c11h8o2

from The Collaborative International Dictionary of English v.0.48

vitamin K3 \vitamin K3\ n.
   menadione, ({C11H8O2}) being chemically
   2-methyl-1,4,-naphthoquinone. Having no side chain in the 3
   position, it cannot exert all functions of a true K vitamin
   and the designation as vitamin K3 has been discouraged
   --[MI11]

   Syn: vitaminK2(0); menaphthone.
        [WordNet 1.5]