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Langmuir 1998, 14, 2656-2661
Effects of Association Colloids on Elimination from 1,2-Dihalo-1,2-diphenylethanes. The Role of Surfactant Structure Lucia Brinchi, Raimondo Germani, and Gianfranco Savelli* Dipartimento di Chimica, Universita` di Perugia, 06100 Perugia, Italy
Nicoletta Spreti Dipartimento di Chimica, Ingegneria Chimica e Materiali, Universita` di L’Aquila, 67010 L’Aquila, Italy
Renzo Ruzziconi Dipartimento di Chimica, Universita` della Basilicata, 85100 Potenza, Italy
Clifford A. Bunton Department of Chemistry, University of California, Santa Barbara, California 93106 Received February 18, 1997. In Final Form: February 9, 1998 Rates of E2 elimination of D,L-1,2dichloro-1,2-diphenylethane (1) and the analogous dibromide (2) were measured in aqueous cetyltrimethylammonium chloride and hydroxide (CTACl and CTAOH), cetyltripropylammonium hydroxide (CTPAOH), didodecyldimethylammonium chloride and hydroxide (DDDACl and DDDAOH), and N,N-dimethyl-N-(2-hydroxyethyl)-n-hexadecylammonium hydroxide (DOH,OH). Micelles or other association colloids of these surfactants increase rates in solutions that contain OH-, and DOH,OH is much more effective than the nonfunctional surfactants. Variations of first-order rate constants, kobs, with [surfactant] or [OH-] are fitted by a pseudophase model with water and micelles as distinct reaction media, to give second-order rate constants in the latter. These rate constants are in the sequence DOH,OH > CTPAOH ≈ DDDACl > DDDAOH ≈ CTAOH ≈ CTACl. Micelles favor reactions of 2 over 1, relative to reactivities in water, due to differences in amphiphilic interactions with the leaving halide ions.
Introduction Aqueous association colloids increase rates of E2 elimination by OH-, and effects of cationic micelles on dehydrochlorination of DDT and related compounds have been studied kinetically.1,2 Analysis of the rate data in aqueous micelles in terms of the pseudophase ion exchange3 or similar (PIE) treatments shows that concentration of reactants in the small volume of the interfacial region is a major factor in giving the rate effect,2 as is general for bimolecular nonsolvolytic, reactions.4 The PIE treatment has been used to estimate second-order rate constants of reactions of OH- with phenethyl bromides in mixed-ion systems of cationic micelles,2a,b which contain mixtures of reactive and inert anions. A similar treatment, with fitting of ion-binding in terms of equations of the form of Langmuir isotherms,5 has been used to analyze (1) (a) Lapinte, C.; Viout, P. Tetrahedron 1979, 35, 1921. (b) Minch, M. J.; Giaccio, M.; Wolff, A. J. Am. Chem. Soc. 1975, 97, 3766. (2) (a) Wilk, K. J. Phys. Chem. 1989, 93, 7432. (b) Wilk, K.; Burczyk, B. J. Phys. Chem. 1989, 93, 3, 8219. (c) Otero, C.; Rodenas, E. J. Phys. Chem. 1986, 90, 5771. (d) Rezende M. C.; Rubira, A. F.; Franco, C.; Nome, F. J. Chem. Soc., Perkin Trans. 2 1983, 1075. (e) Stadler, E.; Zanette, D.; Rezende, M. C.; Nome, F. J. Phys. Chem. 1984, 88, 1892. (3) (a) Romsted, L. S. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 509. (b) Romsted, L. S. In Surfactants in Solution; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1984; Vol. 2, p 1015. (4) (a) Sudholter, E. J. R.; van der Langkruis G. B.; Engberts, J. B. F. N. Recl. Trav. Chim. Pays-Bas 1980, 99, 73. (b) Bunton, C. A.; Savelli, G. Adv. Phys. Org. Chem. 1986, 22, 213. (c) Bunton, C. A.; Nome, F.; Romsted, L. S.; Quina, F. H. Acc. Chem. Res. 1991, 24, 357.
effects of changes in the size of micellar headgroups on these reactions, but excluding inert counterions.6 Rate constants in the micellar pseudophase are calculated by simulation of the rate data in terms of equations that include parameters for the transfer of ionic and nonionic reactants between water and micelles.3-5 For E2 reactions of phenethyl and 4-nitrophenethyl bromide in micelles of cetyltrimethyl- and cetyltripropylammonium hydroxide (CTAOH and CTPAOH), without inert anions, secondorder rate constants in micelles relative to those in water increase with increasing bulk of the headgroup, especially with the 4-nitro compound.6 These headgroup effects are much larger than with SN2 reactions with anions,7 and it appears that rate constants, and their relation to the sizes of headgroups, depend on the extent of charge dispersion in the transition state,6 as for solvent effects in nonmicellar systems.8 We have extended this study to reactions of D,L-1,2-dichloro- and 1,2-dibromo-1,2-diphenylethane (1 and 2, respectively). (5) Bunton, C. A.; Gan, L.-H.; Moffatt, J. R.; Romsted, L. S.; Savelli, G. J. Phys. Chem. 1981, 85, 4118. (6) Brinchi, L.; Di Profio, P.; Germani, R.; Savelli, G.; Bunton, C. A. Langmuir 1997, 13, 4583. (7) (a) Bonan, C.; Germani, R.; Ponti, P. P.; Savelli, G.; Cerichelli, G.; Bacaloglu, R.; Bunton, C.A. J. Phys. Chem. 1990, 94, 5331. (b) Bacaloglu, R.; Bunton C. A.; Ortega, F. J. Phys. Chem. 1989, 93, 1497. (8) (a) Ingold, C.K. Structure and Mechanism in Organic Chemistry, 2nd ed.; Cornell University Press: Ithaca, New York, 1969; Chapter 7. (b) Reichardt, C. Solvent and Solvent Effects in Organic Chemistry, 2nd ed.; VCH: Weinheim, 1988; Chapter 5.
S0743-7463(97)00170-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/24/1998
Elimination Rates of 1,2-Dihalo-1,2-diphenylethanes
These substrates are very hydrophobic and bind strongly to micelles, so there are no uncertainties regarding their distribution, and they form trans-1-halostilbene quantitatively. The reactions can readily be followed spectrophotometrically with dilute substrate. The surfactants used in the present work are cetyltrimethylammonium hydroxide and chloride (CTAOH, CTACl), cetyltripropylammonium hydroxide (CTPAOH), didodecyldimethylammonium chloride and hydroxide (DDDACl and DDDAOH), and N,N-dimethyl-N-(2-hydroxyethyl)-nhexadecylammonium hydroxide (DOH, OH), which generates functional micelles with a reactive alkoxide ion9 (Scheme 1).
Scheme 1
Langmuir, Vol. 14, No. 10, 1998 2657
Figure 1. Values of kobs for reaction of 1 and 2 in CTAOH (9, 0) and CTPAOH (2, 4). Solid and open points are for reactions of 1 and 2, respectively. Solid and open points refer to the righthand and left-hand axes, respectively.
n-C16H33NMe3X, CTAX, X ) OH, Cl; n-C16H33NPr3OH, CTPAOH; (n-C12H25)2NMe2X, DDDAX, X ) OH, Cl n-C16H33N+Me2CH2CH2OH,OH- a DOH,OH n-C16H33N+Me2CH2CH2ODOExperimental Section Materials. The purification of CTABr (Aldrich) and the preparation and purification of various surfactants used were performed by standard methods as described.7 In particular, DDDACl was prepared by quaternizing dodecyldimethylamine with dodecyl chloride;10 CTAOH, DDDAOH, and DOH,OH were prepared from the bromide, via the sulfate as described.5,9,10 D,L1,2-Dichloro-1,2-diphenylethane was prepared from cis-stilbene as described:11 mp ) 90-92 °C (lit. 87-90 °C).11a D,L-1,2Dibromo-1,2-diphenylethane was prepared from cis-stilbene and tetrabutylammonium tribromide in acetic acid; mp ) 112-113 °C (lit. 109.5-111 °C).11b cis- and trans-R-chlorostilbenes were prepared by sodium hydroxide induced dehydrochlorination in ethanol of meso- and d,l-1,2-dichloro-1,2-diphenylethane, respectively.11a For cis-R-chlorostilbene bp ) 111-113 °C, at 0.5 mmHg (lit. 97-99 °C, at 0.25 mmHg),11c and for trans-R-chlorostilbene mp ) 52-53 °C (lit. 52-54 °C).11c Kinetics. Kinetic experiments were carried out at 25.0 °C by following the appearance of the halostilbene at 300 nm by using a Beckman 35K, a Perkin-Elmer 551, or a Canterbury Stopped Flow Spectrometer SF3A or a UV-vis 160 A Shimadzu spectrophotometer. The substrate, typically 4.6 × 10-5 M, was added to the reaction solution in CH3CN so that the final solution contained 0.1% organic solvent. All runs were at least duplicated, and differences were
