Molecules that are considered aromatic have 'special' stability.Which of the following (I - V) DO NOT satisfy the requirements for aromaticity?

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The concept at play here is aromaticity: a molecule must be cyclic, planar, fully conjugated, and follow Hückel’s rule (4n + 2 pi electrons) to be aromatic.

I. I appears as a six-membered ring that is fully conjugated with alternating double bonds around the ring, giving 6 pi electrons. This satisfies 4n + 2 with n = 1, and if the ring is planar, it should be aromatic.

II. II also shows a fully conjugated six-membered ring with six pi electrons, again matching 4n + 2 for n = 1. This pattern is consistent with aromaticity provided planarity is maintained.

III. III resembles a benzene-like ring system with all atoms participating in the conjugation and 6 pi electrons in the ring. This aligns with the aromatic criterion (4n + 2, n = 1) as long as there is no disruption to planarity or conjugation.

IV. IV contains a methyl substituent (CH3) but crucially the ring appears to have a disruption in conjugation (likely due to an sp3 center or a nonplanar geometry at that position), breaking the continuous p-orbital overlap needed for a fully conjugated loop. Without complete conjugation (or with nonplanarity), it cannot be aromatic, even if the total pi count would naively seem favorable. Therefore, IV does not meet the aromatic criteria.

V. V shows a fused bicyclic aromatic system (looks like naphthalene or a similar structure) where the entire system is conjugated and contains 10 pi electrons overall. That fits 4n + 2 with n = 2, which is aromatic.

In summary, the only structure lacking complete conjugation (or proper planarity) and thus failing the aromatic criteria is IV. All the others (I, II, III, V) fulfill the necessary conditions for aromaticity under typical interpretations of these structures.

My LMS Subjects Practice LMS questions, Week 3

Molecules that are considered aromatic have 'special' stability.Which of the following (I - V) DO NOT satisfy the requirements for aromaticity?

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