Terbuthylazine is a selective herbicide. Chemically, it is a halogenated triazine; compared with atrazine (1958 inv., Geigy lab) and simazine, it has a tert-butyl group in place of the isopropyl and ethyl group, respectively.[1][2]The sim-azine molecule with 2 ethyl groups is symmetric and flat (excepting its equal ends). The threefold substituted triazines have resonance of the free (non-bonding, -) electron pairs, resulting in equivalent mesomeric structures.
<Simazine remains active in the soil for 2 to 7 months or longer after application. Atrazine remains in soil for a matter of months (although in some soils can persist to at least 4 years)[3] and can migrate from soil to groundwater.>
^Atrazine: Chemical Summary. Toxicity and Exposure Assessment for Children's Health (PDF) (Report). U.S. Environmental Protection Agency. 2007-04-24. Archived from the original (PDF) on 2012-03-16.