Cyclodecapentaene

Cyclodecapentaene or [10]annulene is an annulene with molecular formula C₁₀H₁₀. This organic compound is a conjugated 10 pi electron cyclic system and according to Huckel's rule it should display aromaticity. It is not aromatic, however, because various types of ring strain destabilize an all-planar geometry. The all-cis isomer (1), a fully convex decagon, would have bond angles of 144°, which creates large amounts of angle strain relative to the ideal 120° for sp² atomic hybridization. Instead, the all-cis isomer can adopt a planar boat-like conformation (2) to relieve the angle strain. This is still unstable compared to the next planar trans, cis, trans, cis, cis isomer (3). Yet even this isomer is also unstable, suffering from steric repulsion between the two internal hydrogen atoms. The nonplanar trans, cis, cis, cis, cis isomer (4) is the most stable of all the possible isomers.

The [10]annulene compound can be obtained by photolysis of cis-9,10-dihydronaphthalene as a mixture of isomers. Because of their lack in stability even at low temperatures the reaction products revert to the original dihydronaphthalene.

Aromaticity can be induced in compounds having a [10]annulene-type core by fixation of the planar geometries. There exist two methods to bring it about. One way is to replace two hydrogen atoms by a methylene bridge (-CH
2-) gives the planar bicyclic 1,6-methano[10]annulene (5). This compound is aromatic as indicated by from lack in bond length alternation seen in its X-ray structure and evidence of telltale diamagnetic ring current in its NMR spectrum.