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

Abstract

We investigated the kinetics and mechanism of the reaction between the 3-methylbenzenediazonium ions (3MBD), and gallic acids (¼ 3,4,5-trihydroxybenzoic acid; GA) in aqueous buffer solution under acidic conditions by employing spectrometric, electrochemical, and chromatographic techniques and computational methods. To discern which of the three OH groups of GA is the first one undergoing deprotonation, the geometries of the resulting dianions were optimized by using B3LYP hybrid densityfunctional theory (DFT) and a 6-31G(þ þ d,p) basis set, and the results suggest that the OH group at the 4-position is the first one which is deprotonated. The variation of the observed rate constant, k obs , with the acidity at a given [GA] follows an upward curve suggesting that the reaction takes place with the dianionic form of gallic acid, GA 2À , and rate enhancements of ca. 23000 fold are obtained on going from pH 3.5 up to pH 7.5. At relatively high acidities, the variation of k obs with [GA] is linear with an intercept very close to the value for the thermal decomposition of 3MBD; however, a decrease in the acidity leads to saturation-kinetics profiles with nonzero, pH-dependent intercepts. The saturation-kinetics patterns found suggest the formation of an intermediate in a rapid pre-equilibrium step, but the nonzero, pHdependent intercepts cause the double reciprocal plots of 1/k obs vs. 1/[GA] to curve. This prompts us to propose an alternative reaction mechanism comprising consecutive equilibrium processes involving the bimolecular, reversible formation of a highly unstable (Z)-diazo ether which undergoes isomerization to the (E)-isomer through a unimolecular step. The results obtained indicate the complexity of reactions of arenediazonium ions with nucleophilic arenes containing three or more OH groups.

Kinetics and mechanism of the reaction between 3‐methylbenzenediazonium ions and catechol

“…We also note that the ratio k 1 / k 2 is very high, indicating that the first equilibria in Scheme is shifted towards the formation of the diazo ether. Curiously enough, the variation of both k 3 and k 1 / k 2 with pH is the same as that found for the reaction with gallic acid in spite of the much lower reactivity of 3MBD with [CAT] than with gallic acid.…”

“…The finding of saturation kinetics where the intercept is different from zero is not common, and contrasts with the experimental behavior found for the reaction with methyl gallate (methyl 3,4,5‐trihydroxybenzoate) or ascorbic acid although were already observed for the reaction of 3MBD with gallic acid (3,4,5‐trihydroxybenzoic acid). It is also worth noting that the effects of [CAT] on k obs , Fig.…”

“…We postulate the O‐coupling reaction mechanism shown in Scheme , which comprises two equilibrium steps involving the formation of a highly unstable Z‐diazo ether, which undergoes isomerization to the E‐isomer, which is similar to that proposed for the reaction of ArN 2 + ions with gallic acid but slightly different to those proposed for the reactions with ascorbic and methy gallate . Broxton et al .…”

“…Equation can be derived from Scheme , where [CAT T ] stands for the total or stoichiometric concentration of [CAT] employed in the experiment. Equation predicts that when [CAT T ] → 0, k obs → k 4 , consistent with the experimental observation of non‐zero intercepts, Fig.…”

Effects of emulsifier hydrophile–lipophile balance and emulsifier concentration on the distributions of gallic acid, propyl gallate, and α-tocopherol in corn oil emulsions

“…4 PG is much more hydrophobic and less soluble in water than GA because of the acylation of the carboxylic group [56]. Note that the working pH = 3.7 is nearly 10 5 times lower than the pK a value of the phenolic group in the 4-position of the pyrogallol moiety, pK a % 8.1 [59,60], which means that the 4-OH group of PG is essentially completely protonated. This is reflected in the calculated P I W value, Table 2, which is higher than that of GA as a consequence of the higher hydrophobicity.…”