Which of the following is an aromatic compound?
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The Correct Option is A
Solution and Explanation
Option (1), cyclopentadienyl anion: all 5 carbons are sp2 (two C=C give 4 \(\pi\) electrons and the carbanion lone pair adds 2), total 6 \(\pi\) electrons, so \(n=1\): aromatic.
Option (2): one C=C and two carbanion centres, but one ring carbon is an sp3 \(CH_2\), so conjugation is not continuous: not aromatic.
Option (3): cyclopentadiene has an sp3 \(CH_2\) in the ring: not aromatic.
Option (4): cyclopentadienyl cation has only 4 \(\pi\) electrons (\(4n\) type): anti-aromatic.
Answer: (1) cyclopentadienyl anion
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