Inchemistry, amethylene bridge is part of amolecule with formula−CH2−. Thecarbon atom is connected bysingle bonds to two other distinct atoms in the rest of the molecule. A methylene bridge is often called amethylene group or simplymethylene, as in "methylene chloride" (dichloromethaneCH
2Cl
2). As a bridge in other compounds, for example in cyclic compounds, it is given the namemethano. However, the termmethylidene group (not to be confused with the termmethylene group, nor thecarbenemethylidene) properly applies to theCH
2 group when it is connected to the rest of the molecule by adouble bond (=CH2), giving it chemical properties very distinct from those of a bridgingCH
2 group.
It is therepeating unit in the skeleton of the unbranchedalkanes.Polyethylene also can be called polymethylene.
Compounds possessing a methylene bridge located between twoelectron withdrawing groups (such asnitro,carbonyl ornitrile groups) are sometimes called active methylene compounds.[1] Treatment of these with strong bases can formenolates orcarbanions, which are often used inorganic synthesis. Examples include theKnoevenagel condensation and themalonic ester synthesis.[2]
Examples of compounds that contain an activated methylene bridge include:
A methylene bridge can be aligand joining two metals.Titanium andaluminum are linked by a methylene bridge inTebbe's reagent.[3]
Methylene bridge is sometimes called amethylene spacer ormethanediyl group.