Which of the following compounds is not aromatic?

We need to identify which compound among the given options is not aromatic. A compound is aromatic if it is cyclic, planar, fully conjugated (every atom in the ring has a p-orbital), and has $$4n + 2$$ pi electrons, where $$n = 0, 1, 2, 3, \ldots$$.

In benzene there are six pi electrons. Applying Huckel’s rule gives:

$$4n + 2 = 6 \Rightarrow n = 1$$

Benzene is cyclic, planar, and fully conjugated, so it is aromatic.

The cyclopentadienyl anion ($$C_5H_5^-$$) has six pi electrons: four from the two double bonds and two from the lone pair on the carbanion. Huckel’s rule gives:

$$4n + 2 = 6 \Rightarrow n = 1$$

It is cyclic, planar, and fully conjugated, so it is aromatic.

Cyclooctatetraene ($$C_8H_8$$) has eight pi electrons from the four double bonds in the eight-membered ring. Huckel’s rule gives:

$$4n + 2 = 8 \Rightarrow n = 1.5$$

Because this is not an integer and $$8 = 4 \times 2$$ indicates 4n electrons (antiaromatic if planar), COT adopts a non-planar tub-shaped conformation to avoid antiaromaticity and is therefore non-aromatic.

The tropylium cation ($$C_7H_7^+$$) has seven carbons in a ring with an empty p-orbital on the carbocation and six pi electrons. Huckel’s rule gives:

$$4n + 2 = 6 \Rightarrow n = 1$$

It is cyclic, planar, and fully conjugated, so it is aromatic.

Hence, the correct answer is Option C: Cyclooctatetraene.