Pentamethylene /(?)/

Pen·ta·meth·yl·ene

Pentamethylene

n.
  1. A hypothetical hydrocarbon, C5H10, metameric with the amylenes, and the nucleus of a large number of derivatives; -- so named because regarded as composed of five methylene residues. Cf. Trimethylene, and Tetramethylene. (Chem.)