Azoaryl alkanes and aryl azoalkanes

The basic problems with which most investigations are concerned are the nature of the transition state, the variations of rate parameters with molecular structure and the question of whether the decomposition occurs in a step-wise manner or simultaneous two bond cleavage. 

The experimental data are discussed in terms of resonance and hyperconjugative effects on the initial and transition states and to a lesser extent in terms of polar factors. Steric considerations are also invoked and interaction among the substituents of each half of the molecule and across the azo linkage are taken into account. Solvent effects appear to play only a minor role in the decomposition kinetics nonetheless, it is not always justifiable to dismiss them a priori. It is 

Important contributions to the elucidation of this problem came from the work of Seltzer et al. who utilized the powerful technique of kinetic isotope effect for probing into the nature of the transition state. For a unimolecular decomposition of the type 

Substitution of either electron donating or withdrawing groups in para or meta position in phenyl azotriphenyl methanes lead to small changes in decomposition rates . Two interpretations have been put forward. According to one, the phenyl radical is stabilized by electron accesssion and destabilized by electron depletion and according to the other, desolvation in the transition state plays a role in the rate controlling step . 

The chemistry of this type of azocompounds is closely tied in with that of the alkyl and aryl type diradicals. Investigations in the field received new impetus during the sixties when interest in the chemical behavior of diradicals became intensified . Perhaps the most interesting and thoroughly studied species are the five membered ring structures, the 1-pyrazolines. 

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