The Collaborative International Dictionary of English v.0.48:

Pentadecatoic \Pen`ta*dec`a*to"ic\
   (p[e^]n`t[.a]*d[e^]k`[.a]*t[=o]"[i^]k), a. [Penta- +
   decatoic.] (Chem.)
   Of, pertaining to, or derived from, pentadecane, or
   designating an acid related to it.
   [1913 Webster]