Question

Which one of the following is aromatic?
$\left( A \right)$ Cyclopentadienyl cation
$\left( B \right)$ Cyclooctatetraene
$\left( C \right)$ Cycloheptatriene
$\left( D \right)$ Cycloheptatrienyl cation

Answer 1

Aromatic compounds are the class of organic compounds that consist of conjugated planar ring systems accompanied by delocalized pi-electrons in place of individual alternating double bond and single bond.

Complete answer:
In the given question we have asked which one of the following is an aromatic compound. There are three conditions for an aromatic compound, all the carbon atoms are $s{p^2}$ hybridized, pi-electron clouds should be delocalized over the ring of aromatic compounds and they should also follow Huckle’s rule.
Now, In the given question only cycloheptatrienyl cation is aromatic because according to Huckle’s rule, an aromatic must have $\left( {4n + 2} \right)\pi$ electron that forms an uninterrupted $\pi$ cloud. It is also cyclic and planar because it has $s{p^2}$ hybridized carbon atoms. So we can say that Cycloheptatrienyl cation is an aromatic compound because it has $6$ electron in its pi system. It has conjugated system and cyclic and planar

So, the correct option is $D.$

Additional information:
Aromatic compounds are also known as arenes are chemical compounds that contain conjugated planar ring systems with delocalized pi electrons and also alternating single bond and double bonds. Most common examples are benzene.
Benzene is an aromatic compound which contains three alternating single and double bonds and each carbon atom has $s{p^2}$ hybridized and possesses a planar ring system. The pi electron of benzene is delocalized over the ring.

Note:
Huckle’s rule stated that all planar aromatic compounds must have $\left( {4n + 2} \right)\pi$ pi-electron system where $n$ can be a whole number. The Huckle’s rule helps to determine whether a planar ring compound possesses aromatic properties or not.