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

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.

supporting

confidence: 86%

supporting

confidence: 85%

supporting

confidence: 63%

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Formation and Decomposition of Diazo Ethers under Acidic Conditions. Effects of Ethanol Concentration, Acidity, and Temperature on the Ethanolysis of 4‐Methylbenzenediazonium Ions

Fernández-Alonso

Bravo‐Díaz

2010

“…Similar situations where found when reacting ArN þ 2 ions with neutral nucleophiles such as MeOH [15] and EtOH [14] and in systems of restricted geometry [13]. We are also interested in determining the distribution of polar organic molecules such as antioxidants in emulsified systems by using arenediazonium ions as chemical probes that react with a given antioxidant, the kinetic results being interpreted in terms of a pseudophase model [24] [25].…”

mentioning

confidence: 92%

Kinetics and Mechanism of the Reaction between an Arenediazonium Ion and Methyl Gallate (=Methyl 3,4,5‐Trihydroxybenzoate) in Aqueous Solution: Evidence for Diazo Ether Formation through an O‐Coupling Reaction

Losada‐Barreiro

Sánchez-Paz

Bravo‐Díaz

2007

We have investigated the kinetics and mechanism of the reaction between 3-methylbenzenediazonium ions (3MBD) and methyl gallate (¼ methyl 3,4,5-trihydroxybenzoate; MG), in aqueous buffer solution by employing spectrophotometric (UV/VIS) and electrochemical (linear-sweep voltammetry, LSV) techniques and computational methods. Because the absorption band of MG overlaps that of 3MBD, the reaction was monitored spectrophotometrically by measuring the changes in absorbance with time due to product formation, and biphasic kinetic profiles, indicative of accumulation of an intermediate in the course of the reaction, were obtained. The formation of an intermediate was confirmed by LSV. The observed rate constants k obs for 3MBD disappearance were obtained by fitting the decrease in the peak current of the first reduction peak of 3MBD with time to the integrated firstorder equation. The variation of k obs with [MG] was determined at different pH values and follows a saturation kinetic pattern. Alternatively, at a fixed [MG], k obs values show an inverse dependence on [H þ ], suggesting that the reactive species is the anion and not the neutral form of MG. To discern which of the three OH groups of MG is the first one undergoing deprotonation, the geometries of the resulting anions were optimized by using B3LYP hybrid density functional theory (DFT) and a 6-31G(þ þ d,p) basis set. The deprotonation energies suggest that the OH group at the 4-position is first deprotonated. The kinetic results can be accommodated, therefore, by assuming two competitive mechanisms, the spontaneous D N þ A N decomposition involving 3MBD, and a mechanism involving an electrophilic attack at the O-atom at C(4) in a pre-equilibrium step, leading to the formation of a transient diazo ether of the type ArÀN¼NÀOÀR which further decomposes. All attempts to isolate and characterize the diazo ether failed.

“…In addition, we recently investigated the reaction with methyl gallate, concluding that this reaction takes place through the formation of an unstable Ocoupling adduct that further decomposes (Scheme 1, b) [36]. Similar O-coupling mechanisms have been proposed before for the reaction between a number of ArN þ 2 ions with neutral nucleophiles such as MeOH [7] and EtOH [37], cationic nucleophiles such as ascorbate ions [4] [6] [38] [39], and with polyalcohols in systems of restricted geometry [5]. The results of the present study should contribute to bridging the gap between the chemical and biological antioxidant activity by exploring the effects of the micro-environment in which the antioxidant is located, as studied by means of membrane-mimetic systems.…”

Section: Introduction -Reactions Of Natural Reducing Agents With Arementioning

confidence: 85%

Micellar Effects on the Reaction between an Arenediazonium Ion and the Antioxidants Gallic Acid and Octyl Gallate

Losada‐Barreiro

Sánchez-Paz

Pastoriza-Gallego

et al. 2008

The effect of sodium dodecyl sulfate (SDS) micelles on the reaction between the 3-methylbenzenediazonium (3MBD) ion and either the hydrophilic antioxidant gallic acid (GA) or the hydrophobic analogue octyl gallate (OG) have been investigated as a function of pH. Titration of GA in the absence and presence of SDS micelles showed that the micelles do not alter the first ionization equilibrium of GA. Analysis of the dependence of the observed rate constant (k obs ) with pH shows that the reactive species are GA 2À and OG À. Kinetics results in the absence and presence of SDS micelles suggest that SDS aggregates do not alter the expected reaction pathway. SDS Micelles inhibit the spontaneous decomposition of 3MBD as well as the reaction between 3MBD and either GA or OG, and upon increasing the SDS concentration, with k obs approaching the value for the thermal decomposition of 3MBD in the presence of SDS. Our results are consistent with the prediction of the pseudophase model and show that the origin of the inhibition for the reaction with GA is different to that for the reaction with OG; in the former case, the observed inhibition can be rationalized in terms of the micelle-induced electrostatic separation of reactants in the micellar Stern layer, whereas the observed inhibition in the reaction with OG is a consequence of the dilution effect caused by increasing SDS concentration, decreasing the local OG À concentration in the Stern layer.