CH2CCH2

CH2CCH2 refers to allene, also known as propadiene, the hydrocarbon with formula C3H4 and the simplest example of a cumulene, containing two cumulated carbon–carbon double bonds (CH2=C=CH2).

In allene, the central carbon is sp-hybridized and forms two perpendicular π bonds to the terminal carbons, which are each sp2-hybridized. The result is a linear C=C=C backbone with the two terminal CH2 groups oriented in perpendicular planes. The molecule exhibits high symmetry, typically described as D2d in the gas phase for the idealized structure.

CH2CCH2 is one of the two structural isomers of C3H4; the other is propyne (methylacetylene), with the formula CH3-C≡CH. Allene and propyne differ in connectivity and reactivity, making them distinct members of the C3H4 family.

Preparation and occurrence: Allene can be formed in small quantities as an intermediate in various organic syntheses and can be generated from propargyl precursors via dehalogenation or rearrangements. It is typically handled and studied under controlled laboratory conditions due to its reactive cumulated double bonds and tendency to participate in further transformations.

Reactivity and applications: The cumulated double bonds endow allene with unique reactivity, enabling cycloadditions and electrophilic or nucleophilic additions at the terminal positions, often under catalysis. Allene derivatives are valuable building blocks in organic synthesis, enabling the introduction of allene moieties into complex molecules and serving as ligands in transition-metal chemistry. Industrial use is limited, but allenes appear as intermediates in the synthesis of other chemicals and materials.