Why is cyclooctatetraene neither aromatic or anti-aromatic?
(I can explain why it isn't aromatic (4n+2 fails), but i can't explain why it isn't anti-aromatic)
Its reactivity (instability) may not be high enough to be classified as anti-aromatic. [Actual data on this are not easily available.]
To determine whether a molecule is aromatic or anti-aromatic, we need to analyze its electronic configuration and the Huckel's rule. Aromaticity refers to a cyclic, planar molecule with a continuous π-electron system and follows the Huckel's rule of having 4n+2 π-electrons, where n is an integer. Conversely, anti-aromatic molecules have a cyclic, planar structure but with 4n π-electrons.
Let's examine cyclooctatetraene (COT) to understand why it is neither aromatic nor anti-aromatic.
To determine aromaticity, we count the π-electrons in the molecule. Cyclooctatetraene contains a conjugated system of eight carbon atoms in a cyclic arrangement, meaning that it has eight π-electrons. Applying the Huckel's rule (4n+2), we expect an aromatic compound if (4n+2) = 8. However, this does not hold true; there is no integer value of n that satisfies the equation. Therefore, cyclooctatetraene is not aromatic.
Now, let's consider anti-aromaticity. For a molecule to be anti-aromatic, it must have a cyclic, planar structure with a continuous π-electron system and 4n π-electrons. In the case of cyclooctatetraene, it also has eight π-electrons, which matches the 4n electron count for anti-aromaticity. However, the molecular geometry of cyclooctatetraene prevents it from achieving anti-aromaticity.
In cyclooctatetraene, the carbon atoms form a tub-shaped structure rather than a flat, planar arrangement. Due to this tub-like distortion, the molecule's π-electron density is not evenly distributed throughout the cyclic system, breaking the continuous π-electron flow required for anti-aromaticity. As a result, cyclooctatetraene is not anti-aromatic either.
In summary, cyclooctatetraene is neither aromatic nor anti-aromatic because it doesn't fulfill the requirements for either category: it has neither the (4n+2) π-electrons needed for aromaticity nor the 4n π-electrons necessary for anti-aromaticity.