Conformational studies by dynamic nuclear magnetic resonance. Part 16. The mechanism of tautomerism in triazenes
Abstract
The kinetics of tautomerism of triazenes, R1NH–NNR2⇌ R1NN–NHR2, have been determined by means of line-shape analysis of the 1H n.m.r. spectra at various temperatures. When R1= R2= Me the equilibrium reaction is second-order whereas when R1= Me, R2= Ph or when R1= R2=p-MeC6H4 the reaction follows a first-order course. Thermodynamic activation parameters have been determined for both situations for a variety of solvents. Theoretical INDO calculations helped in understanding the switch from a bimolecular to a unimolecular reaction pathway when aliphatic are replaced by aromatic substituents. The absence of solvent effects suggests a radical mechanism, in agreement with the conclusion of a theoretical charge population analysis.