Designate whether each of the following compounds is aromatic or not aromatic.

A compound is aromatic only if it satisfies all of the following conditions:

1. It must be cyclic.
2. It must be planar.
3. It must be completely conjugated, with continuous overlap of p-orbitals.
4. It must contain $$(4n+2)$$ $$\pi$$ electrons, where $$n$$ is a non-negative integer.

Systems that are cyclic, planar, and fully conjugated but contain $$4n$$ $$\pi$$ electrons are anti-aromatic and are considered not aromatic in this question.

Now, evaluate each structure:

• (a) Cyclopentadienyl anion
  • $$\pi$$ electrons = $$2$$ double bonds $$=4$$ electrons + one carbanion lone pair $$=2$$ electrons.
  • Total $$\pi$$ electrons = $$4+2=6$$.
  • It is cyclic, planar, fully conjugated, and satisfies Hückel's rule.
  • Result: Aromatic.
• (b) Cyclopentadienyl cation
  • $$\pi$$ electrons = $$2$$ double bonds $$=4$$ electrons + empty p-orbital $$=0$$ electrons.
  • Total $$\pi$$ electrons = $$4$$.
  • It is fully conjugated but contains $$4n$$ $$\pi$$ electrons.
  • Result: Not aromatic.
• (c) Cyclobutadienyl dication
  • $$\pi$$ electrons = $$1$$ double bond $$=2$$ electrons + two empty p-orbitals $$=0$$ electrons.
  • Total $$\pi$$ electrons = $$2$$.
  • It is cyclic, planar, fully conjugated, and satisfies Hückel's rule.
  • Result: Aromatic.
• (d) Cyclobutadienyl dianion
  • $$\pi$$ electrons = $$1$$ double bond $$=2$$ electrons + two lone pairs $$=4$$ electrons.
  • Total $$\pi$$ electrons = $$2+4=6$$.
  • It is cyclic, planar, fully conjugated, and satisfies Hückel's rule.
  • Result: Aromatic.
• (e) Cycloheptatrienyl cation (Tropylium ion)
  • $$\pi$$ electrons = $$3$$ double bonds $$=6$$ electrons + empty p-orbital $$=0$$ electrons.
  • Total $$\pi$$ electrons = $$6$$.
  • It is cyclic, planar, fully conjugated, and satisfies Hückel's rule.
  • Result: Aromatic.
• (f) Cyclooctatetraene
  • $$\pi$$ electrons = $$4$$ double bonds $$=8$$ electrons.
  • If planar, it would possess $$4n$$ $$\pi$$ electrons and be anti-aromatic.
  • To avoid this instability, it adopts a non-planar tub-shaped conformation, destroying continuous conjugation.
  • Result: Not aromatic.
• (g) Cyclobutadiene
  • $$\pi$$ electrons = $$2$$ double bonds $$=4$$ electrons.
  • It is fully conjugated but contains $$4n$$ $$\pi$$ electrons.
  • Result: Not aromatic.
• (h) Cyclopropenyl cation
  • $$\pi$$ electrons = $$1$$ double bond $$=2$$ electrons + empty p-orbital $$=0$$ electrons.
  • Total $$\pi$$ electrons = $$2$$.
  • It is cyclic, planar, fully conjugated, and satisfies Hückel's rule.
  • Result: Aromatic.

Therefore,

• Aromatic: $$a,\ c,\ d,\ e,\ h$$
• Not aromatic: $$b,\ f,\ g$$

Hence, the correct option is (D).