrr½HCOO
16
S.K. Shukla, A. Kumar / Chemical Physics Letters 566 (2013) 12–16
Similar is the case for CH3CH2COOH where due to the lower slope (m < 1) value, increase in acidic strength by PILs than water has been arranged by using intercept values. The order of different PILs in promoting the acidity of CH3CH2COOH than that of water is as,
½HBIm½CH3 COO > ½HBIm½HCOO > ½HPyrr½HCOO > ½HPyrd½HCOO > ½HMIm½HCOO 4. Conclusions In conclusion, we have shown that both water and PILs have different interactions towards proton due to the distinct structural and electronic differences, they behave analogously towards the strength of carboxylic acids. By using linear correlation in Ho values for carboxylic acids between PILs and PIL-water, simple equations can be derived which can further be used for the determination of strength in water and PIL, only by measuring the strength in one of them at different concentration of acid. Acknowledgements S.K. Shukla is grateful to UGC, New Delhi, for awarding him a research fellowship. A. Kumar thanks DST, New Delhi, for a J.C. Bose National Fellowship (SR/S2/JCB-26/2009). The authors acknowledge an anonymous referee whose suggestions improved the quality of this work. References [1] E.J. King, Acid–Base Equilibria, Pergamon Press, Oxford, 1965.
[2] T. Welton, Chem. Rev. 99 (1999) 2071. [3] P. Wassercheid, T. Welton, Ionic Liquids in Synthesis, Wiley-VCH Verlag, Stuttgart, Germany, 2002. [4] D.R. MacFarlane, M. Forsyth, Adv. Mater. 13 (2001) 957. [5] M.C. Buzzeo, R.G. Evans, R.G. Compton, Chem. Phys. Chem. 5 (2004) 1106. [6] N.V. Plechkova, K.R. Seddon, Chem. Soc. Rev. 37 (2008) 123. [7] M. Smiglak, A. Metlen, R.D. Rogers, Acc. Chem. Res. 40 (2007) 1182. [8] H. Tokuda, S. Tsuzuki, M.A.B.H. Susan, K. Hayamizu, M. Watanabe, J. Phys. Chem. B 110 (2006) 19593. [9] M.J. Earle, K.R. Seddon, Pure Appl. Chem. 72 (2000) 1391. [10] R.D. Rogers, K.R. Seddon, ACS Symp. Ser. 856 (2003) 599. [11] K.R. Seddon, R.D. Rogers, ACS Symp. Ser. 901 (2005) 334. [12] J.P. Hallett, T. Welton, Chem. Rev. 111 (2011) 3508. [13] Z. Duan, Y. Gu, J. Zhang, L. Zhu, Y. Deng, J. Mol. Catal. A: Chem. 250 (2006) 163. [14] V.B. Kazansky, Catal. Today 73 (2002) 127. [15] S.K. Shukla, N.D. Khupse, A. Kumar, Phys. Chem. Chem. Phys. 14 (2012) 2754. [16] M.M. Huang, H. Weingartner, Chem. Phys. Chem. 9 (2008) 2172. [17] C. Thomazeau, H. Olivier-Bourbigou, L. Magna, S. Luts, B. Gilbert, J. Am. Chem. Soc. 125 (2003) 5264. [18] D.R. MacFarlane, J.M. Pringle, K.M. Johansson, S.A. Forsyth, M. Forsyth, Chem. Commun. (2006) 1905. [19] K. Fumino, A. Wulf, R. Ludwig, Angew. Chem., Int. Ed. 48 (2009) 3184. [20] G.S. Rai, A. Kumar, Chem. Phys. Lett. 496 (2010) 143. [21] N.D. Khupse, A. Kumar, J. Phys. Chem. B 114 (2010) 376. [22] G.S. Rai, A. Kumar, Chem. Phys. Chem. 13 (2012) 1927. [23] S. Tiwari, A. Kumar, Angew. Chem., Int. Ed. 45 (2006) 4824. [24] N.D. Khupse, A. Kumar, J. Phys. Chem. A 115 (2011) 10211. [25] A. Singh, A. Kumar, J. Org. Chem. 77 (2012) 8775. [26] H. Ohno, M. Yoshizawa, Solid State Ionics 303 (2002) 154. [27] H. Zhou, J. Yang, L. Ye, H. Lin, Y. Yuan, Green Chem. 12 (2010) 661. [28] D. Marx, M.E. Tuckerman, J. Huttler, M. Parrinello, Nature 397 (1999) 601. [29] F. D’Anna, S. La Marca, R. Noto, J. Org. Chem. 74 (2009) 1952. [30] H. Deng, Y. Chu, J. He, J. Cheng, J. Org. Chem. 77 (2012) 7291. [31] J.A. Duffy, M.D. Ingram, J. Am. Chem. Soc. 93 (1971) 6448. [32] C.A. Angell, J. Solid State Electrochem. 13 (2009) 981. [33] M. Salanne, C. Simon, P.A. Madden, Phys. Chem. Chem. Phys. 13 (2011) 6305.
Contents lists available at SciVerse ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Do protic ionic liquids and water display analogous behavior in terms of Hammett acidity function? Shashi Kant Shukla, Anil Kumar ⇑ Physical & Materials Chemistry Division, National Chemical Laboratory, Pune 411 008, India
a r t i c l e
i n f o
Article history: Received 31 October 2012 In final form 16 February 2013 Available online 5 March 2013
a b s t r a c t We address an issue whether the strength of carboxylic acids in water is linear with respect to that in ionic liquids. Strength of carboxylic acids in water and different PILs using Hammett function (Ho) has revealed interesting linear correlation between the Ho function for all acids in PILs and PIL-water in spite of large structural and electronic differences. These observations suggest that different structural and electronic features of PILs and water behave analogously towards Ho. This linearity in Ho functions between PILs and PIL-water systems can be used to develop predictive method to calculate Ho values. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Acidity of a medium is an intrinsic number. However, the strength of an acid in a medium varies with (i) the nature of the acidic species and (ii) the ability of the solvent to dissociate it. Strength of an acid can be used for establishing the reaction mechanism and also in fine tuning the yield and selectivity of acid catalyzed reactions. Determination of strength of an acid in non-aqueous medium by using the same procedure as in aqueous medium cannot be employed satisfactorily due to some practical limitations [1]. The common methods that have been used for the precise determination of acid strength beyond pH range in aqueous/non-aqueous medium are potentiometry and ultraviolet spectroscopy. Measurement of acidic strength using spectral technique depends on the isolation of spectral changes due to the protonated and non-protonated forms of indicator. Ionic liquids (ILs) are molten organic salt which contain asymmetric organic cation and symmetric/asymmetric organic/inorganic anion [2]. Last few decades have witnessed the stupendous increase of ILs for a variety of reactions [3] because of their unique solvation behavior and the presence of electrophilic (cation) and nucleophilic (anion) sites. ILs have been divided into two sub-groups viz. protic ionic liquid (PIL) and aprotic ionic liquid (AIL). The prefixes protic and aprotic designate that quaternization step during synthesis of IL involves proton (H+) and alkyl (–R) group, respectively. IL serves as a potential reaction media for a variety of polar and apolar reactant because of their amphiphilic nature. They have wide applications in electrochemical and biochemical processes [4,5] because of wide electrochemical window, high thermal stability, insignificant vapor pressure, wide liquidus range, non-flammability, recyclability, etc. [6–11]. PILs contain dissociated proton ⇑ Corresponding author. Fax: +91 2025902636. E-mail address: [email protected] (A. Kumar). 0009-2614/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2013.02.029
therefore, act as efficient reaction media for a number of acidcatalyzed reactions [12]. However, the measured acidity level are usually low, suggesting small degree of dissociation (a) of PILs [13]. The efficiency of PILs as catalyzing media can further be improved by adding Brønsted acids. Strength of added acid in a medium depends on the dissociation (a) of acid and the stabilization of the conjugate base [14]. Since different solvents solvate proton (H+) up to different extent therefore, strength of an acid vary with medium. Because of the various specific and non-specific interactions, ILs have unique solvation behavior. However, it is difficult to correlate the solvation behavior of ILs with its solvent properties such as polarity (ENT ) [15] and relative permittivity (er) [16]. Thomazeau et al. for the first time measured the strength of HNTf2 (Bistriflimide) and HOTf (Triflic acid) in alkylimidazoliumbased ILs using Hammett acidity function (Ho) [17]. They observed that C2–H has no influence on the strength of HNTf2 and HOTf. They also noted similar Ho values for HNTf2 and HOTf in [BMIM][NTf2] (1-butyl-3-methylimidazolium bistriflimide) due to the leveling effect of the medium. Higher acidity level for HNTf2 in [BMIM][BF4] (1-butyl-3-methylimidazolium tetrafluoroborate) than that in [BMIM][NTf2] was attributed to the lesser solvation of proton (H+) by BF 4 than by NTf 2 . MacFarlane et al. showed that the relative basicity of IL anion and water is the only criterion in deciding the strength/acidity level of an acid in these media [18]. Though PIL mimics water in a sense of extensive hydrogen bonding [19], they have different structural and electronic features. In our laboratory, we have attempted to correlate the structure– activity relations for different ILs by measuring their physicochemical properties [20–22] and their implications on various chemical reactions [23–25]. Prompted by the similarity between PILs and water in terms of hydrogen bonding, we now address an issue whether PILs and water display analogous behavior with regard to their Hammett acidity functions. For this purpose, we investigate the strength of carboxylic acids (HCOOH, CH3COOH
13
S.K. Shukla, A. Kumar / Chemical Physics Letters 566 (2013) 12–16
and CH3CH2COOH) in water and in different PILs. The PILs of current interest are [HMIm][HCOO] (1-methylimidazolium formate), [HMIm][CH3COO] (1-methylimidazolium acetate), [HBIm][HCOO] (1-butylimidazolium formate), [HBIm][CH3COO] (1-butylimidazolium acetate), [HPyrr][HCOO] (1-methylpyrrolidinium formate) and [HPyrd][HCOO] (1-methylpiperidinium formate). The structures of PILs are shown in Figure 1.
1.5
Absorbance
1.2
2. Experimental section The spectroscopic indicator dye 4-nitroaniline was obtained from M/s Sigma–Aldrich and used as obtained. 1-Methylimidazole, 1-butylimidazole, 1-methylpyrrolidine and 1-methylpiperidine were all purchased from Sigma–Aldrich and were distilled prior to use. Formic acid, acetic acid and propionic acid were provided by Merck and used as obtained. Deionized water having specific conductance <0.055 106 was used wherever needed during experiment. The water content in PILs was measured before the start of each experiment and it did not exceed beyond 50 ppm. PILs employed in the current work were synthesized by the procedures used by Ohno and Yoshizawa [26]. PILs were further purified and freed from other volatile impurities by placing them under high vacuum (102 bar) for 10 h at 60 °C. 1H NMR data of these PILs were found in good agreement with those reported in the literature. Since these PILs were synthesized by halide free pathway they were found devoid of any halide impurities. A stock solution of 4-nitroaniline (104 M) was prepared in methanol and was used for measurement of strength in water and PILs. All the acids were mixed in equimolar proportion with deionized water. A small volume of indicator solution was added in 1 ml of PIL/water after removing methanol. The sample was filled inside the quartz cuvette capped with septum and further protected by parafilm to avoid any contamination. The cuvette was placed inside the UV chamber and absorbance was recorded at 25 °C. The absorbance without any acid content was taken as reference. The reference absorbance was due to only the non-protonated form of indicator. In all the experiment reference value was kept constant around 1.45. Now the calculated amount of acid was added inside the cuvette and absorbance was recorded. These absorbances at different concentrations of acid were compared with reference value to get the relative amount of protonated ([I]s) and non-protonated ([HI+]s) forms of indicator. Strength of an acid solution then calculated by using Hammett equation:
Ho ¼ pKðIÞaq: þ log ½Is =½HIþ s
ð1Þ
where pK(I)aq. is the protonation constant of indicator in aqueous solution. For 4-nitroaniline pK(I)aq. = 0.99. [I]s and [HI+]s indicates the relative population of unprotonated and protonated forms of indicator. From Eq. (1) it is clear that Ho will be lower for strong acids and vice versa. A typical absorption spectra for a series of experiment for an acid/PIL couple is shown in Figure 2.
0.9 0.6 0.3 0.0
320
360 400 440 Wavelength (nm)
Figure 2. A typical absorbance spectra of 4-nitroaniline at various concentrations of an acid in PIL.
Eq. (1) can also be written as,
Ho ¼ log aðHþaq: Þ log ½cðIÞ=cðHIþ Þ log ½CðIÞ=CðHIþ Þ
3. Results and discussion Strength of an acid in ILs using Hammett function is not much discussed. Only few reports are available on the measurement of strength in ILs using Hammett function and their correlation with the variation of cation and anion [17]. Other report includes the measurement of strength and its correlation with catalytic activity for acid-catalyzed reactions [13,27]. Current work includes the determination of strength for weak carboxylic acids in PILs and water using Hammett function. Unlike water, PIL offer different interactions for a released proton (H+) and hence can affect the solvation of H+. In water, solvation of proton is precisely known [28]; however in PILs it is ambiguous. Similarly, the relative tendency of proton solvation of PIL and water cannot be compared due to the unavailability of enthalpic data. Since solvation of proton affect
Anions O
N
N H
N H3 C
C
N H
ð2Þ
where c is the activity coefficient and U is the transfer activity coefficient of protonated (HI+) and unprotonated (I) forms of indicator. In a very dilute solution of indicator log[c(I)/c(HI+)] remain constant. The ratio log[U(I)/U(HI+)] arose during the exchange of indicator from one medium to other medium. To keep this ratio constant, it was considered that the different forms of indicator have the same solvation shell in different medium. However, it is difficult to meet this requirement since different solvent solvates these two forms of indicator up to different level. Therefore, it is assumed that for structurally similar indicators the ratio log[U(I)/ U(HI+)] will remain constant.
Cations
R
480
H3C
H
R
O
R = H, CH3, CH3CH2
R = CH3, C 4H9 Figure 1. Structures of PIL employed in study.
S.K. Shukla, A. Kumar / Chemical Physics Letters 566 (2013) 12–16
ðHoÞ2 ¼ m ðHoÞ1 þ c
2.25
(a)
(b)
2.10
2.3
2.2
(b)
3
2.2
2.0
2.1
2.0
1.8 1.8
1.9
2.0
Ho[HMIM][HCOO]
2.1
1.8
1.9
2.0
2.1
Ho[HMIm][HCOO]
Figure 4. Linear correlation in acidity function of HCOOH between (a) [HMIm][HCOO] vs [HPyrr][HCOO] and (b) [HMIm][HCOO] vs [HMIm][CH3COO].
strength of acid between PILs and PIL-water can be interpreted up to certain extent. The different values of slopes and intercepts for HCOOH, CH3COOH and CH3CH2COOH between different PILs and PIL-water are shown in Tables 1–3, respectively. For convenience, influence of PILs on Ho have been discussed in terms of (1) effect of cation, (2) effect of anion and, (3) influence of water on Ho compared to that of PILs. 3.1. Effect of cation From Tables 1–3 it is evident that the variation of cation has more effect on the strength of carboxylic acids. However, the relative tendency of different cationic core with [HCOO] differs largely in promoting the strength of HCOOH, CH3COOH and CH3CH2COOH. The order of different cationic core in promoting the strength of HCOOH and CH3COOH is in the order,
½HMImþ > ½HBImþ > ½HPyrdþ > ½HPyrrþ whereas, for CH3CH2COOH the order reverse as,
½HPyrrþ > ½HPyrdþ > ½HBImþ > ½HMImþ These observations suggest specific interaction between the cationic backbone and conjugate anion of acid. Similar trend of PILs with [CH3COO] anion for all carboxylic acid was observed. These trends of cationic backbone were estimated from the slope of linear correlation between PILs. The order of strength for HCOOH in different PILs having [HCOO] is shown in Figure 5. 3.2. Effect of anion Unlike cation, anion has a weaker influence on the strength of carboxylic acids. The difficulty in synthesizing PILs with different anions made our work complicated. However, between [HMIm][HCOO] and [HMIm][CH3COO], [HCOO] favor strength of CH3CH2COOH more than that of [CH3COO], while for HCOOH and CH3COOH, [CH3COO] promote acidic strength more compared to [HCOO]. 3.3. Effect of water
2
2
HoH O
HoH O
1.65
1.50
1.35
(a)
ð3Þ
The subscripts 1 and 2 have been used for medium at x and yaxis, respectively. In short m > 1 indicate higher strength of an acid in 1 compared to 2 and vice versa. With the aid of slope and intercept values, possible effects of cation and anion variation on the
1.80
2.4
Ho[HMIm][CH COO]
the strength (which in turn affect Ho value), the relative Ho values in water and in PILs can be used as a measure of solvation of medium. Carboxylic acids (HCOOH, CH3COOH and CH3CH2COOH) have different strength in PILs and water as suggested by different Ho values. The different Ho values in PILs and water for carboxylic acids may be due to the combined effect of the structural and electronic variations of these media on the solvation of protons. The relative influence of structural and electronic differences between PILs and PIL-water can be easily seen by plotting their Ho values against each other for carboxylic acids. Surprisingly a linear relation in Ho values was observed for all carboxylic acids in different media. A linear correlation between the Ho values of HCOOH and CH3COOH in [HMIm][HCOO] and water is shown in Figure 3. Similarly, a linear relation was observed for Ho values of HCOOH in different PILs despite of their different cationic and anionic counterparts (Figure 4). Though the linear dependency between Ho values for carboxylic acids in PILs and PIL-water is surprising, these observations cannot be explained satisfactorily due to the limited knowledge about solvation behavior of PILs. This analogous behavior of PIL and water suggest that though the molecular environment around proton remains different for PIL and water, overall response on the strength of weak carboxylic acids remain same in these media. Similar observations on the protonation constant (pKa) of different carboxylic acids in AILs have been noted by D’Anna et al. [29] and Deng et al. [30]. Optical basicity scale is developed for determining the Lewis basicity of metallic oxide and it primarily depends on the polarizability of the base [31,32]. Madden et al. have calculated the optical basicity (k) for different protic solvents (HF, H2O and NH3) and their binary mixtures [33]. They have measured the Hammett acidity function (Ho) at different compositions of protic solvents and found a linear relation with optical basicity. This correlation can be used to develop the understanding between variation in polarizability of the mixture, optical basicity and Hammett acidity. However, in the present work, linearity in Ho function has been used to develop simple equation between PILs and PIL-water that would be helpful in predicting the Hammett acidity (Ho) at a different composition of acid only by using the Ho value in any of them. A general form of linear equation for Hammett function (Ho) is as,
Ho[HPyrr][HCOO]
14
1.95
1.80
1.8
1.9
2.0
Ho [HMIm][HCOO]
2.1
1.65
1.50 1.65 1.80 1.95 2.10
Ho ([HmIm][HCOO])
Figure 3. Linear correlation between Ho of [HMIm][HCOO] and H2O for (a) HCOOH and (b) CH3COOH.
Unlike PILs, where the relative interactions between cation and anion vary, water differs a lot from PIL in both structural organizations and the absence of distinct charge. Solvent parameters such as polarity (ENT ) and relative permittivity (er) therefore cannot be accounted for this unusual behavior, since both water and PILs have different values. Therefore, at a moment it seems improbable to explain these observations by using a suitable solvent scale. The slope of linear correlations between PILs and water for HCOOH exhibit higher strength in [HMIm][HCOO] and lower in
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S.K. Shukla, A. Kumar / Chemical Physics Letters 566 (2013) 12–16 Table 1 Linear correlations between the Hammett function for HCOOH between PILs and PIL-water. Ionic liquid
Intercept (c)
Slope (m)
R
r
[HMIm][HCOO] vs [HPyrr][HCOO] [HMIm][HCOO] vs [HPyrd][HCOO] [HPyrr][HCOO] vs [HPyrd][HCOO] [HMIm][HCOO] vs [HBIm][HCOO] [HMIm][HCOO] vs [HMIm][CH3COO] [HMIm][CH3COO] vs [HBIm][CH3COO] [HMIm][HCOO] vs H2O [HPyrr][HCOO] vs H2O [HPyrd][HCOO] vs H2O [HBIm][HCOO] vs H2O [HBIm][CH3COO] vs H2O
0.962 0.551 0.250 0.332 0.216 0.515 0.848 0.057 0.298 0.497 0.561
1.54 1.31 0.850 1.276 1.011 1.256 1.282 0.827 0.973 0.996 0.991
0.994 0.993 0.994 0.991 0.990 0.990 0.995 0.993 0.998 0.995 0.996
0.022 0.013 0.018 0.021 0.018 0.022 0.015 0.019 0.010 0.016 0.014
Table 2 Linear correlations between the Hammett function for CH3COOH between PILs and PIL-water. Ionic liquid
Intercept (c)
Slope (m)
R
r
[HMIm][HCOO] vs [HPyrr][HCOO] [HMIm][HCOO] vs [HPyrd][HCOO] [HPyrr][HCOO] vs [HPyrd][HCOO] [HMIm][HCOO] vs [HBIm][HCOO] [HMIm][HCOO] vs [HMIm][CH3COO] [HMIm][CH3COO] vs [HBIm][CH3COO] [HMIm][HCOO] vs H2O [HPyrr][HCOO] vs H2O [HPyrd][HCOO] vs H2O [HBIm][HCOO] vs H2O [HBIm][CH3COO] vs H2O
0.417 0.388 0.026 0.117 0.008 0.048 0.480 0.747 0.731 0.564 0.511
1.268 1.250 0.984 1.142 1.096 1.021 0.812 0.640 0.650 0.710 0.725
0.995 0.997 0.999 0.996 0.998 0.997 0.995 0.999 0.999 0.999 0.999
0.026 0.021 0.007 0.023 0.015 0.018 0.017 0.007 0.008 0.008 0.007
Table 3 Linear correlations between the Hammett function for CH3CH2COOH between PILs and PIL-water. Ionic liquid
Intercept (c)
Slope (m)
R
r
[HMIm][HCOO] vs [HPyrr][HCOO] [HMIm][HCOO] vs [HPyrd][HCOO] [HPyrr][HCOO] vs [HPyrd][HCOO] [HMIm][HCOO] vs [HBIm][HCOO] [HMIm][HCOO] vs [HMIm][CH3COO] [HMIm][CH3COO] vs [HBIm][CH3COO] [HMIm][HCOO] vs H2O [HPyrr][HCOO] vs H2O [HPyrd][HCOO] vs H2O [HBIm][HCOO] vs H2O [HBIm][CH3COO] vs H2O
0.218 0.101 0.152 0.224 0.375 0.063 1.006 0.854 0.946 0.866 0.796
0.818 0.922 1.174 0.914 0.831 1.014 0.573 0.700 0.600 0.627 0.677
0.999 0.999 0.999 1.0 0.997 0.998 0.998 0.998 0.999 0.991 0.998
0.009 0.013 0.017 0.004 0.017 0.015 0.009 0.009 0.006 0.010 0.009
2.4
Ho
2.2
2.0
1.8
1.6
0.6
0.9
1.2
1.5
CH+ (M) Figure 5. Ho vs C Hþ plot for HCOOH in [HMIm][HCOO] (h), [HPyrr][HCOO] (.), [HPyrd][HCOO] (s) and, [HBIm][HCOO] (N).
[HPyrr][HCOO] compared to water, whereas the same strength was noted for HCOOH in [HPyrd][HCOO] and [HBIm][HCOO] when compared with water, probably due to the structural compensation (larger size of [HPyrd]+ and presence of –C4H9 in [HBIm]+).
Similarly, linear correlation for Ho values of HCOOH in [HBIm][HCOO] and [HBIm][CH3COO] with water indicated that basic anion [CH3COO] promote strength of an acid more than less basic [HCOO] anion. These conclusions were drawn from the slope value of linear correlations between PILs and PIL-water. However, the effect of PIL and water for CH3COOH cannot be explained on the similar ground as the observed slope values were unexpectedly very low (m < 1). The lower m value indicates higher strength in water than that in PILs. These dubious observations based on slope value for CH3COOH in water and PILs are in contradiction to that indicated by Ho value. Thus the lower strength of CH3COOH in water than in PILs can only be explained by using unusually large intercept (c) values. Intercept of a linear correlation indicates the starting point at y-axis when the value at x-axis is zero. According to Eq. (1), the value of intercept represents the non-assistance of medium 2 compared to 1. Therefore, the enhancement in the strength of CH3COOH by various PILs compared to that of water is in the order,
½HMIm½HCOO > ½HBIm½CH3 COO > ½HBIm½HCOO > ½HPyrd½HCOO > ½HPyrr½HCOO
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S.K. Shukla, A. Kumar / Chemical Physics Letters 566 (2013) 12–16
Similar is the case for CH3CH2COOH where due to the lower slope (m < 1) value, increase in acidic strength by PILs than water has been arranged by using intercept values. The order of different PILs in promoting the acidity of CH3CH2COOH than that of water is as,
½HBIm½CH3 COO > ½HBIm½HCOO > ½HPyrr½HCOO > ½HPyrd½HCOO > ½HMIm½HCOO 4. Conclusions In conclusion, we have shown that both water and PILs have different interactions towards proton due to the distinct structural and electronic differences, they behave analogously towards the strength of carboxylic acids. By using linear correlation in Ho values for carboxylic acids between PILs and PIL-water, simple equations can be derived which can further be used for the determination of strength in water and PIL, only by measuring the strength in one of them at different concentration of acid. Acknowledgements S.K. Shukla is grateful to UGC, New Delhi, for awarding him a research fellowship. A. Kumar thanks DST, New Delhi, for a J.C. Bose National Fellowship (SR/S2/JCB-26/2009). The authors acknowledge an anonymous referee whose suggestions improved the quality of this work. References [1] E.J. King, Acid–Base Equilibria, Pergamon Press, Oxford, 1965.
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