C4H9.SH

from The Collaborative International Dictionary of English v.0.48

butyl mercaptan \bu"tyl mer*cap"tan\, n. (Chem.)
   A volatile liquid, {C4H9.SH}, having a strong odor like that
   of a skunk; also called {butanethiol}. All three isomers,
   normal, sec-, and tert-butanethiol possess the skunk odor.
   [PJC]