Aromatic compounds follow these criteria (Hückel's Rule):
Cyclic: The molecule must be cyclic (a ring).
Planar: The molecule must be planar (all atoms in the same plane).
Conjugated: The molecule must have a continuous ring of overlapping p-orbitals (alternating single and double bonds).
Hückel's Rule (4n + 2 π electrons): The molecule must have (4n + 2) π electrons in the ring, where n is an integer (0, 1, 2, 3...).
Non-Benzenoid Aromatic Compounds
These are aromatic compounds that do not contain a benzene ring.
Analyzing the Given Compounds:
i (Aniline): This is benzene with an -NH2 substituent. It is a benzenoid aromatic compound.
ii (Phenol): This is benzene with an -OH substituent. It is a benzenoid aromatic compound.
iii (Cyclopentadienyl Anion): This is a cyclic, planar, and conjugated system. It has 6 π electrons (2 from each double bond and 2 from the negative charge), which satisfies Hückel's rule (4n + 2, where n = 1). It is aromatic, and it is non-benzenoid.
iv (Pyridine): This is benzene with one CH replaced by N. It is an aromatic, and it is non-benzenoid.
Therefore, the non-benzenoid aromatic compounds are iii and iv.
The correct answer is (A) iii and iv
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Approach Solution -2
1. Definition of Benzenoid and Non-Benzenoid Aromatic Compounds:
Benzenoid Aromatic Compounds: These are derivatives of benzene (\( \text{C}_6\text{H}_6 \)) that retain the aromaticity of benzene. They have a six-membered ring with alternating single and double bonds, and their resonance structures involve delocalized electrons.
Non-Benzenoid Aromatic Compounds: These are aromatic compounds that do not have the benzene ring structure but still exhibit aromaticity due to the presence of a conjugated system with \( 4n+2 \) π electrons (Hückel's rule).
2. Analyze Each Compound:
(i) \( \text{NH}_2 \)-substituted benzene:
This is aniline (\( \text{C}_6\text{H}_5\text{NH}_2 \)).
It is a derivative of benzene, where the amino group (\( \text{NH}_2 \)) is attached to the benzene ring.
Since it retains the benzene ring structure, it is a benzenoid aromatic compound.
(ii) Phenol:
Phenol (\( \text{C}_6\text{H}_5\text{OH} \)) is a derivative of benzene, where the hydroxyl group (\( \text{OH} \)) is attached to the benzene ring.
Like aniline, phenol retains the benzene ring structure.
Therefore, phenol is also a benzenoid aromatic compound.
(iii) Cyclopentadienide Ion (\( \text{C}_5\text{H}_5^- \)):
The cyclopentadienide ion has a five-membered ring with alternating single and double bonds.
It satisfies Hückel's rule (\( 4n+2 \) π electrons): \( n = 1 \), so \( 4(1) + 2 = 6 \) π electrons.
However, it does not have the benzene ring structure.
Therefore, the cyclopentadienide ion is a non-benzenoid aromatic compound.
(iv) Pyridine:
Pyridine (\( \text{C}_5\text{H}_5\text{N} \)) is a heterocyclic aromatic compound with a six-membered ring containing one nitrogen atom.
It satisfies Hückel's rule (\( 4n+2 \) π electrons): \( n = 1 \), so \( 4(1) + 2 = 6 \) π electrons.
Although pyridine has a six-membered ring, it does not have the benzene ring structure because it contains a nitrogen atom instead of a carbon atom.
Therefore, pyridine is a non-benzenoid aromatic compound.
3. Identify the Non-Benzenoid Aromatic Compounds:
From the analysis, the non-benzenoid aromatic compounds are:
Aromatic hydrocarbons, sometimes known as arenes, are aromatic organic molecules made up entirely of carbon and hydrogen. In aromatic compounds a benzene ring which is named after the simple aromatic chemical benzene, or a phenyl group when part of a larger structure, is the configuration of six carbon atoms.
This reaction involves the replacement of one substituent on the ring of an aromatic hydrocarbon, commonly a hydrogen atom, by a different substituent group.
The common types of aromatic substitution reactions are:
