Cyclononatetraene is an organic compound with the formula C<sub>9</sub>H<sub>10</sub>. It was first prepared in 1969 by protonation of the corresponding aromatic anion (described below). It is unstable and isomerizes with a half-life of 50 minutes at room temperature to via a thermal 6ÃÂ disrotatory electrocyclic ring closing. Upon exposure to ultraviolet light, it undergoes a photochemical 8ÃÂ electrocyclic ring closing to give .
Cyclononatetraenyl anion is a 10àaromatic system. Two isomers of the cyclononatetraenyl anion are known: the trans,cis,cis,cis isomer ("Pac-Man"-shaped) and the all-cis isomer (a convex enneagon). The former is less stable and isomerizes to the latter upon warming from âÂÂ40 ðC to room temperature.
The all-cis isomer of C<sub>9</sub>H<sub>9</sub><sup>âÂÂ</sup> can be prepared by treatment of 9-chlorobicyclo[6.1.0]nona-2,4,6-triene (1) with lithium or potassium metal. Despite the ring strain resulting from having CâÂÂCâÂÂC bond angles of 140ð instead of the ideal 120ð for sp<sup>2</sup> carbon, this species is believed to be planar and to possess D<sub>9h</sub> symmetry. The lithium salt was found to react with cyclopentadiene to give lithium cyclopentadienide, showing that cyclononatetraene is a weaker acid than cyclopentadiene.
Cyclononatetraenyl cation is an 8àsystem. Its intermediacy is implicated in the solvolysis of 1. The facile solvolysis of 1 suggests that the cation is stabilized. Computation and experimental evidence suggest that is a rare example of a ground state species that exhibits Möbius aromaticity. A more recent study, however, suggested that the stability of is not much different in energy compared to a Hückel topology isomer. Laser flash photolysis of 1 identified an intermediate, of half-life 1.5 üs, with UVâÂÂVis spectrum matching the computed spectrum of the Hückel isomer, but failed to detect any intermediates with spectrum matching the Möbius isomer.