Toxicity of pesticides and allied chemicals depends mainly on chemical structure of the substance.
However, a precise co-relation between toxic properties and the chemical structure of substances is not well understood. Since long it has been observed that there are certain chemical groups or atoms which when introduced into the molecule induce or enhance its toxicity. These groups or atoms which may be called toxophoric groups include halogen atoms (F, CI, Br, I), the nitro group, atoms of heavy metals (Hg, Sn, Cu etc.) and the rhodanic group (-SCN).
It is due to the presence of bromine atoms that Methyl bromide, an effective fumigant, is much more toxic than Methane. Presence of nitro group in phenols increases its insecticidal activity sharply. LD50 of phenol for leguminous aphids is only 0.3%, while of 4-nitrophenol is 0.0075% and that of 2-4-dinitrophenol I is 0.001 %. The substitution of chlorine atoms in second position of triazine ring in chlorine-derivatives of 1-3-5- triazines with hydroxyl ions results in complete loss of herbicidal activity.
In many cases toxicity of a substance may increase by addition of chemically inert groups. Addition of methyl group in 2-4-dinitrophenol results into the formation of 2-4-dinitro-5-methyl phenol the insecticidal activity of which is almost twice as strong as that of the parent compound (LD50 for leguminous aphids being 0.0005%).
Substitution of hydroxyl group in DDT causes sharp changes in the spectrum of its activity. The toxicity of a chemical may also depends on the precise spatial arrangement of various atoms or groups. Isomers of hexachlorocyclohexane differ from each other in spatial arrangement of chlorine and hydrogen atoms. It is the gamma-isomer which is the strongest in its insecticidal activity (Lindane).