The Collaborative International Dictionary of English v.0.48:

Pentyl \Pen"tyl\, n. [Penta + -yl.] (Chem.)
   The hypothetical radical, C5H11, of pentane and certain of
   its derivatives. Same as Amyl.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Amyl \Am"yl\, n. [L. amylum starch + -yl. Cf. Amidin.] (Chem.)
   A univalent hydrocarbon radical, C5H11, of the paraffine
   series found in amyl alcohol or fusel oil, etc.
   [1913 Webster]