Abstract
Although detailed examinations have been published on structure-activity relationships of herbicide safeners, only a few data are available on chemical reactivity-safener activity relationships. Chemical reactivity of acetamide type compounds as well as their safener activity against thiocarbamate herbicides change with the number of chlorine substituents in the order: non-chlorinated < monochloro < dichloro. Several compounds of another chemical group, acetals (e.g. MG-191, 2-dichloromethyl-2-methyl-1,3-dioxolane), are also effective safeners for thiocarbamate herbicides. According to our growth room studies, the safener activity of acetals also increases with increasing chlorine content up to two chlorine atoms on the same carbon. A number of differently chlorinated acetals have been synthesized and their acid-catalyzed hydrolysis rate determined in order to establish a relationship between their bioactivity and chemical reactivity. The hydrolysis rate order of acetals containing no, one or two chlorine atoms: non-chlorinated > monochloro > dichloro, is just the opposite than what has been found for acetamides. Thus, safener activity of acetals increases with decreasing chemical reactivity. The opposite reactivity order of acetamides and acetals can be explained by the different mechanisms of their hydrolysis. Dichloroacetals may not be effective safeners in their original structure. In plants, however, they can be biologically activated to active safeners by a transformation other than hydrolysis.