Voltammetric determination of the pk a of various acids in polar aprotic solvents using 1,4-benzoquinone

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1 Journal of Eletroanalytial Chemistry 498 (2001) Voltammetri determination of the pk a of various aids in polar aproti solvents using 1,4-benzoquinone H.-s. Kim a, T.D. Chung b,1, H. Kim a, * a Shool of Chemistry and Moleular Engineering, Center for Moleular Catalysis, Seoul National Uni ersity, Seoul , South Korea b Institute of Medial and Biologial Engineering, Medial Researh Center, Seoul National Uni ersity, Seoul , South Korea Reeived 12 April 2000; reeived in revised form 30 August 2000; aepted 30 August 2000 Dediated to Fred C. Anson for his distinguished ontributions to eletrohemistry Abstrat The effet of various proton-donors on the eletrohemial redution of 1,4-benzoquinone (BQ) was investigated in aetonitrile, dimethylsulfoxide and methylene hloride. Three representative types of aids, protonated amines, arboxyli aids and phenol derivatives, were used as proton-donors to over a wide range of pk a. As a proton-donor is added, a new peak appears and the magnitude of the potential shift ( E p ) is proportional to pk a of the protogeni aid. Plots of E p versus pk a in the aproti solvents employed in this study show good linear relationships regardless of the aid s funtional group. An ECE proess is believed to be responsible for this behavior and the linearity reflets dissoiation of the Br nsted aid. As a onsequene, it is possible to estimate the dissoiation onstants of vaious organi aids in aproti media onveniently and reproduibly by voltammetry. A detailed interpretation of the redox behavior of quinone and some fundamental information for potentially promising analytial appliations are desribed Elsevier Siene B.V. All rights reserved. Keywords: Quinone; Br nsted aids; Dissoiation onstants; Voltammetry; Aproti solvents 1. Introdution The aidity of a protogeni speies in nonaqueous media depends on several fators suh as the properties of the solvent, struture of the aid and the funtional group releasing the proton [1,2]. The relative aidity in aproti solvents may differ markedly from that in water. For instane, aeti aid (pk a =4.8) is muh more aidi than ammonium ion (pk a =9.2) in water while the relative aidity is reversed in dimethylsulfoxide (DMSO, pk a =12.6 for aeti aid and pk a =10.5 for ammonium ion) [2]. The effet is even greater in aetonitrile (AN, pk a =22.3 for aeti aid and pk a =16.5 for ammonium ion) [2]. * Corresponding author. Tel.: : fax: address: hasukim@plaza.snu.a.kr (H. Kim). 1 Present address: Arthur Amos Noyes Laboratory , Division of Chemistry and Chemial Engineering, California Institute of Tehnology, CA 91125, USA. In order to determine the aidity of a protogeni speies in nonaqueous media, several methods have been suggested and used to date. Among them, potentiometri titration, ondutometry and spetrophotometry have been widely aepted and most of the dissoiation onstants (K a ) of aids in the literature were obtained by these methods. Even though potentiometri titration is the most ubiquitous method up to now, it needs repeated measurements and exeptionally prudent handling beause potentiometri signals from glass eletrodes in nonaqueous media are not as reliable as in water, mainly due to high liquid juntion potentials. Besides troublesome proedures inluding standardization and alibration steps, the potentiometri approah annot be extended to high pk a regions, as pointed out by Bordwell et al. [3]. In addition, this method requires a relatively large amount of sample and thereby has not been onsidered as a favorable way to determine the pk a value of newly synthesized ompounds or natural produts. On the other hand, of the three methods spetrophotometry is the most preferred espeially with a small amount of sample. But this /01/$ - see front matter 2001 Elsevier Siene B.V. All rights reserved. PII: S (00)

2 210 H.-s. Kim et al. / Journal of Eletroanalytial Chemistry 498 (2001) method needs hromogeni indiators, whose absorption spetra are sensitive to the proton onentration and requires the prior knowledge of pk a values in a given solvent [4,5]. Moreover, this method may suffer from serious error if the aid exhibits onsiderable hromogeni properties. Condutometry is onsidered inferior to the two methods mentioned above in terms of auray. In addition to the onventional methods, voltammetry an be employed for the same purpose. Voltammetry has valuable advantages over other methods, suh as being relatively simple and onvenient, requiring only a small amount of sample for aurate measurement and overing a wide pk a range. With regard to the voltammetri determination of pk a in nonaqueous solvents, Sawyer and oworkers utilized the half-peak potential of proton redution to determine the effetive aidity of Br nsted aids [6]. Also Vianello and oworkers showed that the eletrohemial redution of NH-proti -bromo amide, whih was used as an additive, an be utilized to determine the pk a of weak aids in dimethylformamide (DMF) [7]. These two reports demonstrated that voltammetry is a useful tehnique to determine pk a values of weak aids and also suggested that the aidity of an intermediate with a short life time an be measured by voltammetry. However ompliated alulations and/or simulations for the hemial reations involved in the eletrohemial proess have limited its appliation. In order to overome the limitation, it is neessary to use a stable additive, whose eletrohemial behavior is simple and well established. In addition, the additive must have at least one voltammetri parameter that is sensitive to the aidity and an be measured easily. A polarographi method to determine the onentration of aids in aqueous medium through the eletrohemial redution of quinone was suggested for the first time in 1963 [8]. Based on this fundamental idea, Takamura and oworkers reported a linear relationship between E 1/2 of methyl-p-benzoquinone and pk a of several arboxyli aids and phenol derivatives in 2- methoxyethanol as well as in aqueous solution [9 11]. Similar work has been reported in DMF by Demange- Guerin [12] and by Kheifets et al. [13,14]. In these studies, only a few aids were tested to demonstrate that quinone an serve as an additive to quantify the mixed aids seletively. Also pk a values of various weak aids were estimated on the basis of the half-wave potential (E 1/2 ) of the new waves by d polarography. In terms of onveniene in the measurement, E 1/2 in d polarography, however, is not as good as the peak potential (E p ) in linear sweep voltammetry. In addition, it should be noted that solid eletrodes are more pratial to use in proton-defiient nonaqueous media ompared to merury as the working eletrode material. Reently, it was reported that quinone derivatives were suessfully employed to quantify the organi aids or amines and to determine the enzymati ativities by amperometry [9,15]. In spite of the well designed study, no attempt was made to determine pk a in nonaqueous media. In the present report, systemati evaluation of the voltammetri behavior of quinone is arried out in the presene of various proton-donors suh as protonated amines, arboxyli aids and phenol derivatives in aetonitrile, dimethylsulfoxide and methylene hloride (MC), whih are different in polarity and basiity. In addition, the feasibility of its pratial appliation to determine pk a and the solvent effet are desribed. 2. Experimental Eletrohemial experiments were performed with a Windows-driven BAS100B/W eletrohemial analyzer (Bioanalytial Systems, West Lafayette, IN) using a onventional three-eletrode ell, with a glassy arbon working eletrode (area; m 2 ), platinum wire ounter eletrode and Ag Ag M AgNO 3 referene eletrode, separated from the solution by a Vyor plug. The surfae of the working eletrode was polished with 0.3 m alumina (Buehler, Lake Bluff, MN), then rinsed with deionized water and washed arefully with the solvent to be used. The supporting eletrolyte was 0.1 M tetrabutylammonium perhlorate (TBAP). The onentration of 1,4-benzoquinone (BQ) was 0.50 mm and an appropriate amount of aid was added with a miro-syringe. All experiments were arried out under a nitrogen atmosphere and the solution temperature was maintained at C by a thermostat (model B, Lauda-Königshofen, Germany). HPLC grade AN, DMF and MC from Fisher and DMSO for spetrosopy from Merk were used as solvents. BQ was purhased from Junsei Chemial. n-butylammonium perhlorate and tri-n-butylammonium perhlorate were synthesized from the orresponding amines and perhlori aid. Other reagents were obtained from Aldrih. Hygrosopi reagents were dried for 48 h under redued pressure at 20 C. 3. Results and disussion 3.1. New redution peak aused by Br nsted aids In the proton-defiient environment, two pairs of reversible redox waves are generally observed by yli voltammetry of BQ [16,17]. These two redution peaks are due to the eletrohemial redution produing a monoanion radial (BQ ) and a dianion (BQ 2 ), respetively. The voltammetri behavior of BQ hanges signifiantly when a Br nsted aid is added to the

3 H.-s. Kim et al. / Journal of Eletroanalytial Chemistry 498 (2001) solution. The redox behavior of BQ strongly depends on the strength of the Br nsted aid. Conerning the effet of the Br nsted aid, Gupta and Linshitz showed that weak hydrogen-bonding agents or weak Br nsted aids suh as alohols (pk a 9.3 in aqueous media) interat with neither BQ nor BQ while BQ 2 assoiates with the aids by hydrogen-bonding [18]. Aording to this report, the seond redution peak produing BQ 2 shifted to the positive diretion and the potential value varied with the onentration of the agents, whereas the first redution peak produing BQ remained intat. The report also demonstrated that the presene of aids with medium or strong aidity ommonly gave rise to a new peak prior to the first redution peak but the mehanism was different, ECE (e H + e ) and prior protonation, respetively. The aidity of the Br nsted aids as well as the basiity of the quinones influened the speies responsible for the new peak. Table 1 lists the pk a values of organi aids that have been tested in the present study. Exept for some protonated alkylamines, pk a values of all the aids are less than 9 in aqueous media. Although protonated alkylamines in Table 1 are weaker aids than arboxyli aids and phenol derivatives in aqueous media, they are omparable or stronger aids in AN and DMSO. Thus, the aids in Table 1 are regarded as intermediate Br nsted aids. In the ase of strong Br nsted aids, it is expeted that both the first and seond redution peaks will be affeted signifiantly and a new redution peak will appear at a more positive potential than that of the original peak. Fig. 1 shows the eletrohemial behavior of BQ in the absene and presene of one equivalent of n-butylammonium ion in AN. A new redution peak appears at a more positive potential, around 0.45 V, than the peak potential due to the first redution of BQ alone. All of the aids in Table 1 exhibit analogous voltam- Table 1 pka and voltammetri parameters of BQ in the presene of Br nsted aids Aids Water DMSO AN MC pk a pk a a E p /mv b pk a a E p /mv b E p /mv b Sulfanili aid d 348 Aniline hydrohloride Pyridine hydrohloride Ammonium perhlorate n-butylammonium perhlorate Tri-n-butylammonium perhlorate Piperidine hydrohloride Guanidine hydrohloride d 24 Trihloroaeti aid d d Dihloroaeti aid ,4-Dinitrobenzoi aid ,6-Dihlorobenzoi aid d Nitrobenzoi aid ,6-Dihydroxybenzoi aid Saliyli aid Nitrobenzoi aid Naphthoi aid d d Benzoi aid Aeti aid Piri aid e ,4-Dinitronaphthol d ,6-Dinitrophenol ,4-Dinitrophenol Nitro-3-CF 3 -phenol Nitrophenol ,3-Dihydroxy naphthalene d Nitrophenol Pyrogallol d 74 a From the literature [19]. b E p =E p2 E p1, where E p1 is the peak potential of the original peak due to the first redution of 0.50 mm BQ and E p2 is the peak potential of the new redution peak of BQ in the presene of 0.50 mm aid. Could not be determined beause of low solubility. d Measured in this work using E p values. e Unusually small E p value and has not been inluded in Figs. 3 and 5. See the text.

4 212 H.-s. Kim et al. / Journal of Eletroanalytial Chemistry 498 (2001) Fig. 1. Cyli voltammograms of 0.50 mm BQ in the absene (solid line) and presene (dotted line) of n-butylammonium perhlorate in AN. San rate; 50 mv s 1, working eletrode; glassy arbon. In CV s, please note that athodi urrents are plotted positive. Case 2: BQ assoiates with the aid by hydrogenbonding and the redution peak is attributed to the redution of aid-bq addut. Case 3: BQ takes a proton from the aid and thus the seond eletron transfer beomes failitated. In this ase, an ECE proess results where E and C refer to the eletron transfer and hemial reation, respetively. Sine BQH + (pk a = 7) is suh a strong aid that BQ is diffiult to protonate in aqueous solution, Case 1 is not reasonable. On the other hand, aids with strong hydrogen-bonding power ould be ombined with BQ by hydrogen-bonding. If hydrogen-bonding between BQ and aid ours, the redox wave should gradually shift to the positive diretion without altering the symmetri shape as the onentration of aid inreases [18]. But the new redox waves in Fig. 1 are totally asymmetri. Moreover, Fig. 2 shows that the new peak grows with a little shift of potential as the onentration of aid inreases. As a onsequene, neither Case 1 nor 2 is responsible for the voltammetri behavior of Figs. 1 and 2. The eletrohemial behavior of BQ in the presene of aids presented in this study is very similar to that observed from the trifluoro aeti aid tetrahloro-1,4- benzoquinone system, whih was reported by Gupta and Linshitz [18]. An ECE (e H + e ) mehanism suggested in the report rationalizes the effet of the aids well. The harateristi E p depending on the pk a of the aids in Table 1 is onsistent with this interpretation. Therefore, the new redution peak in the presene of the aids investigated in this study must be attributed to the ECE proess. The voltammetri responses in DMSO and MC are very similar to those obtained in AN. E p s are smaller in DMSO and larger in MC than in AN for the same aid. Sine DMSO is likely to interfere as a base and ompete with quinone, the effet of protonation is not as great as that observed in AN and vie versa in MC. In the ase of MC, its inertness and low dieletri onstant are believed to failitate the proton transfer substantially following the first eletron transfer. Fig. 2. Cyli voltammograms of 0.50 mm BQ in the absene (solid line) and presene of saliyli aid in DMSO. Conentration of saliyli aid is 0.50 (dotted line) and 1.0 mm (dashed line). Other experimental onditions are the same as those in Fig. 1. metri behavior to Fig. 1 but the individual peak potentials of the new redution peak are different. Similar behavior is observed in DMSO and MC. In order to understand the nature of the new redution peaks, three possible proesses are examined. Case 1: BQ beomes protonated ahead of redution and thus the redution peak is assigned to the redution of BQH Effet of the funtional group and sol ent Sine the redution mehanism of BQ involves a proton, the magnitude of E p, the potential differene between the new peak and the original peak due to the first redution of BQ, is suggested as a voltammetri parameter to estimate pk a values in aproti media. Although the redox behavior of BQ in the presene of aid is not ompletely understood, it turns out to be possible to use the first redution proess to simplify the situation for the pratial appliation proposed in this study.

5 H.-s. Kim et al. / Journal of Eletroanalytial Chemistry 498 (2001) Fig. 3 shows the plots of E p against pk a obtained in aqueous media in three different aproti solvents. As long as the aids have the same funtional group, there is a linear relationship between E p in a given solvent and the pk a obtained in aqueous media. But aids with different groups exhibit different slopes. The E p are in the order of protonated amines phenols arboxyli aids. In partiular, the E p due to protonated amines in aproti media is abnormally large. This is not surprising, onsidering that protonated amines are originally Fig. 4. Relationship between E p and pk a in AN. All onditions and notations are the same as those in Fig. 3. monovalent ations, whereas arboxyli aids and phenol-type aids are neutral speies. As the dieletri onstant of the solvent dereases, harged speies beomes unstable whereas unharged ones are stabilized. On the other hand, a different linear relationship between E p s and pk a for phenol derivatives ompared with that for arboxyli aids appears to be due to the stability of anions. In the ase of neutral aids suh as phenol derivatives and arboxyli aids, the stability of onjugate anioni bases is the key fator determining the proton-donating power of the aid beause the solvation of anions needs more energy in AN, DMSO and MC than in aqueous media [20]. A negative harge on the phenolate ring is more effetively deloalized than that on the arboxylate group and is preferred in less polar media. Similar results are also obtained in other aproti solvents suh as DMSO and MC. The E p values due to the same aid in different solvents are different in the order of MC AN DMSO as demonstrated in Fig. 3. When the solvent is inert and its basiity is extremely small, the proton-donating power of the aids is amplified and, thus, is learly refleted by the magnitude of E p. Sine it is more diffiult to employ onventional methods in less polar solvents, this voltammetri tehnique seems quite useful espeially when the relative proton-donating powers of various aids are ompared and assessed in inert solvents Relationship between E p and pk a Fig. 3. Effet of pk a on E p in AN (a), DMSO (b) and MC (). E p =E p2 E p1, where E p1 and E p2 represent the peak potential due to the first redution of BQ in the absene and presene of aid, respetively. Cirles, retangles and triangles denote protonated amines, arboxyli aids and phenol derivatives, respetively. All pk a values obtained in water were from the literature [18]. When E p data are plotted against pk a in orresponding aproti media, good straight lines with orrelation oeffiients (R 2 ) of 0.99 for AN and 0.97 for DMSO are observed irrespetive of the funtional groups of the aids as shown respetively, in Figs. 4 and 5. The slopes are

6 214 H.-s. Kim et al. / Journal of Eletroanalytial Chemistry 498 (2001) and V per pk a unit in AN and DMSO, respetively. Assuming that E p should be at least 50 mv for two adjaent peaks onsidered to be well separated, Figs. 4 and 5 indiate that pk a values less than 24 in AN and 12 in DMSO an be determined by this method. Based on the proposed ECE mehanism, the eletrohemial and hemial proesses in the presene of aid an be depited as the following sheme: In order to find out the origin of the linear relationship between E p and pk a, a rigorous mathematial treatment with two Nernst equations, two aid dissoiation onstants (aid, K a and protonated BQ anion radial, K aq ) and ompliated quantitative expressions, has to be arried out. It seems unneessary, however, to explore further in detail onsidering the fat that omplex hemistry is involved around the eletrode in an aproti solvent. Instead, a simple derivation with two Nernst equations, aid dissoiation onstants and assuming E o E p and [BQH ]=[A ] when the initial onentrations of HA and BQ are the same provides an interesting expression. E=E 1 + RT F ln [BQ] [BQ (1) ] E=E 2 + RT F ln [BQH ] (2) [BQH ] Fig. 5. Relationship between E p and pk a in DMSO. All onditions and notations are the same as those in Fig. 3. K aq = [BQ ][H+ ] [BQH ] K a = [A ][H + ] [HA] Adding Eqs. (1) and (2), and replaing with two aid dissoiation onstants gives E= E 2+E RT 2 2F (pk a Q pk a )+ RT 2F ln [BQ] 2 [BQH ] 2 (3) Then the potential of the new peak in the presene of aid when [BQ]=[BQH ] (formal potential) an be written as: E new = E 2+E RT(pK a Q pk a ) (4) 2 2F while the formal potential in the absene of aid is E original =E 1 (5) Therefore, the differene in formal potentials whih is related to the potential differene between two peaks an be expressed as the following. E =E new E original = E 2 E RT 2 2F (pk a Q pk a ) (6) Eq. (6) indiates that the differenes in the formal potentials, E o, are diretly dependent on the dissoiation equilibria between BQH and aids. It should be noted that serious error might be involved in the expression beause of over simplified assumptions, for instane, [BQH ] must require more ompliated terms by onsidering all hemial speies involved in the system inluding prior assoiation between BQ and the aid through hydrogen-bonding [18] espeially in less polar solvents. Also, an additional error must be present by taking E E p beause the eletrohemial behavior is not reversible. Nevertheless, good straight lines in Figs. 4 and 5 indiate that there is a linear relationship between E p and pk a with a theoretial slope of V per deade (at 25 C) whih is very lose to the experimentally obtained slope of V per deade in DMSO. It is reasonable to observe an even larger slope in AN ( V per deade) beause of a strong tendeny for assoiation in AN. This omparison onfirms indiretly that the interpretation based on an ECE proess is reasonable to aept. Suh a high sensitivity enables us to utilize the voltammetri method to determine pk a in DMSO and AN with good reproduibility of about 0.1 units of pk a. E p data in Table 1 are used to determine the pk a of several aids inluding trihloroaeti aid and 1-naphthoi aid whih have not been reported in DMSO and AN so far. They are listed in Table 1. Consequently, E p measured by voltammetry is a good parameter to evaluate pk a in aproti solvent in terms of auray, onveniene and the amount of sample that needs to be

7 H.-s. Kim et al. / Journal of Eletroanalytial Chemistry 498 (2001) prepared. In addition, this method is useful in a variety of aproti solvents. 4. Conlusions The voltammetri response of 1,4-benzoquinone in the presene of Br nsted aids with various aid funtional groups was studied in aetonitrile, dimethylsulfoxide and methylene hloride. The magnitude of the potential shift aused by the presene of the aid showed a good linear relationship with the aid dissoiation onstants, pk a, in the orresponding media. The pk a value of organi aids in aproti solvents, therefore, an be determined based on the voltammetri behavior of quinone, whih is very sensitive to the aidity of proton soures. The method proposed in this study is quite useful to assess the relative proton-donating power in non-polar aproti media, espeially when no onventional method an be used to measure the K a. The pk a values of trihloroati aid and 1-naphthoi aid were obtained for the first time by the proposed method. Aknowledgements This work was supported finanially by KOSEF through a grant, and by the Center for Moleular Catalysis, Seoul National University. The Korea Researh Foundation is gratefully aknowledged for a grant in the domesti postdotoral program (T.D.C.). Referenes [1] R.G. Bates, Determination of ph: Theory and Pratie, Wiley, New York, [2] I.M. Kolthoff, M.K. Chantooni, S. Bhowmik, J. Am. Chem. So. 90 (1968) 23. [3] W.S. Matthews, J.E. Bares, J.E. Bartmess, F.C. Bordwell, F.J. Cornforth, G.E. Druker, Z. Margolin, R.J. MCallum, G.J. MCallum, N.R. Varnier, J. Am. Chem. So. 97 (1975) [4] I.M. Kolthoff, M.K. Chantooni, S. Bhowmik, Anal. Chem. 39 (1967) 315. [5] T. Higuhi, C. Rehm, C. Barnstein, Anal. Chem. 28 (1956) [6] J.W.C. Barrette, J.H.W. Johnson, D.T. Sawyer, Anal. Chem. 56 (1984) [7] F. Maran, D. Celadon, M.G. Severin, E. Vianello, J. Am. Chem. So. 113 (1991) [8] J.C. Abbott, J.W. Collatt, Anal. Chem. 35 (1963) 859. [9] K. Takamura, T. Fuse, K. Arai, F. Kusu, J. Eletroanal. Chem. 468 (1999) 53. [10] K. Takamura, Y. Hayakawa, Anal. Chim. Ata 43 (1968) 273. [11] K. Takamura, Y. Hayakawa, J. Eletroanal. Chem. 31 (1971) 225. [12] G. Demange-Guerin, Compt. Rend. 266 (1968) 784. [13] L.Y. Kheifets, V.D. Bezuglyi, Zh. Obshh. Khim. 41 (1971) 514. [14] L.Y. Kheifets, V.D. Bezuglyi, L.I. Dimitrievskaya, Zh. Obshh. Khim. 41 (1971) 742. [15] M.J. Rose, S.M. Lunte, R.G. Carlson, J.F. Stobaugh, Anal. Chem. 71 (1999) [16] D.H. Evans, in: A.J. Bard, H. Lunte (Eds.), Enylopedia of Eletroanalytial Chemistry, vol. XII, Dekker, New York, 1978 Chapter I. [17] M.E. Peover, in: A.J. Bard (Ed.), Eletroanalytial Chemistry, vol. II, Dekker, New York, 1967 Chapter I. [18] N. Gupta, H. Linshitz, J. Am. Chem. So. 119 (1997) [19] K. Izutsu, Aid-Base Dissoiation Constants in Dipolar Aproti Solvents, Blakwell Sientifi Publiations, Oxford, [20] F.G. Bordwell, A. Chem. Res. 21 (1988) 456..