(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic.
(ii) For aromatic molecules, solve for n in Hückel’s rule. For all other molecules, explain which rule of aromaticity is being broken.
(g)

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Identify the molecule: The structure shown is biphenylene, which consists of two benzene rings connected by a four-membered ring.

Determine the number of π-electrons: Count the π-electrons in the conjugated system. Each double bond contributes two π-electrons. Biphenylene has 4 double bonds, contributing a total of 8 π-electrons.

Apply Hückel's rule: For a molecule to be aromatic, it must be cyclic, planar, fully conjugated, and have (4n + 2) π-electrons, where n is a non-negative integer. Check if 8 π-electrons fit this rule.

Check planarity and conjugation: Ensure the molecule is planar and fully conjugated. Biphenylene is planar and conjugated, but the 8 π-electrons do not satisfy Hückel's rule.

Classify the molecule: Since biphenylene is planar and conjugated but has 4n π-electrons (where n=2), it is antiaromatic, as it does not satisfy the (4n + 2) rule for aromaticity.

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Aromaticity

Aromaticity refers to a property of cyclic compounds that exhibit enhanced stability due to the delocalization of π electrons across the ring structure. For a molecule to be classified as aromatic, it must be cyclic, planar, fully conjugated, and follow Hückel's rule, which states that the number of π electrons must equal 4n + 2, where n is a non-negative integer.

Hückel's Rule

Hückel's rule is a criterion used to determine the aromaticity of a compound. It states that a planar, cyclic molecule is aromatic if it contains 4n + 2 π electrons in its conjugated system. This rule helps in identifying stable aromatic compounds, as they tend to have lower energy compared to their nonaromatic or antiaromatic counterparts.

Anti-aromaticity

Anti-aromaticity is a property of cyclic compounds that have a full conjugated π system but do not satisfy Hückel's rule, typically containing 4n π electrons. These compounds are less stable than their nonaromatic counterparts due to the presence of destabilizing interactions among the π electrons, leading to higher energy states and increased reactivity.

(i) Classify the following molecules as aromatic, nonaromatic, or antiaromatic. (ii) For aromatic molecules, solve for n in Hückel’s rule. For all other molecules, explain which rule of aromaticity is being broken.(g)