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 −[C−(CH3)3] in place of the isopropyl −[CH−(CH3)2] and ethyl group, respectively. The sim-azine molecule with 2 ethyl groups is symmetric and flat (excepting its equal NH−C2H5 ends). The threefold substituted triazines have resonance of the free (non-bonding, π {\textstyle \pi } -) 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) and can migrate from soil to groundwater. Terbuthylazine's HRAC classification is Group C1, Group C (global, Aus), Group 5 (numeric), as it inhibits photosynthesis at photosystem II.
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