allenes

Allenes are a class of organic compounds characterized by two adjacent carbon–carbon double bonds that share a central carbon, giving the general formula R2C=C=CR2. The simplest member is propadiene, CH2=C=CH2. In allenes the two pi bonds are orthogonal, so the substituents attached to the terminal carbons lie in perpendicular planes. The carbon framework is typically linear through the central carbon, with the C1–C2–C3 angle near 180° in unsubstituted examples.

Structurally, the central carbon of an allene is sp-hybridized, accommodating the two perpendicular p orbitals that form the cumulated double bonds. This arrangement creates two distinct pi systems that do not conjugate with each other in a simple, planar sense. As a result, the geometry around the terminal carbons can impart notable steric and stereochemical properties, particularly in substituted allenes.

Stereochemistry in allenes is notable: simple allene cores like propadiene are achiral, but many disubstituted allenes are chiral if the substituents on each terminal carbon are all different and the molecule lacks a mirror plane. Such axial chirality arises from the perpendicular orientation of the end groups.

Synthesis methods for allenes include double dehydrohalogenation of vic-dihaloalkanes, rearrangements of propargyl halides (such as the Fritsch–Buttenberg–Wiechell rearrangement), and various transitions from alkynes or propargyl precursors using metal catalysts. Reactions of allenes span electrophilic additions, cycloadditions, and metal-catalyzed couplings, making them useful building blocks in organic synthesis and in the preparation of natural products and functional materials.