An aromatic species must be cyclic, planar, have a continuous ring of p-orbitals, and contain ( 4n+2 ) π-electrons (Huckel's rule). Let's apply these rules to each option (c) Cycloheptatrienyl cation : It is cyclic, planar, and has 6π-electrons. This follows Huckel's rule aromatic. (b) Cyclopentadienly anion: It is cyclic, planar, and has 6π-electrons. This follows Huckel's rule aromatic. (a) Cyclohexadienyl cation: It is not aromatic. It is anti-aromatic as it has 4n electrons, which violates the Huckel's rule for aromaticity. (d) Naphthalene : It is cyclic, planar, and has 10π-electrons. This follows Huckel's rule aromatic. Therefore, the cyclohexadienyl cation is the only species that is not aromatic.
