ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 2, pp. 237–241. © Pleiades Publishing, Ltd., 2008. Original Russian Text © K.G. Bogolitsyn, D.S. Kosyakov, N.S. Gorbova, A.M. Aizenshtadt, N.V. Shorina, 2008, published in Zhurnal Fizicheskoi Khimii, 2008, Vol. 82, No. 2, pp. 303–308.
PHYSICAL CHEMISTRY OF SOLUTIONS
The Acidity and Solvation of Lignin-Related Phenols in Water–1,4-Dioxane Mixtures K. G. Bogolitsyn, D. S. Kosyakov, N. S. Gorbova, A. M. Aizenshtadt, and N. V. Shorina Arkhangel’sk State Technical University, Arkhangel’sk, Russia e-mail:
[email protected] Received April 10, 2007
Abstract—The pKa values were determined for 10 guaiacyl phenols by UV spectrophotometric and potentiometric titration in the water–dioxane binary system over the composition range 0–80% organic solvent. The differentiating action of the solvent and para-substituent effects on the acidity of guaiacol derivatives were analyzed. The dependences of acidity constants on the mixed solvent composition were interpreted using the preferential solvation model. DOI: 10.1134/S0036024408020179
INTRODUCTION Para-derivatives of 2-methoxyphenol (guaiacol) are common in nature. They are important participants of metabolism in plants, precursors in lignin biosynthesis reactions, the products of lignin degradation in nature and industrial technologies for the production of cellulose from wood, and are produced in the conversion of technical lignins into low-molecular-weight aromatic compounds [1]. The occurrence of these processes is to a substantial extent determined by the ability of the phenol hydroxyl group of such compounds to undergo acid ionization with the formation of reactive phenolate anions and quinonemethide structures [2, 3]. The most important factor influencing protolytic equilibria with the participation of monolignols is the nature of the solvent. Changes in solvent composition can be considered a powerful tool for controlling the reactivity of guaiacyl structures. Of great importance in this respect are binary mixtures of water with aprotic solvents extensively used in the chemistry of lignin and related phenols. Components of such solvents interact specifically and nonspecifically with various participants of acid–base reactions [4]. They have a differentiating action on the strength of acids, including action caused by the selective (preferential) solvation of both phenol molecules and conjugated phenolate anions. In [5, 6], we determined the acid dissociation constants for phenols of the guaiacyl series in mixtures of water with dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMFA), and acetone. We found that the pKa values increased virtually linearly as the mole fraction of the nonaqueous solvent grew. The mechanism of the influence of DMSO and DMFA on the acidity of phenol hydroxyls is the destabilization of phenolate anions in media with a low electron acceptor ability [7]. The dissociation of phenols in water–dioxane mixtures is of special interest. This is caused by the
extremely low permittivity of 1,4-dioxane, which can contribute to the role played by preferential solvation and nonspecific interactions with solutes. There is nevertheless only one work [8] concerned with the determination of the acidity constants of some guaiacol derivatives in 50% dioxane. The purpose of this work was to determine the acidity constants of a wide range of para guaiacol derivatives in the water–dioxane system and study the influence of solvent composition and phenol structure on acid–base equilibria. EXPERIMENTAL Experimental studies were performed for 2-methoxyphenol (guaiacol) and nine its para derivatives, including 3-methoxy-4-oxytoluene (creosol), 1-(3methoxy-4-oxyphenyl)-propene-2 (eugenol), 1-(3methoxy-4-oxyphenyl)-propene-1 (isoeugenol), 3methoxy-4-oxybenzaldehyde (vanillin), 3-methoxy-4oxybenzyl alcohol (vanillic alcohol), 3-methoxy-4oxyacetophenone (acetovanillone), 3-methoxy-4-oxybenzoic acid (vanillic acid), 3-methoxy-4-oxycinnamic acid (ferulic acid), and 1-(3-methoxy-4-oxyphenyl)propanol-1 (α-guaiacylpropanol). α-Guaiacyl propanol was synthesized from vanillin according to the Grignard reaction, the purity of the product was controlled by IR spectroscopy and high-performance liquid chromatography. The other phenols were used without additional purification in the form of commercial reagents (purum grade) from Fluka and Sigma-Aldrich. Mixed solvents were prepared from 1,4-dioxane of ch. d. a. (pure for analysis) grade and deionized water. The pKa values were determined by potentiometric (for vanillin and vanillic and ferulic acids) and spectrophotometric titration with a 1 M solution of tetraethylammonium hydroxide (puriss, Fluka); pH of solutions
237
238
BOGOLITSYN et al.
for potentiometric and
pKa 16 3
4
56 7
89
10
(3)
for spectrophotometric titration [13, 14] using the whole titration curve (15–25 values). In (2), V and Ve are the volume of alkali added and equivalent volume, respectively. In (3), D, DPhOH, and D PhO– are the optical
14 2 1 12
10
–I 10% 20% 30% 40% 10 pKa (w)
8 7
D PhO– – D pK a = pa H+ + log ----------------------D – D PhOH
8
9
50% 60% 70% 80%
Fig. 1. Dependences of pKa on pKa(w) for guaiacol para derivatives in the water–dioxane system. The numbers of points (1–10) correspond to the numbering of compounds in Table 1; dioxane content 10–80 wt %, I is mixed solvent 12.
was measured on an Ekspert-001.1.01 (Ekoniksekspert, Russia) high-precision ionometer and an electrode system including ESL-63-07 indicator and EVL1M3 auxiliary silver-chloride electrodes (ZIP, Belarus) calibrated against aqueous standards. The correctness of calibration was controlled directly before measurements. The instrumental pH value obtained was recalculated to the proton activity index in the given medium pa H+ with introducing correction δ for the interphase potential and the energy of proton transfer from water into the mixed solvent [10, 11], pa H+ = pH – δ.
(1)
Because of unstable glass electrode operation in media with low permittivities [12], the working dioxane concentration range was 0–80 wt %. Titration was performed in a glass temperature-controlled (25 ± 0.1°ë) cell of volume 100 ml in the atmosphere of argon under continuous stirring. The UV absorption spectra were recorded using one cm quartz cells with respect to the corresponding solvents on a Specord 200PC spectrophotometer (Analytik Jena AG, Germany). The concentrations were 0.01 mol/l for vanillin and vanillic and ferulic acids and 10–4–10–5 mol/l for the other guaiacol derivatives. The pKa values were calculated by the equations Ve – V pK a = pa H+ + log -------------V
(2)
densities of the solution at the given pa H+ , the nonionized phenol form, and the phenolate anion, respectively, at the wavelength of the maximum of the second benzoid band of the phenolate anion. DPhOH was the optical density before titration, and D PhO– was measured in a 0.01 M solution of Et4NOH. To improve the reliability of the results, the ionization constants of all the compounds studied were also calculated by an independent method based on a comparison of the instrumental measurement results with similar values for a standard substance with a known acidity constant. The standard substance was benzoic acid, whose pKa was determined in [15] by measuring the electromotive force of circuits without transfer. The Henderson equation [16] pK a = pK a ( HBz ) + pH α – pH α ( HBz )
(4)
was used. Here, pHα and pHα(HBz) are the instrumental pH values at which the degrees of ionization α of the phenol studied and benzoic acid are equal in the given solvent. According to statistical data processing [17], the error in pKa was 0.02–0.03 units. RESULTS AND DISCUSSION The pKa values of 10 phenols of the guaiacyl series determined by (2) and (3) and calculated according to Henderson are listed in Table 1. The largest discrepancy between the pKa values obtained by different methods did not exceed 0.3 units, which was quite satisfactory and testified to the correctness of the approaches used. The acidity of all the compounds decreased sharply as the fraction of the aprotic solvent increased. For each binary system composition, the pKa values of phenols linearly depended on the dissociation constant index in aqueous solution pKa(w) (Fig. 1). The slopes of the straight lines (Table 2) increased as the concentration of dioxane grew, which was evidence of the differentiating action of dioxane on the strength of the acids studied [19]. The mechanism of the differentiating action was similar to the mechanism characteristic of water mixtures with DMSO and DMFA. This was the stabilization of anions with the greatest degree of charge delocalization in dipolar aprotic solvents caused by ion– dipole interactions [19]. This conclusion is substanti-
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Table 1. Guaiacyl phenol pKa values in water–dioxane mixtures at 25°C (the values calculated by the Henderson equation are given in parentheses) No.
Compound
0* wt %
10 wt %
20 wt %
30 wt %
40 wt %
50 wt %
60 wt %
70 wt %
80 wt %
1 Vanillin
7.40
2 Acetovanillone
7.90
3 Ferulic acid (−OHph) 4 Vanillic acid (−OHph) 5 Vanillic alcohol
9.15
6 α-Guaiacylpropanol 7 Guaiacol
9.85 10.04
8 Isoeugenol
10.11
9 Eugenol
10.15
10 Creosol
10.40
7.57 (7.48) 8.16 (8.07) 9.56 (9.42) 9.73 (9.62) 10.18 (10.09) 10.19 (10.10) 10.42 (10.33) 10.27 (10.03) 10.47 (10.35) 10.66 (10.46)
7.78 (7.83) 8.53 (8.42) 9.92 (9.81) 10.21 (10.01) 10.51 (10.42) 10.66 (10.49) 10.81 (10.61) 10.68 (10.51) 10.87 (10.84) 11.05 (10.95)
8.12 (7.90) 8.87 (8.61) 10.34 (10.15) 10.53 (10.31) 10.97 (10.74) 11.16 (10.84) 11.26 (11.03) 11.14 (10.80) 11.34 (11.14) 11.47 (11.26)
8.58 (8.37) 9.26 (9.07) 10.77 (10.52) 10.89 (10.61) 11.45 (11.24) 11.68 (11.33) 11.78 (11.57) 11.59 (11.29) 11.90 (11.64) 11.97 (11.78)
9.16 (9.01) 9.89 (9.82) 11.31 (11.18) 11.63 (11.38) 12.06 (11.94) 12.24 (12.17) 12.36 (12.03) 12.23 (12.05) 12.51 (12.42) 12.60 (12.56)
9.87 (9.81) 10.58 (10.53) 11.97 (11.89) 12.31 (12.11) 12.76 (12.71) 12.88 (12.83) 13.11 (13.06) 12.94 (12.79) 13.31 (13.22) 13.40 (13.50)
10.71 (10.47) 11.53 (11.33) 12.79 (12.66) 13.13 (12.90) 13.66 (13.41) 13.81 (13.64) 14.09 (13.76) 13.93 (13.74) 14.24 (14.14) 14.42 (14.29)
11.67 (11.85) 12.48 (12.29) 14.12 (14.06) 14.39 (14.31) 14.86 (14.81) 14.92 (14.83) 15.29 (15.19) 15.16 (15.11) 15.53 (15.47) 15.71 (15.63)
9.40 9.80
* Values for aqueous solutions were taken from [18].
ated not only by the data listed in Table 2 but also by the obvious interrelation between the acidity constant indices and Hammett σ constants of substituents [20] situated para with respect to the phenol hydroxyl (Fig. 2). This interrelation characteristic of all solvent compositions can be written as pK a = pK a – ρσ, g
g
(5)
where pK a is the guaiacol dissociation constant index in the given solvent. Such a behavior of the system is at variance with the mechanism of the differentiation of the strengths of acids in water–dioxane mixtures suggested by MchedloPetrosyan on the basis of the data on the acidity of sulfophthaleine dyes [19]. This mechanism presupposes the selective solvation of hydrophobic molecule parts with dioxane. We might then expect a correlation between acidity changes and phenol polarity rather than the substituent induction effect. For guaiacyl phenols, this effect can manifest itself at higher organic solvent contents; that is, the behavior of dioxane can be similar to that of dipolar aprotic solvents because fairly polar complexes of dioxane with water play the role of solvating particles over the range of medium compositions studied. The dependence of the Hammett reactive deprotonation constants calculated from the slopes of the straight lines drawn in Fig. 2 according to (5) on solvent composition is shown in Fig. 3. Although the ρ values increase considerably as the mole fraction of dioxane grows, the influence of the solvent on the sensitivity of the dissociation of phenols to the properties of the para RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
substituent in the water–dioxane system is noticeably weaker that in mixtures of water with DMSO and DMFA [5]. This can be caused by a weaker solvating ability of dioxane and, accordingly, its lower content in solvation shells compared with more polar solvents. The curve is S-shaped, which is evidence of the complexity of solvation processes and inapplicability of one-parameter models to describing them. One of such models is the Marshall–Quist model [21], which relates the pKa and ρ values to the molar concentration of water. This model was successfully used in [22] to describe a wide range of substituted phenols and pyridinium ions in water–dioxane media. The dependences of pKa on the mole fraction of 0
dioxane in a mixed solvent x 2 (Fig. 4) are nonlinear Table 2. Empirical constants of the pKa = apKa(w) + b equations relating the pKa values of phenol hydroxyl groups of guaiacol derivatives in water–dioxane mixtures to pKa in water x, wt %
a
b
r
10 20 30 40 50 60 70 80
1.05 1.09 1.14 1.17 1.18 1.20 1.23 1.33
–0.09 –0.15 –0.17 –0.02 0.58 1.07 1.67 1.96
0.99 0.99 0.98 0.98 0.98 0.98 0.98 0.98
Note: x is the content of dioxane and a and b are the empirical coefficients; r is the correlation factor.
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BOGOLITSYN et al. 10 9 8
7–5 4
14
3
Water 30% 40% 10% 20% 50%
ρ 3.5
60% 70% 80% 2 1
3.1
12 10 8
2.7
–0.12
0.08
0.28
0.48
0.68
0.88
1.08 σ
0
Fig. 2. Dependences of pKa of phenol hydroxyls on the Hammett constants of para substituents in guaiacyl phenols in the water–dioxane system. See Fig. 1 for notation.
0
(6)
The meaning of the d and p coefficients can be understood using the model of the preferential solvation of the solute by one of the mixed solvent components. The components considered most frequently are pure water (S1) and dioxane (S2) and the product of their interaction (S12), or “mixed solvent” formed in the reaction [23] 2S12. (7) S1 + S2 The necessity of including an additional mixed solvent for describing the behavior of lignin-related phenols was substantiated by analyzing the dependences of absorption bands in the electronic spectra of these compounds on the composition of water–DMFA mixtures [24]. In the water–dioxane system, the equilibrium constant of reaction (7) is very large and can be assumed to equal infinity [25]. The relation between the initial (x0) and equilibrium (x) mole fractions of the components is then described over the composition range studied (at 0 x 2 ≤ 0.5) by the equations 0
0
x 1 + x 2 = x 1 + x 12 = 1,
(8)
0
x 1 = x 1 + x 12 /2,
20
30
40
x20, %
Fig. 3. Dependence of the deprotonation reaction constant on the mole fraction of dioxane in mixed solvents.
and can best be described by the equation d x2 -0 . pK a = pK a ( w ) + ----------------1 + px 2
10
where n1 and n12 are the numbers of molecules of the corresponding solvent components in the solvation shell of the phenol. The solvation shell composition is related to the composition of the solvent via the preferential solvation pKa
10 9 78 6 5 4 3
15
13 2 1 11
9
(9)
0
x 2 = x 12 /2.
(10)
The Gibbs energy of phenol dissociation can be found as the mean of ∆G° values in media S1 and S12, and the pKa value in a mixed solvent is determined similarly, n 1 pK a ( w ) + n 12 pK a12 -, pK a = ----------------------------------------------n 1 + n 12
(11)
7
0
0.2
0.4
x20
Fig. 4. Dependences of the ionization constant of phenol hydroxyls of guaiacol para derivatives on the content of dioxane in mixed solvents. Symbols correspond to the experimental data, and lines were calculated by (13). Line numbers correspond to the numbering of compounds in Table 1.
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THE ACIDITY AND SOLVATION OF LIGNIN-RELATED PHENOLS Table 3. Preferential solvation parameters of guaiacol derivatives in the water–dioxane system Compound Vanillin Acetovanillone Ferulic acid (–OHph) Vanillic acid (–OHph) Vanillic alcohol α-Guaiacylpropanol Guaiacol Isoeugenol Eugenol Creosol
pKa12
f12/1
σ
r
12.18 12.93 14.43 14.76 15.24 15.23 15.69 15.66 15.96 16.23
1.11 1.27 1.36 1.34 1.37 1.56 1.38 1.17 1.29 1.18
0.09 0.05 0.09 0.08 0.03 0.05 0.03 0.05 0.06 0.02
0.9983 0.9992 0.9989 0.9990 0.9998 0.9995 0.9998 0.9995 0.9994 0.9999
Note: σ is the standard deviation, and r is the correlation coefficient.
parameter f12/1, which characterizes the predominant transfer of the “mixed” solvent from bulk solution into the solvation shell [23], n 12 /n 1 -. f 12/1 = ------------x 12 /x 1
(12)
Using (8)–(10) and (12), we can transform (11) to the form corresponding to (6), 2 f 12/1 ( pK a12 – pK a ( w ) )x 2 -. pK a = pK a ( w ) + ----------------------------------------------------------0 1 + 2 ( f 12/2 – 1 )x 2 0
(13)
A regression analysis of the experimental data according to (13) allowed us to calculate the preferential solvation parameter and acidity constant indices of phenols in a hypothetical water–dioxane mixed solvent; the results are listed in Table 3. The correctness of the model used is substantiated by the high correlation coefficients; in the majority of cases, calculated curves (Fig. 4) pass close to the experimental points. An additional argument in favor of the correctness of the method for data processing that we used was correct pKa12 values, which linearly correlated with pKa(w) (Fig. 1). The f12/1 parameter value is larger than one, which means the predominance of water–dioxane complex molecules in the solvation shell. Such selectivity can be explained by the ability of the mixed solvent, as distinct from water, to effectively solvate both hydrophobic molecule fragments and polar groups because of different properties of its components. It should also be borne in mind that the preferential solvation parameter is a complex value that describes the overall effect of the solvation of particles of three types participating in acid–base equilibria, namely, phenol, the corresponding phenolate anion, and proton. ACKNOWLEDGMENTS This work was financially supported by the Russian Foundation for Basic Research (project nos. 06-0332075_a and 05-03-97504_r_sever_a). RUSSIAN JOURNAL OF PHYSICAL CHEMISTRY A
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