Micellar Head Group Size and Reactivity in Aromatic Nucleophilic Substitution

First-order rate constants for reaction of OH- with 2,4-dinitrochloronaphthalene go through maxima with increasing concentrations of quaternary ammonium bromide surfactants. The surfactants are cetyltrialkylammonium bromide (C16H33NR3Br, CTABr, CTEABr, CTPABr, CTBABr, R = Me, Et, n-Pr, and n-Bu), cetyl-N-methylmorpholinium bromide (CMMBr), cetylcyclohexyldimethylammonium bromide (CCHDMABr), and tetradecylquinuclidinium bromide (TDQBr). Competition between Br- and OH- is described in terms of two mass-action-like equations. The ion binding parameter for Br- is much larger than that of OH-, but both follow the sequence CTABr > CMMBr > CCHDMABr =TDQBr > CTEABr > CTPABr > CTBABr. The fitting of the data is satisfactory for up to 0.5 M OH- and second-order rate constants at micellar surfaces are approximately 3 times that in water. The classical pseudophase ion exchange model fits kinetic data only in dilute OH- and requires second-order rate constants at the micellar surface to increase by factors of ca. 2 at high [OH-]. Micellar effects upon the reaction of Br- with methyl naphthalene-2-sulfonate are analyzed by using a mass-action-like equation.