Question

The correct statement regarding cyclic planar molecules is (A) It is aromatic compound if its all bonding molecular orbitals are filled and non-bonding molecular orbitals if present, are empty or completely filled. (B) It is antiaromatic compound if it has electrons in antibonding molecular orbitals or if it has half-filled bonding or a non-bonding molecular orbitals. (C) It is an aromatic compound if it contains \((4 n+2)\) peripheral \(\pi\)-electrons. (D) All of the above

Short answer

The correct statement regarding cyclic planar molecules is (D) All of the above. All three statements accurately describe conditions for aromatic or antiaromatic molecules, including conditions related to bonding/non-bonding molecular orbitals and Hückel's rule (4n+2 peripheral π-electrons).

Step-by-step solution

Understanding Aromaticity, Antiaromaticity, and Cyclic Planar Molecules

Aromatic compounds are cyclic planar molecules with resonance structures and follow Hückel's rule, having (4n+2) π-electrons, where n is an integer (0, 1, 2, 3...). They have delocalized π-electrons that spread across the ring structure. Antiaromatic compounds are also cyclic planar molecules with conjugated π-electrons. However, they do not follow Hückel's rule and have 4n π-electrons instead. Cyclic planar molecules can be either aromatic or antiaromatic based on their molecular orbital structures and electron configurations.

Analyzing Statement A

Statement A says that a cyclic planar molecule is aromatic if all its bonding molecular orbitals are filled and non-bonding molecular orbitals, if present, are empty or completely filled. This statement describes the condition for an aromatic compound, as an aromatic molecule has filled bonding orbitals and, if there are non-bonding molecular orbitals, they must be completely filled.

Analyzing Statement B

Statement B claims that a cyclic planar molecule is antiaromatic if it has electrons in antibonding molecular orbitals or if it has half-filled bonding or non-bonding molecular orbitals. This statement corresponds to the condition for an antiaromatic molecule. Antiaromatic molecules have electron configurations that destabilize the molecule, including electrons in antibonding molecular orbitals or half-filled bonding/non-bonding molecular orbitals.

Analyzing Statement C

Statement C says that a cyclic planar molecule is aromatic if it contains (4n+2) peripheral π-electrons, where n is an integer. This statement refers to Hückel's rule, which is a defining characteristic of aromatic molecules.

Conclusion

After analyzing all three statements, we can conclude that all of them correctly describe conditions for aromatic or antiaromatic molecules. Therefore, the correct answer to the question is (D) All of the above.

Key concepts

Hückel's Rule

Aromaticity is a fascinating concept in organic chemistry, primarily dictated by Hückel's rule. According to this rule, for a molecule to be considered aromatic, it must possess a specific cyclic, planar configuration with a conjugated system of π-electrons.
Hückel's rule states that the compound should have \( (4n + 2) \) π-electrons, where "n" is a non-negative integer, such as 0, 1, 2, and so on.
This equation suggests that compounds with 2, 6, 10, 14, etc., π-electrons are predicted to be aromatic.
These electrons are delocalized, meaning they are spread over the entire ring structure, creating a resonance stabilization effect that significantly lowers the energy of the molecule.
Thus, Hückel’s rule serves as a guideline for determining the aromatic nature of a compound, underpinning its special stability.

Antiaromatic Compounds

In contrast to aromatic compounds, antiaromatic compounds present an intriguing anomaly. These molecules are also cyclic and planar, possessing conjugated π-electrons, yet they defy the stability conferred by aromaticity.
Antiaromatic compounds are defined by having exactly \( 4n \) π-electrons, where "n" is an integer similar to Hückel's rule.
However, their electron configuration does not confer stability. Rather, it promotes instability due to the presence of delocalized electrons that create unfavorable electronic interactions.
These interactions often involve electrons occupying antibonding molecular orbitals or having half-filled bonding or non-bonding molecular orbitals, making the compound less stable than its aromatic counterpart.
Understanding these electron configurations is crucial for recognizing why some cyclic planar molecules do not gain stability even though they follow structural prerequisites similar to aromatic compounds.

Cyclic Planar Molecules

Cyclic planar molecules are the overarching category under which both aromatic and antiaromatic compounds fall. Such molecules are characterized by having atoms arranged in a planar (flat) ring.
For a molecule to be cyclic and planar, each atom within the ring must typically adopt an sp² hybridization, allowing the structure to lay in a single plane.
This configuration allows for the overlap of π-orbitals, creating a continuous cycle of π-electrons, fundamental for the delocalization observed in aromaticity.
However, not every cyclic planar molecule is aromatic.

• If the molecule obeys Hückel's rule and has \( (4n+2) \) π-electrons, it is aromatic and highly stable.


• If it has \( 4n \) π-electrons, it becomes antiaromatic and is typically unstable.

Therefore, while the cyclic planar structure is critical for determining molecule type, π-electron count and configuration are decisive in discerning between stability and instability.