Question

Pentalene is a most elusive molecule that has been isolated only at liquid- nitrogen temperature. The pentalene dianion, however, is well known and quite stable. Explain.

Short answer

Pentalene is antiaromatic and unstable, while its dianion is aromatic and stable.

Step-by-step solution

Understand the Structure

Pentalene is an 8-carbon, nonaromatic hydrocarbon that forms two connected five-membered rings. Its structure often leads to instability in neutral molecules.

Discuss Aromaticity and Stability

Pentalene on its own is antiaromatic as it contains 8 π-electrons. According to Hückel's rule, for a molecule to be aromatic (and thus more stable), it needs to have 4n+2 π-electrons. Since 8 is not of the form 4n+2, neutral pentalene is destabilized by its antiaromaticity.

Dephenolate to Assess Dianion

The pentalene dianion ( ext{C}_8 ext{H}_6^{2-}) is formed by the addition of two electrons to the molecule, which increases the number of π-electrons to 10.

Apply Hückel's Rule to Dianion

With 10 π-electrons, the dianion follows Hückel's rule (since 10 = 4n+2, where n=2). This number of π-electrons allows the dianion to be aromatic, making it energetically stable.

Conclusion

Due to its aromatic nature from having 10 π-electrons, the pentalene dianion is stable, unlike its neutral counterpart which is antiaromatic and thus unstable.

Key concepts

Aromaticity

Aromaticity is a concept in organic chemistry used to describe molecules that exhibit enhanced stability due to their electronic structure. It is a property of cyclic, planar molecules that enables them to possess lower energy levels than other similar structures that lack this feature. Aromatic compounds tend to have high resonance stabilization, making them energetically favorable and quite stable compared to non-aromatic or antiaromatic counterparts.

To be considered aromatic, a molecule must meet certain criteria:

• It should have a cyclic structure, where all atoms in the ring allow for a continuous overlap of p-orbitals.
• The molecule needs to be planar, meaning all the atoms in the cycle must lie in the same plane.
• It must satisfy Hückel's rule, which relates to the number of π-electrons contributing to the aromatic system.

This special aromatic stability occurs due to the delocalization of π-electrons across the cyclic structure, leading to a lower energy state.

Hückel's Rule

Hückel's rule is a fundamental principle in determining the aromaticity of a molecule. It provides a simple formula to predict whether a cyclic planar molecule will possess aromatic properties. According to this rule, a molecule is aromatic if it contains a total of \(4n+2\) π-electrons, where \(n\) is a non-negative integer (0, 1, 2, etc.).

Consider the pentalene dianion. In its stable dianion form, it has 10 π-electrons. Applying Hückel's rule, since 10 can be expressed as \(4(2) + 2\), it fits perfectly into the rule with \(n=2\). Thus, these 10 π-electrons in the dianion form make it aromatic and lend to its stability.

Hückel's rule has been a cornerstone in understanding the stability and reactivity of aromatic systems, guiding chemists in predicting which cyclic molecules will exhibit aromatic characteristics. It distinguishes aromatic compounds from non-aromatic ones, and more importantly, from antiaromatic molecules, which often have exactly 4n π-electrons and are destabilized by this configuration.

Antiaromaticity

Antiaromaticity refers to a situation where a cyclic planar molecule contains 4n π-electrons, a configuration that leads to instability rather than stability, due to the electron arrangement in such a system. Unlike aromatic molecules, which benefit from π-electron delocalization, antiaromatic molecules are often at a higher energy state. This higher energy state is due to the repulsion and lack of electron delocalization harmony seen in aromatic systems.

Pentalene in its neutral form serves as a classic example of an antiaromatic compound since it has 8 π-electrons. Applying Hückel's rule rejects stability for this configuration (\(n=2\) but 8 ≠ \(4(2) + 2\)). Hence, rather than being stabilized, the 8 π-electron configuration leads to electrons experiencing avoidable repulsion within the system, making the molecule unstable and elusive unless it is stabilized by adding electrons or creating its dianion state.

• This means that such molecules often prefer to avoid the antiaromatic form, transforming into other structures or states, such as the pentalene transforming into its more stable dianion form by acquiring additional electrons.

Understanding antiaromaticity is critical for chemists, especially when designing new molecules or studying reaction mechanisms that might involve transient antiaromatic intermediates.