Efficient Factors on the Hydrolysis Reaction Rate of Some Para-Aminophenol Derivatives in Acidic pHs

Abstract

Electrochemical oxidation of some p-aminophenol derivatives (1–5) in acidic solutions has been investigated both experimentally and theoretically to provide insight into the influence of some factors on the hydrolysis reaction rate. The result of this work shows that the electrogenerated p-quinoneimines participate in the hydrolysis reaction and are converted to the p-benzoquinone. The hydrolysis reaction rate strongly depends on the structure of the p-aminophenols and solution's pH. In this work, the observed homogeneous rate constants of hydrolysis (kobshyd) of p-quinoneimines were determined using digital simulation technique. The effect of different parameters such as: change of Gibbs free energy (ΔG) of the electrochemical oxidation of para-aminophenol derivatives (1–5), charge of reaction site, N–C4 bond order (Wiberg Bond Indices, WBIs) and the nature of substituted group, on the hydrolysis rate constant were also studied. All calculations were performed using Density Functional Theory (DFT) both BP86 and B3LYP levels of theory and 6-311G (p,d) basis set. The results showed that the N–C4 bond order and charge on the reaction site play significant roles in hydrolysis reaction's rate.

Mechanistic study of electrochemical oxidation of 2,5-diethoxy-4-morpholinoaniline in aqueous solutions: hydrolysis, trimerization, and hydroxylation processes

“…and formation of 1oxH 2? in this condition cause increasing instability of produced p-quinonediimine [26,28]. In other words, using this data it could be concluded that in the intermediate pHs rate of the dimerization is faster than hydrolysis, and in the lower pH range the rate of the hydrolysis is faster than dimerization.…”

“…Investigations indicated that dimerization, trimerization, hydrolysis, and hydroxylation can occur in electrochemical oxidation of amines which is highly dependent on pH of solution [24,25]. Along with our previous works in electrochemical study of amines [23][24][25][26][27][28], diversity of behavior in the electrochemical oxidation of 2,5-diethoxy-4-morpholinoaniline (2,5-DEMA, 1) at various conditions and also the importance of Schiff base reaction [29] prompted us to studying electrochemical oxidation of 2,5-DEMA at various conditions using cyclic voltammetry (CV) and controlled-potential coulometry. The results show that in all pHs electrochemically generated p-quinonediimine (1ox ?…”

“…). Our reported results show that the N=C bond order (WBI) and charge on the reaction site (C 1 ) have significant roles in the hydrolysis reaction rate [28]. Increasing the positive charge on C 1 atom and consequently increasing its electrophilicity makes it more susceptible to nucleophilic attack by hydroxide ion or water which causes increasing hydrolysis reaction rate.…”

“…1 and 3 shows that instability of the produced p-quinonediimine increases upon decreasing pH. It was reported that based on structures of species, rate of hydrolysis for some amines increases with increasing pH, while it decreased for some others [28]. To explain the observed difference in the electrochemical oxidation behavior in lower pH range and intermediate pHs, computational study was used.…”

“…Increasing the positive charge on C 1 atom and consequently increasing its electrophilicity makes it more susceptible to nucleophilic attack by hydroxide ion or water which causes increasing hydrolysis reaction rate. Also =NR 2 is a leaving group and the hydrolysis rate is inversely proportional to the strength of N=C bonds, in which the lower the WBI of N=C bond, the higher the rate of the hydrolysis reaction [28]. The positive charge on the reaction sites (C 1 ) and WBIs of C 1 =N 1 bonds for 1ox ?…”