H2CCCH2

H2CCCH2 is the shorthand for the hydrocarbon known as allene or propadiene, with the chemical formula C3H4. It is the simplest member of the allene family and is commonly represented by H2C=C=CH2, reflecting its cumulated diene structure.

Structure and bonding: Allene features two perpendicular carbon–carbon double bonds arranged in a linear three-carbon chain. The central carbon is sp-hybridized, while the terminal carbons are sp2-hybridized. Each terminal carbon carries two hydrogens, resulting in a molecule where the two CH2 groups lie in orthogonal planes. In the unsubstituted molecule, this geometry makes allene achiral, though substituted allenes can be chiral when the terminal groups are different.

Nomenclature and relatives: The preferred IUPAC name is propadiene; allene is the common trivial name. H2CCCH2 is a compact shorthand used in some contexts to denote the same structure without explicit bond notations.

Synthesis and uses: Allene is typically prepared and used as a reactive intermediate in organic synthesis. Common preparation methods involve transformations of propargyl derivatives, including base-induced eliminations or rearrangements that generate the cumulated diene. In the laboratory and industry, allenes serve as versatile building blocks for cycloaddition reactions and for the construction of more complex carbon frameworks. They can participate in (4+2) and other cycloaddition chemistry under appropriate conditions.

Physical properties and safety: Allene is a colorless, volatile hydrocarbon that is flammable and should be handled with standard laboratory precautions. Its reactivity stems from the cumulated pi bonds, making it more reactive than many simple alkenes.

See also: Allene, propadiene.