Analytical Azo Coupling Reactions: A Quantum-Chemical Consideration

Abstract

The nitrite ion is a priority environmental toxicant and a precursor of N-nitrosamines, which are carcinogenic and mutagenic agents [1,2]. It interacts with oxyhemoglobin in blood to form methemoglobin, which disturbs cell respiration, and, therefore, the vital functions of the entire organism [3,4]. Nitrites are added to meat foods to improve vendibility. The analytical determination of nitrites and N-nitrosamines in foodstuff and other samples is a topical problem. The simplicity, availability, and satisfactory metrological characteristics of photometry make it one of the most suitable methods for solving this problem [5,6]. The most important photometric reactions for nitrites and N -nitrosamines are the diazotization of primary aromatic amines and the subsequent azo coupling reaction [5,6].In photometric analysis, the following processes in which or N -nitrosamines participate are used: (1) the self-coupling of 1-aminonaphthalene ( I ) through the diazotization of I followed by the coupling of the resulting naphthalene-1-diazonium cation ( II ) with I [5-8] and (2) the analytical Griess reaction involving the diazotization of 4-aminobenzenesulfonic (sulfanilic) acid followed by the azo coupling of the resulting 4-sulfobenzenediazonium with amine I [5,6,9].In an aqueous solution, the former of the processes mentioned above occurs to only a small extent and the product precipitates [7,8]. However, the yield of the analytical form and the stability of the analytical signal over time improve dramatically if the reaction is performed in microreactors [10], such as anionic surfactant (sodium dodecyl sulfate) micelles [7,8]. On the contrary, the Griess reaction is successfully performed in an aqueous medium [5,6], whereas anionic surfactant micelles only slightly enhance the analytical signal NO 2 -(increase the absorbance of the solution of the analytical form) and stabilize it over time [9].Note that the stability of an analytical signal and the thermodynamic stability of molecular systems discussed below are different concepts. The former is characterized by the time in which the maximum intensity of the analytical signal remains virtually constant, whereas the latter is characterized by the enthalpy and the free energy of formation of a certain molecule.In the diazotization-azo coupling tandem, the azo coupling reaction is crucial, because its products are the sources of the analytical signal in the photometric determination of nitrite and N -nitrosamines.According to [11], the self-coupling product of 1-aminonaphthalene I is 4-amino-1,1'-azonaphthalene resulting from the interaction of naphthalene-1-diazonium cation II with I at the 4-position of the amine molecule. At the same time, the possibility is discussed [11] for the formation of 1,1'-diazoaminonaphthalene (triazene) upon the electrophilic attack of the nitrogen atom in molecule I by cation II . The author of [11] reports nothing concerning the detection of 1-amino-2,1'-azonaphthalene (the result of coupling at position 2 of molecule I ).At the same tim...