Reactions of p‐Substituted Phenols with Nitrous Acid in Aqueous Solution

Abstract

The reaction of phenols with nitrite (nitrous acid HONO, or its conjugated base, NO2−) is of importance in stomach fluids (low pH) and in atmospheric hydrometeors (mild acid and basic pH). The initial reaction associated with the oxidation/nitration of 4‐substitued phenols promoted by HONO/NO2 depends on the pH of the solution. At low pH, the initial step involves the reaction between HONO and phenol, whereas at basic conditions this involves an electron transfer from the phenoxy anion to nitrogen dioxide (NO2) producing the nitrite anion. The rate of both processes is determined by the donor capacity of the substituent at the 4‐position of the phenol, and the data obtained at pH 2.3 follow a linear Hammett‐type correlation with a slope equal to –1.23. The partition of the gaseous intermediates (NO and NO2) makes the rate of HONO‐mediated oxidation dependent on their gas–liquid distribution. At low pH, the main process is phenol oxidation, even in oxygen‐free conditions, and the presence of any 4‐substituted phenol decreases the rate of HONO auto‐oxidation.

Kinetics of the Anaerobic Reaction of para‐Substituted Phenols with Nitrogen Dioxide

“…The occurrence of different mechanisms at different pHs is a frequent feature of NO 2 ● + HONO reactions The data of phenol reaction with HONO represented in Fig. show that the fastest rate takes place at low pH according to Reaction . Conversely, the slowest reaction takes place at pH ≈11 involving nitrite and phenolate anions as reactants.…”

“…However, it must be taken into account that both, NO 2 ● and phenols, are, particularly at low pHs, distributed between the gaseous and liquid phases of the reactor (Fig. ), precluding a sound quantitative evaluation of the aqueous phase rate constants .…”

“…However, steady‐state measurements of these reactions are difficult to interpret. This is mainly due to the fast interchange among reactive nitrogen species such as NO ● , NO 2 ● , NO 2 − , NO 3 ● , HONO, N 2 O 3 , and N 2 O 4 . Additionally, the descriptions of such reactions are complex since each reactive species is distributed between the aqueous solution and the gas phase .…”

“…This is mainly due to the fast interchange among reactive nitrogen species such as NO ● , NO 2 ● , NO 2 − , NO 3 ● , HONO, N 2 O 3 , and N 2 O 4 . Additionally, the descriptions of such reactions are complex since each reactive species is distributed between the aqueous solution and the gas phase . To get more insight into the influence of the chemical structure of the phenol on the kinetics of the above‐mentioned reactions, in the present work we studied the consumption of para‐substituted phenols, bearing electron donor and acceptor groups, triggered by NO 2 ● , and the results were compared with those previously reported employing HONO as a reactant.…”

“…To get more insight into the influence of the chemical structure of the phenol on the kinetics of the above‐mentioned reactions, in the present work we studied the consumption of para‐substituted phenols, bearing electron donor and acceptor groups, triggered by NO 2 ● , and the results were compared with those previously reported employing HONO as a reactant. For this purpose, solutions containing phenols were kept at known experimental conditions (concentration, ionic strength, and pH) and bubbled with a continuous flux of a gas mixture of nitrogen and NO 2 ● or exposed to HONO. In addition, and taking into account the importance of cinnamic acid derivatives in the antioxidant field, we compared results obtained employing para‐substituted phenols with coumaric, ferulic, and sinapic acids.…”