The kinetics and mechanism of the reaction between hydrophobic 4-hexadecylarenediazonium ions, 16-ArN R 2 and vitamin C, VC, in a model emulsion prepared by mixing octane, acidic (HCl) water and the non-ionic surfactant hexaethyleneglycol monododecyl ether, C 12 E 6 , were investigated. Because emulsions are opaque, linear sweep voltammetry, LSV, was employed to monitor the reaction. Voltammograms of 16-ArN R 2 in emulsions show two reduction peaks as in aqueous systems. The half-life for the spontaneous decomposition of 16-ArN R 2 in the emulsion was estimated as t 1/2 ¼ 14.5 h at T ¼ 25 -C. Upon addition of VC to the system, the first reduction peak of 16-ArN R 2 disappears almost immediately and a new reduction peak is detected at E p ¼ S0.25 V. Electrochemical titration of 16-ArN R 2 shows that the new peak corresponds to the formation of a 1:1 adduct. The i P (E p ¼ S0.25 V) values can be linearly correlated with [16-ArN R 2 ] and the observed rate constants, k obs , were determined by fitting the (i p , t) data to the integrated first order equation. The variation of k obs with [VC] follows a saturation kinetics profile, consistent with the formation of an intermediate in a pre-equilibrium step. All the evidence is consistent with a reaction mechanism comprising two competitive pathways, the spontaneous D N R A N mechanism and the unimolecular decomposition of a transient diazo ether (DE) formed in a pre-equilibrium step. The data allowed estimations of the interfacial rate constant for the reaction between 16-ArN R 2 and VC S but did not allow the determination of the equilibrium constant for the DE formation.

We have investigated the effects of solvent composition, acidity and temperature on the dediazoniation of 4-bromobenzenediazonium (4BrBD) ions in MeOH-H(2)O mixtures by employing a combination of spectrometric and chromatographic techniques. The kinetic behaviour is quite complex; in the absence of MeOH, dediazoniations follow first-order kinetics with a half-life t(1/2) approximately 3000 min (T = 45 degrees C), but addition of small concentrations of MeOH lead to more rapid but non-first-order kinetics, suggestive of a radical mechanism, with t(1/2) approximately 125 min at 25% MeOH. Further increases in the MeOH concentration slow down the rate of dediazoniation and reactions progressively revert to first-order behaviour, and at percentages of MeOH higher than 90%, t(1/2) approximately 1080 min. Analyses of reaction mixtures by HPLC indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4-bromophenol (ArOH), 4-bromoanisole (ArOMe), and bromobenzene (ArH). At acidities (defined as -log[HCl]) < 2, the main dediazoniation products are the substitution products ArOH and ArOMe but, upon decreasing the acidity, the reduction product ArH becomes predominant at the expense of ArOH and ArOMe, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given MeOH content, the plot of k(obs) or t(1/2) values against acidity is S-shaped, the inflexion point depending upon the MeOH concentration and the temperature. Similar S-shaped variations are found when plotting the dediazoniation product distribution against the acidity. The acid-dependence of the switch between the homolytic and heterolytic mechanisms suggests the homolytic dediazoniation proceeds via transient diazo ethers. The complex kinetic behaviour can be rationalized by assuming two competitive mechanisms: (i) the spontaneous heterolytic dediazoniation of 4BrBD, and (ii) an O-coupling mechanism in which the MeOH molecules capture ArN(2)(+) to yield a highly unstable Z-adduct which undergoes homolytic fragmentation initiating a radical process. Analyses of the effects of temperature on the equilibrium constant for the formation of the diazo ether and on the rate of splitting of the diazo ether allowed, for the first time, estimation of relevant thermodynamic parameters for the formation of diazo ethers under acidic conditions.

We investigated the effects of solvent composition, acidity, and temperature on the dediazoniation of 4-methylbenzenediazonium (4MBD) ions in EtOH/H 2 O mixtures by employing a combination of spectrometric and chromatographic techniques. First-order behavior is found in all solvent composition ranges. HPLC Analyses of the reaction mixtures indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4-cresol (ArOH), 4-phenetole (ArOEt), and toluene (ArH). At acidities (defined as À log [HCl]) < 2, the main dediazoniation products are the substitution products ArOH and ArOEt but upon decreasing the acidity, the reduction product ArH becomes predominant at the expense of ArOH and ArOEt, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given EtOH content, the plot of k obs values against acidity is S-shaped, the inflexion point depending upon the EtOH concentration and the temperature. Similar S-shaped variations are found when plotting the dediazoniation -product distribution against the acidity. The acid dependence of the switch between the homolytic and heterolytic mechanisms suggests that the homolytic dediazoniation proceeds via transient diazo ethers, and this complex kinetic behavior can be rationalized by assuming two competitive mechanisms: i) the spontaneous heterolytic dediazoniation of 4MBD, and ii) an O-coupling mechanism in which the EtOH molecules capture ArN þ 2 to yield a highly unstable (Z)-adduct which undergoes homolytic fragmentation initiating a radical process (Scheme). Analyses of the effects of temperature on the equilibrium constant for the formation of the diazo ether and on the rate of splitting of the diazo ether allowed the estimation of relevant thermodynamic parameters for the formation of diazo ethers derived from methylbenzenediazonium ions under acidic conditions.

We have measured the rates and product yields of dediazoniation of 1-naphthalenediazonium (1ND) tetrafluoroborate in the presence and absence of sodium dodecyl sulfate (SDS) micellar aggregates by employing a combination of UV-vis spectroscopy and high-performance liquid chromatography (HPLC) measurements. Kinetic data were obtained by a derivatization procedure with product yields were determined by HPLC. HPLC chromatograms show that in aqueous acid and in micellar solutions only one dediazoniation product is formed in significant quantities, 1-naphthol (NOH), and the observed rate constants (k obs ) are the same when 1ND loss is monitored spectrometrically and when NOH formation is monitored by HPLC. Activation parameters were obtained both in the presence and absence of SDS micellar aggregates. In both the systems, the enthalpies of activation are high and the entropies of activation are positive. The enthalpy of activation in the absence of SDS is very similar to that in the presence of SDS micelles, but the entropy of activation is lower by a factor of 4. As a consequence, SDS micelles speed up the thermal decomposition of 1ND and increase k obs by a factor of 1.5 when [SDS] = 0.02 M. In contrast, results obtained in the presence of complexing systems such as crown ethers and polyethers show significant stabilization of the parent arenediazonium ions. Kinetic and HPLC data are consistent with the heterolytic D N + A N mechanism that involves the rate-determining fragmentation of the arenediazonium ion into a very reactive phenyl cation that reacts competitively with available nucleophiles.