The rates of geometrical inversion of azobenzene in seven solvents, and of azobenzene together with five $-substituted azobenzenes in benzene, have been studied by dielectric capacity-time measurements. All show Arrhenius equations similar to those previously found applicable to aromatic diazocyanides. A rough connection between the energy of activation recorded for X*C,H,*N2*Ph and pphs is demonstrable via the empirical equation E = E , + 1316(p + 0.226p2). A correlation between the kinetic data and the square roots of the internal pressures of the solvents has been examined, and also one depending on the solute-solvent interaction energy. The second gives indications of being less unsatisfactory than the first. FOLLOWISG an investigation of the effects of substituents and solvents on the velocities of the cis to trans thermal changes of various aromatic diazocyanides (Le F6vre and Northcott, J., 1949, 944) it became of obvious interest to compare our findings with those parallel for the corresponding azobenzenes. Some relevant information had already been recorded by Hartley (J., 1938, 633) for the parent azobenzene and five of its derivatives having respectively the OH, NH,, NMe,, OMe, and +NJle, groups in the 4-position, the majority of these having been studied in light petroleum, benzene, and acetone, although in some cases water, acetic acid, aqueous acids and other mixtures were used.However, since neither these media nor the substituents overlapped extensively with those taken by us for the diazocyanides, and because his measurements had been spectrophotometric a t dilutions of 3 -8 x 10-4 mole/l., we have re-examined, at higher concentrations in benzene, azobenzene and 4-methoxyazobenzene and added 4-chloro-, .i-bromo-, and 4nitro-azobenzene. Halpern, Brady, and Winkler (Cnmd. J. Res., 1950, 28, B, 140) have provided full data for azobenzene in sixteen homogeneous solvents with dielectric constant ranging from that of heptane to that of water.
EXPERIMENTALAzobenzene was twice recrystallised from alcohol after a solution in this solvent had been boiled under reflux for 1 hour. All operations were performed in the absence of direct daylight. The material, m. p. 68O, was stored in an opaque desiccator. The cis-isomer, prepared according to Hartley (Zoc. cit.) and recrystallised from light petroleum by cooling below room temperature, had m. p. 71-72".
