Question

Cyclooctatetraene is not aromatic. The most important reason for this is that: (a) It is a planar molecule (b) It has eight \(\pi\) -electrons (c) Its structure cannot be described by more than the canonical forms (d) Its structure is not that of regular octagon

Short answer

Cyclooctatetraene is not aromatic because it has eight π electrons.

Step-by-step solution

Understand Aromaticity

For a molecule to be considered aromatic, it must meet certain criteria: it should be cyclic, planar, fully conjugated (p orbitals at each atom in the ring), and follow Hückel's rule, where the number of π electrons must be \(4n+2\) (where \(n\) is a nonnegative integer).

Assess Cyclooctatetraene

Cyclooctatetraene is an eight-membered ring with alternating double and single bonds, giving it eight π electrons. Being an even number, it does not satisfy the \(4n+2\) rule (it satisfies \(4n\) instead), which strongly suggests it is non-aromatic according to Hückel's rule.

Analyze Planarity and Structure

For a molecule to be aromatic, planarity is essential so that the p orbitals can overlap effectively. Cyclooctatetraene adopts a non-planar, tub-shaped conformation to relieve angle and torsional strains, which prevents effective delocalization of π electrons.

Conclusion on Choice

The correct explanation as to why cyclooctatetraene is non-aromatic is that it has eight π electrons and doesn't satisfy the \(4n+2\) Hückel's aromatic criteria. Other factors like planarity and shape contribute further, but the primary reason is the electron count.

Key concepts

Hückel's Rule

Aromaticity is a fascinating concept in chemistry that helps us understand why certain molecules exhibit unusual stability. One of the fundamental principles to determine aromaticity is Hückel's Rule. According to this rule, a molecule must have a specific number of π electrons to be considered aromatic. The rule states that the total number of π electrons must fit the formula: 4n + 2, where \( n \) is a nonnegative integer. This provision ensures that the π electrons are completely delocalized across the ring, contributing to the molecule's stability and unique chemical properties.

In simple terms, aromatic molecules enjoy extra stability because their electrons are able to freely circulate around the ring, effectively creating a 'cloud'. This electron cloud leads to what is commonly referred to as resonance energy, which gives aromatic compounds distinctive stability compared to non-aromatic ones. For example, benzene, with its 6 π electrons, adheres to Hückel's criterion when \( n \) equals 1, as 4(1)+2 equals 6. This configuration is not only stable, making it chemically inert in many reactions, but also energetically favorable.

Cyclooctatetraene

Cyclooctatetraene (COT) is an intriguing compound that, despite having an eight-membered ring with alternating double and single bonds (totaling eight π electrons), is not aromatic. This seems counterintuitive but becomes clearer when we evaluate it against Hückel's Rule and consider its geometric structure.

To determine aromaticity, we rely on the 4n + 2 rule. Cyclooctatetraene fits more accurately into the formula 4n, with n equating to 2, giving it 8 π electrons. This count doesn't comply with the aromaticity conditions outlined by Hückel's Rule. Therefore, the electron arrangement isn't suitable for delocalization and stabilization.

Moreover, its physical structure plays a critical role. Cyclooctatetraene naturally adopts a tub-shaped conformation rather than a planar one. This shape choice is driven by the need to relieve angle strain present in what would otherwise be forced into a planar octagon. As a result, the π electrons in cyclooctatetraene are not effectively delocalized, making it unstable and thus non-aromatic.

Planarity in Aromatic Compounds

Planarity is a crucial structural feature for aromatic compounds. It ensures that p orbitals can overlap continuously, allowing the delocalization of π electrons across the entire ring. Without planarity, this overlap is disrupted, and π electrons cannot move freely, undermining the stability and special properties associated with aromatic molecules.

In the case of cyclooctatetraene, its non-planar, tub-shaped conformation negates the possibility of achieving effective orbital overlap. The molecule avoids adopting a planar form due to the substantial angle strain it would otherwise endure, especially given its eight-membered ring, which is rare for aromatic compounds. The lack of planarity prevents cyclooctatetraene from being classified as aromatic, despite having a cyclic structure and conjugated double bonds.

This requirement of planarity is why most well-known aromatic compounds, such as benzene or naphthalene, are entirely flat structures. Their structural geometry plays a vital role in enabling the free movement of their electrons, thus fully realizing the unique stability attributed to aromatic compounds.