In a previous paper, it was determined from spectroscopic measurements that 1,1,1-trifluoro-3-(2-thenoyl)acetone (TTeA) was less than 2% enolized in water; the presence of cationic micelles did not increase the
enol content, but a strong increase of its acid character was observed; e.g., in 0.02 M tetradecyltrimethylammonium bromide (TTABr), the measured pK
a was nearly 3 u lower than that determined in
water (Iglesias, E. Langmuir
2000, 16, 8438). In the present work, the nitrosation reaction of TTeA in both
water and aqueous micellar solutions of the cationic surfactants TTABr and tetradecyltrimethylammonium
chloride is studied. The reaction performed in water at low acidities (e.g., pH > 4) is extremely slow;
conversely, in aqueous micellar solutions of cationic surfactants, the reaction is too slow at high acidities
(e.g., pH < 2). Cationic micelles induce the nitrosation of TTeA at low acidities because of the rapid
formation of enolate anions in the positively charged micellar interface. Rate enhancements higher than
30-fold are observed by the presence of cationic micelles. Kinetic features are mechanistically explained
on the basis of a rate-determining step for the attack of the nitrosating agent to the enolate. Depending
on the experimental conditions, the kinetically detected nitrosating agents are NO+, N2O3, or XNO (X =
Cl, Br); however, so as to observe nitrosation via dinitrogen trioxide, either in water or in the presence
of cationic micelles, it is necessary to work with relatively high [nitrite] (∼10-2 M). When the reaction is
performed in water in the presence of Br-, the only nitrosating agent detected is BrNO, and the observed
experimental facts are explained by postulating the formation in a reversible step of an intermediate in
steady state, which possibly could be the chelate−nitrosyl complex, in analogy with the use of β-diketones
as chelating extractants of metal cations.