Surfactant Effects upon Cyclization of o-(.omega.-Haloalkoxy)phenoxide Ions. The Role of Premicellar Assemblies

Abstract

Aqueous cationic micelles of R'NR3Br (R' = C12H25, R = Me, DoTABr; R' = C16H33, R = Me, CTABr; R' = C16H33, R = Bu, CTBABr) and R ' N M~~( C H Z ) W~~R~B~ ( R = C16H33) increase rates of cyclization of O--OC6&o(CH2)n-~ (X = Br, I) designated PhXn. Rate increases are small, except for Ph17 in CTBABr,l and activation parameters are similar to those in water or 75% aqueous EtOH. In very dilute surfactant there are large rate increases, by factors of up to 39 with PhI16 in CTABr, but CTBABr is less effective. These rate increases are ascribed to formation of substrate-premicellar complexes of PhBrlO, PhBrl2, and PhX16 with all the surfactants. These complexes dissolve into normal micelles at higher [surfactant], and the unusual rate effects disappear.Cyclizations of o-(whaloa1koxy)phenoxide ions, 1 (Scheme 1) are well studied,2 and factors such as ring strain and loss of rotational entropy in formation of the transition state have been explored in detail. Following precedent, we write substrate structure in terms of the size of the ring formed by cyclization.2These cyclizations can be regarded as intramolecular sN2 reactions with varying effective molarities (EM).2 Reactions of the o-(3-bromopropyloxy)phenoxide and 0-(3-iodopropy1oxy)phenoxide (PhBr7 and PhI7, respectively) in aqueous micelles are useful models for micellar effects upon rates of sN2 reactions in the interfacial region at micellar surfaces.' First-order rate constants in this region k", can be compared with those in water, k',, and a t 25.0 "C values of k',/k', range from 1.7 and 3.9 for PhBr7 and Ph17 in CTABr, CH3(CH2)15N(CH3)3Br, to 7.7 and 34 in CTBABr, C H~( C H Z )~~N [ ( C H~)~C H & B~.These relative rate constants decrease modestlywith increasing temperature.3These values of k'm/Kw show that cationic micelles do not significantly control initial state conformations of PhX7 so as to increase rates of cyclization. We were therefore * To whom the correspondence should be addressed. @ Abstract published in Advance ACS Abstracts, September 1, 1994. (1) Cerichelli, G.; Luchetti, L.; Mancini, G.; Muzzioli, M. N.; Germani, R.; Ponti, P. P.; Spreti, N.; Savelli, G.; Bunton, C. A. J. Chem. SOC., Perkin Trans. 2 1989, 1081. (2) (a) Illuminati, G.; Mandolini, L.; Masci, B. J. Am. Chem. SOC. 1974,96,1422. (b) Illuminati, G.; Mandolini, L.; Masci, B. J. Am. Chem. SOC. 1977, 99, 6308. (c) Dalla Cost, A.; Illuminati, G.; Mandolini, L.; Masci, B. J. Chem. SOC., Perkin Trans. 2 1980, 1774. (3) Cerichelli, G.; Luchetti, L.; Mancini, G.; Savelli, G.; Bunton, C. A. J. Colloid Interface Sci. 1993, 160, 85. Scheme 1 1 X = B r , I interested in the report by Wei et al. that, although micelles of CTABr did not significantly assist cyclization of PhBr7, they markedly speeded cyclization of the larger chain homolog^.^ However, rates were measured only at loe3M CTABr, which is very close to the critical micelle concentration, cmc (ca. 8 x M in water at 25 0C),5 so that micelles were in very low concentration. Under these conditions it is necessary to consider the possibi...

Catalytic Effect of Cationic Gemini Micelles on the Rate of Condensation between Glycyl-dl-Aspartic Acid and Ninhydrin in the Absence and Presence of Organic Solvents

“…The characteristics of regions I and II are the same as with conventional surfactants [21,22]. The unexpected behavior in region I, as increasing the rate constant values below geminis' cmc, can be understood by considering presence of premicelles in the environment [23]. This result is confirmed by the small cmc values under reaction conditions (Table 3).…”

Micellar Effects on SN2 Reactions of Alkyl Naphthalene-2-sulfonates:The Role of Hydrophobic Substituents

“…For example, concentration of monomeric surfactant is often treated as a disposable parameter and fitting of data in dilute surfactant is very uncertain, especially with hydrophobic reactants (1d), in addition, in some reactions the kinetics indicate that premicelles are affecting reaction rates (22), although it is often difficult to distinguish between reactant-induced micellization and reaction in premicelles (23). Added electrolytes increase micellar aggregation numbers and may affect micellar morphology.…”

“…For reactions of very hydrophobic substrates rate constants usually increase so sharply in dilute surfactants that the concentration of monomeric surfactant is often treated as an adjustable parameter and is equated with a so-called "kinetic" cmc (1b,c,d). It is equally reasonable to assume that substrate interacts with monomeric or premicellar surfactant, and in a few systems extremes in rate constants are ascribed to formation of premicelles (22). Fits are poor in very dilute surfactant, especially for reactions in hydroxide ion surfactants and we neglect possible interactions of the hydrophobic substrates with monomeric surfactant or premicelles.…”

Surfactant effects on the reaction of 2‐(4‐cyanophenoxy)‐quinoxaline with hydroxide ion

“…The model is known to fail near the critical micelle concentration (cmc) due to the pseudophase assumption [3]. One of the premises of the model is that there should be no interactions between the monomeric surfactant and the substrate, although there are evidences that they interact at surfactant concentrations below the surfactant cmc [1,2,[7][8][9].…”