12cyclohexadiene

1,2-Cyclohexadiene (often written 1,2-cyclohexadiene; molecular formula C6H8) is a cyclic allene in which two double bonds are cumulated at adjacent carbon atoms in a six-membered ring (C1=C2=C3). Unlike the common conjugated isomer 1,3-cyclohexadiene, the cumulated geometry of 1,2-cyclohexadiene imposes significant ring strain and a distorted allene axis, giving the molecule unusual electronic and steric properties.

The compound is thermally and chemically reactive and is typically encountered as a short-lived intermediate rather than an isolable bulk material. It can be generated under controlled conditions from precursor compounds by methods such as dehalogenation, elimination, or photochemical fragmentation, and has been characterized by matrix isolation, low-temperature spectroscopy, and trapping reactions. Computational studies and spectroscopic data indicate deviation from ideal linear geometry at the central carbon of the allene due to ring constraints, which influences frontier orbital energies and reactivity.

Reactivity of 1,2-cyclohexadiene is dominated by addition and cycloaddition processes; it undergoes nucleophilic additions, [2+2] and [4+2] cycloadditions and can serve as a dienophile or dienyl equivalent in synthetic transformations. These reactions have been exploited in organic synthesis for the construction of polycyclic frameworks and strained-ring systems. Because of its instability, practical use typically involves in situ generation and immediate trapping. Interest in 1,2-cyclohexadiene spans mechanistic organic chemistry, physical studies of cyclic allenes, and the development of methods that harness transient reactive intermediates.