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Chlorination of aromatic compounds in micellar media: Regioselectivity
Journal of Colloid and Interface Science 302 (2006) 207–213 www.elsevier.com/locate/jcis
Chlorination of aromatic compounds in micellar media: Regioselectivity B.S. Samant, Y.P. Saraf, S.S. Bhagwat ∗ Chemical Engineering Division, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400019, India Received 4 March 2006; accepted 8 June 2006 Available online 13 June 2006
Abstract Chlorination of phenol and ortho-chlorophenol was studied in micellar media in order to observe the effect on regioselectivity. Hydrogen peroxide/hydrochloric acid–aqueous system, which is environmentally a safer route was employed for chlorination. Selectivity ratio was found to be dependent on the nature and concentration of the surfactant. Ortho/para selectivity ratio up to 12 was realized for the chlorination of phenol. 2,6-/2,4-dichlorophenol ratio up to 1.01 was realized for the chlorination of ortho-chlorophenol. © 2006 Elsevier Inc. All rights reserved. Keywords: Phenol; Ortho-chlorophenol; Chlorination; Micellar catalysis; Regioselectivity
1. Introduction Aromatic substitution reactions generally pose a challenge in terms of separation, owing to selectivity considerations in the product isomer distributions [1,2]. ‘Selectivity Engineering’ for the desired product has been the underlying theme for a considerable amount of research, both for economic reasons, as well as for more facile separations and downstream processing on an industrial scale [3–5]. The monohalogenation of phenol invariably produces a mixture of ortho- and para-isomers, in which the latter predominates Fig. 1. Several techniques have been reported in the literature to improve the regioselectivity in the chlorination of activated aromatic compounds and these include employing solvents [6–8], reagents [9–11] and catalysts [2,3,12,13]. These techniques have some limitations such as the use of strong and non selective chlorinating agents, toxic and expensive reagents, low yields and long reaction times. Micellar aggregates composed of surfactant molecules are the simplest of the aggregates to effect selectivity. It has been ascertained by studies in substitution reactions [14,15] as well * Corresponding author.
E-mail address: [email protected] (S.S. Bhagwat). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.06.007
as by 1 H NMR spectroscopy [16] that a solubilizate such as phenol exists in a preferred average orientation in the domain of the surfactant micelle. Ortho and para substituted phenols can be selectively solubilized by micelles depending on the polarity of the substituent group [17]. It is possible to exploit this orientation to alter the selectivity towards a particular product. The effect of different types of surfactants (i.e. anionic, cationic, non-ionic and zwitterionic) on chlorination of phenol was studied. Onyiriuka and Suckling [18] have reported the selective chlorination of phenol at the ortho position by using tertiary butyl hypochlorite in the presence of a tailored surfactant molecule bearing a tertiary alcohol moiety close to the anionic head group. Chhatre et al. [19] have reported a similar improvement in selectivity in the nitration of phenol by dilute aqueous HNO3 in the presence of sodium di-(2-ethyl hexyl)sulfosuccinate (AOT). The objective of the present work is to study the effect of micellar media on chlorination of phenol and ortho-chlorophenol utilizing a mixture of hydrogen peroxide and hydrochloric acid. This hydrogen peroxide/hydrochloric acid chlorinating system is environmentally safe for chlorination [20]. The reaction between hydrogen peroxide and hydrochloric acid in situ generates positive chlorine species [21,22] in the reaction medium
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Fig. 1. Chlorination of phenol and o-chlorophenol.
which attacks on the substrate. As the rate of chlorine generation is slow, utilization of chlorine by reactant is more predominant with little wastage of chlorine. 2. Materials Phenol, ortho-chlorophenol and hydrochloric acid were obtained from M/s s.d. Fine Chem. Ltd., Mumbai. AR grade hydrogen peroxide was obtained from M/s Merck India Ltd. Electrophoretic grade sodium dodecyl sulphate (SDS), linear alkylbenzene sulphonate (LABS), cetyl trimethylammonium bromide (CTAB) were obtained from M/s Sisco Research Laboratories, Mumbai and lauryl dimethyl benzyl ammonium chloride or benzalkonium chloride (BKC), lauramine oxide (LAO) and cocoamidopropyl betaine (CAPB) were obtained from M/s Galaxy surfactants, Mumbai and their purity was ascertained tensiometrically. All chemicals were used as supplied without any further purification. Distilled water was used for all the reactions. 3. Experimental procedure A two phase mixture was agitated in a 2.5 × 10−4 m3 baffled glass reactor equipped with a six-bladed turbine agitator and 0.6 m i.d. The speed of agitation was maintained at 1.67 Hz. Aqueous phase (4 × 10−5 m3 ) contend of surfactant, hydrochloric acid (5.145 × 10−3 mol l−1 ) and hydrogen peroxide (1.985 × 10−3 mol l−1 ) while the organic phase was pure phenol or ortho-chlorophenol (50 × 10−3 mol l−1 ). Isothermal conditions were maintained at 303 ± 0.5 K. Analysis was performed on gas chromatograph (Chemito 8610) with flame ionization detector. A 4 m long i.d. S.S. column packed with 10% SE-30 on chromosorb WHP was employed for the analysis. Nitrogen was used as carrier gas at a flow rate of 5.0 × 10−7 m3 s−1 .
4. Results and discussion The molecular environment of the solute is altered considerably on its solubilization in the micellar aggregates. As shown in Fig. 2, phenol, when solubilized in the micelle, attains a preferred molecular orientation with the polar-OH group projecting out of the micelle towards the bulk aqueous phase and a relatively non polar aromatic ring penetrated in the hydrophobic core [23]. Hence, it is anticipated that the attack of a strong electrophile such as chlorine at para position of phenol would be suppressed by the non-polar micellar core, while the polar aqueous phase would provide a more favorable locale for the attack of chlorine at the ortho position. Similar effects of orientation on selectivity improvement [24–32] and rate enhancement [33– 40] in micellar and microemulsion media were also reported. 4.1. Chlorination of phenol The stoichiometry of the overall reaction [41] is as given in equation RH + H2 O2 + HCl → RCl + 2H2 O.
Fig. 2. Orientation of phenol in micelle.
(1)
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209
Fig. 3. Effect of HCl on % conversion of phenol.
Table 1 Conversion of phenol Phenol ×103 (mol l−1 )
H 2 O2 ×103 (mol l−1 )
HCl ×103 (mol l−1 )
CCl4 ×103 (mol l−1 )
Surfactant (BKC) ×103 (mol l−1 )
Conversion (mol%) 4h
8h
50 4.17 4.17 50
1.99 1.99 1.99 1.99
5.15 5.15 5.15 5.15
– 6.0 6.0 –
– – 0.5 0.5
35 30 35 40
60 50 50 64
The rate of chlorination at 300 K was found to be dependent on the concentration of phenol and hydrochloric acid. An equimolar proportion of hydrogen peroxide to phenol was found to restrict the chlorination to monochlorination. Fig. 3 indicates that the % conversion of phenol increased with the increasing concentration of hydrochloric acid in the reaction mixture. At all ratios of phenol:hydrochloric acid employed, the selectivity ratio (ortho-chlorophenol/para-chlorophenol = 0.60) remained the same with para chlorophenol as the major reaction product. At 50 × 10−3 mol l−1 phenol and 1.94 × 10−3 mol l−1 hydrochloric acid concentration, the titrimetric analysis of unreacted hydrochloric acid in the reaction mixture, indicated that 15% of the hydrochloric acid reacted with hydrogen peroxide after 8 h to produce an equivalent amount of chlorine. The analysis of organic phase showed that, only 10% of the liberated chlorine was consumed by phenol and thus utilization of chlorine was low. However at 50 × 10−3 mol l−1 phenol and 5.15 × 10−3 mol l−1 hydrochloric acid concentration, 69% of chlorine reacted with ≈ 60% of phenol to produce the corresponding chlorophenols. Hence in the latter case, rate of chlorine generation as well as phenol conversion was faster than that in the first case. The use of carbon tetrachloride as a solvent for phenol results in dilution of phenol in the reaction mixture, but does not affect the rate of reaction significantly. The addition of a surfac-
tant (BKC 0.5 × 10−3 mol l−1 ) leads to a significant increase in the rate of reaction (Table 1). 4.1.1. Effect of nature of the surfactant The attack of chlorine on phenol solubilized in micelle at the para-position was prevented by the non-polar micellar core, while the polar aqueous phase provided a more favorable locale for the attack of chlorine at the ortho-position. Thus, chlorination of phenol solubilized in micelle is expected to be highly ortho selective. The ortho/para selectivity ratio was found to be dependent on the nature of the surfactant employed. SDS (anionic surfactant) was seen to increase the ortho/para selectivity ratio (Fig. 4). Experiments with LABS have shown similar results. Hence, the presence of an aromatic ring in LABS did not have an additional effect on the ortho/para selectivity ratio. In the case of cationic surfactant, BKC was seen to increase ortho/para selectivity ratio (Fig. 5). LAO and CAPB both exibit cationic nature in acidic media. Fig. 6 shows the change in selectivity caused by the surfactants. CAPB contains a total number of 15 carbon atoms in its structure while LAO has a chain of 12 carbon atoms. The hydrophobic chain of CAPB contains an amide linkage within and this alters the polarity in the locale of solubilization of phenol in micelle and hence exhibits a different selectivity in the present reaction than LAO.
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Fig. 4. Chlorination of phenol in micellar solution of sodium dodecyl sulphate.
Fig. 5. Chlorination of phenol in micellar solution of benzalkonium chloride.
The highest selectivity ratio for ortho-chlorophenol at 15% conversion was 12 when the SDS concentration was 8 × 10−3 mol l−1 which decreased to 8 at 60% conversion. This decrease in the selectivity with increase in the conversion is likely to be due to subsequent chlorination of monochlorophenols to di- and tri-chlorophenols. Chlorination in solution of other surfactants such as BKC, LAO and CAPB gave similar decrease in selectivity with increase in conversion as shown in Figs. 5 and 6. 4.1.2. Effect of concentration of surfactant The presence of strong acid decreases the solubility of phenol in the bulk aqueous medium [17]. The large negative
free energy of solubilization of phenol (G = −15.6 kJ/mol) [17] in SDS micelle in the presence of hydrochloric acid shows that phenol is preferentially solubilized in the SDS micelle. The solubility of phenol in micellar solution increased with an increase in the surfactant concentration, resulting in a significant increase in the selectivity ratio. Fig. 4 shows the o/p ratio at various conversion levels of phenol when SDS was used as the surfactant. The presence of hydrochloric acid lowers the critical micelle concentration (CMC) of the surfactant [42]. The presence of strong electrolytes like NaCl, HCl, etc. decrease the CMC of dissolved surfactant by screening the charges of ionic surfactants such as SDS [43] (CMC of SDS is 0.1 × 10−3 mol l−1 at 0.5 mol l−1 of
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Fig. 6. Chlorination of phenol in micellar solution of lauramine oxide and cocoamidopropyl betaine.
Fig. 7. Chlorination of ortho-chlorophenol in solutions of various surfactants.
NaCl) [44]. At SDS concentrations up to 4 × 10−3 mol l−1 , the selectivity ratio remained relatively unchanged at approximately 0.56. This may be due to insufficient number of micelles present in the reaction mixture. An increase in the surfactant concentration favoured chlorination at ortho position. Higher selectivity was obtained at 4–12(×10−3 ) mol l−1 concentration of SDS. The increased rate of reaction can be explained by the higher effective concentration. However, the change in selectivity indicates the role of reaction occurring at micellar interface. At higher concentration of SDS (16 × 10−3 mol l−1 ), an adverse effect on the selectivity was observed.
4.2. Chlorination of ortho-chlorophenol Chlorination of ortho-chlorophenol leads to a mixture of 2,6-dichlorophenol and 2,4-dichlorophenol, as shown in Fig. 1. 2,4-dichlorophenol was found to be the major reaction product (2,6-/2,4-dichlorophenol selectivity ratio = 0.20) in the absence of micelles. This reaction was conducted in micellar environment with a view to increase the selectivity towards 2,6dichlorophenol. 4.2.1. Effect of surfactant concentration and the nature The rate of chlorination of ortho-chlorophenol was less than that of phenol (Table 2). This can be attributed to poor sol-
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Table 2 Conversion of o-chlorophenol o-chlorophenol ×103 (mol l−1 )
H 2 O2 ×103 (mol l−1 )
HCl ×103 (mol l−1 )
Surfactant (BKC) ×103 (mol l−1 )
Conversion (mol%) 4h
8h
50 50
1.99 1.99
5.15 5.15
– 0.5
10 15
18 20
ubility of ortho-chlorophenol in the aqueous acidic medium. Also, the presence of an electron-withdrawing group like chlorine at ortho position, makes ortho-chlorophenol less reactive for chlorination. Mary and Jyothi [45,46] have also found less reactivity of ortho-chlorophenol towards chlorination. The rate of chlorination was marginally higher in the presence of micelles (Table 2). The 2,6-/2,4-dichlorophenol selectivity ratio was found to be dependent on the nature and concentration of the surfactant employed. Fig. 7 gives the change in 2,6-/2,4-dichlorophenol selectivity ratio at varying concentration of different types of surfactants, i.e., anionic (SDS, LABS), cationic (BKC, CTAB), non-ionic (LAO) and zwitterionic (CAPB). The 2,6-/ 2,4-dichlorophenol selectivity ratio was observed to increase to a maximum of 1.01 with increasing surfactant concentration. This indicates that the enhanced formation of the orthochlorinated product was caused by micellar phenomena. At higher surfactant concentration the overall reaction rate was observed to increase; however, a concomitant decrease in the 2,6-/2,4-dichlorophenol selectivity ratio (as seen in Fig. 7) was also noticed. 5. Conclusions Hydrogen peroxide/hydrochloric acid system as a chlorinating agent predominantly lead to formation of monochlorinated product. This system was also found to be environmentally safer with little wastage of chlorine. Ortho selectivity in chlorination of phenol was found to be dependent on the nature and concentration of the surfactant. Ortho/para ratio of up to 12 was observed in the presence of SDS, LABS and BKC. The ratio was higher at low conversion but decreased with an increase in conversion. The selectivity ratio of 2,6-/2,4-dichlorophenol, in the chlorination of ortho-chlorophenol, which is 0.2 in absence of any surfactant was observed to reach ≈1 in 4 × 10−3 mol l−1 BKC. These observations confirm that the selectivity for orthochlorination is significantly improved under micellar-catalyzed conditions and suggest that this method could be applicable to other phenolic compounds as well. Spatial orientation of a molecule in micelles can be exploited to enhance the selectivity of a reaction towards one of the products. This effect can be efficiently employed for various substitution reactions.
CTAB BKC LAO CAPB CMC
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
Appendix A. Nomenclature SDS LABS
sodium dodecyl sulphate linear alkylbenzene sulphonate
cetyl trimethylammonium bromide benzalkonium chloride lauramine oxide cocoamidopropyl betaine critical micelle concentration
[33] [34] [35]
D.E. Pearson, C.A. Buehler, Synthesis 9 (1971) 455. W.D. Watson, J. Org. Chem. 50 (1985) 2145. H.F. Wiegandt, P.R. Lantos, Ind. Eng. Chem. 43 (1951) 2167. R. Taylor, Electrophilic Aromatic Substitution, Wiley, Chichester, 1990. P.B.D. de la Mare, Electrophilic Halogenation, Cambridge Univ. Press, Oxford, 1976. W.D. Watson, J. Org. Chem. 39 (1974) 1160. D.R. Harvey, R.O.C. Norman, J. Chem. Soc. (1961) 3604. V.L. Heasley, G.E. Heasley, M. Ingle, P.D. Davis, T.L. Rol, J. Org. Chem. 38 (1973) 2549. R. Bolton, P.B.D. de la Mare, J. Chem. Soc. B (1967) 1044. R.C. Huston, A.H. Neeley, J. Am. Chem. Soc. 57 (1935) 2176. H.P. Crocker, R. Walser, J. Chem. Soc. C (1970) 1982. L. Delaude, P. Laszlo, K. Smith, Acc. Chem. Res. 26 (1993) 607. L. Delaude, P.J. Laszlo, J. Org. Chem. 55 (1990) 5260. C.A. Dewar, C.J. Suckling, R. Higgins, J. Chem. Res. Synop. 10 (1979) 336. D.A. Jaeger, J.R. Wyatt, R.E. Raymond, J. Org. Chem. 50 (1985) 1467. C.J. Suckling, A.A. Wilson, J. Chem. Soc. Perkin Trans. 2 12 (1981) 1616. Y.P. Saraf, S.S. Bhagwat, Sep. Technol. 5 (1995) 207. S.O. Onyiriuka, C.J. Suckling, J. Chem. Soc. Chem. Commun. 15 (1982) 833. A.S. Chhatre, R.A. Joshi, B.D. Kulkarni, J. Colloid Interface Sci. 158 (1993) 183. P.V. Vyas, A.K. Bhatt, A.V. Bedekar, Tetrahedron Lett. 44 (2003) 4085. N.B. Barhate, A.S. Gajare, R.D. Wakharkar, A.V. Bedekar, Tetrahedron Lett. 39 (1998) 6349. N.B. Barhate, A.S. Gajare, R.D. Wakharkar, A.V. Bedekar, Tetrahedron 55 (1999) 11127. J.J. Jacob, R.A. Anderson, T.R. Watson, J. Pharm. Pharmacol. 23 (1971) 148. S.S. Bhagwat, M.M. Sharma, Chem. Eng. Sci. 43 (1988) 195. J.H.M. Heijnen, V.G. de Bruijn, L.J.P. van den Broeke, J.T.F. Keurentjes, Chem. Eng. Process. 42 (2003) 223. J. You, X. You, X. Su, T. Wang, J. Mol. Catal. A Chem. 202 (2003) 17. X. Yu, D. Scheller, O. Rademacher, T. Wolff, J. Org. Chem. 68 (2003) 7386. T. Dwars, E. Paetzold, G. Oehme, Angew. Chem. Int. Ed. 44 (2005) 7174. F. Wang, L. Hui, L. Cun, J. Zhu, J. Deng, Y. Jiang, J. Org. Chem. 70 (2005) 9424. H. Firouzabadi, N. Iranpoor, A. Garzan, Adv. Synth. Catal. 347 (2005) 1925. M. Pattabiraman, L.S. Kaanumalle, V. Ramamurthy, Langmuir 70 (2006) 2185. U.C. Nagaonkar, S.S. Bhagwat, J. Dispersion Sci. Technol. (2006), in press. H.J. Foroudian, N.D. Gillitt, C.A. Bunton, J. Colloid Interface Sci. 250 (2002) 230. K. Holmberg, Curr. Opin. Colloid Interface Sci. 8 (2003) 187. V.C.R. Reinsborough, Interfacial Catal. (2003) 377.
B.S. Samant et al. / Journal of Colloid and Interface Science 302 (2006) 207–213
[36] M. Graciani, A. Rodriguez, M. Munoz, M. Moya, React. Kinet. Catal. Lett. 78 (2003) 113. [37] G. Sarasan. Asian J. Chem. 15 (2003) 202. [38] R. de la Vaga, P. Paraz-Tejeda, P. Lopez-Cornejo, F. Sanchez, Langmuir 20 (2004) 1558. [39] U. Costas-Costas, C. Bravo-Diaz, E. Gonzalez-Romero, Langmuir 20 (2004) 1631. [40] L. Garcia-Rio, J.C. Mejuto, M. Perez-Lorenzo, J. Phys. Chem. B 110 (2006) 812.
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[41] J. Dakka, Y. Sasson, J. Chem. Soc. Chem. Commun. 19 (1987) 1421. [42] A.T. Florence, D. Atwood, Surfactant Systems—Their Chemistry, Pharmacy and Biology, Chapman and Hall, New York, 1983. [43] E. Dutkiewicz, A. Jakubowska, Colloid Polym. Sci. 280 (2002) 1009. [44] S.H. Yalkowsky, Solubility and Solubilization in Aqueous Media, Oxford Univ. Press, Am. Chem. Soc., New York, 1999, chap. 7, p. 260, Table 7.8. [45] M.C. Mary, K. Jyothi, B.Th. Gowda, Oxid. Commun. 25 (2002) 92. [46] M.C. Mary, B.Th. Gowda, Oxid. Commun. 25 (2002) 286.
Chlorination of aromatic compounds in micellar media: Regioselectivity B.S. Samant, Y.P. Saraf, S.S. Bhagwat ∗ Chemical Engineering Division, Institute of Chemical Technology, University of Mumbai, Matunga, Mumbai 400019, India Received 4 March 2006; accepted 8 June 2006 Available online 13 June 2006
Abstract Chlorination of phenol and ortho-chlorophenol was studied in micellar media in order to observe the effect on regioselectivity. Hydrogen peroxide/hydrochloric acid–aqueous system, which is environmentally a safer route was employed for chlorination. Selectivity ratio was found to be dependent on the nature and concentration of the surfactant. Ortho/para selectivity ratio up to 12 was realized for the chlorination of phenol. 2,6-/2,4-dichlorophenol ratio up to 1.01 was realized for the chlorination of ortho-chlorophenol. © 2006 Elsevier Inc. All rights reserved. Keywords: Phenol; Ortho-chlorophenol; Chlorination; Micellar catalysis; Regioselectivity
1. Introduction Aromatic substitution reactions generally pose a challenge in terms of separation, owing to selectivity considerations in the product isomer distributions [1,2]. ‘Selectivity Engineering’ for the desired product has been the underlying theme for a considerable amount of research, both for economic reasons, as well as for more facile separations and downstream processing on an industrial scale [3–5]. The monohalogenation of phenol invariably produces a mixture of ortho- and para-isomers, in which the latter predominates Fig. 1. Several techniques have been reported in the literature to improve the regioselectivity in the chlorination of activated aromatic compounds and these include employing solvents [6–8], reagents [9–11] and catalysts [2,3,12,13]. These techniques have some limitations such as the use of strong and non selective chlorinating agents, toxic and expensive reagents, low yields and long reaction times. Micellar aggregates composed of surfactant molecules are the simplest of the aggregates to effect selectivity. It has been ascertained by studies in substitution reactions [14,15] as well * Corresponding author.
E-mail address: [email protected] (S.S. Bhagwat). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.06.007
as by 1 H NMR spectroscopy [16] that a solubilizate such as phenol exists in a preferred average orientation in the domain of the surfactant micelle. Ortho and para substituted phenols can be selectively solubilized by micelles depending on the polarity of the substituent group [17]. It is possible to exploit this orientation to alter the selectivity towards a particular product. The effect of different types of surfactants (i.e. anionic, cationic, non-ionic and zwitterionic) on chlorination of phenol was studied. Onyiriuka and Suckling [18] have reported the selective chlorination of phenol at the ortho position by using tertiary butyl hypochlorite in the presence of a tailored surfactant molecule bearing a tertiary alcohol moiety close to the anionic head group. Chhatre et al. [19] have reported a similar improvement in selectivity in the nitration of phenol by dilute aqueous HNO3 in the presence of sodium di-(2-ethyl hexyl)sulfosuccinate (AOT). The objective of the present work is to study the effect of micellar media on chlorination of phenol and ortho-chlorophenol utilizing a mixture of hydrogen peroxide and hydrochloric acid. This hydrogen peroxide/hydrochloric acid chlorinating system is environmentally safe for chlorination [20]. The reaction between hydrogen peroxide and hydrochloric acid in situ generates positive chlorine species [21,22] in the reaction medium
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Fig. 1. Chlorination of phenol and o-chlorophenol.
which attacks on the substrate. As the rate of chlorine generation is slow, utilization of chlorine by reactant is more predominant with little wastage of chlorine. 2. Materials Phenol, ortho-chlorophenol and hydrochloric acid were obtained from M/s s.d. Fine Chem. Ltd., Mumbai. AR grade hydrogen peroxide was obtained from M/s Merck India Ltd. Electrophoretic grade sodium dodecyl sulphate (SDS), linear alkylbenzene sulphonate (LABS), cetyl trimethylammonium bromide (CTAB) were obtained from M/s Sisco Research Laboratories, Mumbai and lauryl dimethyl benzyl ammonium chloride or benzalkonium chloride (BKC), lauramine oxide (LAO) and cocoamidopropyl betaine (CAPB) were obtained from M/s Galaxy surfactants, Mumbai and their purity was ascertained tensiometrically. All chemicals were used as supplied without any further purification. Distilled water was used for all the reactions. 3. Experimental procedure A two phase mixture was agitated in a 2.5 × 10−4 m3 baffled glass reactor equipped with a six-bladed turbine agitator and 0.6 m i.d. The speed of agitation was maintained at 1.67 Hz. Aqueous phase (4 × 10−5 m3 ) contend of surfactant, hydrochloric acid (5.145 × 10−3 mol l−1 ) and hydrogen peroxide (1.985 × 10−3 mol l−1 ) while the organic phase was pure phenol or ortho-chlorophenol (50 × 10−3 mol l−1 ). Isothermal conditions were maintained at 303 ± 0.5 K. Analysis was performed on gas chromatograph (Chemito 8610) with flame ionization detector. A 4 m long i.d. S.S. column packed with 10% SE-30 on chromosorb WHP was employed for the analysis. Nitrogen was used as carrier gas at a flow rate of 5.0 × 10−7 m3 s−1 .
4. Results and discussion The molecular environment of the solute is altered considerably on its solubilization in the micellar aggregates. As shown in Fig. 2, phenol, when solubilized in the micelle, attains a preferred molecular orientation with the polar-OH group projecting out of the micelle towards the bulk aqueous phase and a relatively non polar aromatic ring penetrated in the hydrophobic core [23]. Hence, it is anticipated that the attack of a strong electrophile such as chlorine at para position of phenol would be suppressed by the non-polar micellar core, while the polar aqueous phase would provide a more favorable locale for the attack of chlorine at the ortho position. Similar effects of orientation on selectivity improvement [24–32] and rate enhancement [33– 40] in micellar and microemulsion media were also reported. 4.1. Chlorination of phenol The stoichiometry of the overall reaction [41] is as given in equation RH + H2 O2 + HCl → RCl + 2H2 O.
Fig. 2. Orientation of phenol in micelle.
(1)
B.S. Samant et al. / Journal of Colloid and Interface Science 302 (2006) 207–213
209
Fig. 3. Effect of HCl on % conversion of phenol.
Table 1 Conversion of phenol Phenol ×103 (mol l−1 )
H 2 O2 ×103 (mol l−1 )
HCl ×103 (mol l−1 )
CCl4 ×103 (mol l−1 )
Surfactant (BKC) ×103 (mol l−1 )
Conversion (mol%) 4h
8h
50 4.17 4.17 50
1.99 1.99 1.99 1.99
5.15 5.15 5.15 5.15
– 6.0 6.0 –
– – 0.5 0.5
35 30 35 40
60 50 50 64
The rate of chlorination at 300 K was found to be dependent on the concentration of phenol and hydrochloric acid. An equimolar proportion of hydrogen peroxide to phenol was found to restrict the chlorination to monochlorination. Fig. 3 indicates that the % conversion of phenol increased with the increasing concentration of hydrochloric acid in the reaction mixture. At all ratios of phenol:hydrochloric acid employed, the selectivity ratio (ortho-chlorophenol/para-chlorophenol = 0.60) remained the same with para chlorophenol as the major reaction product. At 50 × 10−3 mol l−1 phenol and 1.94 × 10−3 mol l−1 hydrochloric acid concentration, the titrimetric analysis of unreacted hydrochloric acid in the reaction mixture, indicated that 15% of the hydrochloric acid reacted with hydrogen peroxide after 8 h to produce an equivalent amount of chlorine. The analysis of organic phase showed that, only 10% of the liberated chlorine was consumed by phenol and thus utilization of chlorine was low. However at 50 × 10−3 mol l−1 phenol and 5.15 × 10−3 mol l−1 hydrochloric acid concentration, 69% of chlorine reacted with ≈ 60% of phenol to produce the corresponding chlorophenols. Hence in the latter case, rate of chlorine generation as well as phenol conversion was faster than that in the first case. The use of carbon tetrachloride as a solvent for phenol results in dilution of phenol in the reaction mixture, but does not affect the rate of reaction significantly. The addition of a surfac-
tant (BKC 0.5 × 10−3 mol l−1 ) leads to a significant increase in the rate of reaction (Table 1). 4.1.1. Effect of nature of the surfactant The attack of chlorine on phenol solubilized in micelle at the para-position was prevented by the non-polar micellar core, while the polar aqueous phase provided a more favorable locale for the attack of chlorine at the ortho-position. Thus, chlorination of phenol solubilized in micelle is expected to be highly ortho selective. The ortho/para selectivity ratio was found to be dependent on the nature of the surfactant employed. SDS (anionic surfactant) was seen to increase the ortho/para selectivity ratio (Fig. 4). Experiments with LABS have shown similar results. Hence, the presence of an aromatic ring in LABS did not have an additional effect on the ortho/para selectivity ratio. In the case of cationic surfactant, BKC was seen to increase ortho/para selectivity ratio (Fig. 5). LAO and CAPB both exibit cationic nature in acidic media. Fig. 6 shows the change in selectivity caused by the surfactants. CAPB contains a total number of 15 carbon atoms in its structure while LAO has a chain of 12 carbon atoms. The hydrophobic chain of CAPB contains an amide linkage within and this alters the polarity in the locale of solubilization of phenol in micelle and hence exhibits a different selectivity in the present reaction than LAO.
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Fig. 4. Chlorination of phenol in micellar solution of sodium dodecyl sulphate.
Fig. 5. Chlorination of phenol in micellar solution of benzalkonium chloride.
The highest selectivity ratio for ortho-chlorophenol at 15% conversion was 12 when the SDS concentration was 8 × 10−3 mol l−1 which decreased to 8 at 60% conversion. This decrease in the selectivity with increase in the conversion is likely to be due to subsequent chlorination of monochlorophenols to di- and tri-chlorophenols. Chlorination in solution of other surfactants such as BKC, LAO and CAPB gave similar decrease in selectivity with increase in conversion as shown in Figs. 5 and 6. 4.1.2. Effect of concentration of surfactant The presence of strong acid decreases the solubility of phenol in the bulk aqueous medium [17]. The large negative
free energy of solubilization of phenol (G = −15.6 kJ/mol) [17] in SDS micelle in the presence of hydrochloric acid shows that phenol is preferentially solubilized in the SDS micelle. The solubility of phenol in micellar solution increased with an increase in the surfactant concentration, resulting in a significant increase in the selectivity ratio. Fig. 4 shows the o/p ratio at various conversion levels of phenol when SDS was used as the surfactant. The presence of hydrochloric acid lowers the critical micelle concentration (CMC) of the surfactant [42]. The presence of strong electrolytes like NaCl, HCl, etc. decrease the CMC of dissolved surfactant by screening the charges of ionic surfactants such as SDS [43] (CMC of SDS is 0.1 × 10−3 mol l−1 at 0.5 mol l−1 of
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Fig. 6. Chlorination of phenol in micellar solution of lauramine oxide and cocoamidopropyl betaine.
Fig. 7. Chlorination of ortho-chlorophenol in solutions of various surfactants.
NaCl) [44]. At SDS concentrations up to 4 × 10−3 mol l−1 , the selectivity ratio remained relatively unchanged at approximately 0.56. This may be due to insufficient number of micelles present in the reaction mixture. An increase in the surfactant concentration favoured chlorination at ortho position. Higher selectivity was obtained at 4–12(×10−3 ) mol l−1 concentration of SDS. The increased rate of reaction can be explained by the higher effective concentration. However, the change in selectivity indicates the role of reaction occurring at micellar interface. At higher concentration of SDS (16 × 10−3 mol l−1 ), an adverse effect on the selectivity was observed.
4.2. Chlorination of ortho-chlorophenol Chlorination of ortho-chlorophenol leads to a mixture of 2,6-dichlorophenol and 2,4-dichlorophenol, as shown in Fig. 1. 2,4-dichlorophenol was found to be the major reaction product (2,6-/2,4-dichlorophenol selectivity ratio = 0.20) in the absence of micelles. This reaction was conducted in micellar environment with a view to increase the selectivity towards 2,6dichlorophenol. 4.2.1. Effect of surfactant concentration and the nature The rate of chlorination of ortho-chlorophenol was less than that of phenol (Table 2). This can be attributed to poor sol-
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Table 2 Conversion of o-chlorophenol o-chlorophenol ×103 (mol l−1 )
H 2 O2 ×103 (mol l−1 )
HCl ×103 (mol l−1 )
Surfactant (BKC) ×103 (mol l−1 )
Conversion (mol%) 4h
8h
50 50
1.99 1.99
5.15 5.15
– 0.5
10 15
18 20
ubility of ortho-chlorophenol in the aqueous acidic medium. Also, the presence of an electron-withdrawing group like chlorine at ortho position, makes ortho-chlorophenol less reactive for chlorination. Mary and Jyothi [45,46] have also found less reactivity of ortho-chlorophenol towards chlorination. The rate of chlorination was marginally higher in the presence of micelles (Table 2). The 2,6-/2,4-dichlorophenol selectivity ratio was found to be dependent on the nature and concentration of the surfactant employed. Fig. 7 gives the change in 2,6-/2,4-dichlorophenol selectivity ratio at varying concentration of different types of surfactants, i.e., anionic (SDS, LABS), cationic (BKC, CTAB), non-ionic (LAO) and zwitterionic (CAPB). The 2,6-/ 2,4-dichlorophenol selectivity ratio was observed to increase to a maximum of 1.01 with increasing surfactant concentration. This indicates that the enhanced formation of the orthochlorinated product was caused by micellar phenomena. At higher surfactant concentration the overall reaction rate was observed to increase; however, a concomitant decrease in the 2,6-/2,4-dichlorophenol selectivity ratio (as seen in Fig. 7) was also noticed. 5. Conclusions Hydrogen peroxide/hydrochloric acid system as a chlorinating agent predominantly lead to formation of monochlorinated product. This system was also found to be environmentally safer with little wastage of chlorine. Ortho selectivity in chlorination of phenol was found to be dependent on the nature and concentration of the surfactant. Ortho/para ratio of up to 12 was observed in the presence of SDS, LABS and BKC. The ratio was higher at low conversion but decreased with an increase in conversion. The selectivity ratio of 2,6-/2,4-dichlorophenol, in the chlorination of ortho-chlorophenol, which is 0.2 in absence of any surfactant was observed to reach ≈1 in 4 × 10−3 mol l−1 BKC. These observations confirm that the selectivity for orthochlorination is significantly improved under micellar-catalyzed conditions and suggest that this method could be applicable to other phenolic compounds as well. Spatial orientation of a molecule in micelles can be exploited to enhance the selectivity of a reaction towards one of the products. This effect can be efficiently employed for various substitution reactions.
CTAB BKC LAO CAPB CMC
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
Appendix A. Nomenclature SDS LABS
sodium dodecyl sulphate linear alkylbenzene sulphonate
cetyl trimethylammonium bromide benzalkonium chloride lauramine oxide cocoamidopropyl betaine critical micelle concentration
[33] [34] [35]
D.E. Pearson, C.A. Buehler, Synthesis 9 (1971) 455. W.D. Watson, J. Org. Chem. 50 (1985) 2145. H.F. Wiegandt, P.R. Lantos, Ind. Eng. Chem. 43 (1951) 2167. R. Taylor, Electrophilic Aromatic Substitution, Wiley, Chichester, 1990. P.B.D. de la Mare, Electrophilic Halogenation, Cambridge Univ. Press, Oxford, 1976. W.D. Watson, J. Org. Chem. 39 (1974) 1160. D.R. Harvey, R.O.C. Norman, J. Chem. Soc. (1961) 3604. V.L. Heasley, G.E. Heasley, M. Ingle, P.D. Davis, T.L. Rol, J. Org. Chem. 38 (1973) 2549. R. Bolton, P.B.D. de la Mare, J. Chem. Soc. B (1967) 1044. R.C. Huston, A.H. Neeley, J. Am. Chem. Soc. 57 (1935) 2176. H.P. Crocker, R. Walser, J. Chem. Soc. C (1970) 1982. L. Delaude, P. Laszlo, K. Smith, Acc. Chem. Res. 26 (1993) 607. L. Delaude, P.J. Laszlo, J. Org. Chem. 55 (1990) 5260. C.A. Dewar, C.J. Suckling, R. Higgins, J. Chem. Res. Synop. 10 (1979) 336. D.A. Jaeger, J.R. Wyatt, R.E. Raymond, J. Org. Chem. 50 (1985) 1467. C.J. Suckling, A.A. Wilson, J. Chem. Soc. Perkin Trans. 2 12 (1981) 1616. Y.P. Saraf, S.S. Bhagwat, Sep. Technol. 5 (1995) 207. S.O. Onyiriuka, C.J. Suckling, J. Chem. Soc. Chem. Commun. 15 (1982) 833. A.S. Chhatre, R.A. Joshi, B.D. Kulkarni, J. Colloid Interface Sci. 158 (1993) 183. P.V. Vyas, A.K. Bhatt, A.V. Bedekar, Tetrahedron Lett. 44 (2003) 4085. N.B. Barhate, A.S. Gajare, R.D. Wakharkar, A.V. Bedekar, Tetrahedron Lett. 39 (1998) 6349. N.B. Barhate, A.S. Gajare, R.D. Wakharkar, A.V. Bedekar, Tetrahedron 55 (1999) 11127. J.J. Jacob, R.A. Anderson, T.R. Watson, J. Pharm. Pharmacol. 23 (1971) 148. S.S. Bhagwat, M.M. Sharma, Chem. Eng. Sci. 43 (1988) 195. J.H.M. Heijnen, V.G. de Bruijn, L.J.P. van den Broeke, J.T.F. Keurentjes, Chem. Eng. Process. 42 (2003) 223. J. You, X. You, X. Su, T. Wang, J. Mol. Catal. A Chem. 202 (2003) 17. X. Yu, D. Scheller, O. Rademacher, T. Wolff, J. Org. Chem. 68 (2003) 7386. T. Dwars, E. Paetzold, G. Oehme, Angew. Chem. Int. Ed. 44 (2005) 7174. F. Wang, L. Hui, L. Cun, J. Zhu, J. Deng, Y. Jiang, J. Org. Chem. 70 (2005) 9424. H. Firouzabadi, N. Iranpoor, A. Garzan, Adv. Synth. Catal. 347 (2005) 1925. M. Pattabiraman, L.S. Kaanumalle, V. Ramamurthy, Langmuir 70 (2006) 2185. U.C. Nagaonkar, S.S. Bhagwat, J. Dispersion Sci. Technol. (2006), in press. H.J. Foroudian, N.D. Gillitt, C.A. Bunton, J. Colloid Interface Sci. 250 (2002) 230. K. Holmberg, Curr. Opin. Colloid Interface Sci. 8 (2003) 187. V.C.R. Reinsborough, Interfacial Catal. (2003) 377.
B.S. Samant et al. / Journal of Colloid and Interface Science 302 (2006) 207–213
[36] M. Graciani, A. Rodriguez, M. Munoz, M. Moya, React. Kinet. Catal. Lett. 78 (2003) 113. [37] G. Sarasan. Asian J. Chem. 15 (2003) 202. [38] R. de la Vaga, P. Paraz-Tejeda, P. Lopez-Cornejo, F. Sanchez, Langmuir 20 (2004) 1558. [39] U. Costas-Costas, C. Bravo-Diaz, E. Gonzalez-Romero, Langmuir 20 (2004) 1631. [40] L. Garcia-Rio, J.C. Mejuto, M. Perez-Lorenzo, J. Phys. Chem. B 110 (2006) 812.
213
[41] J. Dakka, Y. Sasson, J. Chem. Soc. Chem. Commun. 19 (1987) 1421. [42] A.T. Florence, D. Atwood, Surfactant Systems—Their Chemistry, Pharmacy and Biology, Chapman and Hall, New York, 1983. [43] E. Dutkiewicz, A. Jakubowska, Colloid Polym. Sci. 280 (2002) 1009. [44] S.H. Yalkowsky, Solubility and Solubilization in Aqueous Media, Oxford Univ. Press, Am. Chem. Soc., New York, 1999, chap. 7, p. 260, Table 7.8. [45] M.C. Mary, K. Jyothi, B.Th. Gowda, Oxid. Commun. 25 (2002) 92. [46] M.C. Mary, B.Th. Gowda, Oxid. Commun. 25 (2002) 286.