What is "pentamethylene"

Pentamethylene \Pen`ta*meth"yl*ene\, n. [Penta- + methylene.] (Chem.) 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.

n. (context organic compound English) A hypothetical hydrocarbon, C₅H₁₀, metameric with the amylenes, and the nucleus of a large number of derivatives.