Micellar Effects on the Reaction between an Arenediazonium Salt and 6-O-Octanoyl-l-ascorbic Acid. Kinetics and Mechanism of the Reaction

Abstract

The effects of 6-O-octanoyl-L-ascorbic acid, VC8, on the reaction with 3-methylbenzenediazonium, 3MBD, tetrafluoroborate were investigated in the absence and in the presence of sodium dodecyl sulfate, SDS, cetyltrimethylammonium bromide, CTAB, and tetradecyltrimethylammonium bromide, TTAB, micelles at different pHs by employing a combination of UV-vis spectroscopy, high-performance liquid chromatography, HPLC, and differential pulse polarography techniques. VC8 behaves as a typical surfactant in aqueous solution giving rise to micellar aggregates at VC8 concentrations above its critical micelle concentration (cmc). This behavior is reflected in the variation of the observed rate constant, kobs, with [VC8]; kobs values increase smoothly upon increasing [VC8] up to a breakpoint at [VC8] ≈ 8 × 10 -3 M after which further addition of VC8 makes kobs values increase sharply. Polarographic titration of VC8 with 3MBD shows that the reaction between 3MBD and VC8 takes place through an inner sphere mechanism leading to the formation of an unstable Z-diazo ether intermediate. All evidence is consistent with a competitive reaction mechanism, that is, the thermal D N + AN dediazoniation and a rate-limiting decomposition of the Z-diazo ether "complex" formed from reaction between 3MBD and VC8ions in a rapid pre-equilibrium step. In presence of SDS, CTAB, or TTAB, at fixed pH and [VC8] , cmc, kobs values increase upon increasing [VC8] up to a maximum after which further addition of surfactant leads to a sharp (SDS) or smooth (CTAB, TTAB) decrease in k obs values. Results are consistent with the predictions of the pseudophase model and are rationalized in terms of micellar-induced concentration-dilution effects (SDS) and reactant and coion (3MBD) incorporation into the micellar Stern layer of CTAB and TTAB micelles. HPLC analyses of the reaction mixtures in the presence of added surfactants show a significant micellar effect on product distribution.

pH Effects on Ethanolysis of Some Arenediazonium Ions: Evidence for Homolytic Dediazoniation Proceeding through Formation of Transient Diazo Ethers

“…Proposed reaction mechanism for solvolysis of ArN 2 ϩ in EtOH/H 2 O mixtures; the reactive EtOH molecule is one of those in the solvation shell of the ArN 2 ϩ ions; k w and k c stand for the thermal rate constant and for the first-order rate constant for the decomposition of the ArN 2 OEt complex, respectively, and K stands for the equilibrium constant for complex formation number of ascorbate ions, for which the formation of such an intermediate was detected experimentally by employment of electrochemical methods. [9,20] The assumption of a rate-limiting decomposition of the diazo ether is also consistent with reported results for other O-coupling reactions [1,3,33,34] and was experimentally examined in reactions of arenediazonium ions where geometric restrictions apply. [21,35] From Scheme 2, Equation (1) can be derived, where k w and k c are the rate constants for the spontaneous thermal decomposition of ArN 2 ϩ and the rate constant for decomposition of the complex, respectively, and…”

“…In the alternative inner-sphere mechanism, reduction of the arenediazonium ion is preceded by the formation of a complex, namely ArϪNϭNϪOϪR, which was detected experimentally in some dediazoniations. [9,20,21] S-shaped curves such as those shown in Figures 1, 2, and 3 (A) are usually observed in reactions of acid-base pairs in which both forms are attainable and show different reactivities. [31] Under our experimental conditions, only two specimens may undergo acid-base processes: the ArN 2 ϩ ions and EtOH.…”

“…These (Z) adducts may undergo subsequent isomerization to the thermodynamically more stable (E) isomers, which in some instances can be isolated, [18] or may eventually give rise to homolytic rupture of the bonds providing the initiation of a radical process. [3,9,19,20] This bond-rotating mechanism to transform the (Z) into the (E) isomers has recently been described for Sandmeyer hydroxylations and chlorination reactions. [19] In most instances analyzed, the nucleophile must possess a charge, such as OH Ϫ , CN Ϫ , RO Ϫ or ascorbate ions, and experimental conditions are chosen such that substantial concentrations of the anionic form of the nucleophile are present, [3,9,19,20] but formation of (Z)-diazo ethers with neutral nucleophiles has also been reported.…”

“…[3,9,19,20] This bond-rotating mechanism to transform the (Z) into the (E) isomers has recently been described for Sandmeyer hydroxylations and chlorination reactions. [19] In most instances analyzed, the nucleophile must possess a charge, such as OH Ϫ , CN Ϫ , RO Ϫ or ascorbate ions, and experimental conditions are chosen such that substantial concentrations of the anionic form of the nucleophile are present, [3,9,19,20] but formation of (Z)-diazo ethers with neutral nucleophiles has also been reported. [5,21] Following previous solvolytic work, [5,26] we have undertaken a study to investigate the effects of pH (0Ϫ7) on the ethanolysis of 2-, 3-and 4-methylbenzcenediazonium ions (2MBD, 3MBD, and 4MBD, respectively) by a combination of spectrophotometric (UV/Vis) and chromatographic (HPLC) techniques to seek for evidence of the nature of the initiation step in solvolytic radical dediazoniations.…”

Reactivity of 3‐Methylbenzenediazonium Ions with Gallic Acid. Kinetics and Mechanism of the Reaction

“…First, to expand our current knowledge on the reactions of arenediazonium ions with polyhydroxylated antioxidants as they have been very much less investigated than those with the mono-and dihydroxylated phenol derivatives [1] [4]. Previous dediazoniation work with antioxidants such as ascorbic acid or methyl gallate, which do not possess structures able to build up very high electron densities at one or more C-atoms, showed that the reactions do not proceed through the C-coupling pathway but via the formation of transient diazo ethers [17] [18] [23] [24].…”