The Collaborative International Dictionary of English v.0.48:

Crotonic \Cro*ton"ic\ (kr?-t?n"?k), a.
   Of or pertaining to, or derived from, a plant of the genus
   Croton, or from croton oil.
   [1913 Webster]

   Crotonic acid (Chem.), a white crystalline organic acid,
      C3H5.CO2H, of the ethylene, or acrylic acid series. It
      was so named because formerly supposed to exist in croton
      oil. Also, any acid metameric with crotonic acid proper.
      [1913 Webster]

   Note: The acid characteristic of croton oil is tiglic or
         tiglinic acid, a derivative of crotonic acid.
         [1913 Webster]