Solvent and Temperature Dependencies of the Rates of Thermal Cis-to-Trans Isomerization of Tetra-(ortho)substituted 4-Aminoazobenzenes Containing 2,6-Dimethoxy Groups.
Sandra Ramírez-Rave, Luis Ramón Ortega-Valdovinos, Ana P González-Castro, Anatoly K Yatsimirsky
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引用次数: 0
Abstract
The kinetics of thermal cis-to-trans isomerization of two tetra-(ortho)substituted 4-aminoazobenzene derivatives containing 2,6-dimethoxy groups in the 4-aminobenzene ring and either 2',6'-dimethyl (1) or 2',6'-dichloro groups in another ring was studied in 16 and 9 solvents, respectively, at room temperature. In addition, the kinetics of isomerization of 1 was studied at variable temperatures in 6 solvents. The solvent effects were analyzed in terms of multiparameter correlations using the Kamlet-Taft, Catalan, and Laurence scales. The temperature dependencies were analyzed in terms of the isokinetic relationship by using Exner's method. The correlation analysis was also extended to several previously reported systems, including isomerization of unsubstituted 4-aminoazobenzene and push-pull 4-NR2-4'-NO2 azobenzenes. It was established that for all 4-aminoazobenzenes in contrast to push-pull azobenzenes, the increase in solvent dipolarity does not affect or even inhibit isomerization rates; however, the increase in solvent acidity induces large rate accelerations. The isokinetic relationship holds only in aprotic solvents, while in alcohols and water, the temperature dependencies do not pass through the common point at T = Tiso, indicating different mechanisms operating in protic and aprotic solvents. The acceleration effect by protic solvents does not represent a type of general acid catalysis as follows from the small solvent isotope effect kH/kD = 1.26 in water. It is suggested that protic solvents induce a change in the reaction mechanism supposedly from inversion to rotation by hydrogen bonding stabilization of the negative charge developed on the azo group in a resonance structure involving a 4-amino group.
期刊介绍:
An essential criterion for acceptance of research articles in the journal is that they provide new physical insight. Please refer to the New Physical Insights virtual issue on what constitutes new physical insight. Manuscripts that are essentially reporting data or applications of data are, in general, not suitable for publication in JPC B.