객관식
Which of the following choices represents a pair of resonance structures?
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1. A Review of General Chemistry
Resonance Structures
객관식
In which of the following cases does a lone pair of electrons on an atom not contribute to the aromatic system of a ring?
A
When the lone pair is on a nitrogen atom that is already involved in a double bond within the ring
B
When the lone pair is on a sulfur atom in a six-membered aromatic ring
C
When the lone pair is on an oxygen atom in a five-membered aromatic ring
D
When the lone pair is on a nitrogen atom in pyrrole
검증된 단계별 안내1
Step 1: Understand that for a lone pair to contribute to a π aromatic system, it must be able to delocalize into the ring's conjugated π system, typically by overlapping with adjacent p orbitals to maintain continuous cyclic conjugation.
Step 2: Recognize that if a nitrogen atom is already involved in a double bond within the ring, its lone pair is typically in an sp2 orbital orthogonal to the π system and thus does not participate in conjugation or aromaticity.
Step 3: For heteroatoms like sulfur or oxygen in aromatic rings, lone pairs can often contribute to the π system if they occupy p orbitals that overlap with the ring's π orbitals, as seen in five- or six-membered heteroaromatic rings.
Step 4: In pyrrole, the nitrogen's lone pair is in a p orbital and contributes to the aromatic sextet, enhancing the ring's aromaticity.
Step 5: Therefore, the key is to identify whether the lone pair is part of the conjugated π system or localized in an orbital that does not overlap with the ring's π orbitals, which happens when the atom is already involved in a double bond.
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