methylene

Methylene is a term used in organic chemistry to refer to the methylene group, a CH2 moiety that can act as a connecting unit between two substituents in a molecule, or, less commonly, to the reactive methylene fragment CH2. The methylene group is divalent, typically forming two single bonds and carrying two hydrogens. In most saturated hydrocarbons, internal -CH2- units link carbon atoms and help define chain length and flexibility. Methylene groups are central to polymers such as polyethylene, where long sequences consist of repeating -CH2- units.

In structural contexts, methylene is best thought of as a linking unit that can bridge two parts of a molecule, or appear as part of a larger framework. The group is generally sp3-hybridized when part of organic compounds, contributing to the three-dimensional arrangement and properties of the substance. The presence and arrangement of methylene units influence melting points, solubility, and flexibility in hydrocarbons and polymers.

The term can also refer to the reactive methylene fragment CH2, a carbene that is highly unstable and short-lived under ordinary conditions. In everyday practice, methylene appears in names such as methylene chloride (dichloromethane), methylene blue (a dye), or in reactions described as methylenation, which introduce a CH2 group into a molecule. Spectroscopically, methylene protons exhibit characteristic signals in NMR and vibrational features in infrared spectra.