Decarboxylation of 6-nitro-5-alkoxybenzisoxazole-3-carboxylate ion (1,OMe and 1,OTD, alkoxy ) MeO, n-C14H29O, respectively) is accelerated by cationic micelles of cetyltrialkylammonium bromide (CTABr, CTEABr, CTPABr, CTBABr, alkyl ) Me, Et, n-Pr, n-Bu respectively). The first-order rate constants kobs for reaction of 1,OMe increase monotonically with [surfactant] and become constant when the substrate is fully bound and kobs ) k′M. The values of kobs for 1,OTD increase sharply with increasing [surfactant] and reach well-defined maxima at [surfactant] at or below the cmc, before decreasing to values corresponding to kobs ) k′M. The magnitude of the rate maxima and the increase in k′M are in the sequence CTABr < CTEABr < CTPABr < CTBABr. The rate maxima are due to formation of premicellar complexes of substrate with one or a few surfactant monomers, but they "dissolve" in micelles at higher [surfactant]. A rationalization is provided for acceleration by very dilute surfactants.
Micelles of cetyltrialkylammonium hydroxide (alkyl = Me (CTAOH),
n-Pr (CTPAOH)) increase rates
of SN2 substitution of methyl benzene- and
4-nitrobenzenesulfonate (1a,b) and E2 elimination from
phenethyl
and 4-nitrophenethyl bromide (2a,b). For the
SN2 reactions in 0.1 M surfactant, rate increases
over
reactions in aqueous 0.1 M OH- are by factors of 6−15,
but for the E2 reactions, increases are by factors
of 17−25 (2a) and 100−240 (2b).
Second-order rate constants in the micellar pseudophase decrease
slightly
in going from CTAOH to CTPAOH for reaction of 1a and
increase for reaction of 1b, 2a, and
2b. Second-order rate constants in the micellar pseudophase increase with head
group bulk for reactions in which
charge is dispersed in the transition state, and charge dispersion in
the transition state formation increases
second-order rate constants in micelles relative to those in
water.
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