2-Amino-5-methyl-1,3,4-thiadiazole and 2-amino-5-ethyl-1,3,4-thiadiazole

The structures of 2-amino-5-methyl-1,3,4-thia­diazo­le, C₃H₅N₃S, and 2-amino-5-ethyl-1,3,4-thia­diazo­le, C₄H₇N₃S, have been determined for comparison with unsubstituted 2-amino-1,3,4-thia­diazo­le. Despite their different space groups (P2₁/n and Pbca, respectively), the packing modes of the methyl and ethyl derivatives are similar, with comparable three-dimensional hydrogen-bonding associations. This is in contrast to the hydrogen-bonding network in 2-amino-1,3,4-thia­diazo­le, which is one-dimensional and has denser packing. It is shown that both packing forms are different polymorphs of a specific subunit of each array.