Terminating reactions, cumylperoxy radical

In recent years much emphasis has been placed on studies of co-oxidations, since they can provide quantitative data about fundamental processes (such as the relative reactivities of peroxy radicals toward various hydrocarbons48-50), which are difficult to obtain by other methods. Co-oxidations are also quite important from a practical viewpoint since it is possible to utilize the alkylperoxy intermediates for additional oxidation processes instead of wasting this active oxygen. That the addition of a second substrate to an autoxidation reaction can produce dramatic effects is illustrated by Russell s observation51 that the presence of 3 mole % of tetralin reduced the rate of cumene oxidation by two-thirds, despite the fact that tetralin itself is oxidized 10 times faster than cumene. The retardation is due to the higher rate of termination of the secondary tetralyl-peroxy radicals compared to the tertiary cumylperoxy radicals (see above). 

Under some experimental conditions, including low hydrocarbon concentration and high rate of radical formation, the interaction of methyl-peroxy and cumylperoxy radicals rather than self-reaction of cumyl-peroxy radicals accounts for most of the terminating interactions. The competing reaction for methylperoxy is abstraction from cumene, viz. 

The term a is the fraction of terminations per self-reaction of two cumylperoxy radicals. When there are no methylperoxy radicals, a equals 0.1 at 60° C. At high rates of initiation and low [RH], the value of a increases and in principle could reach 2.1 if every cumoxy is converted to methylperoxy and terminates with another cumylperoxy radical. Thus kt can range only from fe38 to 2.1 k3S. 

Fiikuzumi and Cuo have very recently concluded that the termination reaction for oxidation of c miene with manganese dioxide or cobalt oxide supported on silica ( ) and during autoxidation of cumene initiated by reaction of cumene hydroperoxide with lead oxide ( ) is strictly first-order with respect to the concentration of cumylperoxy radicals. These workers proposed an unprecedented 1,3-methyl shift followed by 0-0 bond cleavage to account for these inusual kinetics. 

---