Active methylene compound

In organic chemistry, a methylene bridge or methanediyl group is any part of a molecule with formula -CH
2-; namely, a carbon atom bound to two hydrogen atoms and connected by single bonds to two other distinct atoms in the rest of the molecule. It is the repeating unit in the skeleton of the unbranched alkanes.

A methylene bridge can also act as a bidentate ligand joining two metals in a coordination compound, such as titanium and aluminum in Tebbe's reagent.

A methylene bridge is often called a methylene group or simply methylene, as in "methylene chloride" (dichloromethane CH
2Cl
2). However, the term methylene group (or "methylidene") properly applies to the CH
2 group when it is connected to the rest of the molecule by a double bond, giving it chemical properties very distinct from those of a bridging CH
2 group.

Compounds possessing a methylene bridge located between two strong electron withdrawing groups (such as nitro, carbonyl or nitrile groups) are sometimes called active methylene compounds. Treatment of these with strong bases can form enolates or carbanions, which are often used in organic synthesis. Examples include the Knoevenagel condensation and the malonic ester synthesis.

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