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Langmuir 1998, 14, 5098-5105
Effects of Monovalent and Divalent Anionic Dodecyl Sulfate Surfactants on the Dediazoniation of 2-, 3-, and 4-Methylbenzenediazonium Tetrafluoroborate† Carlos Bravo-Diaz,*,‡ Mercedes Soengas-Fernandez,‡ M. Jose Rodriguez-Sarabia,‡ and Elisa Gonzalez-Romero§ Departamento Quimica Fisica y Quimica Organica and Departamento Quimica Analitica y Alimentaria, Facultad de Ciencias, Universidad de Vigo, 36200 Vigo-Pontevedra, Spain Received July 21, 1997. In Final Form: June 10, 1998 We have examined the kinetics and mechanism of dediazoniation of 2-, 3- and 4-methylbenzenediazonium tetrafluoroborate (OMBD, MMBD, and PMBD) in aqueous micellar solutions of sodium dodecyl sulfate and copper dodecyl sulfate at two different NaCl concentrations by combining spectrophotometric and high-performance liquid chromatography (HPLC) measurements. The method allows simultaneous determination of product yields and rates of formation for all dediazoniation products and, indirectly, the rate of decomposition of the diazonium salt. The preferential location of OMBD, MMBD, and PMBD is in the Stern layer, a very anisotropic region. A substantial fraction of diazonium ions are bonded to the micellar aggregates, and formation of strong ion pairs between OMBD, MMBD, and PMBD diazonium salts with the sulfate headgroup of the surfactants is not significant. HPLC analysis of dediazoniation products indicates that, within experimental error, quantitative conversion to products is achieved, with cresol as the major product. Observed rate constants for the dediazoniation reaction in the presence of both monovalent and divalent surfactants are constant for OMBD but decrease slightly for MMBD and PMBD with increasing surfactant concentration. The data are interpreted in terms of the polarity of the environment of diazonium salts in the micellar aggregate. All evidence is consistent with an Dn + An mechanism, i.e., rate-determining formation of an aryl cation that reacts immediately with any available nucleophile.
Introduction Considerable attention has been paid for more than 1 century to diazonium salts.1-3 Their reactions have been widely used in the dyestuff industry,4 as synthetic precursors,2 and as intermediates in many synthetic routes.5 They have been studied in a wide variety of solvents,1-3 water, methanol, tetrahydrofuran, dimethylformamide, trifluoroethanol, dimethyl sulfoxide, hexafluoro-2-propanol, etc., but the bulk of the available kinetic data refers to the arenediazonium ions, probably reflecting their stability and the way they can be handled in relation to their aliphatic analogues.6 Arenedediazoniation chemistry has been subject to great controversy1 because their rich chemistry varies dramatically with experimental conditions,1,2,7,8 changing not only the mechanism of the reactions but also the relative amount of dediazoniation products, and because the many reaction pathways are still not completely understood.1,3 The field is still very * Author to whom correspondence should be addressed. E-mail:
[email protected]. Phone: +986-812303. Fax: +986-812382. † In memory of Professor M. E. Pen ˜ a-Sangil. Thank you, Elena. ‡ Departamento Quimica Fisica y Quimica Organica. § Departamento Quimica Analitica y Alimentaria. (1) Zollinger, H. Diazo Chemistry I, Aromatic and Heteroaromatic Compounds; VCH: Cambridge, 1994. (2) Saunders, K. H.; Allen, R. L. M. Aromatic Diazo Compounds, 3rd ed.; Edward Arnold:, 1985. (3) Hegarty, A. F. Kinetics and Mechanisms of Reactions Involving Diazonium and Diazo Groups. In The Chemistry of Diazonium and Diazo Compounds; Patai, S., Ed.; J. Wiley & Sons: New York, 1978. (4) Zollinger, H. Color Chemistry; VCH: Cambridge, 1991. (5) Wulfman, D. S. Synthetic Applications of Diazonium Ions. In The Chemistry of Diazonium and Diazo Compounds; Patai, S.,Ed.; J. Wiley & Sons: NewYork, 1978. (6) Schank, K. Preparation of Diazonium Groups. In The Chemistry of Diazonium and Diazo Compounds; Patai, S., Ed.; J. Wiley & Sons: New York, 1978.
active. They are currently being used as aryl radical sources for synthetic precursors,1,7,9 and their role in DNA arylations10,11 and DNA deaminations12 is being explored. Scant attention has been given to their reactions in micellar systems.1 Micelles are dynamic aggregates of amphiphilic molecules13,14 that are composed of nonpolar hydrocarbon tails attached to polar, nonionic, zwitterionic, or ionic headgroups.14 Surfactant solutions may spontaneously form aggregates creating a highly anisotropic interfacial region that lines the boundary formed by the highly polar aqueous and nonpolar oil regions, imparting new chemical and physical properties to the system.13,14 Micelles, as well as other association colloids, can act as microreactors concentrating, separating, or diluting reactants, and thereby they may have dramatic effects on chemical reactivity.14 The first kinetic studies involving diazonium salts and micellar systems are those by Poindexter and Mackay15 (7) Zollinger, H. Dediazoniations of Arenediazonium Ions and Related Compounds. In The Chemistry of Triple Bonded Functional Groups; Patai, S., Rappoport, Z., Eds.; J. Wiley & Sons: New York, 1983. (8) Sykes, P. A. Guidebook to Mechanism in Organic Chemistry; 6th ed.; Longman Science and Technical: Essex, 1986. (9) Lloris, M. E.; Abramovitch, R. A.; Marquet, J.; Moreno-Manas, M. Tetrahedron 1992, 48, 6909. (10) Arya, D. P.; Warner, P. M.; Jebaratnam, D. J. Tetrahedron Lett. 1993, 34, 7823. (11) Berth, J. P. J. Chem. Soc., Chem. Commun. 1989. (12) Tannenbaun, S. R.; Tamir, S.; Rojas-Walker, T. D.; Wishnok, J. S. Nitrosamines and Related Compounds-Chemistry and Biochemistry; ACS Symposium Series 533; Leoppky, R. N., Michedja, C. J., Eds.; American Chemical Society: Washington, DC, 1994. (13) Fendler, J. H.; Fendler, E. F. Catalysis in Micellar and Macromolecular Systems; Academic Press: New York, 1975. (14) Bunton, C. A.; Nome, F.; Quina, F. H.; Romsted, L. S. Acc. Chem. Res. 1991, 24, 357. (15) Poindexter, M.; Mckay, B. J. Org. Chem. 1972, 37, 1674-1676.
S0743-7463(97)00814-7 CCC: $15.00 © 1998 American Chemical Society Published on Web 08/08/1998
Surfactant Effect on Dediazoniation Scheme 1. (a) Heterolytic Mechanism for Dediazoniations and (b) Heterolytic Dediazoniation Mechanism in the Presence of SDS and Cu(SD)2 Micellar Aggregates
a
Langmuir, Vol. 14, No. 18, 1998 5099
dediazoniation (see below), so that it provides reliable estimates of the quantity of unreacted arenediazonium ion and the formation of a stable azo dye prevents unwanted side reactions of unreacted arenediazonium ion. Experimental Section
b
who showed that rate constants for the coupling of p-nitrobenzenediazonium with 2-naphthol-6-sulfonic acid (2N6S) are depressed by the presence of anionic, cationic, or nonionic surfactants. Later, Moss16 explained this observation by simple electrostatic arguments: Cationic micelles solubilize the anionic coupling agent but exclude the cationic arenediazonium salt, while anionic micelles solubilize the arenediazonium salt but exclude the coupling agent. However, if the coupling component is not ionic, more dramatic effects occur.17,18 Tentorio18 used 1-naphthylamine as the coupling agent in the presence of SDS micelles, finding that the rate of its coupling reaction with cationic arenediazonium ions increased up to 1100 times when the surfactant concentration was higher than its critical micelle concentration (cmc). Romsted et al.19,20 are currently using arenediazonium salts to probe interfacial compositions, the degree of ionization, and counterion selectivity of cationic micelles and microemulsions. Their work is based on that, to date, all kinetic data available about decomposition of arenediazonium salts in aqueous acid, in the dark, suggest that the reaction goes through an heterolytic mechanism (Scheme 1): Arenediazonium salts decompose by ratedetermining formation of a very reactive aryl cation that reacts competitively with any nucleophile available to form stable products that can be determined quantitatively (the arrow bonding Ar to N2 in Scheme 1A indicates a dative bonding model that represents the actual electron density distribution21 more closely). Here we study the effect of two anionic micelles, the monovalent sodium dodecyl sulfate (SDS) and the divalent copper dodecyl sulfate (Cu(SD)2), on the kinetics and product yields of the dediazoniation of three methylsubstituted diazonium salts, 2-, 3-, and 4-methylbenzenediazonium tetrafluorborate (OMBD, MMBD, and PMBD), by quenching the reaction at selected times with a suitable coupling agent like 1-naphthylamine (1NA). Recently we have reported a new method for monitoring dediazoniations in aqueous systems by combining coupling reactions with high-performance liquid chromatography (HPLC) analysis.22 This coupling step must be faster than (16) Moss, R. A.; Rav-Acha, C. J. Chem. Soc. 1980, 102, 5045-5047. (17) Hashida, Y.; Matsumura, K.; Ohmori, Y.; Matsui, K. Nippon Kagaku Kaishi 1761; Chem. Abstr. 1980, 1979, 92. (18) Tentorio, A.; Gatti, B.; Carlini, F. M. Dyes Pigm. 1985, 6, 197114. (19) Chauduri, A.; Loughlin, J. A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8351-8361. (20) Chauduri, A.; Romsted, L. S.; Yao, J. J. Am. Chem. Soc. 1993, 115, 8362-8367. (21) Glasser, R.; Horan, C. J. J. Org. Chem. 1995, 60, 7518-7528.
Instrumentation. UV-vis spectra and some kinetic experiments were followed on a Beckman DU-640 UV-vis spectrophotometer equipped with a thermostated cell carrier attached to a computer for data storage. Product analysis was carried out on a Waters HPLC system which included a model 560 pump, a model 717 automatic injector, a model 486 vis-UV detector, and a computer for data storage. Products were separated on a Microsorb-MV C-18 (Rainin) reverse phase column (25 cm length, 4.6 mm internal diameter, and 5 µm particle size) using a mobile phase of 65/35 v/v MeOH/H2O containing 10-4 M HCl. The injection volume was 25 µL in all runs, and the UV detector was set at 210 nm (PMBD) and 220 nm (OMBD and MMBD). pH and conductivity were measured using a previously calibrated Metrohm model 713 pH-meter and a Metrohm model 712 conductometer both equipped with temperature sensors. 1H NMR spectra were obtained on a Brucker ARX 400 spectrometer. Materials. Reagents were of maximum purity available and were used without further purification. Cresols, ArOH, chlorotoluenes, ArCl, copper(II) chloride (99.999%), the surfactant SDS (99.9%), and the reagents used in the preparation of diazonium salts (as tetrafluoroborates) were purchased from Aldrich. 2-Naphthol-6-sulfonic acid, sodium salt (2N6S), was purchased from Pfaltz & Bauer. CuSO4‚5H2O was supplied by Fluka and 1-naphthylamine (1NA) by Merck. Other materials employed were from Riedel de Haen. All solutions were prepared by using Milli-Q grade water. Diazonium salts were prepared under nonaqueous conditions23 and were stored in the dark at low temperature to minimize their decomposition. The copper dodecyl sulfate surfactant, Cu(SD)2‚4H2O, was prepared by ion exchange using an acidic Dowex 50WX8 resin activated with concentrated CuSO4 solutions. Once activated, a warm (T ) 30-35 °C, ca. 0.5 M) solution of SDS was added slowly and the eluted solution collected in a flask (the eluted solution showed a blue color, indicative of the presence of Cu2+ ions). This blue solution was selectively precipitated by cooling it down to ca. T ) 21 °C, and a blue crystalline precipitate was obtained (Krafft point for Cu(SD)2 was reported as T ) 23 °C24 or T ) 24 °C25 and that for SDS below T ) 10 °C26 or T ) 16 °C27 or T). The precipitate was filtered under vacuum and washed several times with cool water. cmc for Cu(SD)2‚4H2O was determined at room temperature by a conductometric method yielding cmc ) 1.03 × 10-3 M, in agreement with literature values of cmc ) 1.17 × 10-3 M25 and cmc ) 1.0 × 10-3 M.28 Methods. Kinetic data were obtained both spectrophotometrically and chromatographically. Observed rate constants were obtained by fitting the absorbance-time or percent yieldtime data to the integrated first order eq 1 using a nonlinear least-squares method provided by a commercial computer program, where M is the measured magnitude of the UV-vis absorbance or percent yields (The word “yield” will be used to represent percent yield here and throughout the text, except in figures and equations where we will employ the symbol “Y”).
ln(Mt - M∞) ) ln(M0 - M∞) - k0t
(1)
All runs were done at T ) 60 ( 0.1 °C (PMBD) and at 35 °C (MMBD and OMBD) with diazonium salts as the limiting reagents. (22) Garcia-Meijide, M. C.; Bravo-Diaz, .; Romsted, L. S. Int. J. Chem. Kinet. 1998, 30, 31-39. (23) Doyle, M. P.; Bryker, W. J. J. Org. Chem. 1979, 44, 1572-1574. (24) Miyamoto, S. Bull. Chem. Soc. Jpn. 1960, 33, 371-375. (25) Moroi, Y.; Ikeda, N., Matuura, R. J. Colloid Interface Sci. 1984, 101, 285. (26) Shinoda, K. J. Phys. Chem. 1981, 85, 3311. (27) Armstrong, D. W.; Nome, F. Anal. Chem. 1981, 53, 1662-1666. (28) Miyamoto, S. Bull. Chem. Soc. Jpn. 1960, 33, 375-379.
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Table 1. Values of Parameters a and b (eq 2) and Their Typical Retention Times for the Dediazoniation Products of OMBD, MMBD, and PMBD analyte
tr/min
10-4a
10-9b
OMB-OH OMB-Cl MMB-OH MMB-Cl PMB-OH PMB-Cl
6.1 27.5 5.9 28.0 6.2 27.7
4.19 4.88 -1.58 -6.09 -0.83 2.70
8.11 8.07 8.62 8.90 11.10 20.00
CR Cβ Cγ
Spectrophotometric kinetic data were obtained by following the disappearance of diazonium salt at an appropriate wavelength to minimize interference mainly by chlorocuprate(II) complexes. Beer’s law plots for aqueous PMBD, MMBD, and OMBD solutions up to 2.00 × 10-4 M are linear (correlation coefficient ) 0.999) yielding 310 ) (15.0 ( 0.3) × 102 M-1 cm-1 (MMBD) and 320 ) (19.1 ( 0.5) × 102 M-1 cm-1 (OMBD). Values for PMBD are published.22 Stock solutions were prepared dissolving the diazonium salt in aqueous HCl, to minimize diazotate formation,29 to give final concentrations of about 1 × 10-4 M and [HCl] ) 3.6 × 10-3 M. Stock solutions were generally used immediately or within 90 min with storage in an ice bath to minimize decomposition. Preliminary HPLC experiments showed that only two decomposition products are formed: ArOH and ArCl. Calibration curves for converting HPLC peak areas into concentrations were obtained simultaneously for these dediazoniation products, ArOH and ArCl, by employing commercial samples dissolved in solutions of similar composition to those used in the HPLC analysis of dediazoniation products (see below). Table 1 lists the slopes and intercepts obtained by linear least-squares fits to eq 2 for each product and their typical retention times under our chromatographic conditions
Area ) a + b[analyte]
(2)
Percent yields of the dediazoniation products were obtained from the ratio of the dediazoniation product concentration, [analyte], and the initial diazonium salt concentration (estimated by weight), eq 3. To calculate yields we did not consider intercept values.
Y ) 100[analyte]/[PMBD]
Table 2. Changes (in ppm) in the 1H Chemical Shifts of SDS (∼0.1 M) upon Addition of OMBD, MMBD, and PMBD to a Concentration of ca. 0.001 M
(3)
Chromatographic kinetic data for all dediazoniation products were obtained following a published procedure by22 quenching the dediazoniation reaction at convenient times with an aliquot of a stock quenching solution prepared by dissolving 1-naphthylamine (1NA) in a 0.02 M aqueous SDS solution containing TRIS buffer ([TRIS] ) 0.05 M) to give, after mixing, final 1NA concentrations about 20-fold excess over that of arenediazonium salts and final pH about pH ) 7. This pH was chosen to maximize the rate of azo dye formation1,29,30 because coupling rates change dramatically with pH because free amines are much more reactive than their protonated forms,8,31 but as pH increases, the competing reaction of arenediazonium ions with OH- to form diazotates becomes significant.29 The use of a coupling reaction to stop the dediazoniation reaction requires that its rate is faster than the dediazoniation rate. 1NA was chosen as coupling agent because an extra coupling rate enhancement (micellar catalytic effect) can be obtained since 1NA is sparingly soluble in water but SDS micelles can solubilize it.18 Auxiliary experiments done by following azo dye formation spectrophotometrically show that under our experimental conditions the coupling reaction is essentially over in the time of mixing reagents; i.e., the rate of azo dye formation is, at least, 100 times faster than that of dediazoniation. This procedure also allows us to estimate k0 for the decomposition of arenediazonium salts. Sufficient concentrated HCl (29) Zollinger, H.; Wittwer, C. Helv. Chim. Acta 1952, 35, 12091223. (30) Zollinger, H. Helv. Chim. Acta 1953, 34, 1730-1736. (31) Kaminski, R.; Lauk, U.; Skrabal, P.; Zollinger, H. Helv. Chim. Acta 1983, 66, 2002.
OMBD
MMBD
PMBD
0.1025 0.0799 0.0007
0.0974 0.0833 0.0007
0.1041 0.0933 0.0008
was added dropwise to each volumetric flask to give final [H+] ≈ 0.2 M, and the absorbance of each solution was measured at λmax of the corresponding azo dyes: λmax ) 529 nm (OMBD), λmax ) 484 (MMBD), and λmax ) 500 nm (PMBD). The added HCl ensures that only the protonated form of the azo dye is present and that the measured absorbance is directly proportional to the concentration of the azo dye and therefore the concentration of unreacted diazonium salt.
Results (1) Location of Diazonium Salts. Estimation of Their Association Constants and Investigation for Possible Ion-Pair Formation. Proper understanding of the behavior of substrates in micellar phases requires knowledge of their location in the micelle. A number of methods can be used to locate substrates in micellar aggregates.13,14 We have used in this work 1H NMR spectroscopy, which allows aromatic rings to be located via the upfield shift induced by the ring current (ring current effect) in the signals of neighboring surfactant hydrogen atoms, an effect that depends on the distance between the ring and the proton.32,33 This method has already been used to locate other compounds containing benzene rings in micellar aggregates.33 Table 2 shows that the greatest upfield shift is that for the protons on the R and β carbons of the hydrocarbon chain. It is, therefore, in the neighborhood of these carbons where the aromatic ring of the diazonium salt is predominantly located in SDS micelles and, consequently, very close to the micellar surface. Association constants, Ks, of substrates to the micellar aggregates can be estimated by a number of methods.13,14,34 Ks for OMBD was estimated spectrophotometrically by monitoring changes in the absorbance of its UV-vis spectrum due to its incorporation to the micellar aggregate. This method could not be used to determine Ks for PMBD because its spectral shift gives rise to very low absorbance differences (
