(C2H4NH)2

from The Collaborative International Dictionary of English v.0.48

Piperazine \Pi*per"a*zine\, n. Also -zin \-zin\ . [Piperidine +
   azote + -ine.] (Chem.)
   A crystalline substance, {(C2H4NH)2}, formed by action of
   ammonia on ethylene bromide, by reduction of pyrazine, etc.
   It is a strong base, and is used as a remedy for gout.
   [Webster 1913 Suppl.]