The Effect of Catalysts and Inhibitors

Neither the catalysis by acids nor by bases is conclusive evidence of a non-radical mechanism however. For example, the semiquinone radical is more stable in basic solution where it exists as a negative ion-radical, and the related nitrogen radicals are more stable in acid solution where they exist as positive ion-radicals. In both cases the ion-radicals have symmetrical resonance not possessed by the neutral radical. 

Thus acid or base catalysis of certain oxidation reactions need not mean 

The discovery of diradicals in sulfuric acid or Lewis acid solutions of bianthrone and thianthrene raises the possibility of acid-catalyzed radical reactions for any unsaturated compound.448 Anything that increases the equilibrium concentration of the diradical should promote radical reactions. These results are important because previous to their discovery few chemists would have hesitated to say that a reaction catalyzed by sulfuric acid or aluminum trichloride, for example, was an entirely ionic one. Now we would not want to venture such an opinion without other reasons. 

Catalysis (initiation) by a free radical, on the other hand, is fairly conclusive evidence of a radical reaction, provided it is known that the catalyst is indeed a free radical and that it does not have pronounced polar properties as well. Many classes of compound once thought to decompose exclusively into ions or exclusively into radicals are now known to do both. Peroxides are one well-known example, AT-halo-amides are another. Catalysis by benzoyl peroxide probably does indicate a radical reaction since there is no evidence that this particular peroxide tends to give ions even under the most favorable conditions. But many other peroxides are known to decompose into ions, or at least ion pairs, as well as into radicals. The decomposition of azo compounds can also be either radical or ionic, the dialkyl azo compounds tending to give radicals, the diazonium compounds either radicals or ions. Catalysis by a borderline example of an azo compound would therefore be dubious evidence of either kind of mechanism. The initiation of the polymerization of octyl vinyl ether by triphenylmethyl chloride in polar 

Catalysis by radicals will usually be due to a radical addition or displacement reaction, hydrogen and halogen being the atoms on which the displacement most often occurs. It is usually a chain reaction once the substrate is converted into a radical it carries the reaction to many molecules of substrate. Examples are polymerization and autoxidation. 

---

We conclude that although Scheme 1 represents a basic mechanism for co-oxidation of indene and thiophenol, the system is so sensitive to the effects of catalysts and inhibitors that interpreting its behavior necessitates studying the initiation and termination mechanisms. 

Evaluation of the effects of catalysts and inhibitors on phase stability and thermal decomposition mechanisms. 

---