Cyclotetradecaheptaene, often referred to as 4nnulene, is a

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ...

with molecular formula C₁₄H₁₄, which played an important role in the development of criteria (

Hückel's rule In organic chemistry, Hückel's rule predicts that a planar ring molecule will have aromatic properties if it has 4''n'' + 2 π electrons, where ''n'' is a non-negative integer. The quantum mechanical basis for its formulation was f ...

) for

aromaticity In chemistry, aromaticity is a chemical property of cyclic (ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to sat ...

, a stabilizing property of central importance in

physical organic chemistry Physical organic chemistry, a term coined by Louis Hammett in 1940, refers to a discipline of organic chemistry that focuses on the relationship between chemical structures and reactivity, in particular, applying experimental tools of physical ...

. It forms dark-red needle-like crystals.

Structure and aromaticity

Although the conjugated ring of 4 nnulene contains 4''n''+2 electrons, it only exhibits limited evidence for being aromatic. It does not fully conform to Hückel's rule because none of its ''cis''/''trans'' isomers can adopt a completely planar conformation due to crowding of the interior hydrogens. There is evidence that it has two isomeric forms of comparable stability (''trans'', ''cis'', ''trans'', ''cis'', ''trans'', ''trans'', ''cis''- with four interior hydrogens (shown in the infobox) and ''trans'', ''cis'', ''trans'', ''cis'', ''trans'', ''cis'', ''cis-'' with three interior hydrogens) which rapidly interconvert at room temperature but can be observed at low temperature by NMR. Its ¹H NMR spectrum shows evidence of aromatic ring currents that result in an upfield shift for the interior hydrogens. In contrast, the corresponding 2 and 6nnulenes, which are weakly antiaromatic or nonaromatic, have downfield shifted interior hydrogens. However, unlike the undoubtedly aromatic 8nnulene, 4nnulene does not bear the hallmark aromatic property of chemical stability, and it quickly decomposes when exposed to light and air.

References

Annulenes {{hydrocarbon-stub